The muscular dystrophies are a group of genetically determined, almost exclusively pediatric diseases. Generally, the earlier the age at which symptoms begin, the poorer is the prognosis. Because of a considerable overlap of manifestations and rates of progression and, until recently, the lack of any biochemical test, their classification is still unsettled. As a group, the principal differential diagnosis of the muscular dystrophies is from the muscular atrophies. In the former, the primary defect is in the voluntary muscle fibers; in the latter, it is in the innervation of muscles.

Classification

The most common of the dystrophies and the first to be described was that delineated by Guillaume B. A. Duchenne, a French neurologist, in 1868. Duchenne muscular dystrophy (DMD) is a sex-linked recessive disorder. Consequently, it clinically affects only males and is inherited through female carriers of the gene. Although affected boys have abnormally elevated concentrations of muscle cell enzymes such as creatine phosphokinase in their blood, this abnormality is also found in about three-fourths of the asymptomatic female carriers. DMD appears to have a rather uniform incidence worldwide, with a mean incidence estimated to be about 1 case per 4,000 live male births, or 15 to 33 cases per 100,000. Most surveys have been of predominantly Caucasian populations, but the results of a study in Japan were consistent with the others. A family history of DMD can be obtained in only about one-third of cases. The others are attributed to either a previously unexpressed carrier state or to a new mutation.