Masseter (jaw) muscle rigidity (MMR) denotes trismus to the extent that it is difficult or impossible to open the jaw. Mild and/or transient MMR is a normal response to succinyicholine and is considered to be of no prognostic significance with respect to MH. [Longnecker et al. Anesthesiology. Pg 1969. 2008. McGraw Hill, New York] Approximately 1% of children receiving anesthesia induced by halothane or sevoflurane and then given succinylcholine develop MMR. [Rosenberg, 2007] If a patient has received succinyicholine and his/her jaw cannot be opened or the patient has peripheral muscle rigidity, the clinician should assume this is an MH event and immediately begin MH treatment. Generalized rigidity may not be present, but when it is, it is regularly associated with MH susceptibility.
The actual physiologic changes associated with the onset of MH such as rise in ETCO2 may be delayed for up to 15 minutes after MMR, but will occur if trigger agents are continued. Hence whenever MMR occurs following succinyicholine, elective surgery should be postponed. If the procedure is emergent, the anesthetic may continue with nontrigger agents.
All patients who develop succinylcholine-induced MMR will experience rhabdomyolysis over the ensuing 24 hours. Hence the patient should remain in the hospital and be monitored for signs of rhabdomyolysis such as myoglobinuria and myoglobinemia. CK levels and electrolytes should be checked every 8 hours until returning to normal.