Masseter muscle rigidity and malignant hyperthermia susceptibility.

Seventy-seven patients who developed masseter muscle rigidity (MMR) after receiving succinylcholine to facilitate tracheal intubation were evaluated for malignant hyperthermia (MH) susceptibility by in vitro halothane and caffeine contracture tests. Thirty-nine patients were diagnosed as MH-susceptible. Neither age, sex, nor type of surgery or anesthesia distinguished MH-susceptible from nonsusceptible patients. Two susceptible and two nonsusceptible patients had evidence of a myopathy. Fifty-two patients had serum creatine phosphokinase (CPK) levels measured in the perioperative period. Although all values were above normal, CPK values equal to or greater than 20,000 IU within 24 hr of trismus (in the absence of myopathy) were observed in six of 30 patients diagnosed as MH-susceptible, but were found in none of the nonsusceptible patients. Considering the high percentage of patients exhibiting MMR that are indeed susceptible to MH (approximately 50%) compared to estimates of MH in the population as a whole (approximately 0.005%), MMR should be considered a presumptive sign of MH. Perioperative CPK values greater than 20,000 IU are highly suggestive of MH susceptibility. Patients exhibiting MMR should be evaluated for MH susceptibility and myopathies. Succinylcholine should be avoided for subsequent anesthetics in patients with a history of MMR.