Chapter 31 Metabolic myopathies

Abstract

Metabolic myopathies encompass a large group of hereditary disorders resulting in impaired energy provision for muscle contraction. They can be classified into three major groups: disorders of glycogen metabolism, disorders of lipid metabolism, and disorders of the mitochondrial respiratory chain. The first two groups reflect the importance of carbohydrate and long-chain fatty acids as major fuels for muscle contraction, while third group reflects the crucial role of oxidative phosphorylation to form ATP. This chapter focuses on the disorders of glycogen and lipid metabolism. There is a good correlation between the circumstances triggering exercise intolerance and the different roles of glycogen and lipid metabolism in the provision of energy to contracting muscles. The glycogen metabolism is crucial for anerobic or intense aerobic exercises; patients with glycogenoses usually have symptoms after strenuous bouts of an exercise, and the muscles involved are those that have been exercised. In contrast, patients with disorders of lipid metabolism, such as carnitine palmitoyltransferase II (CPT II) deficiency, usually have no difficulty with short-term intense exercises, and their muscle symptoms follow prolonged moderate exercises, especially when associated with fasting.