Low Vision Flashcards
Definition of low vision
The WHO defines low vision as visual acuity less than 6 / 18
and blindness as Visual acuity less than 3/60 or a visual field < 10 °
What is Disability Glare
Also known as veiling glare
Stray light which interferes with visual performance - normally this glare affects low vision Patients
Predominantly due to scatter and fluorescence in the Ocular media
helped by Suitable filter lenses which reduce shortwave radiation
what is discomfort Glare
Stray light which reduces visual comfort,not responsible forreduction in VA
Normally associated with differing levels of illuminance within the visual field
eg -Performing a visual task under bright direct lightening in a room with poor background illumination
Reflectance
A ratio comparing the lumens of flux leaving a Surface to the lumens of flux that were incident on the Surface
Symbol = P ( rho)
unit =none as a ratio - maybe given as a %( ratio → % X by 100)
optical Surface
P = ( n’ - n )2( Squared)
——–
n’ + n
non optical Surface
P= 0 leaving Surface
————————–
O incident on Surface
Albinism
2 forms
Congenital Condition - lach of pigment or body’s inability to Produce Pigmentation
2 forms
Ocolocutaneous - affect Skin,hair,eyes 1 In 20,000 - VA often worse
Ocular albinism - affects eyes 1 In 40,000 VA often less than 6/ 24
while eyelashes very Pale blue Iris look pink at times - light reflecting of blood vessels
Extreme photophobia,Pendular nystagmus,high refractive error,high with rule of Astigmatism full visual field
no treatment -Improve visual comfort with filters,tints,contact lens
Genetic counseling recommended
Photometry
Luminous Intensity
the Power /strength of a Source in a given direction
Symbol l
unit candela (cd)
I = lumens / w ( Solid angle )
photometry mean spherical intensity
The average intensity of a real world light Source considered to give out light equally in all directions
m. S.I = O (lumens) / 4 (Pie) solid angle Steridians around point)
luminous flux
the rate of light flow from the Source in a given direction
O = I x w
Illuminance
The amount of luminous flux falling on a Surface of unit are
Symbol = E unit lux
luminance
luminance Symbol L is the amount of light per unit area emitted by a surface in a given direction
luminance = illuminance X reflectance
Reflectance
A ratio comparing the lumens of flux leaving a surface to the lumens of flux that were Incident on the Surface
Symbol = P (rho )
unit = ratio= no unit maybe given as % ( x by 100)
luminous effectively
How many lumens of flux as Source produces per watt of electricity used
Inverse Square law
Illuminance at a Point on a surface perpendicular to a source is Inversely Proportional to the Square of the distance from the Source to the surface
E =I / d2 (Squared) (metres)
cosine law of illumination
E = I / d2 x COs O (angle)
the greater the angle between the light Source and the Surface
How many Steradians Surround a point
4 (Pie )
What is meant by the luminance of a Surface
How bright the surface is to the observer
A60 walt tungsten filament lamp has an efficiency of 12 lumens per watt
calculate
a) luminous flux
b) the mean Spherical intensity ofthe lamp
a) 12 lumens X 60 watts = 720 lumens
b) MSI = O / 4 (pie) 720 / 4 (pie) = 57. 30cd
Role of health care practitioner
is ( partially) to increase ‘activity and the participation of our patients
Impairments
problems in body function and structure Such as a significant deviation loss
functioning
an umbrella term for body function and structures it denotes the positive or neutral aspects of the interaction between a persons health conditions and that individuals Contextual factors
Activity limitations
Difficulties a person may have in executing activities
Participant restrictions
problems an individual may experience in involvement in life Situations
low vision definition
the who defines low Vision as visual acuity less than 6/ 18 and blindness as visual acuity less than 3/60 or a visual field < 10 degrees
low vision consensus group 1999
A person with low vision is someone who has impairment of visual function for whom full remediation is not possible by conventional Spectacles contact lenses or medical intervention and which causes restrictions in that persons everyday life
Blind
So blind as to be unable to perform any work for which eye sight is essential
- SSl registrants Can be classified into 3 groups
Groups 1 visual acuity below 3/60 A Cx who is 1/18 cannot be Certified in group 1
Group 2 Visual acuity between 3/60 and 6/60 and is Significantly contracted field of Vision
Group 3 visual Acuity better than 6/60 and have a Severely contracted field of view especially if in the inferior field
Community care act
1948
referral of visual Impairment
RVI form can be sent from a hospital clinic informs Social Services Request assessment states Urgency
benefits of registration
personal independance Payment (pip) free or reduced bus fare free NHS eye exam Assesment by Social Services SI -don't get tax allowance TV license reduction blue badge
Services available multidisciplinary team
low vision Clinic rehab officers mobility officer ECLO G.P occupational therapist audiology education officer
role of dispensing optician
Assist in devices - LVA non optical aid correct addition illumination advice Discuss pathologies refer to multidisciplinary contrast advice local charities
How do we measure contrast sensitivity
Allows us to gauge visual Performance under real world conditions
Allows us to predictthe likely success of LvA’S
consider weather contrast advice + support needed
calculate contrast reserve+ feeds into our prescribing strategies
Pelli robson- chart
All letters same size (2.8 degrees at 1m)
use at 1m
8 rows comprising of 2 triplets of letters
each triplet has Some contrast
Contrast reduces across and down
range 89% to 0.5%
Baily lovie chart
one panel distance acuity one panel low Contrast difference in lines gives low contrast sensitivity value externally illuminated 100% contrast one side 10% contrast other side logmar progression 5x4 grid 6 metre 14 lines of 5 lettersper line ranging from 0.8 logmar to - 0.5 logmar 53x61mm
vistech chat
Five rows Nine columns Sine wave grating/ circular grating Decrease in contrast left to right increase in Spatial frequency top to bottom used at 3m
Rods
Rhodopsin Scotopic low acuity Achromatic High density away from macula
Cones
3 types Red blue green photopic High Acuity Trichromatic Central retina / fovea most dense
Visual field
the extent of the area In space in which object are visible to an eye in a given Position
Scotoma
An area of reduced / no visual acuity Surrounded by an area of relatively normal vision
Absolute Scotoma
Blind spot - no photoreceptors
a defect that is present during all Stimulus
relative scotoma
a defect that Presents when stimulus is weaker
eg reduced contrast Sensitivity
negative scotoma
A Scotoma that’s not generally percieved by Cx
eg blind spot
Positive Scotoma
Percieved by the Patient . Such as black spot in vision
Hemianopia
loss of one 1/2 of your visual field
eccentric fixation
is the process of rotating the eye so light falls on a different less damaged part ( close to macula)
called PRL Preffered retinal location
Steady eye strategy
employed by the patient achieving their PRL keeping eye still and moving print
6 weeks training
AmsIer chart
Gid of lines making Squares Central fixation point Some have diagonal lines to and fixation designed to measure Central10 degrees of visual field well light room held at 30cm wear SVN each eye occluded stare at central fixation Spot report any lines distarted or missing ( emergency referral optom)
Disadvantages of Snellen chart in LV
Too few letters at lower acities non logarithmic design no contrast ratio between letter size non consistant Spacing measures high contrast acuity only designed at 6m
advantages baIie lovie chart in LV
logarithmic design constant ratio of letter size line + letter spacing same any appropriate WD 5 letters per line
magnification
New RIS / Old RIS
BS EN iso 15253
British standard which covers Optical devices for low Vision aids
estimating mag
= Acuity Achieved / Acuity required.
AA / AR
Threshold acuity
the smallest possible size they can read .
fluency
As a print Size reduces towards a patients threshold acuity their reading rate reduces
Hand magnifiers
advantages
Inexpensive cosmetically acceptable can be internally illuminated Portable lightweight trained to be used with Spectacles flexible working distances
Hand magnifiers
Disadvantages
requires steady hand not both hands free Poor Image quality effective magnification less than described needs training due to dynamic System difficult to change batteries for LV Cx
Stand magnification
advantages
very high magnification can be internally illuminated Some can tilt Hands free adjustable
Stand magnifiers
disadvantages
Poor fOv Increased aberrations more conspicuous most require near correction requires flat surface may struggle with body position
Bar devices
reading addition flat Surface tasks place device directly on task good FOV Reducing eye to lens does not effect FoV
Inexpensive
ease of use
cosmetically acceptable
Good FoV
Very limited magnification range
one meridian only - distorts Image
stale the formula for and Conditions under which nominal magnification is Created
Mnom = Fe / 4
object is @ F 1st principal focus
Image@ infinity
state formula for and Conditions under which maximum magnification is created
Mnom = Fe / 4 + 1
Vertex distance = O
Image formed RSD - 25 CM ( reference Seeing distance)
Contrast
is measured of the difference between brightest and darkest Point
Distance Telescope
objective lens
the lens nearer the object
it is the entrance pupil for telescopes
its diameter controls field of Vlew
Distance Telescope
eyepiece lens System
the lens nearer the patient
always the highest power lens
Sometimes a Series of lenses which help control aberrations
Position controls field of View
afocal
A telescope where parallel light enters and leaves the System is referred to as afocal
Distance object
Benefits
only realistic option for Dv mag where object cannot be Moved or changed Good depth of field hands free can be Mono /binocular can be variable focus
Distance telescope
limitations
High cost difficult to use extensive training Very poor Fov Heavy conspicuous Require Very accurate Centration
Types of telescopes
Spotter handheld monocular - hand held non adjustable afocal easier to use,Small
Adjustable handheld Monocular - adjustable tube
Binoculars - wide FOV cheap
Bl optics - Spec mounted on different plane
Spec mounted telescopes
calculating magnification
M = - Fe / Fo. fe = eyepiece fo = objective
Markings
8 X 20 X 7. 5
8: magnification
20: objective Diameter
7. 5: field of View In degrees
field of Vlew
This is Significantly restricted in telescopes
binocularity Is Important to Improve foV
Improves mobility or fluency and visual Comfort
terrestrial telescope
astronomical telescope been modified to produce upright Image
Distance cap
a negative (Concave ) Mounted on the front of a telescope to modify it from near to distance vision
Reading cap
a positive lens mounted on front of a distance vision telescope to modify for near
tele microscope
A distance vision telescope being modified for near with use of reading cap
free working distance
Distance between the most anterior part of System ( front) + object being viewed
Exit image Vergence
Power of light leaving back of System
Explain the stage depression of loss model
Include a minimum of 4 facts
lack of engagement with professional
feeling of worthlessness
poor memory
fee lings of hopelessness at outcome
How will the patients age in f luence the successful use of low vision aids ( 4 facts)
less likely to accept
Dexterity issues
more likely to have additional needs / health issues
difficult to learn new Skills
name a non optical aid that may assist Mrs D Seeing the dials on her Cooker
Stickers or labels @ N18
what is the benefit of testing contrast Sensitivity
15 Mark answer- Allows use to Predict the Likely Success of LVAs
Allows us to gauge weather contrast advice Is warrented
5 mark - gauges visual performance in real world.
state Criteria for registration as Sight Impaired
3/60 → 6/60 full visual field
6/60→ 6/24 moderate field loss
6/18 or better + gross field defect
Symbol cane
symbol cane - foldable White cane Indicates Visually Impaired -extends Cane and holds diagonally across body or keeps folded and holds across chest
low
guide Cane
to find obstacles be .. holds guide cane diagonally across your body.and use it to find Sstacks
long cane
To avoid obstacles you roll or tap It from Side to side as you Walk
Red + white Cane
low hearing and Vision
