eMedicine Specialties > Physical Medicine and Rehabilitation > Muscular Dystrophy
Becker Muscular Dystrophy
Updated: Apr 18, 2008
Introduction
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:
Dystrophinopathies
Muscular Dystrophy
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 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]
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.9 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.10 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.11Ambulatory 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.12
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.
Clinical
History
A typical developmental history of a patient with BMD may include the following:
- Delayed gross motor milestones (eg, late walking, running, jumping, difficulty with stair climbing) may be reported.
- Initially, some children who are later diagnosed with BMD may be called clumsy.
- Increasing numbers of falls, toe walking, and difficulty rising from the floor may be later features.
- Proximal muscle weakness is reported.
- Subclinical cases may manifest later in life; dilated cardiomyopathy can be the first sign of BMD.
- Elbow contractures may be seen later in life.
Physical
- The Gower sign is not a specific finding for muscular dystrophy, but it does point to proximal weakness in the hip extensors, leading to the pattern of movement seen when patients rise from the floor.
- A weakness pattern limited to specific muscle groups may help to differentiate BMD from other muscular dystrophies (such as limb-girdle and Emery-Dreifuss muscular dystrophies).
- Progressive, symmetrical muscle weakness and atrophy with pseudohypertrophic calves may be seen.
- Cases have been described of patients presenting without weakness but with symptoms of cardiomyopathy or cramps as the only indication of a myopathic process. Isolated weakness to the quadriceps femoris may be the only symptom noted.
- Fasciculation or sensory modality abnormalities can exclude the diagnosis of a dystrophinopathy.
- Preservation of neck flexor muscle strength may differentiate BMD from DMD.
Causes
BMD is an X-linked, recessive, inherited disorder. A family history of similarly affected maternal uncles assists the clinician in confirming a diagnosis of BMD.
- A woman is an obligate heterozygote if she has an affected son and one other affected relative in the maternal line.
- A woman with more than 1 affected child and no family history in the maternal line may have a germline mutation or a germline mosaicism.
- An isolated proband without a family history may be explained by a mutation occurring in the egg at or following conception in which only some cells were affected (mosaicism). On the other hand, the proband's mother may have inherited the gene mutation if (1) her mother was a carrier or (2) her mother or father had somatic or germline mosaicism.
- Siblings of the proband are at risk of transmitting the gene defect based on the carrier status of the mother.
- A carrier mother has a 50% transmission rate for the mutation, per pregnancy; daughters inheriting the mutation will be carriers, and sons with the mutation will be affected.
- Mothers with germline and/or somatic mosaicism have a higher risk of transmitting the mutation.
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| Treatment & Medication: Becker Muscular Dystrophy |
| Follow-up: Becker Muscular Dystrophy |
| References |
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References
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Angelini C, Fanin M, Pegoraro E, et al. Clinical-molecular correlation in 104 mild X-linked muscular dystrophy patients: characterization of sub-clinical phenotypes. Neuromuscul Disord. Jul 1994;4(4):349-58. [Medline].
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Ashton EJ, Yau SC, Deans ZC, et al. Simultaneous mutation scanning for gross deletions, duplications and point mutations in the DMD gene. Eur J Hum Genet. Jan 2008;16(1):53-61. [Medline].
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Emery AE, Skinner R. Clinical studies in benign (Becker type) X-linked muscular dystrophy. Clin Genet. Oct 1976;10(4):189-201. [Medline].
Holloway SM, Wilcox DE, Wilcox A, et al. Life expectancy and death from cardiomyopathy amongst carriers of Duchenne and Becker muscular dystrophy in Scotland. Heart. Oct 11 2007;[Medline].
Young HK, Barton BA, Waisbren S, et al. Cognitive and Psychological Profile of Males With Becker Muscular Dystrophy. J Child Neurol. Dec 3 2007;[Medline].
Grootenhuis MA, de Boone J, van der Kooi AJ. Living with muscular dystrophy: health related quality of life consequences for children and adults. Health Qual Life Outcomes. 2007;5:31. [Medline]. [Full Text].
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Further Reading
Keywords
Becker muscular dystrophy, BMD, muscular dystrophy, X-linked dystrophinopathy, childhood muscular dystrophy, Duchenne muscular dystrophy, DMD, DMD-related dilated cardiomyopathy
