[Diagnosis of a myopathic disease in adult]

Strategy for the diagnosis of a muscle weakness includes two steps: to rely the weakness to a muscular origin and to find the aetiology. A muscular deficit is purely motor, without sensory signs, involving mainly axial and proximal muscles. The essential informations for aetiological characterization are: 1st) a family history, indicating a genetic origin, 2nd the chronological profile, 3rd) the clinical pattern (deficit topography, modification of muscle volume, fatigability, contractures, myotonia, oculobulbar, respiratory or cardiac involvement), 4th) investigations (CK level, EMG, muscle imaging muscle, biopsy, genetic testing). Two autosomal dominant myopathies begin in adulthood: Steinert's myotonic dystrophy characterized by myotonia facial and distal weakness and atrophy, plurisystemic involvement and facio-scapulo-peroneal dystrophy with asymmetric facial and scapulo-humeral weakness. If the evolution is rapid and family history absent, a curable myopathy (inflammatory, toxic, iatrogenic, and endocrine) is to be looked for. Inclusion body myositis is the most frequent myopathy after the age of 50 years.