To the Editor:—

The distinguished laboratory at Lille, France, has reported undue sensitivity to halothane 1and caffeine 2in fragments of human masseter muscle that have been chemically skinned and exposed to these agents at temperatures less than 37°C. Reyford et al.  1conclude that this may help to explain causes of masseter spasm in humans who receive halothane and succinylcholine; however, Melton et al.  3,4have contradictory evidence regarding masseter responses. Biopsies of human masseter muscle were taken during complex facial and skull-base surgery, and the dissected bundles were exposed to halothane or caffeine at 37°C using the North American malignant hyperthermia testing protocol. 3,4These bundles were not sensitive to either agent; only two bundles (40- and 50-mg tension) had a contracture after 3% halothane. Furthermore, the mean caffeine concentration producing a 0.2-g increase was 5.5 mM; one caffeine bundle increased tension at 2 mM, but only to 35 mg. Although calcium release in skinned masseter muscle is different from that of skinned vastus when exposed to these drugs, this difference may not directly apply to responses in vivo . We suggest that chemical skinning, perhaps related to use of fragments, may be responsible for this apparent discrepancy. Although the bundle weight in our study was less than that in the usual biopsy, twitch amplitude was excellent. Therefore, our in vitro  results 3,4may more accurately reflect in vivo  responsiveness;i.e. , the individual muscle fiber basis for masseter spasm is not yet explained.

Reyford H, Adnet PJ, Tavernier B, Beague S, Ferri J, Krivosic-Horber RM, Haudecoeur G: Halothane induces calcium release from human skinned masseter muscle fibers. A NESTHESIOLOGY 1999; 90:1019–25
Adnet PJ, Reyford H, Tavernier BM, Etchrivi T, Krivosic I, Krivosic-Horber R, Haudecoeur G: In vitro human masseter muscle hypersensitivity: A possible explanation for increase in masseter tone. J Appl Physiol 1996; 80:1547–53
Melton AT, Antognini JF, Gronert GA: In vitro contracture tests on normal human masseter muscle. Anesth Analg 1997; 84:S368
Melton AT, Antognini JF, Gronert GA: Caffeine- or halothane-induced contractures of masseter muscle are similar to those of vastus muscle in normal humans. Acta Anaesth Scand 1999; 43:764–69