ABSTRACT
An invariable consequence of muscle disease is impaired motor function. In the majority of cases, this impairment is due to weakness caused by loss of muscle mass. In a number of cases, however, symptoms are caused by premature exertional fatigue related to disturbed muscle metabolism or neuromuscular transmission. In these conditions, which include mitochondrial and metabolic myopathies, myasthenia gravis and LambertEaton’s syndrome, symptoms are dynamic and exercise-related, whereas muscle mass is not necessarily affected. Likewise, patients with muscle diseases caused by ion-transport deficiencies across cellular membranes (the so-called channelopathies) also typically have normal muscle mass, but disturbed motor function related to either periodic weakness/paralysis or prolonged duration of muscle contraction (myotonia).
