Increased masticatory muscle stiffness during limb muscle flaccidity associated with succinylcholine administration

The resistance of mouth opening to a constant force of 1.7 N was measured in 44 pediatric subjects anesthetized with enflurane and paralyzed with succinylcholine or vecuronium. Measurements were made during a deep level of anesthesia before relaxant administration, immediately after the loss of the adductor pollicis muscle twitch and 45 s later. In 22 patients receiving succinylcholine, there was a significant reduction in mean mouth opening (from 16.9 +/- 2.8 to 12.6 +/- 4.3 to 13.0 +/- 4.3 mm; P less than 0.0005) and an increase in jaw stiffness (from 102.3 +/- 21.9 to 154.5 +/- 77.4 to 150.5 +/- 77.0 Nm/degree; P less than 0.02) immediately after disappearance of the evoked thenar muscle twitch, as well as 45 s later. In six patients receiving succinylcholine, measurements were continued at 1 min intervals; mouth opening reduction and jaw stiffness increase lasted up to 10 min and extended beyond the return of visible twitch. One patient had a reduction of mouth opening from 20 to less than 1 mm; his corresponding jaw stiffness changed from 83.4 to 3335.4 Nm/degree. This patient, considered by us to have masseter spasm, required several attempts at tracheal intubation due to an increased resistance to mouth opening, as did one other patient. Patients receiving vecuronium showed a significant (P less than 0.02) increase of mouth opening 45 s following loss of twitch (from 19.8 +/- 3.6 to 20.9 +/- 4.1 mm; jaw stiffness changed from 87.0 +/- 15.3 to 83.0 +/- 17.2 Nm/degree). Anesthesia and surgery proceeded normally; in most patients, in excess of 1 h.(ABSTRACT TRUNCATED AT 250 WORDS)