Pathophysiology of malignant hyperthermia

Malignant hyperthermia (MH) is a pharmacogenetic disease in man and animals. It primarily involves skeletal muscle tissue, but other tissues might be affected to a lesser degree. Calcium homeostasis in muscle cells is upset in susceptible individuals, so that various agents and circumstances can increase the free, ionised intracellular calcium concentration to damaging levels. The primary defect is not known at present, but is believed to involve an abnormally sensitive calcium-induced calcium release mechanism. Thus small, localised increases in calcium concentration releases more calcium so that a vicious cycle is triggered. The increased calcium concentration causes multiple effects in the muscles by stimulating contraction and a hypermetabolic state, clinically observed as rigidity and fever. If demands on the homeostatic mechanisms to lower the calcium concentration become exhausted, and metabolism is insufficient to supply enough phosphocreatine and ATP, membrane potentials cannot be maintained, and permeability of the cell membranes increase. This causes loss of phosphate and H+ as well as K+ and Mg++, and later myoglobin and creatine kinase. Thereby oxidative metabolism is further impeded with formation of lactate as a result. The ensuing acidosis stimulates sympathetic innervation, resulting in tachycardia, high blood pressure, and vasoconstriction. Hyperkalemia causes arrhythmia. Dantrolene inhibits the release of calcium and can halt the process if given before depletion of the energy rich phosphates is too advanced.