Distinct pathophysiological characteristics in developing muscle from patients susceptible to malignant hyperthermia.

BACKGROUND

Most patients with malignant hyperthermia susceptibility diagnosed by the in vitro caffeine-halothane contracture test (CHCT) develop excessive force in response to halothane but not caffeine (halothane-hypersensitive). Hallmarks of halothane-hypersensitive patients include high incidence of musculoskeletal symptoms at rest and abnormal calcium events in muscle. By measuring sensitivity to halothane of myotubes and extending clinical observations and cell-level studies to a large group of patients, we reach new insights into the pathological mechanism of malignant hyperthermia susceptibility.

METHODS

Patients with malignant hyperthermia susceptibility were classified into subgroups HH and HS (positive to halothane only and positive to both caffeine and halothane). The effects on [Ca] of halothane concentrations between 0.5 and 3 % were measured in myotubes and compared with CHCT responses of muscle. A clinical index that summarises patient symptoms was determined for 67 patients, together with a calcium index summarising resting [Ca] and spontaneous and electrically evoked Ca events in their primary myotubes.

RESULTS

Halothane-hypersensitive myotubes showed a higher response to halothane 0.5% than the caffeine-halothane hypersensitive myotubes (P<0.001), but a lower response to higher concentrations, comparable with that used in the CHCT (P=0.055). The HH group had a higher calcium index (P<0.001), but their clinical index was not significantly elevated vs the HS. Principal component analysis identified electrically evoked Ca spikes and resting [Ca] as the strongest variables for separation of subgroups.

CONCLUSIONS

Enhanced sensitivity to depolarisation and to halothane appear to be the primary, mutually reinforcing and phenotype-defining defects of halothane-hypersensitive patients with malignant hyperthermia susceptibility.