Rod monochromatism is an autosomal recessive disorder characterised by poor vision from birth, nystagmus and photophobia and is consequent upon mutations in CNGA3 or CNGB3 . Patients usually attain acuities of about 6/60 and have absent colour vision. Most patients are hyperopic. The fundus is usually normal, although some granularity of the central macula may develop with time. The ERG reveals an absent or severely reduced 30-Hz flicker response, but good rod ERGs following even a limited period of dark adaptation (Fig. 9.2 C). Minor reduction in maximum ERG a-wave may reflect the absence of the cone contribution from this mixed response.
S-cone monochromatism is usually an X-linked disorder in which there is absent L- and M- cone function. The presentation in infancy is similar to rod monochromatism but the visual acuity is better (typically between 6/24 and 6/60) and the refractive error is usually myopic. Spectral sensitivity measurement can distinguish S-cone monochromatism from rod mono-chromatism, as can specialised colour vision testing , but these tests are not possible in infancy. ERG abnormalities are similar to those observed in the rod monochromat (Fig. 9.5 C) except that the S-cone-specific ERG, a response to a short-wavelength stimulus recorded in the presence of longer wavelength adaptation, is preserved. A similar electrophysiological phe-notype may be seen in achromatopsia associated with recessively inherited mutation in GNAT2 .
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