Prevention and Management of Chronic Exertional Rhabdomyolysis

Exertional rhabdomyolysis (ER) is a common cause of muscle pain or ‘tying up’. It may occur as an isolated event, however if ER episodes recur despite good management, then chronic ER is present.
Chronic ER has multiple underlying disorders that present as ‘tie up’ episodes. These can be split into 3 groups;

1. Recurrent exertional rhabdomyolysis (RER)
This affects 5–10% of Thoroughbreds (TBs) and most likely affects TB crossbreds, given that in TBs RER is inherited as an autosomal dominant trait. Recurrent exertional rhabdomyolysis is probably the same ER disease in Standardbreds. It is the result of an abnormality in skeletal muscle intracellular calcium regulation, the pathogenesis of which is unknown.

2. Polysaccharide storage myopathy (PSSM)
This was first identified in Quarter Horses, but also affects Draught breeds, Warmbloods, Welsh ponies, Connemaras, Cob types, TB crosses, polo ponies and many other breeds. Horses with PSSM have excess glycogen, and often abnormal glycogen, storage in their skeletal muscle. Discovery of a mutation on the Glycogen Synthase 1 (GYS-1) gene affecting some PSSM horses has divided this into type 1 (have the GYS-1 mutation) and type 2 (don’t have the GYS-1 mutation).

3. Idiopathic disorders
In some horses with chronic ER a muscle biopsy may show nonspecific or no changes. There are likely to be other, as yet undefined, disorders resulting in chronic ER. […]