OPTOM 216A Flashcards
Chart type in research
LogMAR
Most common chart type in clinics
Snellen
Snellen chart design advantage and disadvantage
Simple, cheap and small, most common in clinics but have non-uniform intervals, unequal crowding, different recognisability in letters and inconsistent fonts
Sloan font Design
5x5 px design, non-serif fonts with 10 letter sets, astigmatism detection.
Sloan + LogMAR = ETDRS chart
LogMAR chart design
Each line have same number of letters, 0.0 = 6/6 where lower decimal is better. Each letter scores 0.02 and each line scores 0.1.
Bailey lovie uses LogMAR with different optotype using 5x4 letter design.
LogMAR disadvantage
Mask distance information, Larger, Mesures in LogMAR but in clinics use Snellen fraction
What is VAR
Visual acuity rating
VAR equation
100 - 50LogMAR
What is VAR designed for
Used with BL charts where 6/6 = 100 score. Each letter worth 1 point, each line worth 5 points
Good for low vision patient
Visual efficiency ( % ) equation
0.836^( (1/decimal VA) - 1 )
Gives indication how different vision is to 6/6, 6/12 means a decimal VA of 0.5
Used to compare vision changes and always greater than 0
What chart to use if patient cant speak english
Tumbling E and Landolt C
Kids optotypes versions
Lea and Patti
Lea optotype
Only has 4 optotypes calibrated against landolt c
Patti optotype
Only has 5 symbols as ETDRS
How to measure Near vision
Same as measuring distant vision
Uses words and not letters at 40cm
Measures in M, N and J. Metric, point and Jaeger
N8 = M1.0 where M1.0 = optotype height of 5minarc at 1 meter
Near vision test disadvantages
Font details like upper/lower case, random words and size and crowding
Near vision chart design
LogMAR or Snellen, Can be single letter using ETDRS or BL
Or can use real world stuff like newspapers
What to consider when measuring VA
Can have partial lines ( 6/6+2 )
Metric or imperial
Latin or english
BE different to LE + RE
When to stop testing when errors are made
What is BSV
Binocular single vision, single unified perception of the world with both eyes without double vision that requires all parts of the visual system to function properly
BSV advantages
180 degree visual field, have stereopsis for depth, spatial perception and binocular summation. Also have one spare eye if one is damaged
BSV disadvantages
Entire visual system needs to properly function and can also cause Asthenopia with double or blurred vision
What are the 3 grades of normal BV
Simultaneous perception, Fusion and Stereopsis
Simultaneous perception
See two images fused together, otherwise diplopia
What is motor fusion
Bifoveal fixation on same object
What is sensory fusion
Unify two images into one in visual cortex
What is ocular deviation
When visual axis of each eye is not fixated on the same object
What is stereopsis
Ability to see depth, LE and RE see images differently known as Binocular disparity
What are eye movements for
Look around, locomotion and localisation
What is duction
Movement in one eye
What is a Version
Movement of both eyes in same direction
What is vergence
Opposite movement of both eyes
What is the 3 axis of rotation called
Axes of fick
What is a saccade
Rapid ballistic eye movement
What is smooth pursuit
Slow tracking movement of the eyes
Types of Ductions
Adduction, Abduction, Elevation, Depression, intorsion and extorsion
Types of versions
Laevoversion, Dextroversion, Elevation and depression
Types of vergence
Convergence, Divergence, Vertical vergence and Cyclo-vergence
How to assess eye movements
Double/Broad H or use Prisms
Prisms in optometry
Bend light toward base, measures eye movements, units are prism dioptres or degrees
What can prisms assess
See if pt can detect a change or see if they can move their eyes to the target
Equation for prism dioptres
Image displacement ( cm ) / Viewing distance
What type of vergence occurs with base out prisms
Convergence
Convergence measurement equation
In dioptres or degrees.
Equation = IPD(cm) / viewing distance(m)
What does the pupil do
Limit aberrations and control retinal illumination
What is the normal pupil size
2-9mm
What happens to pupils if they are not drugged
Always in motion, if drugged then no motion
What can observing pupils identify
State or arousal, Sleep state, if on drugs, sleepiness and stroke
What two muscles control pupil size
Radial dilator ( SNS ) and Circumferential sphincter ( PSNS )
Innervation of the pupil
Retina –> Pretectal nuclei –> Edinger westphal nuclei –> Ciliary ganglion –> Retina
Upper thoracic segments of the spinal cord –> Superior cervical sympathetic ganglion –> Carotid plexus –> Retina
Pupil light reflex test
Dont have other light sources
Shine light in eye, look at Direct and consensual for BE as pupil light reflex is bilateral. Then swinging test to check for RAPD then near target response to check near target pupil constriction
How to note a normal pupil light reflex
PERRLA, No RAPD
If anisocoria then how to note
PRRLA, R>L Aniso +0.5mm dim + Light
What can the pupil light reflex also check for
Iridodialysis, corectopia and polycoria
What are 4 pupil abnormalities
Physiological anisocoria, Horners syndrome, Adies tonic pupil and Argyll robertson pupil
What is physiological anisocoria
in 40% of healthy population, No RAPD, no management needed and can spontaneously resolve
Horners syndrome
Sympathetic Efferent Defect with triad symptoms ( Miosis, Ptosis and Anhidrosis ).
No RAPD but direct and consensual effected, defect can be in any of the 3 order of neurons. Syndrome can be isolated using drugs
Adies tonic pupil
PSNS Efferent defect with pupils dilated on presentation. Poor light response. No RAPD but Direct and consensual effected. This is idiopathic and self resolves. Can test for this using pilocarpine
Argyll Robertson syndrome
Bilateral miotic pupils then constrict to near and not to light. Associated with neurological syphylis. Sexual syphilis untreated causes death. This is a lesion in the dorsal midbrain, can be diabetic neuropathy so check eyes for atrophy
What tests do both screening and diagnosis
Cover test and Ocular motility
What is ocular deviation
Unaligned visual axis like Phoria or tropia
What is a phoria
Aligned in normal viewing conditions
What is a tropia
Misaligned axis in normal conditions where pt has no motor fusion
Causes of tropia
Idiopathic, EOM abnormality, Cranial nerve palsy and Head injury
What to record if pt has tropia
Laterality, Direction, Frequency and Magnitude
How to check for a phoria
Remove stimulus in BE and covering one eye
How to record a normal pt if they dont have tropia or phoria
NS, Orthophoria
Hirschberg test
Use a penlight 40cm away, look at the corneal light location and see if it is symmetrical, every 1mm difference is 15 dioptres. This can only check for large tropias.
When to use the hirschberg test
When cover test cant be done like in children
Requirements for cover test
Both eyes need to fixate
Setting up cover test
1 VA line better than patients worse eye but ENSURE they can see the letters with each eye and ask patient to keep their head still
Steps for a cover test
Unilateral, alternating and Phoria recovery ( If they have a phoria )
What can Unilateral cover test check for
Only detect for Tropia or NO tropia
What does the alternating cover test check for
Measures the size of the deviation
What is phoria recovery
Assess patients ability to gain bifoveal fixation on target after dissociation
How to assess ocular motility
Use Broad/Double H test and requires at least 1 eye to see light
What does the H test check for
Restrictions, Jerky or inaccurate movements, Pupil size in eyes and Diplopia
Broad/Double H test instructions
Ask patient to tell if there is any pain and discomfort and also to keep head still
What to record if H test is normal
Motility full + smooth, no diplopia or pain
How to assess vergence
Using NPC ( Near point convergence )
How to perform NPC
Patient needs BSV and ask patient if they get diplopia during the test
Break = Diplopia during the test
Recovery = When BSV is regained\
A break of more than 10cm means convergence problems
How to record NPC results
NPC with RAF rule : 5/7cm, 5/7cm, 5/7cm
IF to nose then record as NPC with RAF rule : To nose
What is accommodation
Far to near focus where there is no accommodation when viewing at optical infinity in emmetropic eye
How does the eye accommodate
Retina detects blur and information send to visual cortex causing ciliary muscles to work inducing a change in lens curvature via Helmholts theory
What is the ciliary muscles main innveration
PSNS and accommodation decreases with age
What is the near triad
Accommodation, Convergence and Miosis when looking at near objects
How to calculate the stimulus to accommodation ( STA )
1/viewing distance with units as Dioptres
How to calculate amplitude of accommodation
1/near point - 1/far point
Pt with refractive error correction has their far point at infinity
How to measure amplitude of accommodation
Use RAF rule –> Bring target closer from clear to blur until first sustained blur and repeat 3 times for RE, LE and BE
Units in dioptres
How to record result for amplitude of accommodation
Amplitude of accommodation with RAF rule push up method : 12D, 12D, 12D
Duane-Hoffstetter equation
Max amplitude = 25 - 0.4 x age
Average amplitude = 18.5 - 0.3 x age
Min amplitude = 15 - 0.25 x age
Factors to consider for accommodation measurements
Uncorrected refractive errors, test conditions, patients motivation to exert effort and their criteria for blur
What is lag of accommodation
When accommodation is less than STA where large lag = blurry vision
What is lead of accommodation
When accommodation is more than STA where accommodation is closer than STA
Presbyopia
40-50 years old and is a natural loss of AoA with age
What is presbyopia caused by
UV, Lens sclerosis and lens hardening
How is Assessment of accommodative facility measured
Measured free space or with lens flippers and patient needs to be fully corrected for refractive error
Lens flipper vs free space
Free space have higher CPM than lens flipper due to less accommodative change and binocular testing helps with free space
Assessment of accommodative facility requirements
Patients accommodative range must be greater than the lens flipper power
Both tests have to be 1-2 lines better than their VA
Using Free space to test for accommodative facility
Distance correction on, the test uses both near and far object at 40cm and 6m. Ask patient to focus on distant until clear and jump to near target until clear. Repeat for 1 min and units in CPM
How to use lens flippers for accommodative facility
Distance correction on and covering the eye not tested. Use default lens flippers at +/-2.00D flippers. If pt cannot focus with it then use lower power flippers
Lens flipper instructions
As pt to focus until clear starting with the plus lens, when clear flip the lens and the focus until clear. Everytime patient clears the image they must say clear.
Also timed for a minute and units in CPM
Recording results for lens flipper
Can write “Showed fatigue”, “Diplopia on minus” or “Slower on plus than minus”
What does perimetry do
Assess the visual field
What is perimetry used for
Diagnosing visual pathway damage, monitor disease/treatment progress. Also a direct measure of visual function
Perimetry advantages
Quantitative, simple and automated
The central visual field
30 degrees out
What measures the central visual field
Visual field analyser
What measured the peripheral visual field
Arc Perimeter
What is the island of vision
3D sensitivity profile across the visual field
What happens if a stimulus is outside of the island of vision
It is not seen
Ways to assess the island of vision
Kinetic and Static strategy
What is the Kinetic strategy
Stimulus moved from periphery to centre and patients reports detection
What machine is used for the kinetic strategy
Goldmann bowl perimeter, different light intensities plots isopters
Disadvantages of the Goldmann bowl perimeter
Time consuming, not automated, rarely used in modern practice
What is the static strategy
Stimulus shown randomly shown 1 at a time, the brightness is increased until detection where the sensitivity is the inverse of the detection luminance
What are the two types of the static strategy
Full threshold and Supra-threshold strategy
What is the full threshold
Threshold determined at all points in the visual field but it is the very time consuming but the MOST accurate
Advantage of full threshold strategy
Monitors change over time
What is the supra-threshold strategy
Stimuli is set at a set just above the expected threshold in their visual field. This means that no further testing is done if stimulus is seen.
If the stimulus is not seen then use staircase method to determine threshold
Advantage of Supra-threshold strategy
Very fast to do
Decibel scale graph for perimetry
0 decibel = 0 log scale = 1000 intensity
10 decibel = 1 log scale = 100 intensity
20 decibel = 2 log scale = 10 intensity
What does Humphrey visual field analyser measure ( HFA )
Measure Centre island of vision
How to use the HFA
One eye is covered and the patient presses a button when they see a light flashing
HFA advantages
Automated, full threshold, most used, gold standard for glaucoma patients, it is a short wavelength automated perimetry ( SWAP )
What is a scotoma
Area of reduced sensitivity
What is diffuse depression
reduced sensitivity is constant on all parts of the retina, can be due to cataracts
Sensitivity profile parameters
Either stimulus or patient parameters
Stimulus parameters to consider
Size, duration, colour and background luminance
How is stimulus size determined
Using Riccos law ; Luminance at threshold x Area = Constant
What is the normal size used in perimetry
26 arcmin
How is stimulus duration determined
Using Bloch’s law ; Luminance at threshold x Duration = Constant
What is the critical duration
0.1s
What is the saccadic reaction time
0.25s
What is the duration of stimulus in perimetry
0.2s
Patient parameters to consider
Dark adaptation level, age and uncorrected refractive error
What should the normal background luminance be
In the low photopic range of 31.5asb
What is asb
Apostilb
Why should background luminance be in the lower photopic range
Shorter adaptation time for the patient and also reduces sensitivity to fluctuations in ambient luminance. Dark adaptation affects island of vision and pt should adapt to background level of the test machine therefore testing with bright lights are not done before visual field testing
Sensitivity decreases with what and at what rate
Decreases with age, after 40 years of age sensitivity decreases at 1dB/decade
What does uncorrected refractive error do to the contrast sensitivity of the fovea, and why
It decreases the contrast sensitivity because defocus raises threshold near the fovea distorting the island of vision reducing the central peak
What is a problem with corrective refractive error during visual field testing
Lens-rim artefacts
Lesions in the visual pathway
Monocular, Binocular lesions and pituitary tumors
Where is Monocular lesion located
Pre-chiasmal
Where is Binocular Lesion
Post chiasmal or symmetrical pre-chiasmal
What is hemianopia
Loss of half of their visual field in ONE eye
What is homonymous
Same side of each eye affected
What is heteronymous
Opposite side of each eye affected
What is Quadrantonopia
1/4 loss of visual field in each eye
What is congruous / Incongruous
Same/Different shape in each eye of visual field loss
Optic fibres crossing patterns
Temporal fibres do not cross but nasal fibres do.
Nasal fibres have an anterior knee of willbrand and posterior knee of willbrand
What is the sella turcica
Houses the pituitary gland
What is macular sparing
Posterior cerebral artery obstructed gives homonymous hemianopia. During SPARING, area 17 is supplied by both the posterior and middle cerebral artery so still receives blood if posterior cerebral artery is blocked.
What part of the retina is affected if the inferior vision has a defect
The Superior retina
How many types of confrontation is there
3
What is confrontation 1
Rapid check for gross field loss in the periphery using confrontation wand in dim illumination
What is confrontation 2
Patients can identify differences in hue, this is done monocularly. Two red targets on either side of fixation and is patient tries to identify if one target is less saturated than the other. If different then visual field defect indicating cone-dominated pathway
What is confrontation 3
Count fingers
What is used to assess the central macular function
Black or white Amsler grid
Amsler grid advantages
Simple, fast and sensitive for little macular defects
How to use the Amsler grid
Used at 30cm where each square is 1 degree on the retina.
If patient describes any distortion, missing or blurry areas then macular defect
Also checks for macular sparing
What is the Friedmann Visual field analyser
Gradient adapted using the supra-threshold strategy and uses multiple stimuli at the same time and pt reports amount of stimuli seen
What is the problem with the Friedmann visual field analyser
Makes process faster, but if the pt gives wrong answer, you dont know which stimulus is not seen
What are the 2 types of test patterns in visual field analysers
24-2 and 30-2
What is 24-2
54 Black dots
What is 30-2
Black and red dots = 76 dots total
What does the blue dot represent
Blind spot
How to maintain patients fixation onto targets during visual field testing
Need a fixation target that is short and random
How to monitor patients fixation onto target
Show stimulus at the blindspot, if seen then the fixation is lost or use the automated gaze tracker
HFA plot graphs
Varies in numbers around visual field in decibels that represent sensitivity
How is data from HFA plotted
Using variable number of flashes to determine threshold at each point
False positives and Negative errors in HFA
False positives = Seeing a non-existent stimuli
False negative = Failing to see repeated stimuli above threshold
Factors for errors in HFA
Patient reliability and attentiveness
What is total deviation in HFA
Point by point difference from expected age-corrected normal values and shows a general depression
Disadvantage of total deviation in HFA
Scotomas may be masked if overall sensitivity is pressured from cataracts
What is the pattern deviation plot in HFA
Most useful plot, defects may not be apparent in total deviation plot but shows up in pattern deviation plot exposing abnormal patterns in the island of vision
Advantage of Pattern deviation in HFA
Confirms scotoma after adjusting for overall depression of island of vision
How many global indices are there
2
What is global indices 1
Has a Mean deviation and pattern standard deviation
Mean deviation is the mean elevation of depression of overall sensitivity compared to the age-corrected normal
Pattern standard deviation measures the amount which the island of vision shape varies from the age-corrected normal where a low PSD = smooth island
What is global indices 2
Has a Short term fluctuation and a corrected pattern standard deviation
Short term fluctuation tests for consistency of patient responses where 10 points in the field are tested twice allows consistency
Corrected pattern standard deviation is the pattern standard deviation minus the estimated scatter shown by the short term fluctuation in patient response
What is the Glaucoma Hemifield test ( GHT )
Compares 5 zones in the upper field with mirror images in the lower field
Disadvantage of GHT
Does not analyse nerve fibre defects serving temporal visual field
How to assess screening results
Study the raw data
Normal points are 6db suprathreshold
Comparing fields of BE to look for asymmetry
Suspicious if there is a cluster of more than 3 points with more than 8dB loss
Results is a FAIL is 15% points are abnormal or if there more than 50% of the points abnormal in any quadrant
How many tests needed to confirm glaucoma
3 needed`
Detecting glaucoma using SWAP
Detects glaucoma 3-5 years earlier than white on white due to having less blue cones in the retina hence less ganglion cells activated. Not that effective though
Detecting glaucoma using FDT ( Frequency doubling technique )
High temporal frequency information carried by the magno-cellular ganglion cells hence Glaucoma indicates magno-cellular cells loss.
Sinusoidal grating flickers at 25hz for 7.2s, Pt sees flicker = normal. In glaucoma the stimulus is not seen
FDT not useful to monitor progression but good to detect glaucoma
What is contrast sensitivity ( CS )
Difference between the brightest and dimmest part of a target
Contrast sensitivity equation
1/contrast
How to convert contrast into a linear scale
Log(1/contrast) via LogCS
Contrast sensitivity table
100% contrast = 1 contrast ratio = 1 contrast sensitivity = 0 LogCS
90% contrast = 0.9 Contrast ratio = 1.11 Contract sensitivity = 0.05 LogCS
Weber contrast ( Luminance )
For Non-repetitive patterns and calculated as background difference / Background brightness
(Lmax - Lmin) / Lmin
What is the weber contrast best for
Optotypes
Michelson contrast ( Modulation )
For repetitive patterns and calculated background difference / Total brightness
(Lmax - Lmin) / (Lmax + Lmin)
Similarity in both contrasts
0% means no contrast and a minimum of 90% contrast required by british standard for high contrast letter acuity charts
Why do we measure contrast sensitivity
VA measures ability to see small and high contrast objects however everyday objects have more contrast than small size
What is contrast sensitivity affected by
Optical Blur, optical media disorders, retina and neural processing
What relationship does contrast and VA have
A proportional relationship where contrast sensitivity function is the area under the functions curve is the functional vision
3 points of contrast sensitivity function are
A, B and C
A is the Peak contract sensitivity of 1% at the peak of the graph
B is the visual acuity at 30CPD at the end of the graph
C is the difference between points B and A that determines the slope in between points A and B
Charts with variable size and contrast
Vistech chart and FACT ( Functional acuity contrast test )
Charts with Fixed contrast and a variable size
BL low contrast chart
Charts with a fixed size and variable contrast
Pelli-robson chart and the Melbourne edge test
VISTECH chart
6 rows and 9 sine wave grating columns
Each row has different spatial frequency of 1-24 PD
Gratings are tilted in three directions or is blank
Patient will indicate is the patch is blank or which direction grating is tilted in
No longer commercially produced
FACT ( Functional acuity contrast test )
Aks VISTECH V2
Has 5 rows and 9 grating patch columns where each row increases with spatial frequency from 1.5-18cpd
Each row decreases in contrast in 0.15 log steps and edges of the patches blend into the grey background
What are the benefits of grating charts
Assess CSF from low to high spatial frequencies used on illiterate children or patients usings hands or pencil to demonstrate orientation of grating
High and Low contrast Bailey lovie charts
Not just a contrast sensitivity chart
Has fixed number of letters per line, log progression of letter size and spacing and has standardised letter set and a standard scoring system
Produced in low contrast 18% weber contrast for clinics and proposes that low contract acuity charts gives same information as contrast sensitivity function testing
The difference in HC and LC VA is constant in all acuity ranges of about 2 lines.
LC BL chart useful as screening tool
Pelli-robson chart
8 lines of letters where each subtends 3 degrees to the patient at 1 metre
2 groups of 3 letter letters per line where letters in each group have the same contrast
Contrast decreases by 0.707
This uses letter rather than sine wave gratings and is a recognition task hence more cortical processing needed compared to a direction detection of sine wave grating
Melbourne edge test advantage
Compact and portable non grating contrast sensitivity test
Melbourne edge test
It is small, has 20 disks each 25mm in diameter
The test is in 4 rows and 5 columns where each disk has an edge that decreases in contrast from the top to the bottom of the chart
Patient indicates direction of edge in each disk
Why is contrast sensitivity used in clinics
Explains poor vision symptoms of poor vision even though they have good VA. Also used after cataract assessments and post-lasik surgery
Also looks for glaucoma, macular degeneration and diabetic maculopathy
What is glare sensitivity
Measure effect of glare on patients visual function
What is the 2 types of glare
Disability and Discomfort glare
What is disability glare
Glare that reduces visual function
What is discomfort glare
Glare that causes discomfort but does not reduce visual function
What type of glare is do we get in everyday life
Both disability and discomfort
Glares effects on vision
Alot of patients with media haze retains good VA but bad visual problems.
Glare also depends on media clarity
How to measure glare
Standard high contrast acuity does not assess glare effect on vision. Disability glare is assessed using a contact threshold value
What is the contact threshold value
When the test target is JUST detectable with and without glare source
What is the Brightness acuity tester ( BAT )
Hand held device that performs Brightness acuity test and the Macular photostress test
How many conditions can the BAT simulate and what are they
3, High direct overhead sunlight, partly cloudy day and a low-bright overhead commercial light
How is the brightness acuity tester used
Tests monocularly using all three settings with the patient at the standard distance from the letter chart, undilated pupils in a dark examination room
Test and record VA at each of the three settings.
What are the possible results that can be recorded for each test with the BAT
No change in acuity, reduction in acuity and improvement in acuity
What is the Berkeley Glare Acuity tester ( BGAT )
A reduced HC and LC Bailey lovie letter chart on an opaque panel in the centre of a plexiglass screen.
The chart is front illuminated with the glare source from trans-illumination of plexiglass panel
BGAT test conditions
High and Low contrast VA measured at 1 metre with and wihtout glare source
The Macular photostress test using the BAT
Not a measure of glare sensitivity. Here the BAT provides the hemispherical lighting to stress the macular region. This test is to detect patients with macular diseases
How is the Macular photostress test done with the BAT
Light is shone for 10s, then measure time taken for VA recovery within 3 letters
How long does it take to recover from the macular photostress test
20-30s, macular diseases takes longer to recover
What is colour vision
Ability to see HSL, Saturation, Brightness, Hue, CMYK and Chroma
Why is evolution an evolutionary advantage
For us to see ripeness of fruit, spot predator or prey and to attract mates
What is young helmholtz theory on colour
There 3 primary colours in vision, originally thought to be RYB but its actually RGB. A combination of these colours can make other colours
Which photoreceptor is for colour
Cones
What are the three types of cones
Long, Medium and Short wavelength cones
What is Herings theory on colour
This theory proposed 4 primary colours of R/G and Y/B in opponent colours. A third pair is the luminance mechanism
What is the luminance mechanism
This mechanism explains no reddish green or bluish yellow colours and explains afterimages
What is the current theory on colour
Both Herings and Helmholtz theory
What does the current theory for colour say
Trichromatic at receptor levels, opponent colours at ganglion cell level onwards and Munsell colours in the Cortexx
What is the chromativity diagram
2D representation of 3D colour space giving a brightness constant
Features of the chromativity diagram
Centre of the diagram is the location of equal energy
the Black line is the colour of blackbody emitters
Can use diagram to find dominant wavelength
Has MacAdam eclipses
What are MacAdam eclipses ( Ovals )
These show the colours the look the same to a normal observer
How many colours are luminance controlled
17000
How many colours are there overall
3 million
What is a Protan defect
Red cone defect so sees more green
What is a Deutan defect
Green cone defect so sees more red
What is a Tritan defect
Blue cone defect
Who are monochromats
People that see in black and white
Problems with monochromats
If no Long or medium cones the VA decreases resulting in photophobia and Nystagmus
Who are Dichromats
People with two types of photopigments with one missing. This condition is luminance controlled and has a confusion loci where different colours appearing the same
What is a trichromat
Have all photopigments aka Normal
What is an Anomalous Trichromat
1 photopigment has a different sensitivity to normal. The most common type of colour deficiency with different levels of severity `
What is a Tetrachromat
Having 4 photopigments like in some animals
What is a confusion loci
This is plotted on a CIE diagram where the isochromatic lines show the colours that look the same
Isochromatic lines are shorter for anomalous trichromats as they dont have a full range of colour compared to a drichromat
What is an inherited CVD
From birth where the type and severity is the same throughout life where VA and VF is normal with mainly red and green defects.
Deutan
Green cone defect, this is the most common followed by protan and tritan
Which colour vision defect is more common than dichromats
Anomalous trichromat, this has a large variation in different ethnic groups
What is the male deutan prevalence in descending order
Europeans, Asians, Africans, Indigenous americans and polynesians
What type of genetic defect are Protan and Deutan
X-linked recessive ; Sons inherit from mother who was a carrier and all daughters inherit recessive gene from a red/green deficient father
What type of genetic defect is tritan
Autosomal dominant with incomplete expression and affects males and females equally
What is a factor that can change colour appearance
Illumination source
Why is illumination during colour vision tests important
Illumination changes perception of colour vision defect tests where people with CVD could pass and people with good colour discrimination make errors
Are cooler colours higher or lower temperature
Higher temperature
What can we use for illumination
Illuminate C for tests needing pigment colours and Macbeth lamp for tungesten illumation
What are the types of colour tests
Pseudoisochromatic plates, Hue discrimination or sorting tests, Colour matching ( Anomaloscope aka Medmont C100 ) and Colour naming ( Falant Lantern )
What can be used to detect colour vision defect
Ishihara test, city university colour vision test, lanthonys trion album and the Farnsworth D15 test
Which colour vision test is the most common in NZ
Ishihara plates
How does the Ishihara test work
Screens for protan and deutan defects only, has a diagnosis plate but not always accurate
Count the number of errors, allowing 3s for response for each plate. Number of errors does not give severity
How many types of plates are there in the Ishihara test
5 –> Demo, Transforming, Disappearing, Appearing and the Diagnostic plates
How far away is the ishihara test done and at what VA
75cm with VA at least VA of 6/60 with Rx
Who can see disappearing plates
People with Protan defects
Who can see appearing plates
People with Deutan defects
What happens if the Ishihara test is failed
Move onto Medmont C100 ( Anomaloscope )
How does the Medmont C100 work
This test separates Protan and deutan, patient will adjust amount of red and green in the mixuture until there is no flicker. 7 readings are taken and then averaged
When is the medmont C100 used
Not a screening tool so used after a colour vision defect is picked up
How does the medmont c100 separate Protan and Deutan defects
Protan defects adds more red due to reduced red sensitivity, Deutan adds more green
What is the gold standard to measure CVD severity
Anomaloscope
Which CVD tests do not give severity
Ishiharam Medmont C100, D15 and the F2 plate
Which CVD tests claim to give severity
City university and Lanthonys tritan album but are inaccurate
What are the problems with the city university colour vision test
1st and 2nd editions found fewer protans failing but more deutan failing compared to the D15
This test is also a forced choice
How does the City university 3rd edition work
First part is screening there you look at a row of 3 spots and identify which spot is different to the other two is any
Second part is detection where you identify which spot is the closest in colour to the centre spot
What does the Lanthonys tritan album detect
Detects tritan defects only
How does the lanthonys tritan album work
There is a demo and 5 test plates with a combo of grey and blue-purple dots of different luminance. The patient indicates if there is a coloured square in one corner
What is the Farnsworth F2 plates
Asks for how many squares are seen and if seen, which is more obvious and indicate where on the plate it is seen. This test does not use colour names and is also no longer produced
What is the D15 test
A sorting test where you arrange caps in their colour sequence starting with the reference cap
What does the D15 test identify
The confusion loci if the test is failed
What is the advantage of this test
Separates mild and moderate defect from a severe defect when results are combined with other test results
How to graph the data for the D15 test
15 colours in a rough circle around a point and connect the dots according to the patients arranged sequence
How to pass the D15 test
Max of 2 transpositions of adjacent caps or one major dirchromatic crossing. Much more Anomalous trichromats pass compared to dichromats
Difference between D15 and D15 desaturated
Desaturated D15 has similar colours but paler using an illumination of 1000 lux
What is the H16 test
Detects only protan and deutan, similar to D15 but different hues are chosen and are more saturated
What is the munsell system
Defines colour by hue, value and chroma
How is hue defined as
There are 10 hues, Red, yellow, green, blue and purple. Hues have 10 steps ( 1RY to 10RY )
What is Value in the munsell system
The amount of light reflected where black is 0 and white is 10
What is chroma in the munsell system
Amount of colour present similar to saturation
What is the Farnswoth-Munsell 100 Hue
A sorting test for hue discrimination, this is not a screening test and used to assess/monitor CVD and identify colour normals with very good colour discrimination
How does the farnswoth-Munsell 100 hue work
4 boxes of colours caps. Each box has 21 caps, first box has 22 caps with colours varied by hue and not chroma
Performance Factors when doing the Farnsworth-Munsell 100 hue
Performance factor is age, learning effects have a 30% improvement in their score
How to calculate Error score in the Farnsworth-Munsell 100 hue
Record order with the patient arranged the caps, compare adjacent caps and find the different between the adjacent caps where normal is a value of 2. Subtract 2 from all results and add the scores together to get a total error score
Total error score for the farnsworth-munsell 100 hue
less than 20 is superior and 20-1– score is average, more than 100 score is a problem
What is the Z score
Relates error score to the population normal
How to calculate Z score
Mean / standard deviation
What does a Z score of 0 mean
The mean for the population
What does a negative Z score mean
Better than population mean
What does a positive Z score mean
Worse than population mean
Computer analysis of sorting tests
Helpful when error score is very large to tell the axis of confusion. This uses vector analysis
What is the angle for major axis of confusion
Horizontal for protan and deutan and Vertical for Tritan
What does the S index measure
Randomness
What does the C index measure
Severity
What is Retinoscopy
An objective refraction, objective because patient does not need to respond
How does the retinoscope work
Has a bulb and a lens system where the lens is moveable and changes the position of the light source where the beam is either convergent or divergent depending on the type of mirror
What are the two types of retinoscopes
Spot and Streak retinoscopes
Spot retinoscope
Patch of light is round with the bulb is a small coiled filament 1-2mm in size
Streak retinoscope
Patch of light is rectangular with the bukb is uncoiled linear filament, bulb is rotated to change the orientation of the projected beam which is helpful to find axis of astigmatism
What are the two types of Retinoscope mirror types
Plane and Concave
Is light convergent of divergent in plane mirror
Divergent
What is the secondary source and how is it made
In the retinoscope the bulb is fixed but the lens distance changes which changes the beams vergence when leaving the mirror producing a secondary source ( S’ ) either behind or infront of the retinoscope
What influences the movement of light on the retina when the retinoscope is tilted
The position of the secondary source
Plane mirror retinoscopy
Sleeve is down decreasing bulb-lens distance decreasing the effective power causing the light to diverge from the retinoscope. Here the secondary source is behind the retinoscope.
A with movement is Hyperopia
An Against movement Myopia
Concave mirror retinoscope
Sleeve is up increasing bulb-lens distance increasing the effective power causing the light to converge from the retinoscope. Here the secondary source is between the retinoscope and the patient.
With movment = Myopia
Against movement is Hyperopia
Which type of retinoscope is recommended to use
Plane mirror retinoscope
Advantages of the plane mirror retinoscope
With movements are easier to see and the sleeve is easier to reach with the thumb to change from plane mirror to concave mirror to confirm
Retinal reflex observations
Observe movement of image from the patch of light where the movement direction of the image depends on the refractive state
What is neutralisation in retinoscopy
Aligning the retina plane with the retinoscope plane
Reflexes to observe in retinoscopy
Direction, Speed, Brightness and Width
Faster the speed the closer to neutral
Width from meridional direction either large or small refractive correction
Observation of Hyperopia
This is with movement
The image patch on the retina is virtual and behind the pt so when face beam is tilted u then the reflex also goes up
Neutralising Hyperopia
Add positive lens toward retinoscope from behind it, when neutral the reflex will flash instantaneously in and out of view
Adding more positive power after neutralisation will have a reversal in movement of reflex so an against movement is seen
Observation of Myopia
This is against movement
Image patch on retina is real and between the pt and the observer where face beam is up and the reflex moves down
Neutralising Myopia
Add negative lens of increasing power, the reflex image will move towards the retinoscope. At neutral the reflex flashes instantaneously in and our of view
Adding more negative power after neutral will reverse reflex movement so a with movement is seen
What does a speed of a reflex tell you
Slower speed means more ametropia
Infinitely fast speed of reflex means neutral
On-Off phenomenon
If the retinoscope is placed in the far point there will not be a with or against motion as the retinoscope is placed at neutrality. This phenomenon is known as the end point when trying to neutralise
What does the brightness of a reflex tell you
Closer to emmetropia has a brighter reflex
In hyperopia the retina is closer to the retinoscope and in myopia the retina is further from the retinoscope
What does the reflex look like in the plane mirror
Reflex smaller and brighter for hyperopia than myopia where the S’ is closer to the hyperopic far point
What does the reflex look like in concave
Reflex is smaller and brighter for myopia than hyperopia where S’ is closer to the myopic far point
Controlling accommodation methods
cycloplegia, Controlling fixation distance, Inserting working distant lens and Checking BE for against movement
How does cycloplegia work
Using drugs to relax ciliary body
How to control fixation distance
Asking patient to look at target at 6m with BE open using a large letter with duochrome overlay. Ask pt to look at the green background. Then scope the RE while the patient observes with their left eye.
Ask pt to indicate if your head is blocking their LE view
Important to keep close to their line of sight you are scoping
How does a working distant lens work
Working distant lens will relax their accommodation and will move their far point to the retinoscope and has their distant vision blurred in an emmetropic eye
In a myopic eye the WD lens moves their far point in front of the retinoscope so their vision is more blurred
In a hyperopic eye the WD lens moves their far point behind the retinoscope and +2 or +1.5 lens may not be enough to cause blur
Checking both eyes for against movement in retinoscopy
If movement is with, add plus lens until BE are blurred making the eye myopic eith their focal plane in front of the retinoscope
What is a working distant lens
Not possible to do retinoscopy from infinite distance so we adapt a working distant of 67cm at +1.50D. The working distance lens adjusts for vergence from near position and helps to relax accommodation
