3.1.4 Identifies abnormal colour vision and appreciates its significance. Flashcards
Difference between color vision types
- Dichromacy is the state of having two types of functioning colour receptors
- Monochromacy is the state of having one type of functioning colour receptors
- People with ‘faulty’ trichromatic vision will be colour blind to some extent and are known as anomalous trichromats. In people with this condition all of their three cone types are used to perceive light colours but one type of cone perceives light slightly out of alignment, so that there are three different types of effect produced depending upon which cone type is ‘faulty’.
- Monochromats are weird but crazy rare! - normally have reduced VA, visual fields, nystagmus, photophobia
- Trichromacy / trichromats = normal CV
Congenital Vs Acquired:
Congenital
8% males (mainly red-green)
0.4% females
Onset at birth
Type & severity constant
Easy to classify
Both eyes effected equally
VA and visual fields normal (except monochromat)
So, the chances are, in a clinic of 12 children (if they were all males) one would have a colour vision defect
Acquired
Equal prevalence male/female
Onset after birth
Deficiency varies
Not easy to classify
Monocular differences
VA and visual fields may be affected
Difference between colour vision deficiency
- Protanopes cannot distinguish between Red-Green-Yellow
- May be able to distinguish between these colours based ONLY on brightness
Reds may be confused with black or dark grey (may not see a red traffic light)
- May be able to distinguish between these colours based ONLY on brightness
- Deuteranopes also cannot distinguish between Red-Green-Yellow
- However, they do not have the dimming effect a protanope has e.g. with reds being confused for black or dark grey
- Anomalous trichromats confuse pale, desaturated colours, but not bright and saturated ones
- Tritanope cannot distinguish between Blue-Green-Yellow
Protan CVD - red = long = 700nm
o Protanopia – L wavelength cone missing
o Protanomaly – L wavelength cone is anomalous/reduced sensitivity
Deutan CVD - Green = medium = 540nm
o Deuteranopia – M wavelength cone missing
o Deuteranomaly – M wavelength cone is anomalous/reduced sensitivity
Tritan – blue = short = 490nm
o Tritanopia = S wavelength cone missing
o Tritanomaly = S wavelength cone is anomalous/reduced sensitivity
Acquired Defects testing/types
City should be used as it is sensitive to blue-yellow defects
Monocular testing – may be unilateral or bilateral/asymmetrical
Secondary to pathology both ocular and systemic disease
Associated with loss of VA and VF
Type 1 (similar to protan) – Red/Green
o Progressive cone dystrophies
Type 2 (similar to deutan) – Red/Green
o Changes at the optic nerve
o Retrobulbar neuritis (common with ON defect)
o MAOIs
Type 3 (similar to tritan) – blue/yellow
o Found in many central and peripheral retinal lesions and lesions of the visual pathway
o Retinal vascular disorders
o Cataract
o Glaucoma (some evidence of red-green also
o Drugs – hydroxychloroquine. NSAIDs
o Alcoholism
describe Ishihara
A Confusion test used to screen for protan and deutan (R-G) CVDs
o No blue/yellow plates
38 or 24 plate version, does not grade severity – pass or fail basis
38 – three fails = pass
24 – two fails = pass
Should be used @ 75cm
Plates should be illuminated by daylight/artificial daylight, illuminant C or D65 luminance in the range 250-600 lux
4 seconds per plate
Px must have a VA of at least 6/24
Colour normal will make immediate decisions, CVD takes time to use brightness cues
Avoid touching plates or leaving in sunlight to prevent fading
Ishihara
Types of plates
- Plate 1 - demonstration plate
o Designed with brightness differences so all px (including CVD and rod monochromats) can see
o If px cannot see this plate they either have poor VA (<6/24) or are malingering - Transformation plates (2-7)
o Colour normal see’s one figure whilst the CVD sees a different one
o The transformation occurs because the colours that transform / disappear are on the same isochromatic confusion line as the characters / background.
o This can therefore make the 74 appear as a 21 in a red-green CVD patient
o These plates are useful as it keeps the patient motivated as they can still see numbers - Vanishing design (8-13)
o Both figure and background lie on an isochromatic confusion line so that the figure cannot be distinguished in CVD, but easily seen by a colour normal - Hidden digit plates (14-15)
o The colour normal sees no figure whereas the CVD will see a figure
o The colours which make up the background are numerous; and all lie on isochromatic confusion lines so will have similar appearance in CVD
o The colours which make up the figure have similar hue but are less saturated than those used for the background, but again lie on an isochromatic confusion line
o One set of dots look the same to make the background and another set to make a figure – a digit will be discerned in CVD
o Works best for dichromats
o Half of normal individuals will fail these plates - Classification plates (16-17)
o Used to differentiate protan from deutan and employ a vanishing design set on a neutral background
o 2 digits one made up in reddish dots – lies on protan isochromatic confusion line, one in purple which lies on deutan isochromatic confusion line - Wavy line plates = used in young children/illiterates, however not recorded on score card. Px asked to trace wavy line, which is not ideal as px has to look at the plate for more than 4 seconds and therefore can use brightness cues.
City
Three editions – edition 3 in practice
This is useful for acquired CVD and to grade the severity of a congenital defect
Confusion test – used for classification
Subjects select the peripheral colour that looks closest to the central spot
o 3 peripheral colours are isochromatic confusions with central spot, the fourth colour is the next colour in the D15 sequence
4 screening plates
6 detection plates
Classification based on number of errors
Advantages = available in book form, not available online so px can’t practice
Disadvantages = px can give mixed response (go for the majority), protan classification poor due to brightness cues, different pages have different classification efficacies.
Use @50cm
Differences between City & Ishihara. Learn front page of Ishihara. Learn how to explain what deutranopia etc is
- City is at different working distance
- City allows identification of tritan defects
- City can grade a defect from mild, moderate & severe more accurately but would still need Ishihara added with it to make the full conclusion
Farnsworth D15
15 loose caps, on fixed cap/reference cap in the box
Colour confusion test (based on isochromatic confusion lines)
Caps have approx. equal hue differences, colours lie on isochromatic confusion lines
Not a screening test
Pass and fail criteria varies;
o 1 minor error (transposition of adjacent caps permitted
o 1 major error (isochromatic confusion) permitted
o 2 major errors (isochromatic confusion) permitted
Farnsworth 100 hue
Colour matching / hue discrimination test
Developed for vocational use
CVD classified from position of errors, does not reliably identify mild CVDs
TES is calculated, perfect observer error score = 0
Anomaloscope
Based on a colour match
Two different light sources must be matched to the same colour
- GOLD STANDARD CV TEST
- Mainly used in research & teaching institutions
- Differentiates dichromats from anomalous trichromats
- Types: Nagel (R-G), Besancon anomolometer (R-G & B-Y), Pickford-Nicolson (R-G & B-Y)
Vocational standards (more in-depth standards can be found on AOPs website)
Police Scotland
o Monochromats rejected
o Mild anomalous trichromats acceptable and treated as normal
o Severe anomalous trichromats and dichromats also acceptable and are to be instructed in coping strategies
o Applicants who show lowered discrimination for blue colours are to be referred to an ophthalmologist for further assessment. This must include a measure of their dark adaptation performance.
Army/RAF/Navy
o Ishihara normal
Fire services
o Minimum standard – pass Farnsworth D15
o Monochromats & dichromats = not accepted
o Anomalous trichromats = occupational testing
Electrical engineers
o No more than two incorrect plates with ishihara
o Holmes-Wright or Giles-archer lantern test
o CAM lantern
Pilots
o Ishihara – 24 plate, must pass first 16 plates
o Ishihara fail
▪ Anomaloscope – Nagel or equivalent
▪ Colour assessment and diagnosis i.e. CAD test: pass if threshold is < 6 SU deutans / <12 SU for protan
When to test Colour Vision:
- Children at school entry age, for advice on colour recognition.
- Children at 11 - 14 years, for advice relevant to prospective careers.
- Any patient reporting changes to their colour vision, monocular or binocular.
- Patients taking drugs known to cause colour vision changes e.g. certain antipsychotic meds like chlorpromazine
- As an aid to diagnosing or monitoring pathology. - optic neuritis, AMD (blue/yellow)
- Trauma to back of head - V4 is there
- Occupational purposes
Hue Discrimination tests:
-
City - NOT a screening test
- Good lighting, book at 35-50cm, 3 seconds per page
- Farnsworth dichotomous D15 test - NOT a screening test
- Allows identification of protan, deutan AND tritan defects
- Moderate-severe defects
- Monitors acquired
- Farnsworth-Munsell 100 hue test
- Time-consuming!
- Direction of the spokes indicates the type of defect. Numerical score indicates severity of the defect
- The colours chosen for the caps are based on the CIE diagram
Genetic Implications:
- Just read the entire lecture, breaks it down very well:
- R-G is X-linked recessive, type & severity same within family
- B-Y is autosomal dominant defect, very rare, carried on chromosome 7. 1 parent needs it then child will get it. Severity can vary within family
