Polysaccharide storage myopathy

PSSM (polysaccharide storage myopathy) is currently divided into PSSM 1 and PSSM 2, although the term actually only applies to PSSM1.

PSSM 1 is a hereditary disease of the skeletal muscles. It is accompanied by severe muscle pain and tension, as the storage of glycogen and presumably also the utilisation of glycogen in the muscle is impaired. It is caused by a mutation of the glycogen synthase gene (GYS1). It was first discovered in Quarter Horses, but has since been detected in around 35% of all horse breeds, particularly in muscular, calm horse types such as Haflingers, Norikers, etc. Symptoms already appear in young horses.

Affected horses show excessive storage of glycogen and abnormally long, complex glycogen chains. These are stored mainly in the fast type II muscle fibres, i.e. the “white muscles” (further information: Specialist article on muscle types). Glycogen likely remains in the muscle, which is unable to generate energy from glycogen, or can only do so incompletely.

While light work is still well tolerated, more strenuous exertion leads to muscle damage and symptoms such as muscle pain, muscle cramps, stiffness, fatigue or complete refusal to perform.

Such horses absolutely require a diet that can be summarised as follows: “high fibre, low grain, high fat”. That means plenty of hay, no grain, little sugar (including from grass) and energy sourced from fat wherever possible. Horses with PSSM show increased insulin sensitivity and can transport carbohydrates into the muscle cell more easily and quickly. Therefore, the main focus is on reducing the carbohydrate content of the feed. A good supply of vitamin E (1,000 to 3,000 mg/day) and selenium (1.8 mg/day, but caution: do not overdose) is also recommended. In addition to the diet, regular light exercise (grazing without stress, not overly rich grass) is important, and box rest should be avoided wherever possible.