Malignant hyperthermia (MH) is a rare generalized muscle disease in which there is progressive hyperthermia (elevated body temperature due to failed thermoregulation that occurs when the horse's body produces or absorbs more heat than it dissipates), hypercapnia (a condition of abnormally elevated carbon dioxide (CO2) levels in the blood) and muscle rigidity associated with certain anesthetic agents or muscle relaxants.

Horses with the MH mutation may intermittently show signs of tying up and high body temperatures, or they may show no signs at all and die suddenly after an episode of tying up.