Becker Muscular Dystrophy

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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]}

After a thorough history has been taken and a physical examination has been performed, a diagnosis of BMD may be confirmed with the following lab study sequence:

Serum creatine kinase shows moderate to severe elevation (that is, 5-100 times the normal level)

Dystrophin gene deletion analysis shows specific exon deletions in about 98% of cases; test methods include the multiplex polymerase chain reaction assay, southern blot analysis, and fluorescent in situ hybridization

Muscle biopsy with dystrophin antibody staining demonstrates the presence of dystrophin in variable percentages; this may be helpful in the young child with no maternal history

Spinal radiographs may be performed to follow the progression of scoliosis, particularly during adolescence.

Because no cure exists for BMD, treatment is focused on controlling a patient’s symptoms. Weakness progresses, and emergencies related to cardiac and respiratory symptoms are hallmarks of advance in the disease process.

The role of physical therapy services is to address the functional needs of the patient as the disease progresses. Early interventions may focus on stretching tight muscles (which may initially be the only therapy goal). As the patient’s weakness progresses, appropriate equipment and assistive devices will be required to enable the individual to maintain functional mobility and independence in daily living activities. Educational objectives include teaching the patient techniques for energy conservation, joint protection, and the prevention of overuse fatigue.

Activities of daily living skills are addressed, depending on the level of impairments, in occupational therapy. Dysphagia concerns may be evaluated by a speech therapist. Specific planning for avocational needs and desires may be coordinated with a recreational therapist.

Progressive scoliosis and contracture formation may require surgical intervention. Spinal fusion to correct scoliosis may be scheduled based on the progression of spinal deformity and the age of the patient. Ankle contractures may be corrected with appropriate heel cord release and lengthening. Muscle transfers, such as with the posterior tibialis muscle, also may be considered to preserve functional mobility.

See also the following related Medscape Drugs & Diseases articles:

Dystrophinopathies

Muscular Dystrophy

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, 10]} This possibly explains why, in comparison with females, males suffer a rapid progression to death.

A study by Nicolas et al suggested that clinical variations in patients with BMD are related to differences in dystrophin mutations, as derived from different in-frame exon deletions. For example, delayed onset of dilated cardiomyopathy seemed to be related to specific exon deletions, as did earlier wheelchair dependency. ^[11]

See also the following related Medscape Drugs & Diseases articles:

Dilated Cardiomyopathy [Cardiology]

Dilated Cardiomyopathy [Emergency Medicine]

Pediatric Dilated Cardiomyopathy [Pediatrics: Cardiac Disease and Critical Care Medicine]

Imaging in Dilated Cardiomyopathy [Radiology]

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. ^[12] 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.

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. ^[13] 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. ^[14]

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. ^[15]

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.

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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Benjamin R Mandac, MD Chief of Physical Medicine and Rehabilitation, Medical Director of Pediatric Rehabilitation, Kaiser Permanente at Santa Clara

Benjamin R Mandac, MD is a member of the following medical societies: American Academy for Cerebral Palsy and Developmental Medicine, American Academy of Physical Medicine and Rehabilitation

Disclosure: Nothing to disclose.

Francisco Talavera, PharmD, PhD Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Received salary from Medscape for employment. for: Medscape.

Kat Kolaski, MD Assistant Professor, Departments of Orthopedic Surgery and Pediatrics, Wake Forest University School of Medicine

Kat Kolaski, MD is a member of the following medical societies: American Academy for Cerebral Palsy and Developmental Medicine, American Academy of Physical Medicine and Rehabilitation

Disclosure: Nothing to disclose.

Stephen Kishner, MD, MHA Professor of Clinical Medicine, Physical Medicine and Rehabilitation Residency Program Director, Louisiana State University School of Medicine in New Orleans

Stephen Kishner, MD, MHA is a member of the following medical societies: American Academy of Physical Medicine and Rehabilitation, American Association of Neuromuscular and Electrodiagnostic Medicine

Disclosure: Nothing to disclose.

Elizabeth A Moberg-Wolff, MD Medical Director, Pediatric Rehabilitation Medicine Associates

Elizabeth A Moberg-Wolff, MD is a member of the following medical societies: American Academy for Cerebral Palsy and Developmental Medicine, American Academy of Physical Medicine and Rehabilitation

Disclosure: Nothing to disclose.

Becker Muscular Dystrophy

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