Background Succinylcholine-induced masseter muscle rigidity (MMR) is a potentially life-
threatening complication of anesthesia and is closely correlated with the heterogeneous
disorder malignant hyperthermia (MH) susceptibility. MMR also is identified with a variety of
neuromuscular disorders, including the myotonias, that are associated with abnormal in vitro
contracture test (IVCT) results. Recently, mutations in the adult skeletal muscle sodium
channel alpha-subunit gene (SCN4A) have been shown to cause generalized …