Becker Muscular Dystrophy
- Author: Benjamin R Mandac, MD; Chief Editor: Denise I Campagnolo, MD, MS more...
Background
Becker and Kiener initially described Becker muscular dystrophy (BMD) in 1955.[1, 2] BMD is an inherited disease with a male distribution pattern and a clinical picture similar to that of Duchenne muscular dystrophy (DMD). BMD is generally milder than DMD, and the onset of symptoms usually occurs later.
The clinical distinction between the 2 conditions is relatively easy because (1) less severe muscle weakness is observed in patients with BMD and (2) affected maternal uncles with BMD continue to be ambulatory after age 15-20 years.
Accuracy of diagnosis has been refined with the recognition of the dystrophin gene defects and with dystrophin staining of muscle biopsy specimens.[3, 4, 5]
See also the following related eMedicine articles:
Pathophysiology
Advancements in the diagnosis of genetic conditions have revealed that BMD is a type of recessive, X-linked dystrophinopathy. Exon deletions exist in the dystrophin gene Xp21 (X-chromosome, short arm p, region 2, band 1). Affected males in approximately 30% of known cases of BMD phenotype do not have a demonstrable mutation/deletion. A reading frame or in-frame mutation hypothesis has been proposed to explain abnormal translation of the dystrophin gene. Abnormal but functional dystrophin may be produced, in contrast to the pathology in DMD, in which a frame-shift mutation essentially leads to failure to produce dystrophin.[6, 7, 8, 9] Dystrophin levels in BMD are generally 30-80% of normal, while in DMD, the levels are less than 5%.[3]
Dilated cardiomyopathy with congestive heart failure presents in males between age 20 and 40 years, but in carrier female carriers it is found later in life.[3] This possibly explains why, in comparison with females, males suffer a rapid progression to death.
See also the following related eMedicine articles:
Cardiomyopathy, Dilated [Cardiology]
Cardiomyopathy, Dilated [Emergency Medicine]
Cardiomyopathy, Dilated [Pediatrics: Cardiac Disease and Critical Care Medicine]
Cardiomyopathy, Dilated [Radiology]
Epidemiology
Frequency
United States
The incidence and prevalence of BMD are lower than those of DMD. The estimated incidence of BMD is 1 individual per 30,000 male births, compared with 1 individual per 3500 male births for DMD.[10] The prevalence of BMD is 17-27 cases per 1 million population.
International
The international incidence is probably similar to that in the United States.
Mortality/Morbidity
A series by Emery and Skinner showed the mean age for symptom onset to be 11 years, with the age range for onset being 2-21 years.[11] The mean age at which affected patients described in the studies became nonambulatory was 27 years, with an age range of 12-30 years. Death usually resulted from respiratory or cardiac failure at a mean age of 42 years, with the age range being 23-63 years.[12]
Ambulatory status and age may differentiate DMD from BMD. In general, an ambulatory patient who is older than 16 years may be classified as not having the Duchenne phenotype, although some subjects with BMD stop walking between ages 13-16 years. Atypical clinical presentations include cramps with exercise, focal myopathy, and isolated cardiomyopathy. Unaffected patients with no evidence of skeletal muscle disease have been classified as having subclinical BMD.[13]
Sex
BMD is an X-linked disorder. Given the transmission pattern, the disease affects primarily males. Translocations may allow the possibility of a female presentation of the BMD phenotype.
Age
The onset of symptoms occurs at a mean age of 11 years, with the age range for onset being 2-21 years.
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