Masseter myodegeneration as a cause of trismus or dysphagia in adult horses

Abstract

The medical records of eight horses with histological evidence of myodegeneration of the masseter muscles were examined. While they were alive their most common clinical signs had included difficulty in eating or opening their mouths, weight loss, difficulty in moving, and noticeable atrophy of the masseter muscles. The serum activities of muscle enzymes were abnormally high in all of the horses. Whole blood and/or liver selenium and vitamin E concentrations were less than the reference ranges in some of the horses. The lesions varied with the stage of the disease and consisted of swelling and discoloration, or muscle atrophy and fibrosis. Histologically, the muscle changes ranged from acute to subacute degeneration, with regenerative changes accompanying ongoing degeneration, to chronic degeneration with fibrotic replacement of muscle tissue. There were changes in the masseter muscle of all the horses, but some had widespread lesions in skeletal muscle, and a few also had myocardial lesions.

Nutritional myodegeneration as a cause of dysphagia in adult horses: three case reports

“…Cases with similar symptoms and pathological findings were reported later in the USA (Freestone and Carlson, 1991;Step et al, 1991;Pearson et al, 2005). Selenium and vitamin E deficiency were considered as a causative factor of muscle degeneration in all of these cases.…”

“…The concentration of selenium was lower than the reference value in the case report of Step et al (1991), and lower values of selenium were documented by Pearson et al (2005) in two out of the eight horses with masseter myodegeneration. In another case from the latter paper, the blood value of selenium was normal but the horse was previously treated with selenium.…”

“…Only Step et al (1991) found a decreased concentration of vitamin E in one horse. The other authors found comparable (Owen et al, 1977) or even higher (Pearson et al, 2005) concentrations of vitamin E than those considered as reference values. Similarly, higher vitamin E concentrations than the reference value for our laboratory were found in all of our cases.…”

“…The concentration of vitamin E in horses with dystrophic myodegeneration was evaluated by Owen et al (1977), Step et al (1991) and Pearson et al (2005). Only Step et al (1991) found a decreased concentration of vitamin E in one horse.…”

“…In the papers cited above only one from a group of five horses (Owen et al, 1977), none from a group of three horses (Freestone and Carlson, 1991) and none from a group of eight horses (Pearson et al, 2005) survived. One case of masseter myonecrosis described by Step et al (1991) also survived.…”