The term congenital myopathy is used to describe a group of heterogenous disorders usually presenting with infantile hypotonia due to genetic defects, caus­ing primary myopathies with the absence of any struc­tural abnormality of the central nervous system or peripheral nerves.

A specific diagnosis of each entity is made on the basis of specific histologic and elec­tron microscopic changes found on muscle biopsy. While patients may be hypotonic during early infancy, they later develop muscle weakness that is generally nonprogressive and static.

The serum creatine kinase values are frequently normal, and the EMG may be normal or may show mild, nonspecific changes, usually of a myopathic character (small-amplitude polyphasic potentials). The only congenital myopathy consistently associated with spontaneous activity is myotubular (centronuclear) myopathy. In this disorder, the EMG reveals myopathic motor unit action potentials with frequent complex repetitive discharges and diffuse fibrillation potentials. These myopathies may be considered primarily struc­tural in nature and thus, patients do not actively lose muscle fibers, as is the case in dystrophic myopathies.