PSSM1 (Polysaccharide Storage Myopathy Type 1) is a genetic muscle disease in horses caused by a single mutation that makes the body store too much glycogen in muscle tissue. The excess glycogen damages muscle fibers and leads to episodes of “tying up,” where muscles cramp, stiffen, and sometimes refuse to function normally. It’s one of the most common inherited muscle disorders in horses, particularly in stock breeds and draft breeds.
The Genetic Cause
PSSM1 traces back to one specific mutation in the GYS1 gene, which provides the blueprint for an enzyme called glycogen synthase. This enzyme’s job is to convert sugar from the bloodstream into glycogen, the stored form of energy in muscle. In a healthy horse, the enzyme switches on and off as needed. In a horse with the PSSM1 mutation, the enzyme is essentially stuck in the “on” position.
The mutation swaps a single amino acid at position 309 in the enzyme’s structure, replacing arginine with histidine. That small change breaks a molecular bond that normally keeps the enzyme in its resting, inactive state. Without that bond, the enzyme defaults to its active form, continuously producing glycogen whether the muscle needs it or not. The result is an abnormal buildup of glycogen and unusual sugar-based deposits called amylopectate inclusions throughout the muscle tissue.
PSSM1 is inherited in an autosomal dominant pattern, meaning a horse only needs one copy of the mutated gene (from one parent) to be affected. Horses with two copies tend to have more severe signs.
Which Breeds Are Affected
The mutation appears most frequently in Quarter Horses, Paints, and Appaloosas, but it’s also well documented in draft breeds like Belgians, Percherons, and Clydesdales. Warmbloods and some light breeds can carry it too. Because the mutation is dominant and has likely been in the horse population for centuries, it has spread across many breed lines, particularly those selected for heavy muscling and calm temperaments.
Recognizing the Signs
The hallmark of PSSM1 is episodes of exertional rhabdomyolysis, commonly called tying up. During an episode, the horse’s muscles (especially over the hindquarters) become painfully stiff and rock-hard. The horse sweats heavily, may tremble in the flank area, and is reluctant or completely unwilling to move. Some horses develop a shifting lameness, tense their abdomen, and appear lazy or resistant under saddle when the real problem is muscle pain.
Episodes typically happen during or shortly after exercise, especially if the horse has had a few days off and then returns to work. The severity ranges widely. Some horses show only subtle stiffness and poor performance that owners attribute to attitude or soreness. Others experience dramatic, painful episodes where the muscles visibly seize. Very young foals with PSSM1 can occasionally show signs of severe muscle pain and weakness even without exercise.
Under saddle, affected horses often resist going forward or collecting. Owners frequently describe the horse as “not wanting to work” before the underlying condition is identified, and many cases are initially misdiagnosed as unexplained lameness or behavioral problems.
How It’s Diagnosed
Historically, diagnosing PSSM required a muscle biopsy, where a veterinarian would take a small sample of muscle tissue and test whether the glycogen in it resisted digestion by a specific enzyme. That method could confirm abnormal glycogen storage but couldn’t distinguish PSSM1 from other forms of the disease.
Today, a simple genetic test is the standard for PSSM1. All it requires is roughly 20 to 40 hairs pulled with the roots attached. The lab analyzes DNA from the hair follicles to check for the GYS1 mutation. Results come back as N/N (no copies, clear), N/P (one copy, affected), or P/P (two copies, affected). The test is definitive: if the mutation is present, the horse has PSSM1. This also makes it a valuable tool for breeding decisions, since carriers can pass the mutation to offspring.
PSSM1 vs. PSSM2
The name “PSSM2” gets used for horses that show similar symptoms and have abnormal glycogen or polysaccharide accumulation on muscle biopsy, but do not carry the GYS1 mutation. PSSM2 is not a single, well-defined genetic condition the way PSSM1 is. It likely represents a group of different muscle disorders that haven’t yet been fully characterized at the genetic level. The key distinction: PSSM1 can be confirmed with a hair DNA test, while PSSM2 cannot. If a horse tests negative for the GYS1 mutation but still ties up or has biopsy abnormalities, the condition may be labeled PSSM2, though the underlying cause could vary from horse to horse.
Managing Diet
Because the core problem in PSSM1 is excess glycogen production, the dietary goal is to reduce the raw materials the enzyme works with. That means cutting starch and sugar in the diet and replacing those calories with fat. The logic is straightforward: less sugar going in means less glycogen being overproduced, while fat provides an alternative energy source the muscles can burn without triggering glycogen storage.
Research published in the American Journal of Veterinary Research found that feeding long-chain fatty acids, such as those in corn oil, significantly decreased exercise intolerance and rhabdomyolysis in affected Quarter Horses. The recommended percentage of total digestible energy from fat ranges from about 12% to 25%, depending on the horse’s workload and individual response. In research diets, fat content ranged from 15% to 30% of digestible energy. Commercially available high-fat, low-starch feeds are formulated specifically for this purpose, and many owners supplement with oil (rice bran oil or corn oil are common choices) on top of a low-starch feed.
The forage base matters too. Affected horses do best on grass hay rather than rich alfalfa or high-sugar pasture. Avoiding grain-heavy meals and sweet feeds is essential. Any dietary transition should happen gradually over a few weeks to let the digestive system adjust.
The Role of Exercise
Consistent daily movement is just as important as diet. Muscle that is working regularly burns through glycogen stores and is less likely to accumulate the damaging excess. The worst-case scenario for a PSSM1 horse is several days of rest followed by a sudden bout of exercise, which is a classic trigger for tying-up episodes.
Turnout is a cornerstone of management. Horses that live in stalls with limited movement are far more prone to episodes than those with access to pasture or a large paddock where they move throughout the day. Under-saddle work should start with a long, gradual warm-up, and the workload should increase slowly over weeks rather than jumping to intense effort. On rest days, some form of light activity (hand-walking, turnout, or lunging at a walk and trot) helps keep glycogen from building up.
What to Do During an Episode
If a horse ties up during exercise, stop working it immediately and move it to a stall or quiet area. Do not force the horse to walk, as continuing to use already-damaged muscles can make things worse. Keep the horse calm, offer water, and contact a veterinarian. Severe episodes can cause significant muscle breakdown, which releases proteins into the bloodstream that can damage the kidneys. A vet will often check blood levels of creatine kinase, an enzyme that spikes when muscle cells are damaged, to gauge severity.
Long-Term Outlook
PSSM1 is a lifelong condition with no cure, but it is highly manageable. Research from Utrecht University found that more than 75% of horses whose diet and exercise routines were properly adjusted had no further tying-up episodes. Many PSSM1 horses go on to have full athletic careers in disciplines ranging from trail riding to competitive events, as long as their management stays consistent.
The horses that struggle most are those whose owners can’t provide daily exercise or whose living situations involve long periods of stall confinement. Flare-ups also tend to happen during transitions: a change in feed, a break from training due to injury, or a move to a new facility where the routine shifts. Keeping the diet low in starch, the fat intake adequate, and the daily movement consistent is the formula that works for the vast majority of affected horses.