Background The metabolic myopathies are a heterogeneous group of muscle diseases. The three primary categories include the mitochondrial myopathies, glycogen storage diseases (GSD), and fatty acid oxidation (FAO) defects. Mitochondrial FAO defects can be difficult to identify because clinical and biological manifestations may be transient. Exercise intolerance, myalgia, proximal muscle weakness, and attacks of rhabdomyolysis have been reported in later life.

Aim To report the clinical, biochemical and molecular studies in adult patients with FAO defects and identify the degree of muscle involvement in this group of disorders.

Methods Patients with confirmed FAO defects under the care of the adult metabolic and neuromuscular teams at the National Hospital for Neurology and Neurosurgery were included in the study. The clinical, biochemical, and molecular details of each patient were collected and management recorded.

Results 15 patients had FAO defects. Four suffered attacks of rhabdomyolysis and three had myalgia or proximal muscle weakness.

Discussion Diagnosis of FAO defects may be achieved by measurement of fasting plasma acylcarnitine profile and confirmed with assay of cultured skin fibroblasts, avoiding the need for muscle biopsy. We must attempt to identify these patients so that they may benefit from dietary intervention to alleviate symptoms and improve morbidity.