The aim of this study was to find, using modern techniques, any histological differences in muscle biopsies between malignant hyperthermia (MH) susceptible (MHS), MH equivocal (MHE) and MH negative (MHN) patients. On the basis of the European MH contracture test carried out in 83 patients, 23 were shown to be MHS, nine MHE and 51 MHN. Four lesions were found with a significantly high frequency in MHS and MHE biopsies: muscle fibre hypertrophy and atrophy, internal nuclei and myofibrillar necrosis. These four lesions were observed together in 35% of MHS but in none of the MHE or MHN biopsies. Three of these lesions occurred together in 57% of MHS, 33% of MHE and 4% of MHN biopsies. Our results support a histological difference between MHE, MHS and MHN biopsies and attempt to contribute towards a better definition of MHE status.
