Bit & Bridle

Equine Muscle Disorders: EPSM

Other Names

  • Polysaccharide Storage Myopathy (PSSM or PSSM2)
  • Tying up
  • Exertional rhabdomyolysis (ER)
  • Azoturia
  • Monday morning disease
  • Set fast
  • Recurrent exertional rhabdomyolysis (RER)

What is EPSM in Horses?

Equine Polysaccharide Storage Myopathy (EPSM) is a metabolic muscle disorder characterised by the abnormal accumulation of glycogen and abnormal polysaccharides within skeletal muscle cells. When muscles cannot correctly process or store energy, they become vulnerable to damage — particularly during exercise — leading to the clinical syndrome known as tying up, or exertional rhabdomyolysis.

EPSM is categorised into two types: Type 1 (PSSM1) is associated with a specific genetic mutation in the GYS1 gene causing overactive glycogen synthesis, most commonly seen in Quarter Horses, Paints, Appaloosas, and some draught breeds. Type 2 (PSSM2) lacks the GYS1 mutation but shows similar muscle biopsy changes, and is associated with other genetic variants affecting muscle function — seen more often in Warmbloods and other light breeds.

The condition can range from subclinical (detected only on muscle biopsy) to severely episodic, with horses experiencing recurrent, painful muscle cramps that interfere with work and welfare. Related muscle and performance conditions are discussed in our overview of Equine Tendinitis.

Causes

PSSM1 is caused by an autosomal dominant point mutation (R309H) in the muscle glycogen synthase gene (GYS1). Horses with even one copy of the mutation (heterozygous) may be affected — homozygous horses typically show more severe clinical signs. The mutation causes continuous, unregulated glycogen synthesis in muscle cells, leading to abnormal polysaccharide accumulation.

PSSM2 is associated with variants in other genes involved in muscle contractile proteins and structure, including MYH1, MYOT, FLNC, and others, though the pathophysiology is less completely understood. Environmental and management factors significantly influence whether genetically predisposed horses develop clinical signs:

  • High-starch, high-sugar diets — grain-heavy feeding fuels excessive glycogen production
  • Inconsistent or insufficient exercise — periods of rest followed by intense work are classic triggers
  • Electrolyte imbalances — sodium, potassium, and calcium dysregulation can exacerbate muscle dysfunction
  • Stress — transport, changes in routine, or concurrent illness can precipitate episodes

Symptoms and Signs

Clinical presentation varies from mild to severe:

  • Mild cases — reluctance to work, stiffness after exercise, subtle gait abnormalities, poor performance
  • Moderate cases — muscle stiffness and cramping during or shortly after exercise, particularly in the hindquarters and back muscles
  • Severe episodes — profuse sweating, severe muscle pain and rigidity (particularly in gluteal and back muscles), reluctance or inability to move, anxiety and distress
  • Dark, coffee-coloured urine (myoglobinuria) — a serious sign indicating significant muscle breakdown and potential kidney damage
  • Elevated heart and respiratory rate during episodes
  • Muscle wasting over time in poorly managed horses

The classic “Monday morning disease” presentation involves a horse that has been rested over a weekend on a full grain ration and then ties up when returned to work on Monday — reflecting the combination of excess glycogen loading and sudden exercise.

Diagnosis

Diagnosis involves a combination of clinical assessment, blood tests, and in some cases genetic testing and muscle biopsy:

  • Serum muscle enzymes — elevated creatine kinase (CK) and aspartate aminotransferase (AST) confirm muscle cell damage. Levels rise within hours of an episode and may remain elevated for days.
  • Urinalysis — myoglobin in urine indicates severe muscle breakdown and requires immediate management to protect the kidneys
  • Genetic testing — a simple blood or hair root sample can be tested for the GYS1 mutation (PSSM1). Available through commercial equine genetic laboratories.
  • Muscle biopsy — definitive diagnosis, particularly for PSSM2. Periodic acid-Schiff (PAS) staining reveals abnormal polysaccharide deposits. Biopsy of the semimembranosus or semitendinosus muscle is standard.
  • Resting enzyme levels — some EPSM horses show chronically mild CK/AST elevations even between episodes, which can be a useful screening indicator

Treatment

Acute episodes:

  • Stop exercise immediately — do not force the horse to move if it is in severe distress
  • Keep the horse warm and quiet — gentle walking may be appropriate if the horse is not severely affected
  • Administer NSAIDs (phenylbutazone or flunixin) for pain management
  • Provide access to fresh water and electrolytes to support hydration and kidney function
  • If myoglobinuria is present or the horse is severely affected, IV fluid therapy is essential — call your vet immediately

Long-term management:

  • Dietary modification — the cornerstone of PSSM1 management. Reduce or eliminate starch and sugar from the diet; replace grain with fat-based energy sources (oil, high-fat feeds, or soaked sugar beet). A diet providing less than 10% of digestible energy from non-structural carbohydrates (NSC) is the target.
  • Consistent daily exercise — regular light exercise is essential. Even 20–30 minutes of daily walking significantly reduces episode frequency by improving glycogen utilisation.
  • Gradual work increases — never increase work intensity rapidly after rest periods
  • Turnout — access to daily movement is preferable to intermittent intense exercise
  • Vitamin E supplementation — supports muscle health and has antioxidant properties that may reduce exercise-induced muscle damage

Prevention

In horses known or suspected to have EPSM:

  • Feed a low-starch, low-sugar diet from the outset — avoid sweet feeds and high-grain rations
  • Maintain consistent daily exercise even if this is just free turnout or gentle hacking
  • Ensure any rest periods are accompanied by reduced caloric intake
  • Genetic test horses before breeding, particularly in breeds with high PSSM1 prevalence, to avoid perpetuating the mutation
  • Manage electrolyte balance carefully, particularly around competitions and transport

Prognosis

The prognosis for PSSM1 horses managed with appropriate diet and consistent exercise is good to excellent. Many horses return to full athletic work with minimal recurrence. PSSM2 horses often respond well to similar management, though the underlying genetic causes are more varied and responses can be less predictable. Horses with recurrent severe episodes despite optimal management may have limited athletic futures, but most can be maintained comfortably as leisure horses.

When to Call the Vet

  • Your horse refuses to move, is sweating profusely, and shows signs of severe muscle pain
  • Urine is dark brown or discoloured — this requires urgent veterinary attention
  • Mild tying-up episodes are recurring despite dietary changes
  • You want to investigate suspected EPSM with diagnostic testing
  • Your horse shows unexplained chronic stiffness, reluctance to work, or poor performance

Frequently Asked Questions

Is EPSM the same as tying up?

Tying up (exertional rhabdomyolysis) is the clinical syndrome of muscle cramping and damage during exercise. EPSM is one specific underlying cause of tying up, but there are others — including recurrent exertional rhabdomyolysis (RER), which is more common in Thoroughbreds and is thought to have a different genetic and physiological basis. Diagnosis requires investigation to distinguish the type, as management strategies differ.

What should I feed a horse with PSSM1?

The ideal diet for a PSSM1 horse is high in fibre and fat, low in starch and sugar. Grass hay, haylage (low-sugar where possible), oil top-dressing, and commercially formulated low-NSC feeds are the building blocks. Avoid sweet feeds, traditional horse cubes high in cereals, and lush spring grass. A nutritionist or vet with expertise in equine metabolic disease can help formulate a bespoke ration.

Can PSSM1 be passed on to offspring?

Yes — PSSM1 is caused by an autosomal dominant mutation, meaning a horse with one copy of the mutation (heterozygous) has a 50% chance of passing it to each foal when bred to an unaffected horse. Two affected horses bred together produce affected offspring in 75% of cases. Genetic testing before breeding is advisable in susceptible breeds. See our horse genetics terminology for more.

How much exercise does a PSSM horse need?

Daily movement is more important than intensity. Even 20–30 minutes of light exercise — hand walking, turnout on a track, or quiet hacking — significantly reduces episode frequency by improving muscle glycogen metabolism. Consistent daily activity is preferable to sporadic intense work followed by days of stall rest. If the horse must rest due to illness or injury, reduce caloric intake accordingly to avoid glycogen overloading.