Viability criterion of muscle bundles used in the in vitro contracture test in patients with neuromuscular diseases.

We have compared the viability criteria of muscle bundles used in the in vitro contracture test for susceptibility to malignant hyperthermia (MH) in a group of 28 patients with various neuromuscular diseases (NMD) and 93 MH-related family patients. In the patients with NMD, this standard test gave one positive, six equivocal and 21 negative results. Compared with MH-related family patients, muscle bundles used had significantly smaller resting membrane potentials and smaller predrug twitch tension amplitudes. Some results from the group with NMD were obtained with muscles which were damaged, more rapidly deteriorating, non-standard or both, and should not be taken to indicate that the patients have the genetic trait for MH. The in vitro contracture test is not always relevant for myopathic muscle (especially dystrophic muscle) and this could explain the lack of specificity for MH.