Assessing loss of colour vision in patients with clinical disorders

Conventional colour assessment (CA) tests rarely isolate the use of Red/Green (RG) colour signals. The successful completion of a visual task cannot therefore be attributed entirely to the use of RG signals, even in normal trichromats. A good example is the Farnsworth D15 test; when no errors are accepted, 6.3% of normal trichromats fail and 50% of deutans and 41% of protans pass. The use of Yellow/Blue (YB) colour signals as well as rod and cone mediated luminance signals contributes significantly to the outcome of the test. Patients with diseases of the retina and systemic diseases that affect colour vision vary in the severity of RG and YB loss, from almost normal responses to complete loss of colour vision. These patients also often exhibit reduced rod and cone mediated luminance contrast sensitivity. When acquired loss, as a result of disease, adds to congenital RG deficiency, CA becomes even more challenging. When taken together, these factors account for the large variability and inconsistency of outcome in conventional CA tests. The recent development of sensitive and specific tests for assessment of RG and YB deficiencies and the establishment of reliable, normal‐age limits have transformed the effectiveness of CA as a clinical tool in the detection and monitoring of diseases such as diabetes, age‐related macular degeneration, glaucoma and a range of other degenerative diseases of the photoreceptors, neural retina and the RPE that often manifest first through a reduction in YB and RG chromatic sensitivity. The latter can range from thresholds that are just above normal age limits to complete loss of colour vision, even when high contrast acuity and other visual functions remain largely unaffected. The loss of colour vision often varies over the retina and in some patients, the loss can be asymmetric and no longer follows the rules of RG and YB chromatic opponency. Such findings have important implications on the processing of chromatic signals.