The effects of succinylcholine on mouth opening.

Mouth opening and the resistance to opening developed by the muscles of mastication were measured in 63 children anesthetized with halothane and relaxed with succinylcholine, pancuronium, or vecuronium. Measurement of mouth opening, induced by a constant test force, was made when each patient was deeply anesthetized, as judged by clinical parameters. Succinylcholine, vecuronium, or pancuronium was then administered. The mouth opening measurement was repeated immediately after the loss of limb muscle twitch response and 45 s following the loss of twitch response. For the 24 patients receiving succinylcholine, there was a significant reduction in mean mouth opening (P less than 0.0001) and a significant increase in jaw stiffness (P less than 0.0001) immediately after limb relaxation. Forty-five seconds after full limb relaxation was attained, the mean mouth opening was still reduced (P less than 0.0001) and the mean jaw stiffness was still increased (P less than 0.0003) in the succinylcholine group. Patients receiving either vecuronium or pancuronium did not show a significant change of mouth opening or jaw stiffness following limb relaxation. Three patients, who received succinylcholine, required several attempts at tracheal intubation due to increased resistance to mouth opening. Anesthesia and surgery proceeded in all patients. None of the patients developed malignant hyperthermia. In view of the fact that a reduction in mouth opening was a constant finding when succinylcholine was administered during halothane anesthesia, the assumption that isolated "masseter spasm" or jaw stiffness heralds malignant hyperthermia should be reconsidered.