Diagnosing Performance-Limiting Muscle Diseases in Horses

There are a number of muscle diseases that can impact performance in horses, from PSSM to MFM.
Warmblood horse with the muscle disease MFM
Warmbloods with exercise intolerance from a specific muscle problem typically don’t have elevated CK or AST, and this syndrome is now referred to as MFM. | Getty Images

At the 2023 AAEP Convention, Stephanie Valberg, DVM, PhD, DACVIM, DACVSMR, professor emeritus at Michigan State University’s McPhail Equine Performance Center, described a number of muscle diseases that can impact performance in horses.


Valberg stressed the importance of measuring creatine kinase (CK) and aspartate transaminase (AST) to confirm myofiber degeneration and guide treatment decisions. It is sometimes necessary to retest for CK if resting values are found to be normal—exercise the horse for 15 minutes and recheck CK four to six hours later. A value that doubles from rest to post-exercise is likely due to subclinical rhabdomyolysis. AST values are indicative of a previous episode of rhabdomyolysis and peak at 12-24 hours with return to baseline two to three weeks later.

Causes of Rhabdomyolysis

Valberg described the many possible causes of exertional rhabdomyolysis (ER):

  • Sporadic ER from excess exercise.
  • Recurrent exertional rhabdomyolysis (RER), common in fit competition horses.
  • Type 1 polysaccharide storage myopathy (PSSM1) in more than 20 breeds—but not Thoroughbreds, Arabians, or Standardbreds—for which there is a genetic test for GYS1 mutation.
  • Type 2 polysaccharide storage myopathy (PSSM2-ER), which tends to occur in Quarter-Horse-related breeds and is identifiable with muscle biopsy of the tailhead.
  • Malignant hyperthermia (MH) in Quarter-Horse-related breeds, which has a genetic test.
  • Myofibrillar myopathy (MFM) in Arabian endurance horses.
  • ER without elevated CK and AST in Warmblood horses, previously referred to as PSSM2 but now called MFM.

For cases of ER with normal CK values, it is important to rule out gastric ulcers, lameness, and primary muscle strain and observe the horse under saddle to rule out rider impact, saddle fit, and horse behavior. Valberg noted that, in general, a horse’s reluctance to go forward or engage the hind end and other variable behavioral issues are often related to pain. However, similar signs might be attributable to a muscle syndrome in Warmbloods described in more detail below. 

Genetic Testing for PSSM

While there is a genetic test for PSSM2-ER, Valberg said even healthy control horses are likely to possess a nucleotide polymorphism P2, P3, or P4 variant similar to what is found in horses with PSSM2. More than one P variant mutation tests positive in 30% of Warmbloods and 60% of Quarter Horses, despite no evidence of muscle disease. PSSM1 is associated with a GYS1 mutation, while PSSM2 is not.

Breed-Specific Approach to Muscle Diseases in Horses

Quarter Horses

A breed-specific approach to PSSM2 is relevant for Quarter Horses that have increased CK and AST and rhabdomyolysis with this syndrome. PSSM2 isn’t a specific disease but, rather, a microscopic description of glycogen accumulation in horses with these clinical signs. Abnormally elevated muscle glycogen is not as high as in PSSM1. The best approach, said Valberg, is to address PSSM2 as a glycogen storage disease by implementing a low-starch diet with low-fat supplements and regular exercise. 


Some lines of accomplished Arabian endurance horses, especially toward the end of a race, experience muscle changes consistent with MFM. In these cases, a low starch-low fat diet is ineffective because these horses do not suffer from glycogen storage disease.


Warmbloods with exercise intolerance from a specific muscle problem are often diagnosed at age 10-11. They typically don’t have elevated CK or AST. Muscle glycogen concentrations are not elevated compared to PSSM1 and PSSM2-ER Quarter Horses, so a low-starch/low-fat diet is not useful in these cases. In normal muscle tissue, electron microscopy shows myofibrils, contractile filaments, and Z discs aligned nicely in uniform configurations. Valberg explained that the Z discs lose alignment in portions of muscle of affected Warmbloods with exercise intolerance and reluctance to engage their hindquarters. Humans experience achy, painful muscles after hard workouts because of this same phenomenon of Z disc disruption, she added, but in normal muscle Z discs restore quickly.

Myofibrillar Myopathy (MFM)

Affected horses have broken myofibrils with proteinaceous material leaking from disrupted Z discs, causing this syndrome to be referred to as MFM. With further investigation, Valberg’s team identified a proteinaceous accumulation of desmin as well as glycogen aggregates. Desmin is a cytoskeletal protein that normally links and holds myofilaments together and links to cell membranes. Because it is easily oxidized, desmin aggregation in affected horses doesn’t get replaced by normal myofilaments. 

This muscle syndrome in Warmbloods is age-dependent, starting at 6-7 years, with desmin aggregating by about 10 years, Valberg noted. 

Cause of MFM

The cause involves a lack of cysteine-based antioxidants in a pro-oxidant environment within the respiratory chain of mitochondria. In normal horses, cysteine is produced from glutathione. But this pathway is lacking in these horses to result in a maladaptive training response in Z disc function. The consequence is constant achy and stiff muscles that affects performance. An affected horse is reluctant to move, doesn’t want to engage the hindquarters, loses push in the hindquarters, has difficulty picking up the canter, displays a short-strided gait, and is stiff and sore, often with back soreness. Performance declines and for horses out of work, muscle mass drops faster than expected, especially in the gluteal and thigh muscles, said Valberg.

Treatment Options for MFM

Treatment relies on “long and low” frame warmups and no more than three days of work with two days off and lots of turnout. Altered training regimens should also allow long periods for recovery. These horses seem to lack energy, so dietary changes include 20-30% of NSC energy requirements as long as the horse doesn’t have underlying metabolic syndrome. Because fat is pro-oxidative, it is best to feed these individuals a low-fat diet < 8%. Amino acids are important to rebuild muscle mass, so Valberg recommends 12-14% protein. 

Cysteine itself is readily oxidized and not absorbed; glutathione fed alone is broken down and not absorbed. However, Valberg explained that N-acetyl cysteine is readily absorbed and, along with coenzyme Q10, can increase glutathione post-exercise. Whey powder is a good source for achieving this objective. Her lab has formulated a palatable MFM pellet that contains branched chain amino acids for rebuilding sarcomeres for contractile function. Many owners adding this supplement to affected horses’ diets report improved performance after four weeks on MFM pellets and exercise management.

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