Congenital muscle fiber-type disproportion

Congenital muscle fiber-type disproportion is a condition that can be defined only in the muscle biopsy by 2 obligatory criteria of “disproportion”: (1) a massive type I myofiber predominance of 80% or more, and (2) myofibers of type I are uniformly smaller than normal for age by 2 standard deviations or more, but are not necessarily angular or rounded as in myofiber atrophy. Internal sarcolemmal nuclei are an inconstant additional feature in a minority of cases, but myofiber necrosis, inflammation, and fibrosis are not typical features. This condition may be isolated as a nonprogressive congenital myopathy inherited as an autosomal dominant or recessive trait; may be associated with other congenital myopathies, such as nemaline rod myopathy, minicore myopathy, and infantile myotonic dystrophy; and may present with a variety of genetic metabolic diseases, including Krabbe leukodystrophy in early stages and insulin-resistant diabetes mellitus. It also is associated with congenital malformations of the brain, particularly cerebellar hypoplasia. Clinically, patients often have dysmorphic facies with facial wasting similar to that of nemaline myopathy or myotonic dystrophy. Serum creatine kinase is normal and EMG is nondiagnostic. Congenital muscle fibertype disproportion, thus, is best regarded as a syndrome, and not a specific disease, except for isolated familial cases.