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Muscle Changes in Horses with Vitamin E-Responsive MyopathyBy Dr. Bryan Waldridge · May 7, 2013

Vitamin E-responsive myopathy has been identified in horses that show clinical signs of equine motor neuron disease (EMND) such as weakness, muscle loss, and weight loss but do not have the microscopic changes in tailhead muscle that are seen with EMND. Instead, horses with vitamin E-responsive myopathy have muscle atrophy (decrease in size or wasting) and abnormal special staining for mitochondrial function. Mitochondria are important organelles inside cells that produce energy.

Three of eight horses affected with vitamin E-responsive myopathy had clinical signs for only two days to three weeks, and these included excessive time lying down, muscle tremors, hind limb stiffness, shifting weight, difficulty standing, low head carriage, and weight loss despite normal appetite and feed consumption. All three horses had access to pasture for at least six hours per day. Five horses had clinical signs for at least two to three months. Chronically affected horses had progressive weight loss, muscle atrophy, and decreased performance. Serum vitamin E concentrations were low in all three acutely affected horses and three of five chronically affected horses. All chronically affected horses had low muscle concentrations of vitamin E. Two horses had low muscle vitamin E concentrations despite having either normal or high serum vitamin E concentration.

Horses were treated symptomatically and with supplemental vitamin E. Horses were discharged from the hospital with instructions to continue oral vitamin E supplementation and became clinically normal two to twelve months later.

Horses with EMND develop weakness, lowered body condition, and muscle loss because long-term vitamin E deficiency causes oxidative damage to the nerves that transmit motor movements to muscles. The special stain used in this study to identify mitochondrial damage is not commonly used diagnostically. It is currently unknown if vitamin E-responsive myopathy is a unique disease or an early manifestation of EMND. Vitamin E-responsive myopathy is believed to occur because of reversible oxidative stress to mitochondria that responds to vitamin E supplementation.

It is possible that vitamin E-responsive myopathy occurs due to dietary deficiency of vitamin E or some abnormality that interferes with vitamin E entering muscle from the blood. Measurement of muscle vitamin E concentration may be more diagnostic than serum to diagnose vitamin E-responsive myopathy.

Bedford, H.E., S.J. Valberg, A.M. Firshman, M. Lucio, M.K. Boyce, and T.N. Trumble.  2013. Histopathologic findings in the sacrocaudalis dorsalis medialis muscle of horses with vitamin E-responsive muscle atrophy with weakness. Journal of the American Veterinary Medical Association 242:1127-1137.