Lesion topography and new histological features in feline taurine deficiency retinopathy

Previous studies of feline taurine deficiency retinopathy have established that the retinopathy begins as a focal lesion at the area centralis (feline central retinal degeneration) and progresses to form a horizontal band of retinal degeneration which, in some cases, eventually involves the whole retina. Several theories have been proposed to account for the strikingly unusual geographic distribution of lesions in this disease, including a preferential cone photoreceptor effect, and damage by exposure to light. In this paper evidence is provided that the regional lesions in advanced cases of taurine deficiency retinopathy extend circumferentially to form a peripheral annular band-like arrangement around the retinal extremity. The circumferential annulus is confluent with both temporal and nasal extremes of the horizontal band-shaped lesion. The topographic arrangement of the advanced lesion thus approximates the shape of the Greek letter theta, theta. A previously unreported sclerad displacement of photoreceptor cells into the subretinal space was commonly observed histologically. Radially sectioned material also revealed pathological changes in photoreceptors over a much more widespread region than the focal lesions observable ophthalmoscopically and in whole-mounts of the retina. The issue is therefore to account for the advanced photoreceptor cell loss in the focal lesions. A preferential effect on cone photoreceptors is insufficient to explain the lesion distribution, and unilateral tarsorrhaphy experiments exclude light damage as a major factor. Some other factor, or combination of factors, is most likely involved in determining the unique geographic distribution of lesions in feline taurine deficiency retinopathy.