Hyperthermia, malignant: A series of potentially fatal problems that can occur during surgery, malignant hyperthermia is caused by a reaction to anesthesia. The body's metabolism rises suddenly, causing a sudden jump in body temperature and muscle rigidity. The result can be damage to tissues and organs (including the brain), or death. The propensity to malignant hyperthermia is inherited. Treatment is by administering dantrolene sodium (brand name: Dantrium) and rapidly cooling the patient.