Colour Vision Assessment

Different parts of the retina are particularly sensitive to certain hues. The majority of nerve fibres of the anterior visual pathways (optic nerve, optic chiasm and optic tract) serve our central vision, which is particularly sensitive to red and green light. Hence, diseases of the optic nerve (optic neuropathies) will cause red-green colour desaturation.

Technique

• In the specialist out-patient clinic, pseudo-isochromatic color plates like Ishihara charts can be used to both identify and quantify red-green colour desaturation.
• At the bedside, relative colour desaturation can be assessed using a bright red or bright green object. The patient is asked to compare the colour of the object between eyes. In the eye with optic nerve disease, the object will appear less bright. This however only works when there is unilateral disease or bilateral disease with asymmetrical involvement.

Clinical importance

• 10% of men (and 0.4% of women) have x-linked red-green colour blindness.
• Red-green desaturation is disproportionately severe (compared to other visual function parameters) in optic neuritis. The majority of patients with optic neuritis have the retro-bulbar subtype, where the optic nerve head is not inflamed. Hence, optic disc swelling is an uncommon sign in optic neuritis. Early detection of optic neuritis is thus dependent on colour vision testing. In Hong Kong, a common cause of secondary optic neuritis is ethambutol-related toxicity. This drug is an important first line agent in the management of active tuberculosis. Cessation of ethambutol when red-green desaturation is detected prevents permanent visual loss.