Equine motor neuron disease is an acquired neurodegenerative disorder of the somatic lower motor neurons.1,2 Dysfunction or death of parent motor neurons results in secondary degenerative lesions in peripheral myelinated motor axons and neurogenic atrophy of highly oxidative type 1 muscle fibers.1 When 30% of motor neurons become diseased or dysfunctional,2 clinical signs of weight loss, muscle atrophy, and weakness occur. Weakness manifests clinically as muscle fasciculation, excessive recumbency, frequent shifting of body weight between the hind limbs, and abnormally low head and neck carriage.3,4
Diagnosis of EMND is made on the basis of relevant clinical signs, low serum concentration of α-tocopherol (< 1 μg/mL), and presence of neurogenic angular atrophy of predominantly type 1 and some type 2 myofibers with adjacent myofiber hypertrophy on histologic evaluation.5 The diagnostic sensitivity of neurogenic atrophy in the sacrocaudalis dorsalis medialis muscle for EMND is reported to be > 90%.4,6,7 Limb muscles in EMND may also have fiber size variation, centrally located nuclei, and alterations in mitochondrial staining, such as dense peripheral rims and redistribution of mitochondrial enzyme stains (moth-eaten appearance). These findings have been considered occasional and nonspecific for EMND.5
Clinical signs and neuropathologic lesions of EMND have been produced experimentally by feeding a vitamin E–deficient diet for 44 months.2 Thus, it is believed that EMND is related to vitamin E deficiency, although < 45% of affected horses are reported to respond to vitamin E treatment, with 69% euthanized within 0 to 3 months following onset of clinical signs and diagnosis.8 Other studies1–4,9 suggest that factors such as genetics, environment, management, and diet may also play a role. We identified a specific subset of vitamin E–deficient horses with clinical signs of EMND that lacked angular neurogenic atrophy in sacrocaudalis dorsalis medialis muscle typical of EMND but rather had characteristic myopathic changes in mitochondrial staining and anguloid myofiber atrophy. These horses responded dramatically to vitamin E treatment, and sacrocaudalis dorsalis medialis muscle biopsy findings did not have antemortem diagnostic features of EMND. The purpose of the study reported here was to describe the signalment, history, clinical signs, and myopathic features of sacrocaudalis dorsalis medialis muscle and gluteal muscle biopsy specimens from vitamin E–deficient horses with clinical signs of muscle atrophy and weakness with favorable response to vitamin E treatment.
Equine motor neuron disease
Nicotinamide adenine dinucleotide tetrazolium reductase
iSolution Lite, IMT, North Hollywood, Calif.
Equine Senior, Purina Mills LLC, Gray Summit, Mo.
Assurance Grass Hay Ration Balancer, Farmer's Mill and Elevator Inc, Castle Rock, Minn.
E-SE, Merck/Schering-Plough, Whitehouse Station, NJ.
Vital E-500, Stuart Products Inc, Bedford, Tex.
Horse Guard Vitamin E 5000, Equine Nutrition Inc, Redmond, Ore.
Horse Guard Maintenance, Equine Nutrition Inc, Redmond, Ore.
SafeChoice, Purina Mills LLC, Gray Summit, Mo.
Elevate liquid suspension, Kentucky Equine Research, Versailles, K y.
Re-leve, Kentucky Equine Research, Versailles, Ky.
Vitamin E 5000, Med-Vet Pharmaceuticals, Eden Prairie, Minn.
Elevate Vitamin E Powder, Kentucky Equine Research, Versailles, K y.
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