Clinical characteristics: Malignant hyperthermia susceptibility (MHS) is a pharmacogenetic disorder of skeletal muscle calcium regulation associated with uncontrolled skeletal muscle hypermetabolism. Manifestations of malignant hyperthermia (MH) are precipitated by certain volatile anesthetics (i.e., halothane, isoflurane, sevoflurane, desflurane, enflurane), either alone or in conjunction with a depolarizing muscle relaxant (specifically, succinylcholine). The triggering substances cause uncontrolled release of calcium from the sarcoplasmic reticulum and may promote entry of extracellular calcium into the myoplasm, causing contracture of skeletal muscles, glycogenolysis, and increased cellular metabolism, resulting in production of heat and excess lactate. Affected individuals experience acidosis, hypercapnia, tachycardia, hyperthermia, muscle rigidity, compartment syndrome, rhabdomyolysis with subsequent increase in serum creatine kinase (CK) concentration, hyperkalemia with a risk for cardiac arrhythmia or even cardiac arrest, and myoglobinuria with a risk for renal failure. In nearly all cases, the first manifestations of MH (tachycardia and tachypnea) occur in the operating room; however, MH may also occur in the early postoperative period. There is mounting evidence that some individuals with MHS will also develop MH with exercise and/or on exposure to hot environments. Without proper and prompt treatment with dantrolene sodium, mortality is extremely high.

Diagnosis/testing: The diagnosis of MHS is established with in vitro muscle contracture testing by measuring the contracture responses of biopsied muscle samples to halothane and graded concentrations of caffeine. The diagnosis of MHS can also be established by identification of a pathogenic variant in CACNA1S, RYR1, or STAC3 on molecular genetic testing.

Management: Treatment of manifestations: Early diagnosis of an MH episode is essential. Successful treatment of an acute episode of MH includes: discontinuation of potent inhalation agents and succinylcholine; increase in minute ventilation to lower end-tidal CO₂; use of MHAUS helpline; administration of dantrolene sodium intravenously; cooling measures if body temperature is >38.5° C; treatment of cardiac arrhythmias if needed (do not use calcium channel blockers); monitoring blood gases, serum concentrations of electrolytes and CK, blood and urine for myoglobin, and coagulation profile; treatment of metabolic abnormalities.

Prevention of primary manifestations: Individuals with MHS should not be exposed to potent volatile agents and succinylcholine. Individuals undergoing general anesthetics that exceed 30 minutes in duration should have their temperature monitored using an electronic temperature probe. Individuals with MHS should carry proper identification as to their susceptibility.

Agents/circumstances to avoid: Avoid potent inhalation anesthetics and succinylcholine. Calcium channel blockers should not be given together with dantrolene because of a potential cardiac depressant effect. Serotonin antagonist (5HT3-antagonist) antiemetics should be used cautiously. Individuals with MHS should avoid extremes of heat, but not restrict athletic activity unless there is a history of overt rhabdomyolysis and/or heat stroke. Strenuous activities at high ambient temperatures should be avoided or performed with caution. In individuals with MH undergoing cardiac bypass surgery, aggressive rewarming should be avoided, as it may be associated with development of clinical signs of MH.

Evaluation of relatives at risk: If the MHS-causative pathogenic variant in the family is known, molecular genetic testing can be used to established increased risk of MH in a heterozygous individual; molecular genetic testing alone cannot be used to identify family members who are not at increased risk for MH because of other possible genetic risk factors. If the pathogenic variant in the family is not known or if an at-risk relative is found to be negative for a familial pathogenic variant, muscle contracture testing can be used to assess susceptibility to MH.

Pregnancy management: If a pregnant woman with MHS requires a non-emergent surgery, a non-triggering anesthetic (local, nerve block, epidural, spinal anesthesia, or a total intravenous general anesthetic) should be administered. Continuous epidural analgesia is highly recommended for labor and delivery. If a cesarean delivery is indicated in a woman who does not have an epidural catheter in place, neuraxial (spinal, epidural, or combined spinal-epidural) anesthesia is recommended (if not otherwise contraindicated). If a general anesthetic is indicated, a total intravenous anesthetic technique should be administered, with an anesthesia machine that has been prepared for an MH-susceptible individual.

Genetic counseling: Malignant hyperthermia susceptibility (MHS) is an autosomal dominant disorder. Most individuals diagnosed with MHS have a parent with MHS, although the parent may not have experienced an episode of MH. The proportion of individuals with MHS caused by a de novo pathogenic variant is unknown. Each child of an individual with MHS has a 50% chance of inheriting a causative pathogenic variant. Prenatal testing for a pregnancy at increased risk is possible if there is a known MH pathogenic variant in the family.