Disorders of muscle lipid metabolism: diagnostic and therapeutic challenges

Disorders of muscle lipid metabolism may involve intramyocellular triglyceride degradation, carnitine uptake, long-chain fatty acids mitochondrial transport, or fatty acid β-oxidation. Three main diseases leading to permanent muscle weakness are associated with severe increased muscle lipid content (lipid storage myopathies): primary carnitine deficiency, neutral lipid storage disease and multiple acyl-CoA dehydrogenase deficiency. A moderate lipidosis may be observed in fatty acid oxidation disorders revealed by rhabdomyolysis episodes such as carnitine palmitoyl transferase II, very-long-chain acyl-CoA dehydrogenase, mitochondrial trifunctional protein deficiencies, and in recently described phosphatidic acid phosphatase deficiency. Respiratory chain disorders and congenital myasthenic syndromes may also be misdiagnosed as fatty acid oxidation disorders due to the presence of secondary muscle lipidosis. The main biochemical tests giving clues for the diagnosis of these various disorders are measurements of blood carnitine and acylcarnitines, urinary organic acid profile, and search for intracytoplasmic lipid on peripheral blood smear (Jordan's anomaly). Genetic analysis orientated by the results of biochemical investigation allows establishing a firm diagnosis. Primary carnitine deficiency and multiple acyl-CoA dehydrogenase deficiency may be treated after supplementation with carnitine, riboflavine and coenzyme Q10. New therapeutic approaches for fatty acid oxidation disorders are currently developed, based on pharmacological treatment with bezafibrate, and specific diets enriched in medium-chain triglycerides or triheptanoin.