Polysaccharide storage myopathy associated with recurrent exertional rhabdomyolysis in horses.

The research examines a type of myopathy, linked to polysaccharide storage, found in horses that have a history of repeated exertional rhabdomyolysis. The identified myopathy is characterised by high muscle glycogen concentrations and certain unique inclusions found in the muscle fibers.

Polysaccharide Storage Myopathy in Horses

• The study addresses a certain type of muscle disease in horses, specifically Quarterhorses, Quarterhorse crossbreds, American Paints, and Appaloosa horses.
• This condition is related to the storage of polysaccharides, a form of carbohydrate, in the muscles.
• The horses affected by this condition had a history of recurrent exertional rhabdomyolysis, a syndrome involving muscle breakdown with exertion.

Characteristics of the Myopathy

• The muscle biopsies obtained from the horses showed high concentrations of muscle glycogen, a stored form of glucose.
• Up to 5% of type 2 muscle fibers contained inclusions which reacted positively to the periodic acid Schiff (PAS) stain, a method used to detect polysaccharides and mucosubstances.
• These inclusions were classified as an acid mucopolysaccharide due to their staining characteristics.

Ultrastructural Studies and Observations

• Ultrastructural studies revealed the inclusions were composed of beta glycogen particles interspersed with filamentous material.
• Many of the type 2 fibers had multiple vacuoles (small cavities) under the sarcolemma (the cell membrane of a muscle cell).
• These vacuoles stained lightly with eosin, a type of stain, but did not react with the PAS stain.
• Biopsies from some horses showed centrofascicular atrophy and necrosis, or death, of scattered type 2 fibers.

Glycogen Metabolism and Enzyme Activity

• No deficiencies were identified in any glyco(geno)lytic enzyme using a biochemical screening test for anaerobic glycolysis, a method of breaking down glucose without oxygen.
• The researchers attempted to measure branching enzyme activities in both affected and control samples but were unsuccessful using methods developed for human muscle.

Conclusion of the Study

• The researchers concluded that the polysaccharide accumulation in the horses possibly suggests a yet undefined metabolic disorder of skeletal muscle.