Low Vision Notes Flashcards
Definition of low vision
Any chronic visual condition not correctable by glasses, CL or medical intervention that impairs everyday function.
What are the 3 categories of sight?
Normal vision, low vision and blindness
What are the uses of LVA?
To maintain independence and make best use of remaining vision
Types of LVA (optical)
Magnifiers and telescopes
Types of LVA (NON-OPTICAL)
-Electronic vision enhancement software
-Apps and software
-lighting
-Tints
-large print
-talking books
-environmental design features
-sensory substitution (braille, echolocation, white canes)
-guide dogs
Who are LVA assessment (optical) carries out by?
Optometrists and dispensing opticians within the HES
In private practices for HES and health boards (local schemes)
Private LVA Assessments
Who are LVA assessments (non-optical) carried out by?
Social services or charities
Who is able to certify who is blind or partially sighted (severely sight impaired Vs sight impaired)?
Severely sight impaired (blind):
VA BELOW 3/60
VA BETWEEN 6/60 AND 3/60 (WITH CONSTRICTED FIELD OF VISION
VA BETTER THAN 6/60 (CONTRACTED FIELD OF VISION ESPECIALLY IF IN LOWER FIELD)
Partially sighted (sight impaired):
VA of 3/60 to 6/60
Upto 6/24 with moderate contraction of visual field, opacities in media or aphakia
6/18 or even better if there is gross field defect e.g. heminopia of glaucoma
What are other definitions of visual impairment?
Functionally blind (CANT see to read/write with use of LVA and CANT move in unfamiliar surroundings without use of GUIDE DOG/CANE)
Functionally sighted (CAN read/write with use of LVA and CAN move in unfamiliar surroundings without Guide dog/Cane)
Functionally sighted with aided mobility ( CAN read/write with use of LVA but CANT move in unfamiliar surroundings without Guide Dog/Cane)
Functionally sighted without sighted literacy (CAN move around in unfamiliar surroundings without Guide Dog/Cane but CANT Read/Write even with use of LVA)
Definitions of Disorder, Impairment, Disability and Handicap, and difference between them?
Disorder- Deviation from ‘normal’ of any bodily structure
Impairment- An interference with a bodily function (e.g. VA or VF’s)
Disability- Lack, loss or reduction of an individual’s ability to perform certain tasks
Handicap- an individual’s perceived or actual disadvantage with respect to the expectations of the society in which they live and limits choice or independence
A disorder may cause impairment and the treatment of the disorder aims to prevent impairment. Disorder & Impairment are judged from a medical viewpoint while disability & handicap are social concepts. The aim of proving a patient with an LVA is to overcome impairment & prevent it causing disability.
Determination of degree of disability or handicap must take into account the requirements and expectations of the individual.
Disorder = ARMD
IMPAIRMENT = Reduced distance VA
DISABILITY= Loss of independence
Handicap = Inability to drive
Scotland Registration Process & Forms
Certification: Patient is referred to ophthalmology department by GP, optometrist (via GP) or social worker (via GP) and CVI is completed by a consultant ophthalmologist.
LVL (low vision leaflet): Issued to patient by optometrist (or supplied by social service departments) , self-referral form, access to social services
CVI Form (Certificate of Vision Impairment)
Certifies patient as blind, completed by consultant ophthalmologist
-Copies are sent to GP, local social services department + Information to census office for research
-social worker reaches out to discuss if wish to be registered or need community care assessment
-patient can then receive benefits of registration
Access to help is not dependent on registration
Paediatric CVI Form
This is to be completed in addition to the standard form and contains details about the disorder(s) resulting in visual impairment of the paediatric patient.
-Children registered blind after 4 years unless obviously no sight
Why should a patient get registered?
To determine funding & resource allocation it is important that accurate statistical information about the number of blind & PS patients is available nationally & for each local authority area.
Estimates suggest that up to 50% of patients who are eligible are not registered.
For the patient the main benefit of registration is that it allows access to services and benefits,
for example:
Financial help (extra benefits and concessions e.g. income tax relief & VAT exemptions)
50% off TV license for blind, not PS!
Free BT-directory enquiries service
Free GOS sight test (England)
Transport (free in some areas, disabled car badge)
RNIB- talking books & many other very useful services
British wireless for the Blind Fund- radios
Registration – Disadvantages and problems with the process
Loss of hope & self-esteem as many patients (and some professionals) see this as the ‘end of the road’, when nothing else can be done.
Problems
Health care & other professionals may not tell patients about registration and it is often done as a last resort by ophthalmologist
Poor communication between professionals involved- ophthalmologist, social workers, optometrists etc.
Long waiting times
Not enough social workers - may not be trained in dealing with LV patients
Budget cuts & lack of funding
What to consider before starting a LVA assessment?
Here are some points to consider before you start:
LVA assessments are time consuming – set aside sufficient time, especially for history (checklist/questionnaire)
What does the patient want you to do (and is this possible)? Px expectations may be too high or very low
Px may be distressed, disillusioned & tire easily
Poor motivation
The patient must agree with what you want them to do - they make the final choices!
What to consider during LVA assessment?
The Adjust your speed to the patient (e.g. Elderly, children) and avoid unnecessary tests/procedures
Keep talking to the patient. Don’t use gestures which they might not see!
Get all their current/past spectacles and magnifiers if possible
Encourage carers/family to be present unless patient objects
Px may have other disabilities e.g be prepared to cope with hearing loss as well
General observation of the patient from the moment you meet them or when you collect them from the waiting room can provide additional information. Here are some points to consider:
Are they bothered by bright light?
Any physical infirmities, which restrict range of activities they need to undertake and limit
their visual requirements, but also restrict the ability to handle LVAs.
Do they look straight at you when talking, or use eccentric viewing?
Can they navigate independently, or need to be guided? If guidance is required offer your
arm and use the correct sighted guide technique.
How to correctly guide a VIP?
– VIP follows half a pace behind
– VIP takes guide’s arm slightly above elbow
– Change to holding wrist if space narrow (or guiding child)
– Crowds: in single file Steps
– keep up a running commentary
– say if they go up or down Doors
– VIP on same side as hinge
– Guide opens door and “hands it” to VIP Chairs
– approach from behind and put VIPs hand in centre of back-rest, or
– approach from in front, and put their hand on arm-rest, or seat
– VIP stands with back of knees touching edge of seat before sitting
– Guide should say if there are any obstructions round the chair
History & Symptoms
The case history is a very important part of the LVA assessment. You must find out what the patient wants & what they require. This may not be the same thing!
Visual Status
You need to find out as much as you can about the patient’s lifestyle, their daily living requirements and specific hobbies and determine the impact of the visual impairment.
Some practitioners advocate the use of questionnaires (Figure 2) (Wolffsohn and Cochrane 2000).
Ask specific questions related to;
Distance and Near Vision (ability to read newspaper, watch TV etc)
Mobility
Everyday tasks (e.g. kitchen)
Hobbies or interests
Different lighting conditions
Examples:
How did they get there: public transport?
Can they see the bus numbers, departure boards etc.? Can they cross the road safely?
Does bright sunlight bother them?
Can they see the TV?
How close do they sit? Could they sit closer?
Do they read?
What? Books, newspapers or just bills etc?
What size print can they see? Small print or headlines? How long can they read for? What sort of lighting?
Where is the light positioned?
Can they still write?
Do they use a computer/tablet the internet?
Do they manage about the house?
cooker dials? food on a plate? labels on cans etc?
Do they get home help, Social Service help?
Do they have a pastime; knitting/sewing, gardening, music?
Often patients will tell you that they can’t see/read anything. What they actually mean is that they don’t enjoy reading, because it is difficult. It is important to find out what size print they would be able to manage (with effort), as this information is useful when calculating their magnification requirements!!!
Ocular & General Health History
This is similar to a routine eye test and you usually go through this before examining the patient. However, often patients don’t remember any specific details about procedures/treatments they have received.
If the information that the patient provides seems inconsistent you may interrupt politely and come back to the question during or after the eye exam. In some cases the amount of useful information that you’ll be able to extract may be limited e.g. if the patient suffers from dementia.
Try and establish the following:
Onset of impairment, duration and circumstances
Stability of vision, difference between the eyes
Ocular condition if known
Cause, duration & onset (sudden onset patient may be too upset for assessment)
Stability & prognosis (deteriorating, improving or stable)
Current, past & future treatment
General health and medication
Physical infirmities (e.g. Arthritis or Parkinson’s - can’t hold hand magnifier.)
Registration of vision impairment
Previous & Current Low Vision Aids
It is important to find out as much as you can about current and previously used LVAs. For example, if a patient used to be able to read newsprint with a particular magnifier and now they can only manage headlines it is likely that their eye condition has deteriorated. Ask questions regarding:
Previous LVA assessments and previous LVAs used?
Important: Have they previously tried LVAs that they didn’t find useful and Why?
Current LVAs & spectacles used?
Record VAs and details of currently used LVAs (e.g. power /magnification, how old?, condition etc. )
Occupation & Education
For patients of working age this is relevant as it will help you establish target VAs, required working distances and magnification needs.
Patients Motivation & Expectations
Patient motivation has a major effect on the outcome of an LVA assessment. If a patient is positive and willing to try new things and accepts the limitations of LVAs, they are much more likely to succeed using them. Establish the following:
What does the patient hope you can do for them?
Is their expectation realistic?
If their expectation is unrealistic try to let them down gently, but do not promise things
you can’t deliver!!
Social Circumstances & Family
You may sometimes need to initiate a referral to social services/voluntary organisations:
Live alone?
Family & friends?
Attitude of patient (angry, denial, confident, unconfident, depressed or accepting)
Attitude of family (caring & supportive, over-helpful or uncaring)
Requirements & Priorities
It is important to prioritise. It is unlikely that you’ll be able to offer a solution for all of the patient’s visual problems.
Start and find out what task is most important to the patient?
Often, different LVAs will be required for different tasks and the Px may end up with several LVAs. Patients often have to accept that not all tasks can still be done (e.g. driving although some states in the USA still allow driving with a distance telescope!).
Sometimes a non-optical aid may be more helpful. Consider the patient’s individual circumstances, listen to the Px and ‘think outside the box’.
Psychological Aspects
As mentioned above MOTIVATION is very important! “What do you want me to do for you?” Patients must be ready and willing to accept help.
Beware of those who don’t want help because they feel that they benefit from their disability e.g. make the family feel guilty or like to have the grandchildren around to read letters to them etc.
Psychological Adjustment to Vision Loss
It is generally accepted that the loss of ability (loss of sensory [e.g Vision], motor [ e.g. loss of a limb], intellectual or reproductive functioning) is comparable to a form of bereavement with a similar response sequence. The adaptation to the loss has been extensively described (Kubler-Ross 1975) and is a gradual process involving a series of 5 stages (loss model).
The psychological adjustment to loss involves perceptual, behavioural, cognitive and emotional adjustment.
Loss Model
The Loss Model is probably most appropriate to sudden visual loss. The five stages are:
1) Denial & shock
2) Grief
3) Anger
4) Depression & apathy
5) Acceptance
Shock
Sudden loss, eg wet AMD or stroke
Disbelief,
Patient doesn’t appear to comprehend and doesn’t appear to be listening
Denial
Not the same as disbelief
Refuses to admit that they have a problem (good example is driving with reduced VA
against the advice of eye care professionals); Most of the public think that ‘blind’ =
complete darkness
May refuse LVA’s (“I just need stronger glasses”)
Anger
May feel that there is lack of or improper treatment
Px may stop taking prescribed medication
Depression & apathy
Hopeless situation, Going to get worse
Nothing can be done and LVA assessment is a waste of time
May need counselling or psychotherapy
Realistic acceptance
Final stage
Understands and accepts the condition
Makes effective use of remaining VA
Uses LVA’s, eccentric viewing, white stick etc.
Happy to use aids in public
Psychological Adjustment to Vision Loss-treatment
Treatment
Prevent loss of competence- start rehabilitation as soon as possible
Goal is to get them performing old tasks at a very early stage
Simple techniques with positive feedback
Tasks which show rapid progress
Attribute failure to external influence and success to their own efforts
NB. Environmental factors can affect the process of adjustment to the disability e.g.
Death of partner, relative or close friend
Patient is carer for someone else
Financial problems
Reaction of relatives eg over helpful or don’t care
Measuring Visual Acuity
Measuring Visual Acuity
1. Measure current distance best corrected VA (BVA) & Near BVA with aids & specs. It is often not useful to measure vision (abnormal relationship between vision and VA).
use current correction if apparently correct
beware of inappropriate balance lenses
give the patient impression they are doing well – cooperation is key!
if VA unknown start at a close distance (0.5 or 1m) and move back if necessary
record test distance e.g. 1/36 ;
DO NOT USE counting fingers (CF) – move chart closer instead
VA notations if <0.5/60: HM, LProj, LP, NLP
increase or decrease illumination if necessary
e.g. rod monochromat-lights down, media opacities – may need additional
lighting recording:
monocular & binocular VA as usual
type of chart and test distance e.g. Snellen 1/36
light level
head turn/eccentric viewing etc. what
Refract if needed
Objective – important because subjective often very difficult/variable.
Subjective
Think about ‘Just Noticeable Difference’, DO USE LARGE STEPS!!!!!
Sph: ± 0.5DS, ±1.00DS. ±3.00DS
X-cyl: ± 0.50 or ±1.00
DO USE bracketing techniques
Direct the patient to single letters or lines and use comparisons e.g.: “Clearer with or without?”
DO NOT ask open ended questions like: “How is it with this lens?” or “What can you read with this lens?”
Reduce testing distance if required (adjust Rx for distance!)
Px should be able to see about 4 lines of the test chart
- Establish distance magnification requirement
Identify the visual task & estimate the VA that is required for the task (target VA [TVA])
Calculate magnification requirement based on BVA and TVA
Check if px achieves this with LVA
Often more important for younger patients or children (ability to see board etc.)
- Near magnification requirement
Identify the visual task & estimate TVA as above
Measure BVA (usually at 25cm with the appropriate near add of +4.0 DS) & estimate the
magnification required to achieve TVA
Assess central fields- if a defect is suspected (Amsler) before prescribing magnifiers; may
need to consider eccentric fixation
Try predicted magnification – check VA with magnifier; modify magnification as required to
achieve target acuity
start with simple magnifiers: high add, hand or stand magnifier
Illumination
Prescribe the LVA with clear instructions!!
Other tests
Establish binocularity (e.g. Cover test) and eye dominancy (many LVAs are monocular)
Contrast sensitivity if indicated
Fields (Amsler may be most useful in LVA assessment)
Ophthalmoscopy/fundus photography, OCT, IOPs etc Spend your time wisely and do not do unnecessary tests
Advice & follow up
Important- patients often forget what the aid was intended for or how to use it
Ideally review patient in 2-4 weeks
may need to change LVAs
Communication
Speak slowly & clearly
o May have hearing loss - speak to best ear
Be encouraging & positive
Don’t rush the patient, but stay in control
Snellen Chart
Advantages
Well known, commonly used standard clinical measure of VA & vision
Very sensitive to blur and uncorrected refractive error Good size (portability)
Easy to move closer to patient Has letters ‘O’ suitable for x-cyl
Snellen chart disadvantages
Unequal numbers of letters on each line (crowding not constant)
No relationship between line size & number of letters - from 1 to 8
Poor control of ‘contour interaction’
Unequal letter size progression- 1.2X increase from 6/5 to 6/6, 1.67X
increase from 6/36 to 6/60
Designed to measure ‘normal’ acuity
Scale intervals change at non-standard distances (crowding inconsistent)
Using Snellen charts at a reduced viewing distance:
Example: A 6 metre letter letter viewed at a distance of 6 m (6/6), has a minimum angle of resolution (MAR) of 1arcmin (gap size or limb width; MAR =1as 6/6=1’).
A 24 metre letter is 4 times larger so MAR=4’ if viewed at 6m (24/6 = 4’)
If a Px with VA 6/24 views the chart at 3m they should be able to see the 12m letter (written as VA of 3/12) because this subtends the same visual angle as the 24m letter viewed at 6 m (MAR 12/3=4’).
Bailey-Lovie (logMAR) chart (Bailey and Lovie 1976)
Advantages
Size, letter and line spacing equivalent throughout chart
Equal numbers of letters per row (5)
Sensible progression of letter size (0.1 log or 1.25X)
All letters have equal legibility
Constant crowding for all VA levels
Final score takes into account all letters that have been read successfully
Bailey-Lovie (logMAR) chart (Bailey and Lovie 1976) Disadvantages
LogMAR not used routinely as a clinical measure of VA
Scoring and conversion not as easy as Snellen
there’s no O letter for X-cyl
A bit on the big side and may be hard to illuminate
Using logMAR charts at a reduced viewing distance:
Each letter = 0.02 log (5 letters per line)
Rule of thumb:
Every time the viewing distance is halved add 0.3 to the logMAR -score.
Example (logMAR chart designed for 6 m)
Px views chart at 6 m
Reads the 0.8 line + 2 letters on the line below
logMAR VA is 0.8 - 0.04 = 0.76
Px views chart at 3 m
Px now reads 0.5 line + 2 letters on the line below
logMAR VA is 0.5 - 0.04 = 0.46 + correction factor of 0.3 for
moving from 6 to 3 metres 0.46 + 0.3 = 0.76
Near Vision Charts
The Faculty of Ophthalmologists Times New Roman near chart
N-Point Notation
Standard reading chart widely used
Based on printing standards
1 point is 1/72”
Top of ascending to bottom of descending limb of letter
Times Roman type face
Easy to understand and convert e.g N24 = twice size of N12
Blocks of text
Bailey-Lovie Word-Reading chart
LogMAR chart
2to6wordsonaline
Unrelated words, no guessing
Limited number of words on line - good test of
reading
Difficult for children, those with poor English or poor cognition
Prescribing optical LVAS Distance Formula:
E.g. telescopes
BVA … Threshold (Best corrected)
VA TA … target acuity
M… required magnification
M = BVA/TA
Example:
Px can see 6/60, needs to see 6/12
M = BVA / TA = 60/12 = 5
5 x magnification required
Prescribing Optical LVAs - Near: Formula
Use N – point notation and test patient’s acuity at a distance of 25cm (+4.00 D Near Add if required)
M = BVA / TA
BVA … Threshold (Best corrected) VA
TA … target acuity
M… required magnification
Example=
Px can see N24 at 25 cm with the appropriate add of + 4.0 D?…needs to see N12
M = BVA / TA = 24 / 12 = 2
2 x magnification required
Available LVAs:
Magnifiers
High add
Spectacle
Hand held
Stand magnifier
Electronic LVAs
Choice of Magnifier
Keep it simple!
Start with low magnification if possible
Illumination!
Consider
Task & age of patient
Physical infirmities e.g. can the patient hold the magnifier/task steady?
Cost
What is magnification and when needed?
Will be needed if the prescription does not create enough improvement and if VA is insufficient for identified tasks
Need to make retinal image size greater hence easier to see
How can magnification be achieved?
- Increase size of object
- Decrease viewing distance (move closer)
- Real image or transverse magnification (E.g. zoom camera)
- Telescopic magnification
Increasing size of object formula?
M = new object size / old object size
Examples: Large screen TV, Large print (books, newspapers, telephone dials, medicine labels, bills, computers and mag software, labelling machines)
Decreasing the viewing distance formula:
(M) magnification = old viewing distance / new viewing distance
- very simple and effective
works at distance or near if sufficient accommodation – hence used frequently by
children and young adults with good accommodation; myopes can remove their
spectacles
Using a short viewing distance without the necessity for accommodation requires the
use of a plus lens (magnifier).
Examples:
- Distance: viewing TV from 6 feet rather than 12 feet gives 12/6 = 2x magnification
- Near: bring print to 5cm rather than 30cm gives 30/5=6x magnification
Real image/transverse magnification (camera zoom)
(M) magnification = size of image/size of the original object
Not an optical method – see electronic LVAs
Advantages include the ability to adjust the magnification factor and manipulate contrast and colour
4.Telescopic magnification (angular magnification)
magnification = angle subtended by telescope image (0’) / angle subtended by original object (0)
- theta angle on the retina?
- Optical method
- Versatile (used at distance, intermediate and near)
Different methods can be combined and the total magnification is found by multiplication.
Example:
What is the magnification obtained by changing print size from 6mm to 15mm, and viewing distance from 30cm to 10cm?
M1 = new object size / old object size = 15 / 6 = 2.5x
M2 = old viewing distance / new viewing distance = 30/10 =3x
The total magnification Mt=M1 x M2 =2.5 x 3 = 7.5x
Effects on field of view, working space and distance of task (N, Intermediate or Dis) for diff types of magnification?
Increase size = no change F.O.V, Working space = no change, Distance of Task = Nr (Int or Dis limited)
Decrease distance = no change F.O.V, Working space = decreased , Distance of Task = Nr, Int or Dis
Using a plus lens = Increase size = F.O.V decreased , Working space = decreased , Distance of Task = Nr
real image (camera) = F.O.V decreased , Working space = no change, Distance of Task = Nr (Int or Dis limited)
Telescopic = F.O.V decreased, Working space = no change, Distance of Task = Nr, Int or Dis
Calculating magnification for Plus lenses
Magnification (M) is relative. Let’s compare two lenses F2 and F1 of different powers, placed at the eye’s nodal point (Figure 8).
- Lens F1 “standard” lens is taken to be +4.00 D (f=0.25 m, “standard” viewing distance if
the object is placed at the focal point of a plus lens) - Lens F2 (magnifier) whose magnification is unknown
- Lens F2 has a magnification of M= F/4 relative to lens F1
-If a patient does not achieve the magnification stated on a magnifier their current reading specs may not have
a +4.00 add (viewing distance 0.25 m) and the baseline acuity may therefore be incorrect. Æ measure vision at
25cm/with +4.00 add to determine a correct baseline acuity.
Image at infinity – no near add or accommodation required as parallel light leaves the
lens and enters the eye
Eye to magnifier distance has no effect on magnification
Called “Nominal magnification”
If the patient does not place the reading material at the focal point of the lens we could end up with two situations: (magnification)
- General case:
M = qL / 1- dL’
q … ‘nearpoint’ or ‘least distance of distinct vision’, usually assumed to be 0.25m
d …distance lens to eye
L… incident vergence
L’…emergent vergence
Note: M depends on the distance of the lens from the eye and the distance of the object from the lens
- Image at near point
M = ( F / 4 ) + 1
Object between F and Lens
Image produced by magnifier at near point (e.g. q=25 cm) -> + 4 m-1 add or accommodation required
Magnifier held close to Px’s eye (d=0) see Figure 12
called “Maximum magnification” or “Isoaccommodative magnification”
The field of view of plus lens magnifiers
Reading speed is greatly affected by the extent of text seen through a magnifier and hence it is
desirable for LVAs to provide as large a field of view (FoV) as possible. The usable FoV mainly
depends on three factors: lens power, lens size and aberrations.
Field of view = Diameter of magnifier / (d x Power of magnifier)
Where d is the magnifier-to-eye-distance and D is the diameter of the magnifier.
Example: How does the field of view change when a patient holds a round 90 mm +6 DS magnifier 25 cm from his eye and then brings it up to a distance of 6 cm from his eye?
D= 90 mm; F=+6 m-1, d1=25 cm; d2=6 cm
FoV1 = 0.09/(0.25*6) = 60 mm
M=F/4 = 1.5x
FoV2 = 0.09/(0.06*6) = 250 mm
It can be seen that the magnifier must be held at a close distance to the eye to maximise the field
of view.
Example: How does the field of view change when a patient holds a round 90 mm +6 DS magnifier 25 cm from his eye and then brings it up to a distance of 6 cm from his eye?
D= 90 mm; F=+6 m-1, d1=25 cm; d2=6 cm
FoV1 = 0.09/(0.25*6) = 60 mm
M=F/4 = 1.5x
FoV2 = 0.09/(0.06*6) = 250 mm
why do magnifiers have to be held close to eye?
To maximise the field of view
Equivalent Power of Hand Held Magnifiers ( + near add e.g. bifocals)
The power of a thick lens or a lens system consisting of two thin lenses can be calculated as:
Feq = F1 + F2 - ( d x F1 x F2)
Feq = power of thick lens or system
F1 = power of 1st surface or lens
F2 = power of 2nd surface or lens
d = distance between lenses (surfaces)
e.g. the patient wears add +2.50 DS and holds a +10 DS magnifier 25 cm from their bifocal
(or reading specs). (With nominal magnification: M = F/4 = 10/4=2.5X magnification)
FMag= +10 m-1
FAdd = +2.5 m-1
z =25 cm (this is d) = 0.25m
Hence: FEQ = 10 + 2.5 –(0.25m *10m-1 *2.5 m-1)
FEQ = +7.5 m-1
So the magnification this patient achieves is: MEQ = FEQ / 4 = 7.5 / 4 = 1.9 X
Conclusion:
When a magnifier is used together with a near add and the magnifier to eye distance is:
1. less than the magnifier’s focal length the equivalent magnification will be higher than the
magnifiers nominal magnification
2. greater than the magnifier’s focal length the equivalent magnification will be lower than the
magnifiers nominal magnification
3. equal to the magnifier’s focal length the equivalent magnification will be equal to the
magnifiers nominal magnification
Key Point: Using a magnifier with reading Rx requires a short magnifier to eye distance. Long
magnifier to eye distances can result in very low enlargement ratios.
e.g. the patient wears add +2.50 DS and holds a +10 DS magnifier 25 cm from their bifocal
(or reading specs). Whats the total mag?
FEQ = F1 + F2 –d F1 F2
FMag= +10 m-1
FAdd = +2.5 m-1
z =25 cm (this is d) = 0.25m
Hence: FEQ = 10 + 2.5 –(0.25m *10m-1 *2.5 m-1)
FEQ = +7.5 m-1
So the magnification this patient achieves is: MEQ = FEQ / 4 = 7.5 / 4 = 1.9 X
What happens if you calculate the magnification provided by a plus lens using the power established by focimitery
The magnification will be overestimated.
Plus lens magnifiers, what are the 3 types?
- Handheld
- Spectacle mounted
- Stand mounted
Hand-held magnifiers advantages
-Familiar to patients, most common type of magnifier, inexpensive
-easy to prescribe, can be used with Nr or Dis rx
-Compact, lightweight and easy to carry around
-Can have long eye-to-mag distance
-Useful for px with reduced peripheral fields
What are the different types of powers of hand-held magnifiers and what happens as the power increases?
Low powered (4x)
Medium powered (4x-8x)
High Powered (8x+)
As increase power, field of view increases (40mm F.O.V. low powered - 25 mm F.O.V. for high powered)
Disadvantages of hand-held format magnifier
-Can be ineffective is used incorrectly with reading add
-Difficult to maintain correct position for extended periods
-Poor field-of-view if long eye-to-magnifier distance
-increased distortions as lens held further from eye
Patient instructions for hand-held magnifier
-Lay magnifier on page and slowly pull away from page until image clear
-advise on spex to be worn
-best to use distance rx
-Nr rx only for shorter mag-to-eye distance
-Obtain largest FOV by holding magnifier as close to eye as possible
-most curved (steeper) part towards patients eye for least distortion
-Hold magnifier parallel to reading material
-Move head and magnifier as a unit together
-
What are the available formats of spectacle-mounted magnifiers?
-On SV or Bifocals
-Paired lenses with base-in prism in standard frame
-Lenses glazed to special mounting or carrier
-Clip-on lenses
-Other LVAs intended for occupational use
Binocular or monocular use for spectacle mounted magnifiers?
Binocular gives:
-Larger F.O.V
-Greater depth of field
-Better acuity
-Depth perception
-Monocular overlap in case of field loss
-conventional + psychologically better appearance
Contra-indications for binocular magnifiers
-Px has no demonstratable binocularity at distance required for magnifier
- greater than 2x diff in VA between two eyes
-Central distortion in previously dominant worse eye
-Bin VA worse than Mono VA
-too great convergence demand, resulting in diplopia or discomfort (balancing/occlusion may be necessary)
centring a BIN near correction (near centration distance formula)
NCD = (PD x WD) / (WD + a)
-Where NCD is the near centration distance
-WD is the working distance
-a is the distance from the Centre of rotation to the spectacle plane
- Patient’s axial length = 24mm, PD = 30/30, vertex distance = 13mm, Add +3.00, requiring a
working distance of 33 cm
NCD = (PD x WD) / (WD + a)
NCD = (2 x 30 x330) /
(330 + 13 + (24/2))
NCD = 57mm
2.Patient’s axial length = 24mm, PD = 30/30, vertex distance = 13mm, Add +20.00, requiring a
working distance of 5 cm
NCD = (PD x WD) / (WD + a)
NCD = (2 x 30 x 50) /
(50 + 13 + (24/2))
NCD = 40mm
- Patient’s axial length = 24mm, PD = 30/30, vertex distance = 13mm, Add +3.00, requiring a
working distance of 33 cm.
What’s the patients Near centration distance?
- Patient’s axial length = 24mm, PD = 30/30, vertex distance = 13mm, Add +3.00, requiring a
working distance of 33 cm
NCD = (PD x WD) / (WD + a)
NCD = (2 x 30 x330) /
(330 + 13 + (24/2))
NCD = 57mm
2.Patient’s axial length = 24mm, PD = 30/30, vertex distance = 13mm, Add +20.00, requiring a
working distance of 5 cm
What’s the patients Near centration distance?
NCD = (PD x WD) / (WD + a)
NCD = (2 x 30 x 50) /
(50 + 13 + (24/2))
NCD = 40mm
What are one of the “rules of thumb” that create base-in prismatic effect to reduce convergence demand?
For “normal” working distances NCD approx. 2-5mm less than distance PD (e.g. inset of bifocal segment is usually 2-2.5mm for each eye). For the very short working distances required for magnifiers, lenses need to be decentred by a much larger amount.
Remember to decentre the trial frame when testing patients with a high binocular add!
Even if the eyes look through the optical centre, the convergence requirement is considerable
(eg: approximately1’ base out for each mm of PD to view at 10 cm), which is why the patient usually requires additional base in prism.
There are various clinical “Rules of Thumb” to determine the NCD for example:
- Fonda’s recommendation
Total decentration for near = 2 x WD (D)
For the example on the previous page this would give 2*5 =10 mm
NCD = PD – Decentration = 60 -10mm = 50mm
Advantages and disadvantages of Spec-mounted magnifiers?
Advantages of spec-mounted magnifiers:
- hands free, useful for prolonged reading
- widest field-of-view (because of short eye-to-magnifier distance)
- cosmesis - similar appearance to “normal” spectacles
- can incorporate astigmatic correction
- maybe used for patients with hand tremors –e.g. with a reading stand
Disadvantage’s of spec-mounted magnifiers
- close working distance
- illumination may be difficult
- fatigue , HA’s, dizziness
- reduced reading speed generally occurs with very close WD
- limited magnification binocularly
Because of the problems (convergence demand and viewing distance) associated with high near
adds, binocular adds are up to which mag?
Because of the problems (convergence demand and viewing distance) associated with high near
adds, binocular adds are:
easy up to +6.0
tricky up to +8.0
risky up to +10.0
highly unlikely to be successful if +12.0 or greater
What’s the formula for calculating the depth of field of plus lens magnifiers
Change in l (triangle l) =
( - triangle L’) /
(4xM)^2
e.g. Calculate depth of field i.e. How much can the reading material be moved before the patient notices defocus if the patient’s
depth of focus is 0.8 dioptres (+- 0.40 DS) and a simple magnifier of + 5 DS is used?
Triangle l = ( - triangle L’) /
(4xM)^2
(M = F/4 = 5/4 = 1.25)
Triangle l = 0.8 / (16 x 1.25^2) = 0.032 = 32mm
Key point- Magnifiers have to be held very steadily to achieve fluent reading.
e.g. Calculate depth of field i.e. How much can the reading material be moved before the patient notices defocus if the patient’s
depth of focus is 0.8 dioptres (+- 0.40 DS) and a simple magnifier of + 5 DS is used?
Triangle l = ( - triangle L’) /
(4xM)^2
(M = F/4 = 5/4 = 1.25)
Triangle l = 0.8 / (16 x 1.25^2) = 0.032 = 32mm
What are the 2 types of stand magnifiers ?
- Fixed Focus
- Variable focus
What are key points to consider about stand magnifiers?
Key Point: When prescribing or dispensing stand magnifiers ensure that the patient has adequate
accommodation or suitable reading glasses.
The higher the near add + the closer the eye-to-mag distance = the higher the mag achieved will be
How can the emergent vergence of a stand magnifier be determined (in practice without
calculation)?
- View object through magnifier and find the maximum plus lens which when held up against the magnifier still gives a clear view
- As in 1.) Use a telescope to eliminate the effect of accommodation
- As in 1.) but reverse light path and focus a distant object on the object plane of the magnifier
Advantages of stand magnifiers:
– Good for patients with hand tremors
– Most are small and portable
– Relatively inexpensive
Disadvantages of stand magnifiers:
– Limited FoV
– Accommodation or add required
– Difficult to determine equivalent power
– Posture & fatigue problems
– Illumination can be problematic unless
built-in light source
what to remember about Flat-field and bar magnifiers
- An interesting feature of the design is that it is self-illuminating.
- The thicker a flat-field magnifier is (in relation to r), the greater the magnification (usually < 3X)
- Image is formed very close to original object so there is no “relative distance
magnification” - Patient must wear reading correction or accommodate
- Can have bar magnifier with magnification in only one direction (Plus cylinder lens)
Advantages of Flat-field and bar magnifiers
- Normal reading posture possible
- Very bright - light gathering
- Useful for patients with hand tremor etc.
- Clear image across the whole lens – aberrations have minimal effect
- Combination of bar and spectacle mounted magnifier can increase the
magnification while maintaining a long viewing distance
Disadvantages of Flat-field and bar magnifiers
- Large lenses - heavy
- Magnification typically low
- Reading material has to be flat and on firm surface
What distance can telescopic magnifiers be used ? + what forms can they be used in?
Telescopic magnification is an optical method of magnification, which is more versatile than a
plus lens because the magnification can be produced at any viewing distance:
- Distance (street signs, bus numbers, blackboard, theatre)
- Intermediate (playing cards, music, TV, computer)
- Near (reading, writing)
Telescopes can be hand-held (for spotting) or spectacle mounted, but it is rarely possible to wear
them constantly or whilst mobile.
General Formulae for telescopic magnification
M = Fe / Fo
t = f ‘E + f ‘ o
2 types of telescopes
Astronomical (Keplerian) telescope :
- Fe and Fo positive
- M negative, image inverted,
- prism/mirror erecting system required
- both f’e & f’o positive so telescope long
- prism allows folding of light path to reduce length
Optical System of Galilean telescope in normal adjustment:
- FE negative, FO positive
- M positive so image erect – (no erecting system required)
- one focal length negative - short system length
Telescopic comparison
Galilean: Lighter, shorter length, exit pupil inside, image erect, mag low (up to 4 x), FOV smaller, image quality poorer + cost lower
Keplerian - Heavier, longer, exit pupil outside, image inverted, mag upto 10x + ,
What is the size and location of the exit pupil for a 3x24 - Galilean telescope with an objective
lens of +10m-1?
(3x24 = The first number gives the magnification and the second number refers to the diameter of the
objective lens in millimetres)
M = Fe / Fo
Fe = - 30 m^-1
1/l’ = 1/l + F
=(1/-0.0667m ) - 30 m ^-1
telescopes are labelled as follows: 8 X 40, meaning?
The first number gives the magnification and the second number refers to the diameter of the
objective lens in millimetres.
- Which of the following telescopes has the largest exit pupil?
a) 7 x 50
b) 8 x 35
c) 10 x 20
a) 7x50 50/7 = 7.14 mm
b) 8x35 35/8 = 4.38 mm
c) 10x20 20/10 = 2.00 mm
Answer: a followed by b and c.
- A patient has an 8 mm pupil and looks through the following telescopes. Which will appear the
brightest?
a) 7x50
b) 8x35
c) 10x20
d) 10x40
Answer: a first, followed by b, d then c
a) 7x50 50/7 = 7.14 mm
b) 8x35 35/8 = 4.38 mm
c) 10x20 20/10 = 2.00 mm
d) 10x40 40/10 = 4mm
Question: Draw a ray diagram of a 3x24 - Galilean telescope with an objective lens of +10m-1.
Find the size and position of the exit pupil using construction rays
Factors affecting a telescope’s field of view
- Magnification - use minimum power
- vertex distance: 5mm = closest distance without specs + 20mm = likely with specs and rubber eyecup
- object distance: not practical to change this
-objective diameter: telescope becomes heavier and more difficult to handle
To optimise the field we need to match size of exit pupil to the patient’s pupil, but if the alignment slips part of the field will go dark. It is therefore actually better if exit pupil > patient’s pupil, which allows some misalignment, but obviously means some loss of field.
Another point to consider is that the calculations of the field of view do not accurately reflect the actual patient experience, because the patient’s pupil is the limiting aperture rather than the telescope’s objective lens. The effect of aberrations is also not accounted for by FoV
calculations based on Gaussian optics.
How much accommodation is required when a patient uses a 3x telescope for reading at 25 cm?
required accommodation= ~ L’
L’ = M^2 x L = (3)^2 x 4 = 36 m-1
Conclusion: Even young patients viewing objects at intermediate distances do not have
sufficient accommodation and need to modify the telescope
Adapting telescopes for near and intermediate viewing (3)
- full correction for viewing distance over objective (Fo)
- increased correction for viewing distance over eyepiece (Fe)
- increasing the separation of Fe and Fo
what is equation for the total magnification of a telemicroscope taking into account the magnification reading cap (plus lens) and the magnification of the telescope?
Mtotal = Mtel x Mrc
Mrc = Frc (power of plus lens) / 4
1.What magnification is achieved when using a telemicroscope, which consists of a 3x
telescope with a +4.00 DS reading cap? What is the working space?
MTOTAL = 3 x 4/4 = 3x
working space: fRC = 1/4m^-1= 25cm
What magnification is achieved when using a telemicroscope, which consists of a 3x
telescope with a +4.00 DS reading cap? What is the working space?
Mtotal = Mtel x Mrc
Mrc = Frc (power of plus lens) / 4
1.What magnification is achieved when using a telemicroscope, which consists of a 3x
telescope with a +4.00 DS reading cap? What is the working space?
MTOTAL = 3 x 4/4 = 3x
working space: fRC = 1/4m^-1= 25cm
2.What magnification is achieved when using a telemicroscope, which consists of a 2x
telescope with a +10.00 DS reading cap? What is the working space?
Mtotal = Mtel x Mrc
Mrc = Frc (power of plus lens) / 4
MTOTAL = 2 x 10/4 = 5x
working space: fRC = 1/10m^-1 = 10cm
So why use a telemicroscope instead of simple spec-mounted plus lens?
Working space is greater with telemicroscope (25cm versus 8 cm)
Practical considerations using full correction for viewing distance over objective (Fo) telescope
Practical considerations:
even if working space is 2x larger, not useful unless at least 5cm increase
increased working space makes binocular viewing more practical
need to be able to angle telescope tubes appropriately (convergence)
overall working distance increased by length of telescope (minor point)
using Increased correction for viewing distance over eyepiece (Fe) telescope
- Not a practical method unless patient already has high-powered reading add for other purpose
- Magnification difficult to calculate unless all system parameters known
Method 3: Increasing the separation of Fe and Fo (focusing) telescope
- A practical method, but more difficult in spec-mounting = can become “front-heavy” as tube length increases + difficult to angle correctly if binocular
- More common in astronomical telescope
= usually hand-held anyway
= image quality better maintained
What effect does focusing have on the overall magnification of the device and is it better to use
this, or to add reading cap?
Conclusion:
For working distances (greater than 25cm) it is always better to refocus a telescope than to
add a reading cap, as this will provide higher magnification. Practical limitations include the
increase in telescope length. The closest possible focusing distance corresponds to the
LONGEST position of telescope (t=max) so there are practical/mechanical limitations and
sometimes the tubes cannot expand far enough.
Wy use telescopes for near vision?
The major reason for using telescopes for near tasks is the increase in working space compared to magnifiers. (2x telescope increases by factor 2; 3x telescope increases by factor 3 etc.). This is usually needed for manipulative tasks. but: The field of view is very small.
Compensating telescopes for ametropia (3)
3 Methods:
1. full correction for refractive error behind eyepiece (Fe)
2. partial correction for refractive error over objective (Fo)
3. changing the separation of Fe and Fo
- full correction for refractive error behind eyepiece (Fe)
= Simplest method: telescope clipped on to, or held over, spectacle correction
= Or correcting lens can be fitted into holder behind eyepiece if required
= No effect on magnification because telescope still afocal (see Figure 30)
Figure 30 Schematic diagram of a Galilean telescope with Rx behind eyepiece for a hyperopic eye (left); clip-on
Galilean telescope (right) - partial correction for refractive error over objective (Fo)
= Use partial correction to achieve some divergence or convergence of light entering telescope, which would then be amplified by telescope to the required amount
= Complex calculation to work out effect on magnification
= Impractical, and never attempted - changing the separation of Fe and Fo
= shorten to correct myopia and lengthen for hypermetropia
= length change needed obviously depends on Rx
= effect on magnification
What are the magnification and length of a 3x astronomical telescope (Fe = +60, Fo = +20),
used by -10.00 DS myope with correction behind eyepiece
MTEL = -Fe/Fo = -60/20 = -3x (as for emmetropic user because telescope is afocal)
t = fo’ + fe’ = 50 + 16.7 = 66.7mm
Example 2:
What are the magnification and length of a 3x astronomical telescope (Fe = +60, Fo = +20),
used by uncorrected -10 DS myope and focused to compensate for the ametropia?
t = fo’ + fe’ = 1/Fo’ + 1/Fe’
= 1/20m-1 + 1/(60-(-10)m-1)
= 64.3 mm
MTEL = -Fe/Fo = -70/20 = -3.5x
Mfocused > Mnormal
Example 3:
What are the magnification and length of a 3x astronomical (Fe = +60, Fo = +20) telescope, used
by corrected /uncorrected +10 hyperope?
- If hypermetrope is corrected -> MTEL = 3x as for corrected myope, or emmetrope, length as in
normal adjustment - If hypermetrope is uncorrected and telescope focused to correct hyperopia:
t = fo’ + fe’ = 1/Fo’ + 1/Fe’
= 1/20m-1 + 1/(60-(+10)m-1)
= 70 mm
MTEL = -Fe/Fo = -50/20 = -2.5x
Mfocused < Mnormal
What will be the effect of a myope on astronomical telescope vs Galilean telescope?
Astronomical = MTEL higher with focusing telescope
Galilean = MTEL higher with correction behind eyepiece
What will be the effect of a hyperope on astronomical telescope vs Galilean telescope?
Astronomical = MTEL higher with correction behind eyepiece
Galilean = MTEL higher with focusing telescope
Plus lens magnifiers vs Telescope! Example:
If a patient has a depth of focus of 0.5 dioptres a change in emergent vergence (L’ for a plus lens
and L’2 for a telescope) of more than 0.50 DS would result in the perception of blur. What change in incident vergence (L) or object distance (l) can the patient tolerate if he uses a +10 D magnifier or a 2.5x telescope with +4.00 DS reading cap?
- For a plus-lens magnifier (2.5x) of powerFEQ=+10m-1:
if object at FLens then:
l = f = -0.1m -> L = 1/-0.1= -10 m-1; L’ = 0
now let the emergent vergence (L’) be +0.5 m-1 L’=L+F , L1 = +0.5-10 = -9.5 m-1
now let the emergent vergence be -0.5 m-1
L’=L+F , L2 = -0.5-10 = -10.5 m-1
Change in l = (1/-9.5) - (1/-10.5) = 1 cm
- For a 2.5x telescope with +4.00 reading cap
Emergent vergence (L’TEL) = M^2 x incident vergence (LTEL)
LTEL = L’TEL / M^2
working distance = 25cm, vergence at plus lens ->
L= -4.00 m-1, L’ = 0
vergence entering telescope LTEL = 0
calculate LTEL if L’TEL= +/-0.5m-1
LTEL = +/-0.50m^-1 / 2.5^2
=0.08m^-1
Incident vergence at reading cap: L = 1/-0.25m = -4.00m^-1
Change in L = -4.00m-1 +/- 0,08m-1
change in l = (1/(-4+0.08m-1) - (1/(-4-0.08m-1)
= 1cm
Conclusion:
It appears that both systems have the same depth
of field.
However, subjectively patients usually feel the
DoF is smaller for telescopes. One possible
explanation is that the DoF of a telescope
corresponds to a smaller percentage of the
working space.
Types of telescopes: + varients
Generally 3 types: Hand held, Clip on and Spectacle mounted. You could further distinguish
monocular versus binocular telescopes and focusable v fixed focus devices.
There are special variants:
- Autofocus
- Contact lens telescope (with CL or with IOL)
- Intra-ocular lens telescopes (Peli 2002)
- IOL VIP system – combination of negative and positive IOL (Orzalesi, Pierrottet et al. 2007)
- BTLT - Behind the Lens Telescopes (Spitzberg, Jose et al. 1989)
- Bioptic or BITA (Bi-level Telemicroscopic Apparatus)
For patients who fulfil certain Visual Field and Acuity requirements driving with BiOptics is legitimate in some States in the US
Driving with BiOptics is not legal in the UK hence there is limited interest in the technology
Contact lens telescopes
( + fitting)
Galilean system
- Eyepiece created by (high-powered) negative contact lens
- Objective created by (high-powered) positve spectacle lens
- Vertex distance of spectacles is equal to length of telescope (sum of focal lengths)
- Field of view better than with conventional telescope because objective & exit
pupil close to eye, but the FoV also depends on the diameter of the objective lens (use blended aspheric to avoid ring scotoma)
- Rarely used because of the many practical and cosmetic disadvantages
- Fitting of CL telescopes:
- Check that conventional telescope improves acuity as expected
- Check nystagmat for oscillopsia
- Fit contact lens, maximise BVD of trial frame, perform over-refraction
- Contact lens must be stable!!
- Vergence amplification means separate reading specs are required
- Uncorrected ametropia changes the effective eyepiece power
= +10 hypermetrope with -30 contact lens, effective eyepiece is -40
= -10 myope with -30 contact lens, effective eyepiece is -20
Advantages + disadvantages of CL telescopes
Advantages :
- In theory patients would be allowed to drive under UK law if VA and field
requirements were met
- Can get acuity much better than expected in congenital nystagmus
- Intra-ocular lens could be used instead of contact lens giving a longer vertex
distance so better magnification but IOL can’t be changed if acuity worsens
Disadvantages:
- Low magnification,so only works for moderate
acuity loss
- Patient needs to be adapted to contact lens wear first
- CL difficult to insert–poor near acuity with highly negative lens power
- Need to wear system regularly for prolonged
periods - Adaptation to spatial distortion required
- Poor cosmesis, specs with long BVD and high plus lenses
Verification of telescopes: If the parameters of a telescope are unknown, how do you calculate type and magnification ?
If the parameters of a telescope are unknown, its type and magnification can be estimated by
direct observation or by locating and measuring the size of the entrance/exit pupil
Determination of Magnifying Power:
- Direct Comparison
- The magnified view seen through the telescope with can be compared with the non-magnified view seen by fellow eye.
- Use tile pattern etc (something you can count). as target (Figure 39) - Estimating based on Exit pupil measurements
- Measure size of exit pupil e.g. using mm scale
on band magnifier (Figure 40)
- Measure size of objective lens (entrance pupil)
-> M = entrance pupil/exit pupil
Determination of telescope type by exit pupil method:
- In general, Keplerian (astronomical) telescopes are longer than Galilean telescopes, but the use of erecting systems which incorporate mirrors or prisms may be misleading.
The exit pupil movement can be used to distinguish telescope types: Looking into the eyepiece observe the apparent movement of the exit pupil while moving the eyepiece lens from side to side.
- Astronomical gives Against (2 A’s)
- Galilean gives with
Telescopic refraction
When viewing a distant object an uncorrected simple myope will shorten the length of an afocal
telescope, while an uncorrected simple hypermetrope will increase the tube length, such that
the image produced by the telescope will form at the far point hence his/her refraction will
determine the tube length of the telescope.
A calibrated telescope as a simple and inexpensive instrument to determine spherical refractive
errors:
- may be applicable to populations in under-served areas where electricity and
modern equipment are not available.
- alternative subjective refraction method in low vision population Æ patients can be refracted at a ‘normal’ 6m testing distance.
- may be applicable to patients with extreme Rx e.g >25 DS and/or patients where retinoscopy or autorefraction is difficult/inaccurate due to media opacities or extremely small pupils and a starting point for subjective refraction is needed
One could also refract through an afocal telescope using a distant test chart and alter the trial lenses behind the eyepiece e.g. in the back pocket of the trial frame if a patient’s best corrected acuity is low and they cannot be moved closer to the chart.
Prescribing Magnification
Distance tasks
- Measure BVA
- Think about target acuity e.g. Tv about 6/18 or street signs <6/12
- Calc magnification, try predicted Mag, modify
- Decreases viewing distance (e.g. go closer to tv) OR LVAid (monoc / binoc)
- VA expected = trial px with instrucyions and follow up appt OR
VA <expected = Focussing/ alignment? M calc incorrect? contrast sensitivity? Scotoma? cosmesis?
If the patient’s VA (with LVA) does not improve as expected don’t just try a different LVA. Find
out what the issue is and choose your next aid systematically to overcome the problem. It is very frustrating for the patient if you get them to try five different LVAs and they can’t see with any of them.
Focussing and alignment problems (incorrect viewing distance, incorrect Rx) can be checked
easily.
Make sure that your best-corrected baseline VA (BVA) measurement is accurate. While this is
straightforward for a visually normal patient, a number of factors can make the BVA variable for
visually impaired patients.
Examples include:
Factors impacting on VA testing:
- Illumination, for example:
= Rod monochromats – may have better acuity in dim illumination
= Retinitis Pigmentosa – may have better acuity in bright illumination
-Visual field status
= Peripheral field loss may result in inability to see larger targets but good response to
see smaller ones
= Central scotoma may require eccentric viewing, isolation of optotypes (crowding)
- Measurement technique
= Patient & practitioner motivation is important
= Different acuity charts may give different VAs, depending on:
->Contrast, (e.g. projector, computer vs printed charts)
->Crowding (number of optotypes and lines etc)
-> Testing distance
a number of factors can make the BVA variable for
visually impaired patients?
Examples include:
Factors impacting on VA testing:
- Illumination, for example:
= Rod monochromats – may have better acuity in dim illumination
= Retinitis Pigmentosa – may have better acuity in bright illumination
-Visual field status
= Peripheral field loss may result in inability to see larger targets but good response to
see smaller ones
= Central scotoma may require eccentric viewing, isolation of optotypes (crowding)
- Measurement technique
= Patient & practitioner motivation is important
= Different acuity charts may give different VAs, depending on:
->Contrast, (e.g. projector, computer vs printed charts)
->Crowding (number of optotypes and lines etc)
-> Testing distance
Contrast sensitivity & contrast reserve
Optimum contrast reserves are in the region of…
reading/watching a film is about…
The ability to read fluently or scan a visual scene quickly depends on the contrast reserve (CR)
CR is the ratio between the patient’s contrast threshold (minimum contrast required
to see targets of a certain spatial frequency) and the target contrast (e.g. book or TV-screen).
Optimum contrast reserves are in the region of 20:1, while the minimum requirement for reading
or watching a film is about 10:1.
Testing the patient’s contrast sensitivity (for example with a Pelli-Robson chart) will help deciding whether lighting enhancement (>6%) or contrast enhancement (>4%) would be
beneficial.
If the patient’s CS is very poor, they will not be able to see with an optical LVA.
Field Loss & Scotomas
Scotomas, which are extremely common in ARMD, reduce VA but also affect the accuracy and
efficiency of eye movements. Poor eye movement control reduces the ability to scan a visual
scene quickly and affects reading speed and reading comprehension.
Calculating Distance Magnification
Snellen notation:
M = TA (target acuity) / BVA
Example: Snellen BVA=6/60 TA= 6/12
M = (6/12) / (6/60)
= 60/12
= 5x
logMAR notation:
M= (1.25)^number of steps
each step = 0.1 log unit = 1.25x
Example: BVA= 1.0 , TA = 0.3
M= (1.25)^7 = 4.77
M = 5x
Options available for improving distance vision
- Spectacle lenses
- Contact lenses
- Coatings & tints
- Telescopes
- Multiple pinhole lenses
Patient instructions for Distance magnifier…
When the magnification need has been established and it has been the decided that the patient requires a telescope, a trial with an appropriate device is carried out:
- Focus LVA yourself for appropriate distance
- Ask patient to locate and read chart
- Show them how to optimise focus
- Note handling ability, reaction to FoV and cosmesis
If the VA achieved with the device matches your prediction, the patient needs to try this in reallife circumstances. This requires good instructions:
- explain exactly how it works
- what it can and cannot do (e.g a focusable distance telescope for bus numbers can also be used for shop windows and a spec-mounted telescope is not for walking around in!)
- need to practice with the device
- how to care for it
sometimes work there are good reasons for NOT using distance acuity to
determine near magnification needs:
Accommodation at near
Pupillary constriction at near
Lighting at near maybe different
Myopia at near (retinal image size greater without Rx)
Corneal or lens opacities:
o Posterior subcapsular cataract – tend to have poorer near vision than distance vision
o Nuclear & diabetic cataract - tend to have better near vision than distance vision
It is more accurate to assess the patient’s near acuity using a near chart and calculate the magnification required based on this measurement. Generally, this is done at 25cm viewing
distance. A near add needs to be provided if the patient has insufficient accommodation.
Near VA testing procedure:
for mag
As with distance VA, assess near VA:
- monocularly and binocularly
fogging or occlusion required?
different levels of illumination affect VA?
difference between letter VA and text VA?
A well designed near chart combines long and short words which are unrelated (Bailey-Lovie near). If this proves too difficult for the patient use a chart design with related words but
constant crowding (IOO chart). Assess near vision with the best near Rx and with the patient’s own LVAs. Record chart type, viewing distance (e.g. N8 @ 25cm), eccentric viewing position and illumination level.
Determining target acuity (TA) for nr mag
Target acuity can be estimated. As a general rule newspaper print is about N 8 while books and magazines are about N10, for other approximate target acuities see appendix on page 148.
If the exact print size is not known ask the patient to bring a sample and measure the height of the lower case letters. This can be converted into N point or logMAR
calculating the mag for nr tasks.
When both TA and BVA have been determined the required magnification can be calculated. If the patient wants to carry out several different near task with different magnification, the patient will require a number of different near aids. Magnification is calculated as follows:
M = BVA / TA
Example:
BVA = N32 @ 25cm (our ‘standard’ near viewing distance)
Required VA for newsprint about N8 (TA)
M = N32 / N8 = 4x
The patient requires 4x magnification in order to see newsprint.
The Equivalent Viewing Distance (EVD) for nr
The patient in the above example can see N32 print at 25cm. We have just calculated that he needs 4x magnification to see N8, in other words if we make N8 print 4x larger the patient will be able to read it.
Figure 47 shows that N8 print which is 4 times smaller than N32 will have to be brought 4 times
closer to subtend the same visual angle as N32. This patient’s equivalent viewing distance for N8 is therefore 6.25cm (25/4) to achieve 4x magnification.
If the patient wanted to use a simple plus lens magnifier what power would this have to be and where should the reading material be placed?
We know that M = F’/4
So the required lens power would be F =M x 4 = +16m-1
We also know that the reading material should always be placed at the magnifier’s focal point, so we need to find this magnifier’s focal length, which is
f’ = 1/16m-1 = 0.0625m
You can see that the equivalent viewing distance is really the same as the focal length of the magnifier which is required to see a particular print size.
What dioptre lens would be required to read a novel which has 60 letters and spaces in a line measuring 3 inches? The patient’s best acuity is N24 at 27cm
- 60/3=20
- 144/20=7.2 -> N8
- 24 / 8 =3x; -> 3x closer than 27 cm = 9cm
- 1/0.09m = +11.1 m-1
Answer: The patient should be able to read the novel with a + 11 DS lens
If you want to use logMAR scores to calculate magnification requirements, the chart will have to be used at the correct viewing distance, usually 0.25m. (near)
Magnification =
1.25 ^number of steps
eg: BVA: logMAR 1.2 (N32); TA: logMAR 0.6 (N8)
-> 6 lines difference
Mag = 1.25^6 = 4x mag
Reading Rate and Acuity Reserve
A number of more recent studies have demonstrated that it is not possible to read fluently if the print size is close to threshold level (Cheong, Lovie-Kitchin et al. 2002). This has been found for both, normally sighted and visually impaired patients. It implies that the print size of the task
must be larger than threshold to achieve fluent and comfortable reading.
Acuity reserve is the ratio of print size that the patient can read fluently, compared to the threshold print size that the patient can just resolve (text VA):
Px can just see N6 but reads N12 print fluently.
AR = 12/6 = 2:1
Reading rate
Reading rates (RR) vary greatly with age, education, difficulty of reading material and viewing conditions like print size, lighting & contrast etc.
To read paragraphs of text satisfactorily RRs between 80 and 160 wpm are required. Patients with low vision will often read much more slowly.
For spotting tasks like reading price labels or phone numbers, RRs as low as 40 wpm are usually sufficient.
Calculating magnification based on AR and RR?
- Magnification requirement based on an assumed ‘fixed’ acuity reserve: In most cases it will be sufficient to use a fixed AR between 1.5:1 & 2:1 when calculating magnification needs.
Example:
A patient can just see N24 @ 25 cm with his bifocal lenses +2.00 DS / Add 4.00.
The patient requires a portable hand held magnifier to read magazines on the train and in waiting rooms.
What dioptric power should the magnifier have?
BVA = N24; TA=N8; AR= 1.5
N8/1.5 = TA new = N5
M= BVA/TA = N24 / N5 = 4.8 (approx 5x magnification needed)
M=F/4 , F = M x 4 , 5 x 4 = 20 m-1 -> Patient should have a +20DS magnifier.
- Magnification requirement and acuity reserve are calculated individually. In some cases, where RR is insufficient using the fixed AR approach, reading can be improved by
calculating AR for the patient individually.
This is done as follows:
1a. VA assessment: establish threshold acuity using a near chart at a known distance.
2a. RR assessment
-With the patient’s habitual near Rx in place, a text reading chart e.g.
MNREAD or digital app is used.
-The patient reads sentences out aloud and the time taken is recorded
- RR is calculated for a range of print sizes taking into account any errors made
RR(wpm) = ((n. of characters read - errors made) x 60) /
(reading time in seconds x 6)
- CPS can then be established
3a. Magnification can be calculated as:
Required EVD = (TA/CPS) x current viewing distance
Example:
A patient can just see N24 @ 25 cm with a near add of +4.00 DS.
How much magnification does the patient need, to be able to read novels containing N12 print
fluently if their CPS is N64? What power magnifier should be given?
BVA = N36; TA=N12
Required EVD = (N12/N64) x 25cm = 4.7cm ~ 5cm -> Mag 25/5 = 5x
Px requires 5 x mag (+20 m-1 magnifier) to read N12 fluently
25cm 4.7 5cm
N64
N12 Required EVD ¸u | ¹
· ¨
©
§ Î magnification 25/5 = 5x
The patient would require 5x magnification ( + 20 m-1 magnifier) to read N12 fluently.
why need mono LVA over BIN LVA?
- Very close working distance -> too high a convergence demand
- Limited aperture of high-powered lenses
- Limited convergence of binocular telescopes
What happens If the patient wants to perform several different near tasks
(e.g. reading, writing, sewing etc.)
they may need a different device for each task.
Types of aids to consider for improving near vision:
x Hand held magnifiers
x Stand magnifiers
x Spec-mounted magnifiers
x Telescopes/ Telemicroscopes
x EVES
As for distance vision, it is important to verify whether the calculated (theoretical) magnification
produces the predicted VA. If the magnifier does not produce the expected VA find out why.
Check that it is:
focused correctly
positioned correctly
baseline VA measurement is correct
lighting is adequate
If no magnification achieves the required VA the
patient may have
e a central scotoma or poor contrast sensitivity (CS).
CS can be tested with a Pelli-Robson chart at a
reduced viewing distance (e.g. 1m) or other suitable CS tests that do not require good resolution (for example the MET test)
Other reading tests
The Low Vision Reading Comprehension Test
Pepper Visual Skills for Reading Test (PVSRT)
Patient instruction and follow- up
It is important to instruct the patient which spectacles (Table 12) and which eye to use and make sure they understand that it is important to have the correct working distance.
Patients often don’t know how to clean a magnifier or change bulbs and batteries.
As discussed before, if possible, a follow up should be conducted 2-6 weeks after the initial
appointment to find out if the LVA is suitable. Possible questions discussed during the follow up:
x Have you been able to use the aid?
x How long/often?
x What for?
- Is it suitable for the intended use?
- Have requirements/priorities changed or been added?
x What spectacles did you use?
x “Show me what you have been doing?”
x Are there any problems using it?
- May need a different aid?
- Extra training/instruction?
- Non-optical aids? Lighting?
Rx to be used with diff LV aids: Telescope ,
Hand held magnifier ,
Stand magnifier ,
Spec mounted plus lens
magnifier
Telescope = Distance Rx used
Hand held magnifier = Distance, sometimes Near rx used
Stand magnifier = Near rx used, separate add may be necessary
Spec mounted plus lens
magnifier= Distance if clip on; otherwise take into account dist rx e.g. -4.00 myope with 3x mag (F/4) -> nr add = +12.00
-> final lens +8.00
Field loss can be the result of a lesion of any part of the visual pathway. Depending on the site of
the lesion it is generally divided into 2 types:
- peripheral loss, where the patient may have good central visual acuity
- central loss, which will always result in poor visual acuity and is very disabling for the patient.
Common conditions causing peripheral field loss:
- Glaucoma (very gradual onset)
- Retinitis pigmentosa (very gradual over many years, overall constriction (tunnel vision)
- Cerebrovascular accident (hemianopia)
- homonymous hemianopia or quadrant defects
- usually sudden onset due to stroke, seldom gradual onset due to tumour
- other associated problems hemiparesis, dysarthria (impaired speech)
Any patient who has lost peripheral vision has lost rod photoreceptors and will therefore have poor night vision.
Functional effects of a limited field
In early stages of field loss patients usually compensate by making eye movements and scanning the visual scene, but when the visible field has been reduced to 10 degrees or less the loss significantly impairs a patient’s mobility.
Symptoms produced by field loss will affect the patients vision at any viewing distance,
examples include:
Distance :
- bumping into obstacles = poor mobility
- not being aware of layout
of environment
Intermediate :
difficult to get whole
task into view simultaneously (TV, computer)
can’t find objects put down on cluttered table/desk
Near:
Page navigation - doesn’t realise reached end of line;
difficulty in finding start of line without missing one
Importantly, a brain lesion which causes hemianopia can have other effects such as:
Hemiplegia:
This will restrict the patient’s ability to position and grip magnifier and task.
Loss of “higher visual functions”:
e.g agnosia (inability to recognise objects, faces etc.) alexia (inability to read) etc.
Oculomotor problems
e.g. squint, decompensated phoria or impaired convergence
Personality/behaviour/attention changes
can also be drug-induced
- Management of peripheral field loss
Depending on the cause of the VF loss, management strategies include:
- Illumination & glare control
- Optical methods –
2.1. Visual field (VF) enhancement for sighting where a minified image is produced, so that
more information can be squeezed into the patient’s remaining field of vision
2.2. VF enhancement for mobility where objects from the missing part of the field are projected into unaffected parts of the field.
Unfortunately, optical VF enhancement can be quite confusing for the patient, which is why its success rate in practice is relatively low.
Field enhancement
: For Sighting
Reverse telescope –‘field expanders’
Amorphic lens
Concave lens
Convex mirror
Field enhancement:
For Mobility
Mirror system (hemianopia mirrors)
Prism system (Fresnel, Inwave lens)
Scanning
Electronic systems
Reverse telescopes
for tunnel vision
NOT SUITABLE for hemianopia
x magnification <1.0 (minification)
increased FoV equal to power
VA is reduced in proportion
x Therefore, unlikely to be able to wear permanently
hand-held for intermittent spotting tasks
bioptic mounting, clip on
x Door peep-hole viewer (substantial field expansion (>100 deg))
Magnification of reverse telescopes:
Example: Galilean reversed Fo= -50 m-1 , FE= +25 m-1
M = -Fe / Fo = -25/-50 = 1/2 x
-> 2 x minification
Amorphic lens
minifies only in the horizontal plane-expands the field horizontally (where it will be most useful),
preserves VA – no size change vertically
image distortion
Every Day Uses:
1. Scanning.
3. Bike riding.
13. Watching tv/ sports.
15. To see a large group of people all at once
. Reasons for a lack of success included dizziness, distortion, confusion caused by changes in the depth of field, excessive weight, and cosmetic factors.
driving with extensive field defects is illegal in the UK!
Hand-held minus lens
Holding a negative lens 30-50cm from the eye produces a minified image and expands the visual
field.
Since for all telescopes = Fe / Fo ; increasing the power of the negative lens (FO) increases the minification effect, which leads to a bigger field of view, but poorer acuity. In practice, minification values between 3x and 8x seem to be most effective.
A presbyopic patient (Add +2.0) with peripheral field loss has retained a circular field of vision of 10 degrees. If he wanted to use a negative lens as a field expander what lens power would be required to double the size of the field?
At what distance should the patient hold the lens and
what would be the optimum lens diameter?
If the patient’s VA is 6/6, what would it be with the
field expander?
FE (Add) = +2 m-1;
M =0.5 to double the available FoV; VA =6/6
M= -Acc / Fo ;
Fo = -2m-1 / (1/2) = 4m-1
d=f’E+f’o = 0.5m-0.25m = 0.25m
D = d x a / 60
= (250 x 10) / 60 = 42mm
VA = 6/6 x 0.5 = 6/12 (0.5)
Answer:
The patient needs to hold a -4.00 DS lens of 42 mm diameter 25 cm from his eye. The VA with the lens would be 6/12.
Convex mirrors
Convex mirrors give a wider field of view than plane mirrors and can be used for field enhancement. Placed in kitchens, hallways, public places etc. they can help detect people, open
doors and other obstacles. Note the minification effect
Prism systems for patients with RP & bitemporal hemianopia and
patients with less than 20 deg field and acuity better than 6/30
The goal is to stop the patient from “bumping into things”. As a general
guide use as much prism as possible (15-30’) with the base towards the field defect (non-seeing area). The system can be customised for general field constriction or smaller defects.
The disadvantages of prisms systems mainly relate to the “jump” at the prism apex, which is proportional to the power of the prism (1o for
each 1.75’) and the corresponding prism scotoma at the apex. If
Fresnel stick-on lenses are used this will lead to a reduction in VA.
Where should the edge of the prism be placed?
In practice, the edge of the visible VF can be found by moving a card across the across the FoV from the temporal side across the FoV until the patient just sees it. The spectacle lens can be marked about 1mm
temporal to the edge of the field and the patient is asked to walk around to ensure that they don’t notice the mark before the prism is put in place.
Monocular or binocular?
The conventional approach is binocular which has
the disadvantages mentioned above (jump, scotoma)
Monocular prism placement
has been suggested (Peli 1998, Peli 2000).Two separate prism segments are placed monocularly at the level of the limbus, and cover the width of the spectacle lens.
Full-aperture binocular prisms of powers 6’ to 20’ with base towards field defect are put in place.
This does not achieve field expansion and merely produces a small field shift, which would easily be achievable by making a small eye or head movement. It may be useful as a last resort in
desperate situations.
Field expansion for reading and intermediate tasks
Possible strategies involve minification by increasing the viewing distance.
M = old object distance / new object distance
e.g from 35cm to 60cm =35/60 –>M=0.6
Referral for additional Professional Services
Due to the relatively low success rate of field expansion devices, the complex nature of field loss
and its associated complications many patients will benefit from appropriate referrals to
additional professional services like:
- Counselling
- Rehabilitation teaching
- Occupational therapy
- Physical therapy
- Auditometry
- Other support groups
Ocular diseases affecting central VA include:
x ARMD- most common, mostly elderly but may affect younger patients
x Macular holes
x Central serous retinopathy
x Cystiod macular oedema
x Toxic maculopathies etc.
Functional effects of central field loss include:
x Acuity loss: Mainly impaired reading & face recognition
x Distortion
x Photostress
Impaired Dark & Light adaptation
x Photophobia
better vision at night
x Impaired depth perception
x Charles Bonnet Syndrome
Visual Hallucinations after sudden loss of vision
-13% AMD px with CBS
Central field loss and retinal locus
If the VF loss is unilateral, the patient will usually just ignore the affected eye and may sometimes not realise that the condition is present. Occasionally, the affected eye needs to be occluded, for example if it was previously the dominant eye. Therefore, central field loss is only a problem if the condition is bilateral.
Because of the foveal involvement the patient will always suffer severe acuity loss in affected eye. The fovea is also the oculomotor reference for eye movements. In the presence of an absolute macular scotoma the patient will notice a blank patch covering the object of regard and may try to re-fixate using a parafoveal point, just off edge of the scotoma. This is called the patient’s Preferred Retinal Locus (PRL). Images formed away from the fovea are therefore a stimulus to “re-foveate”.
Due to the acuity falloff away from the fovea one would expect the patient to use a PRL as close as possible to the edge of the scotoma, and if the size of the scotoma is known, the patient’s VA should be predictable.
Older patients are less likely to change their oculomotor reference.
In practice, an Amsler grid or field screener (e.g. macular VF on HFAII) can be used to locate the scotoma and identify the EV direction which gives the best vision.
Sometimes, an area which is not the patient’s PRL may be identified and give better VA or higher reading speed than the PRL. In this case the patient could be taught to use this trained retinal
locus (TRL). For reading the TRL should preferably be above or below the macula.
Eccentric Viewing Training in Glasgow:
The New View Project is run by Visibility. Before
entering the training programme patients attend for an LVA assessment at Gartnavel General Hospital, Royal Alexandria Hospital or the Low Vision Clinic at GCU. This mainly involves mapping out the scotoma and assessing the patient’s magnification needs.
Suitable patients enter the programme and receive 1 hour of instructed
training per week, usually between 5 and 8 weeks. This is combined with 5-10 min of daily practice.
Patients are also given advice regarding suitable lighting and the use of the steady eye strategy.
Prism Relocation
The idea behind this technique is that a prism could be used to move the retinal image out of
the scotoma and place it on the optimum PRL without the need for the patient to re-fixate.
Surgical treatment of field loss
Macular translocation surgery has been carried out on patients with wet macular degeneration, where impaired retinal pigment epithelium (RPE) function leads to a loss of macular photoreceptors.
The procedure involves detaching and rotating the sensory retina and reattaching it so that the
macular receptors can be supplied by intact RPE, which will preserve macular function.
technique is still in the research phase.
Real Image/transverse Magnification (Electronic Vision Enhancement Systems (EVES))
-The most common example of Real image magnification
An advantage of electronic magnifiers is that the magnification produced is aberration-free, but image quality depends on the screen resolution.
While the maximum magnification achievable with optical magnifiers is about 20x (80 D), this can be up to 70x or higher with electronic aids, but a limit of about 30x is common.
Zoom control allows variable magnification (important for page navigation: probably only 4 characters required for reading but up to 15 required for optimum page navigation).
Variable magnification may also be useful if the patient’s eye condition changes. Other advantages include the possibility of binocular viewing and variable
camera-to-task and eye-to-screen distances.
Disadvantages of EVES:
x Cost - expensive to buy and service/repair
x Size depending on type/model
x Image quality – depending on screen
The persistence of the image on the screen’s phosphor causes blurring/fading if the
reading material is moved too quickly. Not an issue with LED/LCD.
Patients with distance VA of better than 6/90 may gain little benefit from using EVES
types electronic magnification systems:
CCTV – probably still one of the most commonly used designs
x each component fixed and mounted vertically “in-line”
x working space fixed or limited
x for distance need second camera to be added
x !! reading material needs to be flat – depth of focus !!
x hand held/portable (P-EVES)
greater versatility than CCTV
x television/USB readers
low cost
x head-mounted video magnification devices:
-reading, dealing with correspondence/filling in forms, school tasks, viewing photographs)
?ability to wear permanently?
Distance and near magnification
Independent of external
illumination/reduced glare
Visual performance with EVES compared to optical
LVAs
It has been suggested that more practice is required to achieve optimum reading speed.
– 15 to 20 sessions of daily practice) than with optical aids, but small print (approx. <N8) can be read better and faster with EVES than with optical magnifiers.
How does the patient obtain an electronic LV aid?
Optical aids are provided free of charge through the Hospital Eye Service (HES).
The aids are given to the patient on a “permanent loan” and there is no formal limit to the number of devices. Aids can be changed as required.
There is no equivalent in GOS, although local schemes exist where patients can have an LVA
assessment in the community (usually by a participating optometrist) and the Health Authority
pays for aids and assessment.
If bought privately, patients who are registered VIPs are exempt from VAT.
EVES are available to use in local libraries and resource centres and patients may be able to borrow them from Social Services. They are also available through charities (e.g. “Electronic Aids for the Blind”) and there are special schemes for education and employment.
Lighting
To improve the visibility of objects for visually impaired patients adequate lighting has to be provided in appropriate places. General lighting should be distributed evenly to avoid glare and extra lighting should be provided for hazardous areas.
Light coloured walls & ceilings improve the light distribution in buildings and non-reflective surfaces (matt paint) reduce glare.
Particularly patients with reduced macular function require substantially increased light levels for reading (Figure 76).
While some patients and of course retailers tell you that “daylight” bulbs (these contain a blue filter to simulate the appearance of
natural light) improve reading performance,
there is no scientific evidence for this.
Visual performance improves with increasing light level, but does not depend on colour temperature (Figure 75). Reading lights frequently recommended to VIPs are fluorescent anglepoise or LED lamps.
Cold light sources can be brought very close to the patient’s head and reading material which maximises luminance.
Glare
Discomfort Glare:
Subjective visual discomfort
The patient feels visually uncomfortable or fatigued by this type of glare. Symptoms can range
from mild to severe, but VA is not affected by and symptoms are usually relieved by tints. Examples include uveitis, ocular albinism, cone-rod dystrophy, retinitis pigmentosa.
Disability Glare:
Loss of retinal image contrast as a result of intraocular light scatter, or straylight.
Reduces visual performance (VA)
Severity depends on the angle between task and glare source & relative luminance
Media opacities lead to intra-ocular light scatter which creates a veiling luminance across all parts of the retinal image. This reduces the retinal image contrast.
Conditions which induce disability glare include ageing, cataract, posterior chamber IOL , posterior capsule opacification, keratoconus , corneal oedema, Radial Keratotomy (RK), vitreous opacities.
Measurement of glare
Simple clinical method for assessing disability glare:
Measure VA or low contrast VA without glare source
Re-assess with glare source (e.g. penlight)
Standardise the angle and distance of light to eye
typically 10cm and 30 degrees.
drop in VA of two lines or more - significant degree of glare
Reducing Disability Glare
Glare reduction includes environmental measures discussed before e.g.:
the avoidance of shiny surfaces,
use of blinds,
appropriate task lighting (angle!),
visors, hats
tints
Due to Rayleigh scattering, short wavelengths produce more glare than long wavelengths in patients with media opacities. This would suggest that glare can be reduced by wearing (yellow)
filters. When prescribing tints, objective measures of visual performance like VA with high and low contrast charts or under different light levels can be taken into account.
However, research
has shown that filter colour does not have a significant effect on visual performance (Figure 78)
and most tints are prescribed by ‘trial and error’ based on the patient’s subjective preference.
Non optical and sensory LVAs- 2 types
Daily living aids:
-Reading/writing
-kitchen/around house
-medical/personal care
Travelling/Mobility:
-canes
-dogs
-electronic aids
Non optical aids also include lighting & contrast enhancement, tints and
environmental design features
Daily Living Aids: Non optical and sensory LVAs
Self threading needles
x Liquid level indicators
x Talking microwaves/kettles
x Talking watches etc
x Large number telephones, microwaves etc
x ‘Bump ons’ or ‘Hi marks’
x Coloured stickers and buttons
Reading: Non optical and sensory LVAs
large print books, bank statements & bills, clocks/watches/telephones
x Talking; books, watches, clocks, newspapers
x Typoscopes- for cheques, reading or writing
x yellow paper/lined paper
Braille
3x2 pattern of 6 dots
26 letters + contractions st, ch etc and punctuation
Can be generated by computer- keyboard or
screen
x Disadvantages:
Thick paper with raised dots - thick & bulky books
Not good for elderly due to loss of sensitivity in finger tips
Mobility Aids
These mainly include:
x Canes & sticks
x Electronic aids (e.g. ultrasound, sonar)
x GPS systems
x Talking signs
x Built environment
x Dogs & sighted guides (people)
There are four types of white sticks used by VIPs.
Symbol cane ( lightweight, folding cane + indicates that the user is visually
impaired. )
White walking stick (as above but aids support)
Long cane (made of light-weight aluminium)
Guide cane ( shorter and stronger than the long
cane + back up for those with residual vision)
A range of electronic mobility aids is available. Most of these use laser beams or ultrasonic
sensors to detect obstacles and vibrate or emit a sound to warn the patient.
Canes are available from Social services, but can be bought from various charities like the RNIB. If a white cane has red stripes this indicates that the patient also has impaired
hearing.
Guide Dogs
Guide Dog Assessment (3 stages)
Before pairing a dog with a potential owner, a
guide dog assessment is carried out. This usually involves 3 stages:
- General Information Visit
The owner receives information on general services and mobility training. - Mobility Assessment
Medical status and cause & degree of vision impairment are assessed and the possibility of additional non-guide dog training or mobility aids like a long cane explored. This is followed by an initial general suitability assessment. Contra indicators to guide dog ownership are sufficient sight, dislike of dogs and age <16 years. There is no upper age limit. - Guide Dog Assessment
This involves an assessment by a guide dog mobility instructor and if the applicant is suitable
they will be trained to work and walk with the dog, recognise and follow its movements and develop effective & consistent dog control and geographical awareness. The training starts in their home environment but usually extends into their neighbourhood (shops and other frequently visited places).
Environmental Design
Simple things
x Move TV away from windows to avoid glare
x sit close to TV/ near blackboard
x Location of lights for reading (contrast, glare)
x Extra task lighting where required (kitchen, bathroom)
x May need to have room lights on during the day
x Sit on one side of teacher/TV etc. for hemianopic patient
Architectural design:
x Suitable colour schemes
Matt light coloured paint on walls
Dark paint round doors, contrast surrounds to switches, plugs
Lighter door with contrasting handle
Dark flooring
Hard flooring- sound
Contrasting skirting boards
x Features to aid VIP’s
Wide doors (guide dog access)
Sliding doors
Auditory indicators (e.g. lifts)
Handrails
Enclosed staircases
Tactile elements (e.g. floors)
No projections (radiators, fire extinguishers)
Strip lighting
Tactile and auditory signs, maps and floor plans in public buildings
x Features that hinder VIP’s
These include things like glass doors, open staircases and poor signage
Prognostic factors in predicting successful use of LVA’s
The measurement of success of LVA provision is important to establish if healthcare is effective
and to assess if LVA’s benefit the patients. The question is though, what constitutes a successful
LVA assessment what does the patient perceive as success?
x Increase in VA?
x Increase in patient confidence?
x Improved performance doing required tasks?
x Improved reading speed/accuracy
x Frequency of use of LVA
It is important to remember that perceived success varies and depends on the individual’s goals and expectations. Patient expectations must be realistic – if you know that they can’t be met, be
honest with the patient, but try to let them down gently.
What do patients like best?
Most importantly, patients prefer simple LVA’s!!
x Spectacle prescriptions (high adds)
x Hand magnifiers
x Stand magnifiers
x Illuminated magnifiers
