7 Degenerative/necrotizing myopathies

Although degeneration and necrosis are a feature of a number of different myopathies, this section describes myopathies that do not have a congenital, nutritional, toxic, or infectious etiology.

7.1 Exertional myopathies

An exertional myopathy is defined as myofiber damage occuring as the direct result of exercise. Note that both PSSM and equine malignant hyperthermia are induced by exercise, and can thus be considered exertional myopathies. These notes are organized primarily by underlyign etiology, rather than inciting cause, and thus these conditions are often found elsewhere in these notes, despite their relationship to exercise. They are included elsewhere.

7.1.1 Equine exertional rhabdomyolysis

Synonyms: blackwater, Monday morning disease, set fast, paralytic myoglobinuria, azoturia, tying-up

These synonyms broadly refer to the clinical syndrome of exhibited by horses that experience muscle injury (necrosis, degeneration) following exercise. As noted above, several have already been discussed. Here we will discuss equine recurrent exertional rhabdomyolosis.

7.1.1.1 Equine recurrent exertional rhabdomyolosis

This condition is primarily noted in young, excitable, Standardbred or Thoroughbred fillies. It presents as acute onset of muscle pain, secondary to degeneration and necrosis, often 15-30 minutes into submaximal exercise. Between episodes of muscle pain horses are typically asymptomatic. The cause is poorly described, but is believed to be the result of unknown genetic abnormality concurrent with environmental factors leading to an abnormality in intracellular calcium regulation. Gross and histologic findings are typical of rhabdomylosis and are not specific for RER.

Risk factors (beyond breed and temperment) have been identified. They include diets high in grain, a parent with the condition, and increased periods of rest between exercise.

There is evidence that horses with PSSM are predisposed, but the exact mechanism is unclear.

7.1.2 Canine exertional rhabdyomyolysis

As one might expect, this syndrome appears most frequently in racing dogs, namely Greyhounds and sled dogs. It is not particularly well understood, but thought that separate mechanisms cause the syndrome in sprinting versus endurance racing. In Greyhounds, clinical signs and symptoms are similar to those noted for horses, while in sled dogs may present with sudden death.

7.1.3 Capture myopathy

This is a syndrome seen both in free and captive wildlife. The stress of capture, which is usually preceeded by a chase and/or struggle in these animals, results in a combined massive release of catecholamines and overexertion that is frequently fatal. Muscles may be pale and edematous or may show pale streaks with hemorrhage. Degeneration and necrosis is frequently observed histologically. This condition is extremely important in zoological collections, and as such great care is often taken when tranquilizing or restraining animals for examination.