3.1.4: Identifies abnormal colour vision & appreciates significance Flashcards
What % of males have CVD? What is the breakdown of the %?
8% of males have CVD
5% are deuteranomolous trichromat
1% are deuteranope
1% are protanomalous trichromat
1% protanope
Why is it important to test colour vision?
Careers: pilots, armed forces, police, electrical engineers
Safety: traffic lights, electrical wiring
Pathologies
What is colour theory?
- responses from 3 types of photoreceptor are transformed by complex neural network in retina so at level of ganglion cells colour info is coded into:
- 2 opponent colour channels: red v green, blue v yellow
- 1 opponent luminance channel: black v white (brightness/luminance)
- Human eye can see 380nm - 780nm using (tricolour) three photoreceptors (S, M, L cone) of overlapping spectral sensitivity (Receptor level).
- Cones S, M, L function at high level illuminance, rods fnction at low level illumminance
Describe vision being trichromatic?
Each photopigment is maximally (but not exclusively) sensitive to a different region of visible EM spectrum
- Red (long wavelength 620nm - 720nm)
- Green (medium wavelength 500nm - 575nm)
- Blue (short wavelength 450nm - 490nm)
* Blue cones are absent in central fovea
* Red/green cone ratio vary greatly from person to person
Describe congenital CVD?
- Higher incidence in men than women as X-linked autosomal recessive trait (1 in 12 males, 1 in 250 females)
- Protan: confuse reds and greens, both appear desaturated yellow
- Deutan: confuse reds and greens, both appear desaturated yellow
- Tritan: confuse blues, yellows & greens, appear pink/turquoise
Describe monochromats?
- One functioning cone
- Cone monochromat: single functioning cone
- Rod monochromat: no functioning cones, leads to:
o decreased VA
o photophobia
o nystagmus
o Only percieve brightness variations
Describe dichromacy (absent cone)?
- 2 active cones, 1 non-active
- Protanope 1% (L cone missing, red)
- Deuteranope 1% (M cone missing, green)
- Tritanope 0.001% (S cone missing, blue)
Describe anomalous trichromacy (defective cone)?
- All 3 cones are active but 1 or more has abnormal sensitivity
- Protanomaly 1% (abnormal red cone sensitivity, red defect)
- Protanomalous: red cone sensitivity is shifted towards green cone (shorter wavelengths)
- Deuteranomaly 4.9% (abnormal green cone sensitivity, green defect)
- Deuteranomalous: green cone sensitivity shifted towards red cones (longer wavelengths)
- Tritanomaly (abnormal blue cone sensitivity, blue defect)
- Tritanomalous: blue cone sensitivity shifted towards green cone (longer wavelengths)
What are the colour confusion that protans, deutans or tritans may mix up?
Protan:
- yellow-red/yellow/green-yellow
- violet/blue-green
-purple/grey/green
- brown/green
- red/black
Deutan:
- yellow-red/yellow/green-yellow
- violet/blue-green
- purple/grey/green
- brown/green
- green/black
Tritan:
- blue-green/green
- purple/red
- yellow/white
- violet/grey/yellow-green
- dark blue/black
Describe acquired CVD and the 3 types?
- Secondary to pathology (ocular and systemic), drug linked, defects fluctuate in severity, associated with decreased VA & VF
- Usually monocular & asymmetrical
- Usually blue-yellow
- Type 1: similar to protan - red/green - found in macular dystrophy/hydroxychloroquine retinal toxicity (rheumatoid arthritis)
- Type 2: similar to deutan - red/green - found in retrobulbar optic neuritis (common with ONH defect)
- Type 3: similar to tritan - blue found in many central & peripheral retinal lesions & lesions of visual pathway - POAG, AMD, DR
When should you test colour vision?
- children at 1st eye test - especially boys
- family history of CVD especially mother
- sxs suggestive of impaired colour discrimination
- occupational purposes
- aid in diagnosis or investigation of ocular or systemic disease
- suspected as side effect of medication or injury
What should you consider when using a colour vision test?
Illumination
* results are only valid if the test is administered under the recommended
illumination conditions
o appearance of spectral colours changes with spectral content of the
illuminant (daylight).
Refractive correction
* perform at end of test with appropriate refraction in place
Viewing distance
* Needs to be exact to ensure observations are being made with the central rod
free retinal area
Viewing time
* Short (few seconds) to limit the effects of colour adaptation
Monocularly
* To prevent influence from the fellow (potentially unaffected) eye
Describe the Ishihara colour vison test?
- 38 or 24 plate version (14 screening plates, 2 classification plates, pathway plates non-verbal or numeric)
- Does not grade severity - no. of plates missed does not grade severity
- Pass or fail
- Classification indicate type of defect
- 3 fails pass on 38, 2 fails pass on 24, 6 fails or more then FAIL
- Viewing time is 4 seconds per plate
- Viewing distance 75cm
- Illumination needs to be at least 250 lux - daylight best - Tungston not allowed due to bias, fluorescent is allowed
- Ishihara can ONLY detect protan & deutan –> a tritan will pass it
Describe the plate types in Ishihara test?
- First plate is a test plate always - only a brightenss difference
- Transformation plates - normal sees one number, CVD sees another
- Vanishing plates - camouflage principle, normal sees number, CVD does not
- Hidden digit plates - only seen by CVD px
- Classification plates - only used if screening plates failed to identify defect
- Non-numeric - should be traced <10secs but do not touch the page
24 plate:
Plate 1 demo, P2-9: screening type transformation, P10-13: vanishing, P14-15: hidden dig, P16-17: classification plates, P18-23: tracing (if px can’t read no.s), P24 (trace everyone can do)
38 plate:
P1: demo, P2-9: transformation, P10-17: vanishing, P18-21: hidden digits, P22-25: classification, P26-38: tracing (if px can’t read no.s)
What are the +ves and -ves of Ishihara test?
+ves:
- quick and efficient
- simple to use
- high sensitivity and specificity when performed correctly
-ves:
- no tritan detecting ability
- does not diagnoses severity
- screening plates are very sensitive - risk of false +ves
Describe City Universitiy colour vision test (1st and 2nd edition)?
- Passed by colour normals & mild CVDs
- 10 plates - 5 colour targets per plate subtending 1.5 degrees at 35cm
- Can be used to detect protan, deutan and tritan defect
- Grades severity
- Pages should be shown for 3 secs at 35cm
- Control page first: everyone answers correctly
Section 1: page 1-6, ‘Chroma 4’: 4 dots surrounding 1 in middle, 8mm. “Which is most similar?”
Mild CV defects will not struggle much with this
Section 2: page 7-10, ‘Chroma 2’: 4 4mm coloured spots surrounding central spot. “Choose spot which most closely matches central spot.”
More difficult to distinguish closest colour, so even mild CVD will struggle.
Describe City colour vision test 3rd edition?
- no valuated data - don’t know how good it is
- only 4 plates equivalent to 2nd edition
- criterion (>4 errors) not validated
- 3 coloured spots comparing to left to right row, 10 pages
Describe Farnsworth D-15 test for colour vision?
- Arrangement test - sort coloured samples in order of hue/shade appearance
- 15 Munsell coloured caps & 1 reference cap (starting point)
- Arrange 15 coloured caps into natural colour sequence
- 50cm viewing distance, allow 1-2mins
- Optom plots order on recording sheet using no.s on base of caps (“join the dots” style recording sheet - shape shows defect present)
- Not a screening test
- Detects ~5% of CVD in males, 3% of males w/ CVD will pass it
What does Farnsworth D15 test classify and what are major and minor errors?
Classify: type of CVD and severity
Determines type of defect: protan, deutan or tritan - compare cross over lines with colour confusion axes
Failure: single error of 2 steps or more
Major errors: crossing errors where px cross circle - severe CVD will make these - if px mkes 1 or more then mod-severe CVD
Minor errors: px gets adjacent caps mixed up - can occur in colour normals (some pxs not good at arranging colours) - colour normal or mild CVD will make 1 or 2 minor erros
What is desaturated D15 test used for?
to classify mild CVD, acquired CVD or to detect & monitor pathology
What are the +ves and -ves of D15 colour vision test?
+ves:
- grades severity
- simple & quick
- can classify protan, deutan & tritan defects
-ves:
- not suitable for young children
- does not formally identify anomalous trichromats vs dichromats
What are the occupational vision standards in terms of colour vision?
- Army: person able to still distinguish between red/green may still be allowed for certain roles
- Air service/Pilots: ishihara 38 plates - 9 or more wrong on (1-21), 24 plates - 7 or more wrong on (1-15)
- Fire service: no monochromats or dichromats. Anomalous trichromats need testing (protanomalous likely fail, deutranomalous may pass)
- Police: no monochromats. Mild/severe anomalous trichromats & dichromats allowed but need coping strategies.
- Electrical engineers: need to pass a Clinical Aviation Marine (CAM) lantern test - no official standard but likely no CVD allowed
Name other CVD tests that are available?
Farnsworth Munsell (FM) 100 hue test - takes ages, larger version of D15
HRR - similar to Ishihara in principle but can classify tritan
Nagel Anomaloscope - produces a quantitative measure of the type and degree of a colour deficiency. The gold standard and most commonly used is the Nagel anomaloscope which assesses red/green deficiency
