monochromacy

Monochromacy is a rare visual condition in which functional color vision is absent because the retina relies on a single type of photoreceptor. In humans, this typically refers to rod monochromacy, in which cones are nonfunctional, and cone monochromacy, in which only one cone photopigment type is active.

Rod monochromacy is characterized by severely reduced visual acuity, pronounced photophobia, and nystagmus from infancy, with good sensitivity to dim light but poor color discrimination. Cone monochromacy produces limited color perception and reduced acuity, but retains some rod function; affected individuals commonly experience photophobia and impaired vision in bright light, though the exact pattern varies from rod monochromacy.

Causes and inheritance: Monochromacy is usually genetic. Cone monochromacy and rod monochromacy are most often inherited in autosomal recessive patterns, with many cases due to mutations in cone phototransduction and development genes such as CNGA3, CNGB3, GNAT2, PDE6C, and PDE6H. Some cone monochromacy cases involve variants affecting single cone-type genes or cone development. Rarely, other inheritance patterns have been reported.

Diagnosis and tests: Diagnosis combines clinical history with specialized testing. Color vision testing is abnormal or absent; electroretinography shows absent or severely reduced cone responses with preserved rod responses in rod monochromacy, whereas cone monochromacy shows reduced cone responses with relatively preserved rod function. Genetic testing confirms the underlying variants.

Management and prognosis: There is no cure. Management focuses on maximizing visual function and comfort through low-vision aids, tinted lenses to reduce photophobia, and adaptation strategies. Genetic counseling is often offered. Research includes gene therapy approaches in animal models and humans aimed at restoring cone function.

See also: achromatopsia; color vision deficiency; photophobia.