PSSM in Horses: Polysaccharide Storage Myopathy Signs, Testing, and Management

Quick Answer
  • PSSM is a muscle disorder that causes abnormal sugar storage in muscle, often leading to stiffness, cramping, poor performance, or tying-up episodes.
  • Type 1 PSSM can be confirmed with a DNA test from hair or blood. Type 2 is not confirmed by the same mutation test and may require muscle biopsy plus your vet's exam and lab work.
  • Common signs include reluctance to move, firm painful muscles, sweating, shifting lameness, weakness, and dark urine during severe episodes.
  • Management usually focuses on a low-starch feeding plan, regular daily exercise, turnout, and avoiding long periods of rest when your vet says movement is safe.
  • See your vet immediately if your horse cannot walk normally, is trembling, has severe pain, or passes dark brown or red urine.
Estimated cost: $150–$1,200

What Is PSSM in Horses?

Polysaccharide storage myopathy, or PSSM, is a muscle disease in which a horse stores abnormal amounts or forms of glycogen in muscle. Glycogen is the stored form of sugar muscles use for energy. When that storage process is abnormal, muscles can become painful, stiff, weak, or prone to episodes of exertional rhabdomyolysis, often called "tying up."

Vets usually divide PSSM into type 1 and type 2. PSSM1 is linked to a known mutation in the GYS1 gene and has been identified in many breeds, especially Quarter Horse-related breeds. PSSM2 describes horses with similar muscle signs but without the GYS1 mutation; diagnosis often depends on muscle biopsy findings and ruling out other muscle disorders. Some horses with suspected PSSM2 may ultimately have a different myopathy, so careful veterinary workup matters.

Many horses with PSSM can do well with long-term management. The goal is not a one-time cure. Instead, your vet helps build a plan around exercise consistency, forage-first feeding, starch restriction, and monitoring for flare-ups.

Symptoms of PSSM in Horses

  • Muscle stiffness after light work
  • Reluctance to move forward or shortened stride
  • Firm, painful muscles over the hindquarters or back
  • Sweating, tremors, or cramping with exercise
  • Poor performance or difficulty building topline
  • Shifting lameness or weakness behind
  • Repeated tying-up episodes
  • Dark brown or red urine
  • Unable to stand or unwilling to walk

Mild cases may look like soreness, resistance under saddle, or inconsistent performance. More severe episodes can include sweating, trembling, hard painful muscles, and refusal to move. Some horses mainly show chronic poor performance instead of dramatic tying up.

See your vet immediately if your horse has severe pain, cannot walk comfortably, seems weak enough to go down, or has dark urine. Those signs can mean significant muscle damage and possible kidney complications.

What Causes PSSM in Horses?

PSSM1 is caused by an inherited mutation in the GYS1 gene. This mutation makes glycogen synthase overly active, so affected horses store too much glycogen in muscle. Insulin can further stimulate this process, which is one reason high-starch or high-sugar meals can worsen signs in some horses.

PSSM2 is more complicated. Horses with PSSM2 do not have the GYS1 mutation, and the underlying cause is not fully defined. In practice, this means your vet may need to sort through several possibilities, including other exertional myopathies, myofibrillar myopathy, training issues, pain elsewhere in the body, or metabolic contributors.

Breed matters. PSSM1 is especially recognized in Quarter Horses and related breeds, with lower prevalence in some other populations. Warmbloods are commonly discussed in connection with PSSM2-like presentations. Even so, breed alone cannot confirm the diagnosis. A horse's history, exam, exercise pattern, and test results all need to fit together.

How Is PSSM in Horses Diagnosed?

Diagnosis starts with your vet's history and physical exam. They will ask about tying-up episodes, exercise schedule, days off, diet, breed, and whether signs happen after rest or grain meals. Bloodwork often includes muscle enzymes such as CK and AST, which commonly rise after muscle injury.

For PSSM1, a genetic test on hair roots or blood is the usual confirmatory test. For PSSM2, there is no equivalent single mutation test used the same way. If a horse has compatible signs but tests negative for PSSM1, your vet may recommend a muscle biopsy to look for abnormal glycogen storage and to help separate PSSM2 from other muscle diseases.

Diagnosis is also about ruling out look-alikes. Your vet may consider recurrent exertional rhabdomyolysis, myofibrillar myopathy, electrolyte problems, pain-related poor performance, or other neuromuscular conditions. Because biopsy interpretation and exercise history matter, referral to an equine internal medicine or sports medicine service can be helpful in unclear cases.

Treatment Options for PSSM in Horses

Spectrum of Care means you have options. Here are treatment tiers at different price points.

Budget-Conscious Care

$150–$450
Best for: Horses with mild or intermittent signs, pet parents who need to start with practical management changes, or cases where PSSM1 is already known from prior testing.
  • Farm call or exam with history review
  • Basic bloodwork with muscle enzymes such as CK and AST
  • Immediate rest from work during an episode, then guided return to movement when your vet says it is safe
  • Forage-first diet changes with grain reduction or removal
  • Turnout and a structured daily exercise routine
  • Targeted pain control only if your vet recommends it
Expected outcome: Many horses improve noticeably when exercise becomes consistent and starch intake is reduced. Mild cases may return to useful work with careful management.
Consider: This approach may not fully define whether the horse has PSSM1, PSSM2, or another myopathy. It relies heavily on response to management and close follow-up.
Most Common

Recommended Standard Treatment

$300–$900
Best for: Most horses with repeated tying up, poor performance, or breed risk where a clear first-line workup is needed.
  • Complete veterinary exam and exercise history
  • CBC/chemistry and muscle enzyme testing
  • PSSM1 genetic test from hair or blood
  • Diet formulation focused on low starch and adequate fat calories when appropriate
  • Written conditioning plan with daily exercise and turnout goals
  • Recheck bloodwork or clinical follow-up after management changes
Expected outcome: Good long-term control is possible in many horses, especially when feeding and exercise are consistent. Some horses remain sensitive to missed workouts or dietary changes.
Consider: Requires more upfront testing and ongoing management discipline. Feed changes and regular exercise take planning and may increase monthly care costs.

Advanced / Critical Care

$800–$2,500
Best for: Horses with severe pain, dark urine, inability to stand or walk normally, recurrent unexplained episodes, or negative PSSM1 testing with persistent signs.
  • Hospitalization or emergency care for severe tying-up episodes
  • IV fluids when muscle breakdown raises concern for kidney injury
  • Sedation or muscle-relaxing support if your vet feels it is needed
  • Muscle biopsy for horses negative for PSSM1 but still strongly suspected of having a muscle disorder
  • Specialist referral in equine internal medicine, neurology, or sports medicine
  • Expanded workup for other myopathies or poor-performance causes
Expected outcome: Short-term recovery can be good with prompt care, but long-term outlook depends on the exact myopathy, severity of muscle damage, and how well the horse responds to management.
Consider: Higher cost range and more intensive diagnostics. Advanced testing may clarify the diagnosis, but it can also reveal a different muscle disease rather than PSSM.

Cost estimates as of 2026-03. Actual costs vary by location, clinic, and individual case.

Questions to Ask Your Vet About PSSM in Horses

Bring these questions to your vet appointment to get the most out of your visit.

  1. Do my horse's signs fit PSSM1, PSSM2, or another muscle disorder?
  2. Should we run CK and AST now, and when should blood be drawn after exercise or an episode?
  3. Is a PSSM1 genetic test appropriate for my horse's breed and history?
  4. If the PSSM1 test is negative, when would muscle biopsy be worth considering?
  5. What forage, concentrate, and treat changes do you recommend for my horse specifically?
  6. How much daily turnout and exercise is ideal, and how should I handle days off?
  7. What warning signs mean I should stop work and call you right away?
  8. What is the realistic monthly cost range for long-term management in my horse's case?

How to Prevent PSSM in Horses

Prevention looks different for PSSM1 and PSSM2. For PSSM1, breeding decisions matter because the condition is linked to a known inherited mutation. Genetic testing before breeding can help reduce the chance of passing the mutation forward. For horses already living with PSSM1, prevention means preventing flare-ups rather than preventing the gene itself.

Day-to-day management is the biggest tool. Many horses do best with consistent daily exercise, regular turnout, and a low-starch feeding plan built around forage. Long periods of stall rest, sudden grain increases, and abrupt schedule changes can trigger problems in some horses. Your vet may also recommend reviewing hay nonstructural carbohydrate levels and adjusting calories so the horse stays fit without becoming overweight.

Because PSSM2 does not have the same straightforward genetic screening approach, prevention focuses on early recognition and thoughtful management. If your horse repeatedly gets stiff after work, struggles after days off, or has unexplained tying-up episodes, involve your vet early. A tailored plan can reduce episodes and help protect both performance and comfort.

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