**2.5 Clinical presentation**

The commonest clinical presentation of MH is a hypermetabolic state in a genetically susceptible individual in response to certain anesthetic agents, notably succinylcholine or halogenated volatile anesthetics. One of the earliest clinical signs is MMR after succinylcholine. *In vitro* muscle testing in patients who have developed this sign alone reveals that 28–50% are susceptible to MH. In the full-blown syndrome there is a rapid and sustained rise in body temperature, without shivering, either in the operating theatre or in the recovery room, in the absence of any obvious cause such as infection or a hot and humid environment. Tachycardia, cyanosis, generalized muscle rigidity, and cardiac arrhythmias are common clinical signs. There may be heating and rapid exhaustion of the soda-lime canisters. Acidosis is an early finding and there may also be hyperkalemia, hyperphosphatemia, and hypocalcemia from muscle-cell breakdown. Rhabdomyolysis is an important feature of the syndrome and is best demonstrated by measuring serum CK, which usually peaks on the second or third day after the reaction. Tenderness and swelling of muscles may develop, especially in the thighs. Myoglobinemia and myoglobinuria are common and renal failure may result from the rhabdomyolysis. Another complication is disseminated intravascular coagulation1.

In less obvious cases, MH may present with one or any combination of the above clinical signs. The first indication of MH may be an unexplained cardiac arrest or cardiac arrhythmia. A rise in end-tidal CO2 is often the earliest indication of MH, and now that this is widely measured in clinical anesthesia MH may be picked up before the more florid signs develop. Previously apparently uncomplicated anesthesia with halothane and/or succinylcholine does not exclude the diagnosis of MH on a subsequent occasion. Factors such as the concentration of the anesthetic drugs used, the duration of the anesthesia, and the degree of MH susceptibility of the patient may explain why one anesthetic procedure is uneventful while another in the same patient is not1.

When MH was first recognized as a complication of anesthesia the case-fatality rate was 70%. Today, with the use of a specific drug for MH and the introduction of an *in vitro* muscle-contracture test59 to identify susceptibility to MH in individuals and their relatives, the case-fatality rate is only 5%1.
