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Implications for our understanding of the mechanisms of early development of the visual pathway
Our high resolution, three dimensional view of the world depends on the presence of foveas to give high acuity, on an optic chiasm to direct information from the corresponding areas of nasal and temporal retina to the same hemisphere and on the alignment of the visual axes to allow corresponding features in the image from each eye to be aligned and combined into a single stereoscopic percept. Coordinating the development of these different components into an integrated binocular visual system poses a considerable challenge and it is not surprising that defects arise from time to time.
The most common genetic condition to affect chiasmal development is albinism, in which the fovea is hypoplastic and the fundus is hypopigmented. There is misrouting of retinal ganglion cell axons at the optic chiasm and binocular function is reduced or absent. It has generally been assumed that the foveal hypoplasia and excessive crossing of retinal axons at the optic chiasm resulted from deficient synthesis of melanin, although the mechanisms involved have not been determined.1 In this issue of BJO (p 1098) van Genderen et al2 describe three patients with foveal hypoplasia and electrophysiological findings typical of the chiasmal misrouting found in albinism, but with normal fundal pigmentation. Two were sisters who had been considered to have congenital idiopathic oculomotor nystagmus, while the third had been diagnosed with Kartagener syndrome (primary ciliary dysgenesis and situs inversus) with nystagmus. The observation that foveal hypoplasia and chiasmal misrouting can occur in the presence of normal pigmentation is of considerable interest.
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