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Blue cone monochromatism

Blue cone monochromacy, S cone monochromatism, S cone monochromacy, X-linked incomplete achromatopsia, Color blindness, blue moncone monochromatic type, BCM, CBBM, X-chromosome-linked achromatopsia, Incomplete achromatopsia X-linked, X-linked achromatopsia incomplete, Achromatopsia incomplete X-linked

Overview

Blue cone monochromatism is an inherited vision disorder. In this condition, the light sensitive cells in the eye used for color vision (cones) are affected. There are three types of cones that respond to one of three colors: red, green, and blue. When people have blue cone monochromatism, both the red and green cones do not function properly, while the blue cones work normally. Blue cone monochromatism is a recessive X-linked disease due to dysfunction of the red (L) and green (M) cone photoreceptors,characterized by severely impaired color discrimination, low visual acuity, nystagmus, and photophobia.  BCM is as an incomplete form of achromatopsia. Signs and symptoms may include impaired color vision, low visual acuity (clarity or sharpness), photophobia (light sensitivity), myopia (nearsightedness), and nystagmus (fast, uncontrollable movements of the eye). 

Symptoms - Blue cone monochromatism

Blue cone monochromatism manifests in early infancy and predominantly affects males.

Symtoms are:

  • Impaired color vision and low visual acuity (only rod and blue cone function is preserved).
  • Photophobia, myopia, and pendular nystagmus are commonly observed.
  • Nystagmus may wane with time.

Causes - Blue cone monochromatism

BCM is caused by mutations in the red and green opsin gene cluster OPN1LW and OPN1MW (Xq28) and thus affect the corresponding cones. These mutations include deletions of the locus control region that is critical for expression of both genes. These deletions may also extend to parts of or the whole opsin gene cluster. Genomic rearrangements (unequal crossing-over) can result in single red and/or red/green hybrid genes carrying deleterious point mutations. The c.607T>C p.C203R missense mutation is commonly observed, but other missense and nonsense mutations have also been reported.

Prevention - Blue cone monochromatism

Not supplied.

Diagnosis - Blue cone monochromatism

  • Clinical ophthalmological examination,
  • Electrophysiological: electroretinography/ ERG
  • Psychophysical testing: color vision, dark adaptometry
  • BCM patients show no response to red and green light but normal response to blue light.
  • Mutation screening can confirm the diagnosis.

Prognosis - Blue cone monochromatism

BCM is usually a stationary disease, in rare cases, macular degeneration can occur in older patients.

Treatment - Blue cone monochromatism

There is no cure for this condition; however, there may be ways to manage the symptoms, such as using special glasses or contact lenses and low vision aids. Therapy is symptomatic and includes regular ophthalmological follow-up examinations. Patients should be informed about the possibility of using filtering glasses or contact lenses (red tinted or brown) to reduce photophobia and to improve contrast sensitivity. Low-vision aids include high-powered magnifiers for reading.

Resources - Blue cone monochromatism

  • NIH
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by Robert Derham
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