PSSM, or polysaccharide storage myopathy, is a muscle disorder in horses where abnormal amounts of sugar (glycogen) accumulate in muscle tissue. This buildup interferes with normal muscle function and causes episodes of muscle cramping, stiffness, and “tying up,” a painful condition where muscles seize during or after exercise. PSSM has been documented in at least 30 horse breeds, and it comes in two distinct forms with different causes and diagnostic paths.
How PSSM Affects Muscle Tissue
In a healthy horse, muscles store glycogen as a ready fuel source and break it down during exercise. In horses with PSSM, muscles accumulate far more glycogen than they can use, along with an abnormal form of polysaccharide (a complex sugar). This excess storage damages muscle fibers over time and makes them prone to breaking down during work, releasing enzymes into the bloodstream that signal muscle injury.
The result is what horse owners commonly call “tying up,” where the horse becomes reluctant to move, develops a stiff or stilted gait, and may sweat excessively or seem distressed. Episodes range from mild stiffness that resolves quickly to severe cramping where the horse can barely walk. The hindquarters are most commonly affected, and some horses will show a hunched posture or try to stretch their back legs.
Type 1 vs. Type 2: Two Different Conditions
PSSM is split into two types that share similar symptoms but have fundamentally different causes.
PSSM Type 1 (PSSM1) is caused by a specific genetic mutation in the GYS1 gene, which controls glycogen production in muscle. This mutation essentially tells the muscle to keep making and storing glycogen beyond what’s normal. Because the cause is a single known gene variant, PSSM1 can be definitively diagnosed with a genetic test using a hair or blood sample. The condition is inherited, meaning affected horses can pass it to their offspring. Prevalence is highest in draft breeds and lowest in warmbloods, though it appears across a wide range of breeds including Quarter Horses, Paints, and Appaloosas.
PSSM Type 2 (PSSM2) also involves abnormal glycogen storage, but affected horses do not carry the GYS1 mutation. The cause remains unknown, and researchers believe there may actually be multiple underlying causes grouped under this label. PSSM2 is not so much a single disease as it is a microscopic description of glycogen accumulation in horses that don’t test positive for Type 1. In warmbloods, the condition has increasingly been reclassified as myofibrillar myopathy (MFM), which reflects a different pattern of muscle damage. Warmbloods with this form of muscle disease are often diagnosed around age 10 to 11 and typically don’t show the same blood enzyme elevations seen in PSSM1 horses.
Signs to Watch For
The hallmark sign of PSSM is episodes of tying up, but the condition doesn’t always present dramatically. Some horses show subtler signs that owners may initially chalk up to behavioral issues or poor fitness. These include:
- Muscle stiffness or reluctance to move, especially early in a workout or after a day off
- A short, choppy stride that doesn’t improve with warm-up
- Excessive sweating disproportionate to the level of work
- Hard, painful muscles in the hindquarters or back
- Reluctance to engage the hindquarters, sometimes mistaken for laziness or a training problem
- Dark or discolored urine after an episode, which signals muscle protein being filtered through the kidneys
Episodes are often triggered by a return to work after time off, which is one of the most frustrating aspects of the condition. A horse that’s been resting for several days is more likely to tie up than one that’s been exercised consistently. Stress, high-starch meals, and sudden increases in workload can also set off an episode.
How PSSM Is Diagnosed
For PSSM1, diagnosis is straightforward. A genetic test for the GYS1 variant uses hair follicles or a blood sample, and results are definitive. If your horse tests positive, you have a clear answer.
PSSM2 is a different story. There is currently no scientifically validated genetic test for it. Commercial labs do offer tests for variants labeled P2, P3, and P4, but a recent study found no significant association between those variants and a confirmed PSSM2 or MFM diagnosis based on muscle tissue analysis. The gold standard for diagnosing PSSM2 remains a muscle biopsy, where a small sample of muscle tissue is examined under a microscope to evaluate damage and measure glycogen levels. This is more invasive and expensive than a genetic test, but it’s the most reliable path to a PSSM2 diagnosis at this point.
Your veterinarian may also check blood levels of muscle enzymes (CK and AST) during or shortly after an episode. Elevated levels confirm muscle damage is occurring, though they don’t specifically point to PSSM over other causes of tying up.
Dietary Management
Diet is one of the two pillars of PSSM management, and the goal is simple: reduce the amount of sugar and starch your horse takes in while replacing those calories with fat. Horses with PSSM1 have muscles that are already overloaded with stored sugar, so feeding a high-starch grain only adds fuel to the problem.
Hay and any forage-based feeds should have nonstructural carbohydrate (NSC) levels below 12%. If you’re unsure about your pasture, grass samples can be tested at a forage lab. Many owners switch to a low-starch, high-fat commercial feed or add oil (such as rice bran oil or vegetable oil) to boost fat calories. The idea is to train the horse’s muscles to burn fat for energy instead of relying so heavily on glycogen.
One important caveat: this low-starch, high-fat approach is well supported for PSSM1 and for Quarter Horse-type breeds with PSSM2. However, warmbloods diagnosed with MFM (the condition increasingly distinguished from PSSM2) don’t typically have elevated muscle glycogen levels, so a low-starch, high-fat diet may not help them. This is one reason getting an accurate diagnosis matters before overhauling your feeding program.
Exercise: Consistency Is Everything
Regular, consistent exercise is just as important as diet for managing PSSM, and excess time off is one of the biggest risk factors for triggering an episode. Horses with PSSM benefit from daily movement of some kind, whether that’s riding, lunging, or simply having ample turnout.
The general guidelines emphasize high-intensity, short-duration workouts of no more than 20 minutes, paired with a proper warm-up, cool-down, and stretching. Conditioning should increase gradually rather than jumping straight into hard work. Daily turnout on pasture (assuming the grass meets NSC guidelines) gives the horse baseline movement throughout the day, which helps keep muscle metabolism running smoothly.
If your horse has recently tied up, exercise should resume within a few days, or as soon as residual pain subsides. This may feel counterintuitive, but getting the horse moving again relatively quickly helps prevent the cycle where rest leads to another episode. Start with light walking and build back up slowly.
Long-Term Outlook
PSSM is a lifelong condition with no cure, but most horses can be managed successfully with consistent dietary and exercise changes. The key word is consistent. Horses that stay on a low-NSC diet and maintain a regular exercise schedule often experience a significant reduction in tying-up episodes and can continue performing at a useful athletic level. Horses that go through cycles of work and long layoffs, or that return to high-starch feeds, tend to relapse.
Because PSSM1 is inherited, breeding decisions matter. A horse with one copy of the GYS1 mutation has a 50% chance of passing it to each foal. Horses with two copies (homozygous) tend to have more severe symptoms and will pass the mutation to every offspring. Genetic testing before breeding can help reduce the prevalence of PSSM1 in affected breeds over time.