Metabolic myopathies: a practical approach

Metabolic myopathies are a diverse group of rare genetic disorders and their associated muscle symptoms may be subtle. Patients may present with indolent myopathic features, exercise intolerance or recurrent rhabdomyolysis. Diagnostic delays are common and clinicians need a high index of suspicion to recognise and differentiate metabolic myopathies from other conditions that present in a similar fashion. Standard laboratory tests may be normal or non-specific, particularly between symptomatic episodes. Targeted enzyme activity measurement and next-generation genetic sequencing are increasingly used. There are now specific enzyme replacement therapies available, and other metabolic strategies and gene therapies are undergoing clinical trials. Here, we discuss our approach to the adult patient with suspected metabolic myopathy. We outline key features in the history and examination and discuss some mimics of metabolic myopathies. We highlight some disorders of glycogen and fatty acid utilisation that present in adulthood and outline current recommendations on management.