Forschungs- & Entwicklungsinformationsdienst der Gemeinschaft - CORDIS

Final Activity and Management Report Summary - SVS (Strabismus and visual suppression)

The objective of this study was to further the understanding of a disease that affects vision in 2-5 % of all children. Strabismus (commonly known as squint) is a failure to develop normal perfect alignment of the eyes required to allow binocular vision and stereoscopic depth perception. There are few known causes of strabismus, including infantile hyperopia (farsightedness) and in most cases the origin remains obscure. Since a child with strabismus has misaligned eyes, they must choose with which one to look at (fixate) a target. Some strabismic children will develop a preference for one eye over the other while others will fixate alternately between their eyes. Interestingly, strabismus sufferers rarely report double vision (diplopia). Instead, their brains adapt by suppressing (i.e. concealing from perception) conflicting information from one eye or the other. Diplopia is an unpleasant visual phenomenon, but it does provide a powerful cue to realign the eyes. Once diplopia is eliminated by suppression, spontaneous recovery from strabismus is impossible.

The foremost concern in treating the strabismic child is to halt the progression of a secondary disease known as 'lazy eye', or amblyopia. Amblyopia is not a disease of the eye; it is caused by errant brain development. It occurs when a child develops a predilection for fixating with one eye alone. Amblyopia robs the suppressed eye of visual acuity and in extreme cases causes complete monocular blindness. It is treated by patching the child's preferred eye, forcing them to fixate with their habitually suppressed eye, thus balancing the flow of visual information to the two eyes and restoring normal experience-driven brain development. Patch therapy prevents amblyopia, but prevents further the chances of spontaneous realignment of the eyes by eliminating binocular visual experience altogether. Eye muscle surgery performed in children or adults can straighten the eyes, but it is a cosmetic operation that is not expected to produce normal binocular vision with stereopsis. Suppression is a key player in strabismus and amblyopia. We therefore set out to first characterise suppression behaviour in strabismus, then to search for brain activity that reflects the perceptual experience of suppression.

The data from this study are in the form of maps that show which regions of the visual field are perceived by each eye under both left and right eye fixation conditions. The most important scientific result of this mapping experiment was the discovery that suppression does not work on a whole-eye basis. Instead, non-overlapping regions of each retina are suppressed to produce a complete visual field, any part of which is 'seen' by a single eye alone. This is the means by which diplopia is eliminated in the whole visual field in strabismus. Furthermore, contrary to clinical dogma neither fovea is suppressed. The strabismic subject is effectively endowed with two regions of maximal visual acuity in their visual field.

These novel findings are of critical importance for the clinical treatment of strabismus. The fact that the deviated fovea is not suppressed, as was previously believed, means that patching therapy is not being used in an optimally effective way. Instead of patching the whole eye, for example, it might be more effective to restrict a child's vision in a more piecemeal manor by applying regions of frosting that correspond the patterns of suppression to a pair of spectacles. It is possible that such strategies would allow children to avoid amblyopia yet maintain a binocular view of the world that could encourage spontaneous realignment of their eyes. In any case, the results of this project will prompt a re-evaluation of traditional approaches for the prevention and treatment of strabismic amblyopia.

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