British Journal of Anaesthesia 1994; 72: 210-216 REVIEW ARTICLE Anaesthesia and myotonia

The myotonias are a group of uncommon disorders. All display a characteristic electromyographic pic-ture and some of the disorders have systemic manifestations. They are important to the anaes-thetist for several reasons. Anaesthetic and surgical interventions may induce myotonia which, when initiated, may complicate the course of anaesthesia and be difficult to abolish. Furthermore, patients often display extreme sensitivity to some drugs and abnormal reactions to others. During anaesthesia, the extramuscular manifestations that occur in some myotonic disorders may assume greater importance than the myotonia, but may be masked before operation by the patient's reduced exercise tolerance. Finally, patients with myotonic disorders may be susceptible to malignant hyperpyrexia and have an increased incidence of malignant hyperpyrexia-like reactions. Most myotonic patients survive into adult life with little impairment and it is common for them to conceal their symptoms. They may present for totally unrelated surgery and the diagnosis may therefore not be considered. MYOTONIA Myotonia is a clinical sign or symptom which occurs in several disorders (table I). The term describes a persistent contraction of a muscle observed after cessation of voluntary contraction or stimulation of the muscle. In "action myotonia", the patient is unable to relax the muscle for some seconds after use. This is demonstrated classically in handgrip myotonia, in which patients are unable to release their grip after shaking hands. "Percussion myo-tonia " describes the sustained contraction of a muscle body either after it is trapped directly or after stimulation of the tendon reflex. In some subjects, for example apparently unaffected siblings, the phenomenon may only be elicited and detected electromyographically. In the non-dystrophic myotonias, in which myo-tonia is the only manifestation of the condition, the patient's main complaint is that of stiffness. This tends to be most prominent when the muscle is exercised after a period of rest, and improves with repetitive muscle activity (the so-called "warm-up&quot