Colour Vision Testing

Colour vision testing examines the ability to tell colours apart. It finds inherited colour deficiency, which is common, and acquired colour loss, which may be an early sign of disease of the retina or optic nerve, or of drug toxicity. It also decides fitness for jobs that need good colour discrimination.

Colour vision depends on three types of cone cells in the retina: long wavelength (red), medium wavelength (green) and short wavelength (blue) cones. Inherited red green deficiency is X-linked, and affects about 8% of men and 0.5% of women of Northern European ancestry, with lower rates (about 3 to 5% of men) in people of African and East Asian ancestry.

It is divided into protan (red) and deutan (green) types, and into anomalous trichromacy (reduced sensitivity) and dichromacy (absence of one cone type). Blue yellow deficiency (tritan) is rare and is usually acquired. Total colour blindness (achromatopsia) is rare.

Several tests are used. Pseudoisochromatic plates show numbers or shapes made of dots in a colour that people with deficiency confuse with the background colour, and are the most used screening test. Arrangement tests (hue arrangement with 15 or 100 caps) ask the patient to place coloured caps in order, and show the type and severity of the defect.

The anomaloscope, the most precise test, asks the patient to match a yellow light with a mixture of red and green. Lantern tests are used for occupational purposes. Computer based tests are used increasingly. The screening test takes 2 to 5 minutes, and the arrangement tests 5 to 15 minutes.

Indications.

• Screening of children (from about 4 to 5 years), so that learning difficulties can be understood and career advice given.
• Family history of colour deficiency, and assessment of the type.
• Occupational and fitness testing: aviation, rail and road transport, the armed forces, electrical work, and some medical and laboratory jobs.
• Suspected acquired loss: optic neuritis, optic neuropathy, glaucoma, macular and retinal disease, diabetic retinopathy, and compressive lesions of the visual system.
• Monitoring for drug toxicity: ethambutol, hydroxychloroquine, amiodarone, vigabatrin, digoxin, and others.
• Toxic exposure (solvents, heavy metals), and chronic alcohol or nutritional neuropathy.
• Neurological disease: stroke affecting the visual cortex (cerebral achromatopsia), multiple sclerosis, and Parkinson disease.
• Unexplained complaints of washed out colour, or a difference in the colour of an object between the two eyes.

Not a substitute for tests of acuity and visual fields, and not reliable in patients with very poor vision.

Contraindications and precautions.

• No contraindications.
• Each eye is tested separately for acquired disease, since the loss may be present in one eye only.
• The test is done under proper lighting (a daylight equivalent lamp or natural light, and not coloured or dim light), with the correct reading distance (about 40 cm) and the usual reading glasses, since poor lighting changes the results.
• The plates are not touched or exposed to sunlight and are kept closed, since the colours fade.
• Patients with a learning difficulty, language problems, or illiteracy: plates with symbols or shapes are used.
• Tinted lenses and contact lenses for colour correction are removed before testing, and any known eye disease is noted.

Preparation and technique.

The patient is seated under a good, even daylight type light. Visual acuity is checked first, with glasses. For plates, the booklet is held at about 40 cm, and the patient identifies the number or traces the path on each plate, within about 3 seconds, with each eye tested. The number of correct plates is counted, and the screening plates separate normal from abnormal.

The diagnostic plates, when present, give the type and severity. For arrangement tests, the caps are arranged in a sequence between two fixed reference caps within a time limit, and the order is plotted on a score sheet to show the axis of confusion. In the anomaloscope test, the patient adjusts a red green mixture to match a yellow, and the range of matches shows the type and severity.

Normal appearances.

A person with normal colour vision reads all the screening plates correctly (usually 0 to 2 errors out of the screening set, depending on the set) and arranges the caps with only small, random errors, without a pattern along a colour confusion axis. In the anomaloscope, a normal person accepts a narrow range of red green mixtures to match the yellow. Colour vision in children reaches normal levels by about 5 to 6 years, but may be hard to test earlier.

Interpretation.

• Inherited red green deficiency: errors on the screening plates, with a pattern along the red green axis on arrangement tests. Protan defects also show a darkening of red, and deutan defects do not. Severity is mild, moderate or severe, and the extent matters for occupation.
• Inherited blue yellow deficiency: rare, dominant, and not sex linked. Failure on blue yellow plates, and arrangement errors along the blue yellow axis.
• Acquired defects: blue yellow loss is typical of retinal disease, glaucoma, diabetes, and some drug toxicity, while red green loss is typical of optic nerve disease (optic neuritis, compression, toxic and nutritional neuropathy). Defects may be asymmetric and change with time, and are often out of proportion to the loss of acuity in optic nerve disease.
• Achromatopsia: very poor results on all colour tests, with poor acuity, photophobia and nystagmus in congenital forms. Cerebral achromatopsia: colour loss in both eyes after occipital lesions, with normal acuity.
• Optic neuritis: red desaturation, with a difference in the colour of a red object between the two eyes, and loss of colour in the affected eye.
• Drug toxicity: ethambutol and hydroxychloroquine cause new colour loss early, before acuity falls.
• A new loss in one eye should lead to a search for optic nerve or retinal disease.
• Result interpretation is combined with acuity, visual fields, fundus findings, and the history.

Critical values and findings needing immediate action.

• New colour loss in one eye with reduced vision: urgent assessment for optic neuritis, compressive lesions, and other optic neuropathies.
• New colour loss in a patient on ethambutol: stop or review the drug with the prescriber on the same day, since damage may be permanent.
• New colour loss with headache or visual field loss: urgent imaging.
• Colour loss with pain on eye movement in a young adult: urgent review for optic neuritis and neurological assessment.
• Detection of deficiency in a person in a safety critical job: advice on fitness, with formal occupational testing.

Limitations and artefacts.

• Poor or wrong lighting, wrong distance, faded or dirty plates, reading glasses that are not used, and tinted lenses give false results.
• Plates are good for screening but cannot grade severity reliably, and some mild deficiencies are missed, while some people with normal vision fail one or two plates.
• Learning the plates by memory or by a trick (looking at the edges, using brightness cues) gives false normal results, and a changed set avoids this.
• Poor visual acuity, cataract (yellowing of the lens gives blue yellow errors), and media opacities reduce colour discrimination.
• Fatigue, lack of attention, anxiety, and young age lower performance.
• Different tests detect different defects, so normal results on one test do not exclude a defect, and occupational tests need specific, validated tests.
• Colour deficiency has no treatment, though tinted lenses and filters can improve discrimination in some tasks, and do not restore normal colour vision.
• Language and cultural differences in naming colours affect results on naming tests.

Special groups.

• Children: early testing prevents learning problems and misunderstanding at school, and teachers are informed. Plates with shapes are used for those who cannot read numbers.
• Boys with a family history: the chance of deficiency is 50% if the mother is a carrier.
• Women: carriers are usually normal, and an affected woman needs an affected father and a carrier or affected mother.
• Patients on ethambutol, hydroxychloroquine or similar drugs: baseline and regular colour testing.
• Patients with diabetes: blue yellow loss may appear early in retinopathy.
• Older patients: lens yellowing causes mild blue yellow loss.
• Regions: simple plates are cheap, and screening can be done by trained health workers and teachers. Many countries have no occupational testing programmes. Colour vision matters for the interpretation of medical tests (urine dipsticks, skin signs, stains), so doctors with deficiency are aware of the limits.

Follow up tests.

• Visual acuity, visual fields and pupil examination (for a relative afferent defect).
• Optical coherence tomography of the nerve fibre layer and macula.
• Fundus examination and photography.
• MRI of the brain and orbits when optic neuritis or a compressive lesion is suspected.
• Electroretinography for suspected retinal dystrophy, and genetic testing for inherited defects.
• Review of the drug list, and blood tests for nutritional causes (vitamin B12, folate).
• Formal occupational colour vision assessment, when needed for a job.