Equine polysaccharide storage myopathy (PSSM or EPSM) is a genetic condition in horses that affects how muscle cells partition, generate, and store energy. This can lead to exercise intolerance, stiffness, and an abnormal gait in your horse. After diagnosis, owners often notice signs, such as tail swishing, high resting muscle tone, behavioural changes, and slight performance reductions indicating mild tying-up.

Horses with PSSM are at higher risk of tying-up, also known as exertional rhabdomyolysis. These episodes cause significant pain and are characterized by stiff, firm muscles along with profuse sweating and reluctance to move. In severe cases, the breakdown of muscle tissue can affect kidney function and can be fatal.

PSSM is most common in Quarter Horses and large draft breeds. It is estimated that up to 10% of Quarter Horses and 36% of draft horses have PSSM. [1]

Horses with this condition fall into two categories: PSSM1 and PSSM2. PSSM1 horses have a specific genetic mutation in the gene for glycogen synthase 1. PSSM2 horses show the clinical features of PSSM without this specific genetic mutation. [2]

The best way to manage PSSM horses is to provide a forage-first low-sugar and starch diet with careful attention to salt, mineral and vitamin needs. Additional calories can be supplied by dietary fat, or high-energy soluble fibre sources like beet pulp and linseed cake meal.

A consistent exercise routine can also help promote better energy generation in the muscle. Acetyl-L-carnitine supplementation also promotes efficient energy generation.

Wondering how to best feed your PSSM horse? Our equine nutritionists can look at your horse’s diet and help you come up with an appropriate feeding plan for free.

Equine Polysaccharide Storage Myopathy

PSSM is a muscle disorder characterized by abnormal accumulation of glycogen (the storage form of glucose/sugar) in skeletal muscle cells.

Horses with PSSM have more glycogen or glycogen in abnormal locations in their muscles. Some also form abnormal polysaccharide molecules that are not available as a fuel for the muscle cells.

Types of PSSM

There are two main types of PSSM: PSSM1 and PSSM2. [4] PSSM1 is known to be caused by a mutation in the glycogen synthase 1 (GYS1) gene. [2]

PSSM2 refers to horses with abnormal glycogen storage that do not have a mutation in the GYS1 gene. In most of these horses, the underlying cause has not yet been identified.


PSSM1 was first identified in 1992 in Quarter Horses that were affected by recurrent exertional rhabdomyolysis. Researchers analyzed muscle samples from these horses and found higher amounts of glycogen than normal and an abnormal polysaccharide composed of ubiquitin and polyglucosan. [14]

Further research identified the GYS1 mutation in Quarter Horses, related breeds (Paints, Appaloosas) and some draft breeds.

In all horses, the glycogen synthase 1 enzyme is important for making glycogen in muscle cells. In horses with the GYS1 mutation, this enzyme is more active than normal resulting in more glycogen being made. [2]

Diagnosis of PSSM1

An exercise tolerance test is the first step to determine whether your horse has a muscle issue. However, this test will not provide a definitive answer as to whether they have PSSM1.

Your veterinarian will take a blood sample 4 – 6 hours after a light exercise session of no more than 15 minutes. Levels of muscle enzymes, such as creatinine kinase (CK), are typically higher after exercise in PSSM horses than normal horses.

The exercise tolerance test will indicate whether the horse’s tying-up episode can be explained by an issue in their muscles. If the test does not show elevated levels of muscle enzymes, the tying-up episode was likely due to their diet or exercise management at the time and not an underlying genetic condition.

Genetic Tests

Genetic tests are a non-invasive way to definitively diagnose PSSM1. These