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Preservation of visual field in albinism despite misrouting of the optic nerve
Congenital visual abnormalities can lead to profound differences in the organisation of the visual pathways compared with normal developmental organisation. Knowledge of different cortical organisations is important for understanding the plasticity of the developing brain. Recent advances in functional brain imaging have allowed insights into such functional changes in humans. For example, a large‐scale developmental reorganisation of the visual pathway was shown in rod monochromats (achromatopsia, colour blind people who lack cone photoreceptor function). In rod monochromats, the cortical region that normally responds only to cones during functional magnetic resonance imaging activation responded powerfully to rod‐initiated signals.1 Another fascinating example is that the visual cortex can be activated by Braille reading in blind people.2 One of the most profound reorganisations of the visual pathway occurs in albinism. In this issue of British Journal of Ophthalmology, Hoffmann et al3(see page 509) report on their investigation of visual field function of abnormally wired optic nerve fibres in patients with albinism.
In patients with oculocutaneous and ocular albinism, there is a continuous range of hypopigmentation.4 Accordingly, patients have variable degrees of reduction in visual acuity, nystagmus, iris transillumination, hypopigmentation of the retinal pigment epithelium, foveal hypoplasia, optic nerve head anomalies and abnormal crossing of optic nerve fibres. Normally, the decussation between nasal and temporal retinal nerve fibres is located in the vertical meridian of the fovea. In albinism, this line is shifted temporally, causing the contralateral primary visual cortex to receive more input from the retina. Increased crossing of optic nerve fibres in the chiasm is one of the most consistent findings in albinism in all species. In humans, it has been found to be highly specific and asymmetries in visual evoked potentials are very helpful as a diagnostic tool for albinism.5 The degree of asymmetries is also variable,6 estimated to extend between 2° and 15°.
Anatomical and electrophysiological studies have established that abnormalities of the visual pathway of many different albino mammals extend from the retina to the cortical level. For example, a detailed study in the albino green monkey7 has shown no foveal pit in the retina, with a high area of abnormally large ganglion cells in the central retina, increased fibres crossing in the optic tract with regions of the lateral geniculate nucleus and the superior colliculus, normally innervated by ipsilateral afferents, being innervated exclusively by crossed afferents. The laminar pattern of the lateral geniculate nucleus was abnormal, with magnocellular layers more profoundly affected than parvocellular layers. In the primary visual cortex, mapping of the contralateral visual field was normal, whereas the central parts of the temporal retina mapped abnormally in the contralateral visual cortex, forming a monocular map of the central parts of the visual field. The normal map of the contralateral hemifield formed columns alternating with the abnormal map of the ipsilateral hemifield. However, studies indicate that the reorganisation of the cortical topographical representation varies widely in albino animals. Some albino mammals—for example, the Siamese cat or the ferret—have been found to have reordered retinotopic projection with contiguous cortical representation (the “Boston” pattern).8,9 However, other studies have shown an absence of any reordering of geniculostriate reorganisation in Siamese cats,10 called the “Midwestern” pattern. Animals with the Midwestern pattern showed little evidence of activity of cortical cells corresponding to the misrouted fibres. Profound visual field defects have been found—for example, in the misrouted (nasal) retinal areas of albino ferrets.11
In human albinos, functional magnetic resonance imaging studies have shown that the input from the temporal retina is not substantially suppressed and forms a retinotopic mapping that is superimposed on the mapping of the nasal retina in striate and extrastriate areas.12 Monocular activation in patients with albinism has shown contralaterally dominated activation of the visual cortex which correlated with the clinical severity of albinism.13 The extent to which the increased crossing of optic nerve fibres influences visual perception is unclear. In this issue of British Journal of Ophthalmology, Hoffmann et al3 aimed to elucidate whether the sensitivity of the abnormally projected temporal retina is normal. They compared temporal and nasal light spot detection sensitivities in the nasal and temporal visual field in 15 patients with albinism. In these patients, sensitivity of the abnormal projecting part of the retina was not reduced. They conclude that in humans, mechanisms of cortical self‐organisation make the abnormal crossing fibres available for visual perception. These results, together with functional magnetic resonance imaging investigations mentioned earlier, underline cortical reorganisation and adaptation in human albinism.
However, some earlier investigations showed that visual fields were constricted and had reduced central sensitivity to various degrees in patients with albinism.14,15 In a small group of patients with albinism, reduced contrast sensitivity of the temporal visual field has been found. This underlines the fact that some patients with albinism may have reduced visual function in the visual field corresponding to the abnormally crossing fibres.15 More sophisticated psychophysical tests including motion sensitivity to test the possibility of a more affected magnocellular pathway may show more subtle anomalies in the misrouted retinal fibres. It is also important to investigate a larger number of patients to see whether there are groups with nasal visual field defects or other psychophysical defects more pronounced in the area corresponding to the abnormally crossing fibres. Patient with albinism probably have a spectrum of differences in cortical organisation similar to the Siamese cat. This is supported by a wide range of visual deficits in patients with albinism, which correlate with the cortical reorganisation on functional magnetic resonance imaging data.4,13
The results of Hoffmann et al3 in this issue of British Journal of Ophthalmology are interesting because they show the developmental plasticity of the visual system establishing compensatory mechanisms to preserve visual function in congenital visual pathway disorders.
Competing interests: None declared.