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Blue cone monochromatism
Blue cone monochromatism (BCM) is a recessive X-linked disease characterized by severely impaired color discrimination, low visual acuity, nystagmus, and photophobia, due to dysfunction of the red (L) and green (M) cone photoreceptors. BCM is as an incomplete form of achromatopsia (see this term).
ORPHA:16Classification level: Disorder
- Atypical X-linked achromatopsia
- Blue cone monochromacy
- Color blindness, blue monocone monochromatic type
- S cone monochromacy
- S cone monochromatism
- X-linked incomplete achromatopsia
- Prevalence: 1-9 / 100 000
- Inheritance: X-linked recessive
- Age of onset: Infancy
- ICD-10: H53.5
- OMIM: 303700
- UMLS: C0339537 C2931753
- MeSH: C536238 C538165
- GARD: 917
- MedDRA: -
The prevalence is estimated to be 1/100,000 worldwide.
BCM manifests in early infancy and predominantly affects males, with severely impaired color vision and low visual acuity (only rod and blue cone function is preserved). Additionally, photophobia, myopia, and pendular nystagmus are commonly observed. Nystagmus may wane with time.
The disorder 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.
The diagnosis of BCM is achieved by clinical ophthalmological examination, electrophysiological (i.e. electroretinography/ ERG) and psychophysical testing (i.e. color vision, dark adaptometry), where BCM patients show no response to red and green light but normal response to blue light. Mutation screening can confirm the diagnosis.
Differential diagnosis includes achromatopsia, Leber congenital amaurosis, various types of cone dystrophies (see these terms) and cerebral achromatopsia.
Prenatal diagnosis may be offered by specialized laboratories for at-risk couples. The use of prenatal diagnostic testing in BCM will vary depending on national customs and ethics.
BCM is inherited in an X-linked manner. Genetic counseling is mandatory. A carrier female has a 50% risk of transmitting the mutated allele to her offspring. Penetrance is complete with little variability in disease expression.
Management and treatment
There is no specific therapy available. Management 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.
BCM is usually a stationary disease, yet in rare cases, macular degeneration can occur in older patients.
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