Becker Muscular Dystrophy (BMD)
Incidence
It follows an X-linked pattern of inheritance. The combined incidence of DMD and BMD is approximately 33/100,000, or about 1 in 3000 male births.
Clinical Characteristics
BMD is a milder form of DMD with later onset and slower progression.The muscular dystrophies form a group of genetically determined, progressive, primary disorders of muscle. The various forms can be distinguished by a combination of clinical, genetic, and pathologic criteria. BMD is a form of muscular dystrophy that follows an X-linked pattern of inheritance. The gene responsible for Duchenne and Becker muscular dystrophies (the DMD gene) maps to the short arm of the X chromosome at band Xp21. Positional cloning has resulted in the identification of the gene and its protein product, dystrophin. Dystrophin, the primary product of the DMD gene, is a high-molecular-weight (427 kDa) cytoskeletal protein that belongs to the spectrin family of proteins. In muscle it is localized at the inner surface of the sarcolemmal membrane. The biological function of dystrophin remains unknown. Dystrophin is not confined to muscle. In the milder disease BMD, dystrophin may be reduced in amount or altered in size. Becker and Duchenne muscular dystrophies were distinguished originally on clinical grounds. The main difference is in age of onset and rate of progression, although close questioning may reveal weakness before the age of 5 in BMD patients. The most widely accepted difference is that BMD patients remain ambulatory beyond the age of 16. Muscle pain related to exercise is a prominent feature of BMD. Mental retardation is not part of the clinical picture. Overall, the physical picture resembles that of DMD, with a combination of axial weakness, a tendency for toe walking, and muscular hypertrophy. Laboratory studies are also similar, including elevation of CK and myopathic changes on EMG. The muscle biopsy features are somewhat different, although variability in fiber size, muscle fibrosis, and necrosis as well as basophilia are still noted. Internal nuclei are much more numerous, and the distinction between type I and type II fibers, often obscured in the DMD patient, is quite clear in BMD. Fewer BMD patients require orthopedic surgery either for release of joint contractures or for scoliosis, perhaps because muscle weakness is less severe and occurs at a later time, when skeletal maturity has been reached. The natural history of BMD has not been well defined, but survival may be prolonged into the fourth decade and beyond.
Precipitants
no
Provocation Tests
no
Diagnostic Procedures
Diagnostic evaluation of DMD and BMD patients now routinely includes analysis of dystrophin in muscle biopsies-western blot analysis to estimate quantity and molecular weight; immunolabeling of sections to determine the distribution at the muscle membrane. Diagnostic evaluation also includes Southern blot or PCR analysis of DNA to determine the nature of the mutation, since this information is of prognostic value for the patient and is a necessary prerequisite for carrier identification and prenatal diagnosis in the family.