Equine Polysaccharide Storage Myopathy

  Equine polysaccharide storage myopathy (EPSM) is a form of rhabdomyolysis classified as a metabolic disease that results in the accumulation of high muscle glycogen and abnormal polysaccharide in skeletal muscles.

  The occurrence has presently been documented in Quarter horses, American Paint Horses, Quarterhorse crosses, warmbloods, draft horses and draft crosses.  Presently, EPSM is believed to be transmitted as an autosomal recessive disorder with mares being more frequently diagnosed than geldings.  Geldings may be more commonly affected with non-diagnosed clinical cases.

  Horses that are affected generally are referred to as having a calm demeanor and being heavily muscled.  Signs often occur 10-20 minutes after light work in 2-4 year olds starting training, but signs can also occur at any stage of life.  The episodes may occur once or twice a year to every time the horse is exercised.  A common complaint is that horses are exercise intolerant, especially at high speeds.  In mild cases, horses show a tucked up abdomen, muscle fasiciculations in the flank, and a camped-out stance.  If exercise is continued, profuse sweating, front and hindlimb gait asymmetry, and reluctance to move are seen.  In severe cases, horses may refuse to move forward, buck and lie down to avoid exercise.  When returned to the stall they may show signs of colic, such as rolling or pawing.  Rarely, myoglobinuria can be seen.  The gluteal, biceps femoris, semitendinosus, and epaxial muscles may be tense and painful when palpated.

  Draft breeds tend to show more severe episodes with recumbency often leading to death.  In all breeds, chronic cases can show generalized poor muscling or subtle to severe muscle wasting commonly involving the rump and proximal thigh musculature.

  EPSM is characterized as a glycogenosis or glycogen storage disease.  Affected animals have muscle glycogen levels 1.5-4 times higher than normal animals and abnormal polysaccharide present in skeletal musculature.  It is proposed that affected horses have an abnormality of glycogen synthesis regulation that leads to storage of unmetabolized carbohydrate in the muscles.  However, further research is needed to determine the exact etiology.  The weakness and muscle fiber necrosis evident in EPSM affected animals may be due to an energy “crisis” that results from the inability of the muscle to use the abnormal polysaccharide and result in a catabolic state that leads to the inability to maintain normal muscle fiber size.

  The enzymes of creatininekinase (CK), lactate dehydrogenase (LDH), and aspartatetransaminase (AST) are elevated and increases parallel the severity of the episode.  Serum enzyme levels can range from 2000 to 200,000 U/L.  With typical rhabdomyolysis high CK levels decline to normal within days of resolution, but in cases of EPSM, CK level may remain elevated after the horse is rested and levels can increase due to subsequent subclinical episodes.

  Exercise testing is an excellent screening test and can be performed by testing blood enzyme levels after 15 minutes of trotting on a longe line.  Usually, horses with EPSM show increases of CK of greater than 1000 U/L 4-6 hours after exercise.  Surgical or needle muscle biopsies provide a definitive diagnosis.  The preferred sites for surgical biopsy are the semimembranosus or semitendinosis muscles.  These sites provide easy access, a high prevalence of abnormal polysaccharide, and a poorly visible scar.  The needle biopsy can be taken from the gluteus medius muscle but requires a Bergstrom needle; however, this technique is easy and leaves no scar.

  The hallmark of EPSM is PAS-positive inclusions scattered in fast twitch muscle fibers found in horses showing recurrent episodes of rhabdomyolysis or exercise intolerance.  Unlike normal glycogen, the majority of the PAS-positive inclusions resist digestion with amylase.  Other findings that support EPSM are increased staining of normal glycogen, necrotic fibers, rimmed and subsarcolemmal vacuoles, and centrofascicular atrophy and regeneration of muscle fibers.  False negatives can occur and are usually the result of a small biopsy size of less than 200 fibers or the absence of abnormal polysaccharides in young animals since they tend to form in the later stages of glycogenoses.  False positives typically

result from diagnosing EPSM solely on the basis of increased glycogen without the presence of abnormal polysaccharides.

Treatment consists of dietary management and implementation of a defined exercise regime.  The goals of dietary therapy are to minimize dietary carbohydrates and maximize fat intake by providing 20-25% of the dietary caloric requirements from fat.  All grain, sweet feed, and molasses should be eliminated from the diet and replaced with high quality forages such as alfalfa hay or grass-alfalfa hay mix diets.  Additional calories are generally necessary due to the strict exercise regimen needed to control clinical signs; therefore, fat is added to the diet to provide an alternative energy source.  Fat supplements of vegetable oil, powdered animal fat, or corn oil can be used.  The recommended 1 lb. of fat/1000 pound horse can be accomplished with 2 cups of oil mixed with alfalfa cubes for palatability.  These recommendations must be modified depending on the individual caloric needs of the patient.

  Exercise therapy consists of daily turnout and as little stall rest as possible.  The horse should be longed for 15 minutes a day and, if no increases in CK are evident, the workload can be gradually increased.  When the horse can be worked for 30 minutes without difficulty, active riding can be initiated.

  EPSM horses respond most favorably to both diet and exercise change.  Typically, after 3-6 months of therapy, post-exercise serum concentrations of CK are within reference limits.  Improvements in muscle function are proposed to be the result of segmental necrosis of fiber segments containing unmetabolized carbohydrates.

  The prognosis varies among horses, but dietary and exercise changes almost completely control signs in most EPSM affected animals.  The exception occurs in horses of the draft breeds that have become acutely recumbent; 50% do not survive despite aggressive treatment.  Currently, affected horses are retired to breeding.  Since ESPM may be a heritable condition, this practice should be discouraged.

  -By Timothy Galusha, Class 2000

  -Edited by Christine Hanika, DVM, PhD