Blue cone monochromatism (BCM) 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.
Blue cone monochromatism manifests in early infancy and predominantly affects males.
- 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.
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.
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.
BCM is usually a stationary disease, in rare cases, macular degeneration can occur in older patients.
There is no specific treatment available. 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.