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Malignant Hyperthermia

What Is It?

  • Malignant hyperthermia (MH) is a genetic disorder (autosomal dominant trait), and may occur in conjunction with type 1 PSSM.
  • MH occurs in Quarter Horse bloodlines (with a high frequency in two specific lines), and horses are generally mature before exhibiting clinical signs.

Presenting Symptoms

  • Clinical symptoms include elevated body temperature (may be life-threatening) along with an episode of tying up or under anesthesia, metabolic failure, and death following a bout of tying up, particularly in horses with type 1 PSSM in addition to MH.


  • Turn the horse out for 2 weeks.
  • Longe once daily for 3 to 5 minutes at a walk and trot. Gradually increase by 2 minutes per day.
  • If stiffness is observed, stand the horse for 1 to 2 minutes and then resume walking to see if the stiffness persists.
    • If stiffness persists, stop; if not, resume walking for 2 minutes and then resume trotting.
  • When the horse can trot for 15 minutes, provide a 5-minute break at a walk and gradually increase walking and trotting after this.
  • Once the horse has reached 30 minutes of trotting on a longe line (with a break at 15 minutes), then begin to ride for 20 to 30 minutes and gradually increase duration or intensity of exercise.
  • It should take at least three weeks of exercise before the horse is ridden.
  • Keeping horses aerobically fit increases oxidative metabolism and is the best prevention, in concert with an appropriate diet, for further episodes.

Dietary and Management Recommendations

  • Nutritional and management recommendations for Malignant Hyperthermia mirror those for horses with PSSM.
  • The majority of the diet should be provided by a consistent supply of high-quality forage such as grass or grass/legume mix hays with controlled starch and sugar content. Hay analysis is strongly recommended as nutrient content of forage cannot be determined otherwise.
  • Maintaining a balanced diet while minimizing total dietary (hay or forage + grain or supplement products) starch and sugar intake and maximizing fat and fiber intake is recommended. This can be achieved by providing a majority of the daily calories from fat and digestible fiber and limiting energy sourced from non-structural carbohydrates. A low calorie protein, vitamin and mineral ration balancer may be used to ensure all nutrient requirements are met without introducing excess energy.
  • Fat supplements of vegetable oil, rice bran or corn oil can be used. 1 lb of fat/1000 lb horse can be accomplished with 2 cups of oil mixed with a soluble fiber such as alfalfa cubes or non-molassed beet pulp. These recommendations must be modified depending on the individual caloric needs of the horse.
  • For horses that are obese, minimizing the caloric density of their ration is needed to facilitate weight loss, which can be difficult with high levels of dietary fat. For these horses, rather than providing high levels of supplemental fat to their diet, fasting prior to exercise (~6 hrs) helps promote increases in plasma-free fatty acids and may help alleviate challenges with energy metabolism in these horses.
  • Dietary supplementation with vitamin E (600 to 2000 IU/day) may be beneficial.
  • Due to insulin hypersensitivity, chromium supplementation to PSSM horses is contraindicated.
  • Changes in diet should be made gradually, over a minimum of two weeks, to allow for adaptation and reduce the risk of digestive upset.
  • Salt should be available free choice. Minerals may be offered.
  • Minimize stress and provide regular routine with exercise, turnout and feeding.
  • Consumption of high levels of fructans (plant sugars) can exacerbate clinical symptoms; therefore, horses should be kept off “lush” pastures, new pasture growth (leaves less than 6” high from the ground) and pasture that has been under stress (drought, frost). Depending on management or facility logistics, horses can be muzzled or turned out in a dry lot to limit grass intake and facilitate voluntary exercise.
  • In addition to nutritional intervention, an appropriate turnout and regular exercise program are essential to successful management of horses with PSSM.
  • Minimize stress and provide regular routine with exercise, turnout and feeding.
  • Turn out in large areas, preferably with other horses.
  • Exercise therapy consists of daily turnout and as little stall rest as possible. Exercise should be introduced gradually, starting with 3 to 5 min. of walk/trot on a longe line or under saddle, working up to 15 min. If no increases in creatine kinase (CK) are evident, the submaximal workload can be gradually increased.
  • When the horse can be worked for 30 minutes without difficulty, active riding can be initiated.


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