Why We’re Talking About PSSM
Horses are strong, fast, and full of energy. But even healthy-looking horses can develop muscle problems that make it hard for them to move. One such condition is PSSM, short for Polysaccharide Storage Myopathy. Many horse owners also call it “tying‑up” because of the way it causes severe muscle stiffness and pain.
In this guide, we’ll explain what PSSM is, what causes it, how to recognize the signs, and how to care for a horse so it can live a comfortable, active life.
What Is Polysaccharide Storage Myopathy (PSSM)?
PSSM is a muscle disorder in which a horse’s skeletal muscles store sugar (glycogen) in an abnormal way. This excess or improperly stored glycogen disrupts normal muscle function.
Instead of using glycogen efficiently for energy, the muscles store too much of it. This leads to muscle stiffness, pain, and fatigue — often triggered by exercise. Over time, this can cause repeated tying‑up episodes, where the horse becomes reluctant to move because of painful muscle cramps.
What Does “Tying‑Up” Mean?
“Tying‑up” refers to an episode of severe muscle cramping and pain. It’s similar to a human charley horse but much more intense and can involve the entire body.
When a horse ties‑up:
- Muscles become tight and painful
- The horse may stop walking or move very stiffly
- Sweating may occur even without hot weather
These episodes can be alarming, but with the right management, they can be reduced or prevented.
Causes of PSSM
PSSM has several possible causes:
- Genetic Mutation (PSSM Type 1): Caused by a mutation in the GYS1 gene, which controls glycogen production. Horses with this mutation store energy abnormally in their muscles.
- Other Muscle Changes (PSSM Type 2): Likely genetic but not yet fully understood. Researchers are working to identify the mutations involved.
- Diet and Exercise Factors: Diets too high in sugar and starch, combined with irregular exercise, can make symptoms worse. Poor management may lead to exertional rhabdomyolysis, a type of muscle damage caused by intense exercise.
Breeds at Risk
PSSM can affect many breeds, but certain horses are more prone:
- Quarter Horses: Up to 11% may carry the genetic condition.
- Draft Breeds: Belgian and Percheron horses are often affected.
- Warmbloods: Especially those bred for athletic performance.
- Related Breeds: Paint Horses, Appaloosas, and others linked to Quarter Horse bloodlines.
Knowing if a horse is at higher risk helps owners make informed choices about testing and management.
The Genetics Behind PSSM
In PSSM Type 1, a mutation in the glycogen synthase 1 (GYS1) gene causes an overactive enzyme that stores too much glycogen in muscle cells. The mutation is autosomal dominant, meaning only one copy is needed from either parent for a horse to be affected.
For PSSM Type 2, the exact genetic cause is still unknown, but researchers believe multiple genes and environmental factors may be involved.
Diagnosis for Type 1 can be confirmed through genetic testing from a blood or hair sample. Type 2 is diagnosed based on symptoms, muscle biopsies, and ruling out other muscle disorders.
Signs and Symptoms
Horses with PSSM may show:
- Muscle stiffness, especially after rest
- Reluctance to move forward under saddle
- Sweating during light exercise
- Trouble walking or moving after mild work
- Fatigue or low energy
- Poor performance compared to expectations
- Mild, subtle signs that may be easy to overlook
Some horses show signs only occasionally, while others experience them more often.
Diagnosis
To confirm PSSM, a veterinarian may:
- Take a muscle biopsy to examine glycogen storage in muscle tissue
- Conduct genetic testing (especially for suspected PSSM1)
- Check muscle enzyme levels (CK and AST) through bloodwork
Early diagnosis allows for better long-term management.
How to Help a Horse With PSSM
With proper care, many horses with PSSM can live active, happy lives.
1. Diet
- Feed a low-sugar, low-starch diet (avoid sweet feeds, corn, oats)
- Use healthy fats like rice bran for extra calories without increasing sugar
- Provide low-sugar hay and monitor forage carbohydrate levels
- Supplement with vitamin E to support muscle health
2. Exercise
- Exercise daily to help muscles use stored glycogen
- Keep workouts consistent and avoid long breaks
- Warm up slowly before more intense work
3. Turnout
- Provide ample turnout time so the horse can move freely
- Limit stall confinement unless necessary
Living With PSSM
A PSSM diagnosis doesn’t mean the end of riding or performance. Many horses manage well with the right combination of diet, exercise, and turnout.
The key is consistency — feed the correct diet every day, maintain regular activity, and work closely with a veterinarian to adjust care as needed.
PSSM is a muscle disorder that causes abnormal glycogen storage and can lead to painful tying‑up episodes. While it’s genetic in many cases, careful management can keep horses comfortable and active for years.
If you suspect your horse has PSSM, consult your veterinarian for testing and a tailored care plan. With the right approach, horses with PSSM can still thrive.
