Congenital myopathies are primary muscle disorders that are present from birth, although their expression may be delayed until later in infancy or childhood. These conditions are rare. The most common are nemaline myopathy, central core disease, centronuclear/myotubular myopathies, and congenital fiber type disproportion (table 1).
The specific disorders are characterized on the basis of their histologic and histochemical features. These conditions are thought to be caused by genetic abnormalities of muscle development [1,2]. They are distinct from the metabolic myopathies, in which deficiencies of energy production in muscle result from defects in lipid metabolism, glycogenosis, or other metabolic pathways. (See "Causes of metabolic myopathies" and "Overview of inherited disorders of glucose and glycogen metabolism".)
Congenital myopathies share common features. All present with hypotonia and weakness that is greater proximally than distally. Tendon reflexes are decreased in proportion to the weakness. No specific treatment is available for these disorders. Management consists of physical therapy, nutritional support, assisted ventilation if indicated, and genetic counseling.
Nemaline myopathy derives its name from the characteristic rod bodies in muscle that appear threadlike in longitudinal section.
Clinical features — The clinical expression of nemaline myopathy is variable. The presentation in affected newborns can be severe or relatively mild. In the former, profound generalized weakness and hypotonia involving the face, bulbar, and respiratory muscles is seen; the eye muscles are spared . The milder form, with relatively less facial weakness and diaphragm impairment, also can present in children or adults. Intermediate forms also are identified. In a series of 143 cases, presentation was typical congenital, severe congenital, intermediate congenital, childhood onset, and adult onset in 46, 16, 20, 13, and 4 percent, respectively .