The serum CK values are 25 to 200 times normal, which, with the EMG and muscle biopsy findings, help exclude spinal muscular atrophy. The EMG shows fibrillations, positive waves, low-amplitude and brief polyphasic motor unit potentials, and, sometimes, high-frequency discharges. The female carrier may occasionally display the same abnormalities, but to a much milder degree. The molecular and genetic bases of the disease are discussed further on.
This milder dystrophy is closely related to the Duchenne type clinically, genetically, and ultrastructurally, involving the same DMD gene as in the Duchenne type. It had long been noted that mixed with the Duchenne group were relatively benign cases. In 1955, Becker and Keiner proposed that the latter be separated as a distinct entity, now called Becker muscular dystrophy. The incidence is difficult to ascertain, but it has been estimated as 3 to 6 per 100,000 male births. Like the Duchenne form, it is an X-linked disorder, practically limited to males and transmitted by females. It causes weakness and hypertrophy in the same muscles as Duchenne dystrophy, but the onset is much later (mean age: 12 years; range: 5 to 45 years). While boys with Duchenne dystrophy are usually dependent on a wheel-chair by early in the second decade, it is not uncommon for those with Becker dystrophy to walk well into adult life. In comparison to Duchenne dystrophy, those with Becker and intermediate types retain their ability to raise the head fully off the bed. We have, for example, encountered patients who served in the military with the disease undetected. If maternal uncles are affected by the disease and are still walking, the diagnosis is relatively easy. Mentation is usually normal and cardiac involvement is far less frequent than in Duchenne dystrophy, but there are cases that present with a cardiomyopathy and we have been made aware of 2 brothers who had cardiac transplantation before the disease was detected. Kuhn and associates have reported a genealogy in which early myocardial disease and cramping myalgia were prominent features. The molecular-genetic basis of this form is discussed below.
Pathology of Duchenne and Becker Dystrophies
In the early stages of Duchenne dystrophy, the most distinctive features are prominent segmental degeneration and phagocytosis of single muscle fibers or groups of fibers and evidence of regenerative activity (basophilia of sarcoplasm, hyperplasia and nucleation of sarcolemmal nuclei, and the presence of myotubes and myocytes). The necrosis excites a regenerative or restorative process, which explains the forking of fibers and clustering of small fibers with prominent nuclei. The necrotic sarcoplasm and sarcolemma are removed by mononuclear phagocytic (macrophage) cells. There may also be a few T lymphocytes in the region, suggesting inflammation. There is a hyalinization of the sarcoplasm of many degenerating and nondegenerating fibers. In longitudinal sections these are seen as “contraction bands,” expressive of the irritability of dystrophic muscle. This phenomenon may be present before there is any significant degree of degeneration and is more extensive in Duchenne than in any of the other dystrophies. Eventually, there are histologic changes that are common to all types of advanced muscular dystrophies: loss of muscle fibers, residual fibers of larger and smaller size than normal, all in haphazard arrangement, and the secondary reaction of an increase in lipocytes and fibrosis.