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. 
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. 
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
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. 
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. 
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 are a non-invasive way to definitively diagnose PSSM1. These are available to specifically detect the R309H-GYS1 variant that causes PSSM1.
A blood or hair root sample can be taken to isolate DNA and analyze the sequence of the GYS1 gene. There are two copies of this gene (one from the mother and one from the father).
PSSM1 is an autosomal dominant condition, meaning a horse with just one copy of the variant gene from either the mother or father can show signs of the disorder. In addition, foals that have one parent with PSSM have a 50% chance of carrying the variant and developing PSSM.
The genetic test will give one of three results:
- PSSM1 n/n: two normal copies of the GYS1 gene (negative for PSSM1)
- PSSM1 n/P1: one normal copy of the GYS1 gene and one variant copy (positive for PSSM1)
- PSSM1 P1/P1: two variant copies of the GYS1 gene (positive for PSSM1)
- Presence of amylase-resistant, periodic acid-Schiff (PAS)-positive, abnormal polysaccharide inclusions
- Glycogen levels that are 1.5 – 2 times higher than normal
- Higher amount of damaged or necrotic (dead) muscle fibers
PSSM2 horses show the same clinical signs as PSSM1 horses, including muscle stiffness, pain, and reluctance to work. Muscle biopsies taken from these horses also show abnormal accumulation and/or location of glycogen.
However, these horses do not have the GYS1 mutation and the precise genetic cause is currently unknown. It is possible that several abnormal genes can contribute to this trait.
PSSM2 is more commonly found in warmblood and Arabian horses.
In MFM, there is structural damage to the muscle fibers caused at least in part by severely decreased antioxidant activity.  This damage allows glycogen to pool between damaged fibers leading to the PSSM2 diagnosis. To distinguish from PSSM, MFM can be properly diagnosed by staining for accumulations of the muscle protein desmin.
Diagnosis of PSSM2
There are commercially available genetic tests that claim to identify PSSM2 genetic variants, however, research showed no difference in frequency of those variants in horses with or without PSSM2.
Healthy control horses were just as likely to have the P2, P3 or P4 variants as horses with PSSM2 or myofibrillar myopathy (MFM), another muscle disorder.
This means that the currently available genetic tests for PSSM2 are inaccurate. Note that there are no regulatory requirements for commercial genetic tests in horses. 
Instead of relying on genetic tests, horses should be diagnosed with PSSM2 if they show the features of PSSM1 without the GYS1 mutation. If your veterinarian suspects PSSM and has ruled out PSSM1, this is often when a muscle biopsy is recommended.
Diagnosis is typically based on chronic tying-up with or without elevated muscle enzyme levels and abnormal findings in muscle biopsies. During non-genetic disorder tying-up, muscle enzymes are still elevated but not typically during a short exercise tolerance test.
Signs of PSSM
Your veterinarian might suspect PSSM if your horse is showing one or more of the following signs: 
- Chronic exertional rhabdomyolysis (tying-up)
- Poor performance and lack of energy
- Muscle wasting and weakness
- Gait abnormalities
- May show signs of back pain or whole body pain
- Behavioural issues when saddled, reluctance to stand for the farrier
These symptoms can range from mild muscle cramping and stretching out to severe muscle pain with tissue breakdown. In the most severe cases, the horse is unable to rise to stand. In draft horses, the primary sign can be profound weakness.
Clinical Signs in PSSM Horses
If your horse has already been diagnosed with PSSM and shows any clinical signs (mild to severe), contact your veterinarian. Blood samples can be run to determine if your horse is experiencing a clinical case.
Addressing PSSM when symptoms are mild reduces the risk of severe problems (including kidney damage and muscle necrosis) and improves recovery time.
Collecting blood samples when the horse has been symptom-free for a longer period (1-2 months) will help your veterinarian determine your horse’s baseline for muscle enzyme levels. Enzymes are often slightly elevated compared to non-PSSM horses.
Nutritional and Exercise Management
There is currently no cure for PSSM1 or PSSM2. However, adopting an appropriate nutrition and exercise program can reduce the incidence of severe symptoms, such as severe tying-up.
Feeding and exercise guidelines have primarily been researched in horses with PSSM1. However, a recent study in warmbloods with PSSM2 showed that the same strategies outlined below are effect