Masseter muscle rigidity or spasm may occur in children and other patients undergoing general anesthesia. Some patients have evidence of malignant hyperthermia.

Clinical features:

(1) administration of a volatile agent (halothane, sevoflurane) plus succinylcholine

(2) development of masseter muscle spasm, which in some patients may progress to severe rigidity


Features suggesting risk of malignant hyperthermia:

(1) generalized rigidity (rather than limited to the masseter muscles)

(2) severe masseter muscle rigidity

(3) marked increase in expired carbon dioxide (end tidal CO2)  and/or hyperthermia

(4) positive caffeine-halothane test on a skeletal muscle biopsy

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