38 - Myopathy

Abstract

Myopathies present as pure motor syndromes with no disturbance of sensory or autonomic function. Muscle disorders can be categorized as (1) muscular dystrophies, (2) inflammatory myopathies, (3) necrotizing autoimmune myopathies (4) endocrine associated myopathies, (4) drug-induced and toxic myopathies, (6) metabolic myopathies, (7) congenital myopathies, and (8) myopathy associated with periodic paralysis. In the evaluation of patients with suspected myopathy, molecular genetics has supplanted electrodiagnostic (EDX) studies or muscle biopsy in many patients with suspected inherited conditions. In patients with suspected myopathy and no evidence of an inherited condition, a muscle biopsy is ultimately required for definitive diagnosis, regardless of the findings on EDX studies. Despite these facts, EDX studies, especially needle electromyography (EMG), continue to play an important role in the evaluation of suspected myopathy. EMG can often confirm the presence of a myopathy and add diagnostic information if certain types of spontaneous activity are present. EMG may also be helpful in suggesting alternate diagnoses that can mimic myopathy clinically. The EMG examination also has the advantage of sampling multiple muscles and sites easily and can often suggest a suitable muscle to biopsy. EMG examination may be especially difficult to interpret in mild cases of myopathy. Some myopathies, such as steroid myopathy, may have minimal or no changes on EMG. Some disorders of neuromuscular junction (NMJ) may present with similar clinical and EDX findings. Close attention to clinical detail, and often further EDX studies, including repetitive nerve stimulation and single-fiber EMG, may be required to differentiate a myopathy from an NMJ disorder. Neuromuscular ultrasound can add key information in selected cases of myopathy.