Ocular manifestations of equine motor neuron disease

The characteristics of the ocular manifestations of equine motor neuron dis ease (EMND) are described. Forty-two horses with histories, clinical signs and necropsies compatible,vith EMND were the subjects of this study. Ophthalmoscopic lesions that varied in severity were found in 40 of 42 hors es 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 hyperreflectivi ty formed mosaic patterns in the tapetal fundus. ERG B-wave amplitudes were usually at least 50% reduced and many animals showed extinguished amplitud es. 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 la yer chromatography (TLC) analysis of the RPE and neural retina identified b oth green and orange emitting fluorescent compounds not found in normal hor ses. All unsupplemented horses had plasma vitamin E levels <1.0 mu g/ml. The potential significance of this report is the pathognomonic role the ocu lar manifestations exhibit in helping to diagnose equine motor neuron disea se.