The neuroretinal cells concerned with processing vision are known as rods and cones, the latter being concerned with colour vision. There are three types of cones:
- Red cones (74% of cones).
- Blue cones (16%).
- Green cones (10%).
Each type of cone has a different range of light sensitivity. Stimulation of the cones in different combinations enables the perception of colours - eg, the perception of yellow results from a combination of inputs from green and red cones and relatively little input from blue cones. If all three cones are stimulated then white is perceived.
Types of colour vision defect
Colour blindness is the inability to distinguish certain colours. It occurs when one or more of the cone types are absent or present but defective and unable to send correct signals to the brain. If a pigment is deficient, a patient is said to have a protanomaly and, if they are entirely absent, they are said to have protanopia.
- Trichromatic: all three cone pigments are present and colour vision is normal.
- Dichromatic: complete deficiency in one cone pigment but the remaining two are normal. There are three types of dichromatism depending on which one of the three normal pigments is missing:
- Protanopes are the most common and lack red sensitive receptors.
- Deuteranopes lack green receptors and tritanopes (rare) lack blue sensitive receptors.
- Protanopes and deuteranopes cannot distinguish red from green light.
- Tritanopes cannot distinguish blue from yellow.
- Monochromatic: only one cone pigment. Tends to be associated with reduced visual acuity (usually 6/60), photophobia, nystagmus and sluggish pupil reflex to light.
- Achromatic functioning cones: can only see black, white and shades of grey. Typical complete achromatopsia tends to be associated with reduced visual acuity, photophobia and pendular nystagmus.
- Most cases of colour blindness are hereditary but are occasionally acquired as a result of eye disease.
- The ability to distinguish colours deteriorates with ageing.
- The most common hereditary colour vision defect is failure of red-green discrimination (prevalence is 8% in males and 0.5% in females). The gene for red-green colour blindness is X-linked recessive.
- Failure of blue-yellow is rare and is more commonly acquired.
- Monochromatic: relatively rare (incidence of about 1 in 30,000).
- Colours which look different to people with normal colour vision can look the same to people with defective colour vision - eg, grass may appear green with normal colour vision but appear to be the same colour as orange for people with certain colour-defective vision.
- There is also a marked reduction in the number of separate colours that can be distinguished in the spectrum.
The Ishihara plates are designed to provide a screening assessment of red-green colour vision:
- The Ishihara plate is a crude test but can be easily used in general practice or casualty.
- It consists of a series of 16 plates with a matrix of dots arranged to show a number.
- It was designed to screen for congenital red and green cone abnormalities and therefore is somewhat limited in use for more complex diagnoses.
- Sit the patient in a well-lit room and ask them to put their reading glasses on. Get them to cover one eye and flick through the plates allowing for about five seconds per plate. Repeat with the other eye.
- The first plate is a test plate that colour-blind patients can see and therefore helps identify malingerers.
- Other causes for not being able to read the numbers easily include poor visual acuity, illiteracy (get them to trace out with their finger on the plate, the figure that they see; there are also picture plates) and severely restricted visual fields. Optic nerve lesions will also affect performance.
- Colour vision is often affected by optic nerve disease:
- Colour vision is affected before visual acuity but will not be noticed by the patient until relatively late and usually after visual acuity is affected.
- For this reason it is very important to test colour vision in a suspected optic nerve lesion and also in suspected thyroid eye disease.
- If you do not have an Ishihara plate, get the patient to look at a bright red object (eg, a child's toy) and compare one side with the other.
- Descriptions of things looking washed out should ring alarm bells.
- Macular disease tends to produce blue-yellow visual defects. The vision may be blurred and there is particularly a straight line distortion.
- Loss of colour vision has been reported in association with diabetes mellitus, cataracts, glaucoma, macular degeneration, Alzheimer's disease, Parkinson's disease, leukaemia and sickle cell disease.
- Colour vision can also be affected by some medications - eg, tamoxifen.
- Some chemicals may cause loss of colour vision - eg, carbon disulphide, mercury, fertilisers.
- No treatment can correct or prevent inherited colour vision deficiencies.
- Treatments that slow or reverse the course of an eye disease may also improve colour vision.
Disorders of colour vision cause difficulty at school and affect the choice of certain jobs and careers that require some degree of colour identification. The list of affected careers includes:
- Certain grades within the armed forces.
- Civil aviation: pilots, engineers, technical and maintenance staff, air traffic controllers.
- Customs and excise officers.
- Railways: drivers, engineers and maintenance staff.
- Fire service officers.
- Hospital laboratory technicians and pharmacists.
- Workers in paint, paper and textile manufacture, photography and fine art reproduction.
Colour blindness need not prevent someone from driving because traffic lights can be distinguished by the position of the light.
Further reading and references
Colour vision examination: A guide for occupational health providers; Health and Safety Executive (HSE) Guidance Note MS7 (Third edition)
Neitz J, Neitz M; The genetics of normal and defective color vision. Vision Res. 2011 Apr 1351(7):633-51. doi: 10.1016/j.visres.2010.12.002. Epub 2010 Dec 15.
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