Few mitochondrial myopathies have been described in horses. In a reported case, a deficiency of mitochondrial complex I (NADH coenzyme Q oxidoreductase)—an enzyme catalyzing the first step in the mitochondrial respiratory chain—was identified in a young Arabian filly with clinical signs similar to those of exertional rhabdomyolysis. However, this horse showed no changes in serum CK activity after exercise. A marked lactic acidosis developed even with light exercise, and maximum oxygen consumption was greatly decreased, resulting in marked exercise intolerance. Histologic evaluation of muscle biopsies showed an abnormal increase in mitochondrial density, and biochemical analyses revealed a complex I deficiency. The horse showed slowly progressive clinical signs of muscle atrophy; however, it otherwise remained healthy at rest.
Glycogen Branching Enzyme Deficiency
Glycogen branching enzyme deficiency (GBED) is a glycogen storage disorder causing abortion, seizures, and muscle weakness in Quarter Horse–related breeds caused by an autosomal recessive mutation in the glycogen branching enzyme (GBE1) gene. The mutation is carried by 9% of Quarter Horses and Paint horses, and at least 3% of abortions are attributed to GBED in Quarter Horses.
Most foals with GBED show hypothermia, weakness, and flexural deformities of all limbs at 1 day of age. Ventilatory failure may also be a clinical sign, in addition to recurrent hypoglycemia and collapse. Foals with this condition are euthanized because of muscle weakness or die suddenly because of apparent cardiac arrhythmia. Persistent leukopenia, intermittent hypoglycemia, and high serum CK (1,000–15,000 U/L), AST, and gamma-glutamyltransferase activities are features of affected foals. Gross lesions are not evident, and routine H&E stains of tissues may be normal or show basophilic inclusions in skeletal muscle and cardiac tissues. Frozen sections of muscle, heart, and liver show a notable lack of normal periodic acid-Schiff (PAS) staining for glycogen as well as abnormal PAS-positive globular or crystalline intracellular inclusions. Branching enzyme activity is minimal in skeletal and cardiac muscle as well as liver.
A diagnosis is best obtained by confirming the presence of the genetic mutation in tissue samples or by identifying typical PAS-positive inclusions in muscle or cardiac samples. There is no successful treatment.