Ocular manifestations of equine motor neuron disease

Abstract

Summary The characteristics of the ocular manifestations of equine motor neuron disease (EMND) are described. Forty‐two horses with histories, clinical signs and necropsies compatible with EMND were the subjects of this study. Ophthalmoscopic lesions that varied in severity were found in 40 of 42 horses and appeared as a distinct pigmented reticulated pattern at the tapetal‐nontapetal junction or throughout the fundus, depending upon severity. The pattern colours ranged from yellow brown to black. Areas of hyperreflectivity formed mosaic patterns in the tapetal fundus. ERG B‐wave amplitudes were usually at least 50% reduced and many animals showed extinguished amplitudes. None of the horses had apparent visual impairment. Histopathologically, all 42 horses had retinal pigment epithelial (RPE) congestion with ceroid‐lipofuscin. Retinal degeneration was variable even within the eyes. Thin layer chromatography (TLC) analysis of the RPE and neural retina identified both green and orange emitting fluorescent compounds not found in normal horses. All unsupplemented horses had plasma vitamin E levels <1.0 μg/ml. The potential significance of this report is the pathognomonic role the ocular manifestations exhibit in helping to diagnose equine motor neuron disease. In memory of John Cummings, we dedicate this manuscript.

Vitamin E deficiency and risk of equine motor neuron disease

“…Other evidence of hypovitaminosis E was found on direct and indirect ophthalmoscopic examination; affected horses reveal a pigmentary retinopathy which involves the retinal pigment epithelium. While the vision of affected horses appears normal, there are changes in the electroretinogram [13,17]. Furthermore, electron microscopic studies on the spinal cords of EMND animals have consistently demonstrated the presence of large endothelial accumulations of lipopigment granules [18].…”

Nutritional considerations in grass sickness, botulism, equine motor neuron disease and equine degenerative myeloencephalopathy

“…Fundic examination may reveal a mosaic pattern of dark yellow to dark brown lipofuschin deposits in the tapetum and at the tapetal-nontapetal junction (Riis et al 1999). Consequently, horses with EMND are unable to lock their passive stay apparatus, and appear weaker when standing than when moving.…”

Neurodegenerative Disorders

“…29 Massive accumulations of lipofuscin were found in the pigmented epithelium of rats lacking various antioxidants including vitamin E 49 ; such deposits are thought to be the end products of peroxidation of membrane polyunsaturated fatty acids. 50 Low blood and spinal cord vitamin E concentrations have consistently been found in horses with EMND in the United States and an epidemiologic study linked low serum vitamin E to EMND risk (see Epidemiology section). The typical history for EMND cases also includes lack of access to green forage, so vitamin E deficiency is thought to be involved in the pathogenesis of the disease.…”

“…57 In some 40% of cases, fundoscopic examination reveals a horizontal band of pigment above the optic disc at the tapetal-nontapetal junction ( Figure 19.6). 40,50 The pigment may be brown-black to yellow-brown and has a reticulated or mosaic appearance. Surprisingly, problems with vision have not been reported even in EMND horses with severe pigment retinopathy.…”