Blue cone monochromatism (BCM) is an XL recessive disorder, affecting fewer than 1 in 100,000 individuals, in which affected males have normal rod and blue (S) cone function but lack red (L) and green (M) cone function. The clinical features are similar to incomplete achromatopsia. Affected infants are photopho-bic and develop fine rapid nystagmus in early infancy. In keeping with achromatopsia, the nystagmus often reduces with time. They are usually myopic (a common finding in XL retinal disorders), with a visual acuity of 6/24-6/36. By comparison, subjects with achromatopsia more commonly have a hypermetropic refractive error. Fundi are usually normal in BCM or have changes consistent with myopia. Female carriers of BCM may have abnormal cone ERGs and mild anomalies of colour vision.
Achromatopsia and BCM may be differentiated by the mode of inheritance, findings on psychophysical and electrophysiological testing, and via molecular genetic analysis. In contrast to achromatopsia, there is some preservation of the single flash photopic ERG in BCM and normal S-cone function can be demonstrated using specialised spectral ERG techniques. On colour vision testing, good residual tritan discrimination is consistent with BCM, with the Farnsworth 100-Hue test, Berson plates, Hardy, Rand and Rittler plates all having been used successfully.
BCM is generally a stationary disorder but progressive deterioration, with or without the development of macular pigmentary changes, has been reported in some patients [3,13,27,30].
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