Diseases of Muscle

Diseases of muscle can be inherited or acquired. Inherited disorders include muscular dystrophies, congenital, metabolic, and mitochondrial myopathies, and channelopathies. Acquired disorders include inflammatory and toxic myopathies and myopathies associated with endocrinopathies or other systemic conditions. Symmetric limb-girdle proximal weakness is the most common and least specific presentation of myopathies, yet several myopathies show more characteristic patterns of weakness with more limited differential diagnoses. The clinical diagnostic approach to myopathies focuses on history, especially the timing and acuity vs chronicity of symptom evolution, family history, medications, and toxic exposures, and also on recognition of characteristic patterns of weakness. Measurement of muscle enzyme levels, EMG, imaging, genetic testing, and, in selected cases, muscle biopsy will lead to the correct diagnosis in most cases. Accurate diagnosis of the cause of a myopathy is of paramount importance, as many acquired myopathies are treatable. In addition, the tremendous progress in genetics, molecular biology, and disease modeling over the last decades has led to the recent FDA approval of specific treatments for Pompe disease (enzyme replacement therapy) and Duchenne muscular dystrophy (antisense oligonucleotides and AAV-based gene therapy) and has raised hope that similar treatment options will become available for many other dystrophies and inherited myopathies in the near future.