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Becker muscular dystrophy
Becker muscular dystrophy (BMD) is a neuromuscular disease characterized by progressive muscle wasting and weakness due to degeneration of skeletal, smooth and cardiac muscle.
ORPHA:98895Classification level: Disorder
BMD primarily affects males with an estimated incidence of 1/18,000 to 1/31,000 male births. Females are usually asymptomatic but a small percentage of female carriers manifest milder forms of the disease (symptomatic form of muscular dystrophy of Duchenne and Becker in female carriers; see this term).
Onset usually occurs in childhood, frequently by 11 years of age. BMD can present in several ways: in children the first feature may be a toe walking gait or exercise-related cramps with or without myoglobinuria. As the condition progresses, muscle weakness leads to functional difficulties (difficulty climbing stairs or rising from a chair). A reaction similar to malignant hyperthermia (see this term) following general anesthesia can be the initial presentation in a previously undiagnosed patient. Rarely, cardiomyopathy may be the presenting feature. Clinical examination reveals muscle pseudohypertrophy affecting the calf muscles and there may be atrophy of more proximal muscles such as the quadriceps. There is symmetrical and proximal muscle weakness, with the lower limbs being more severely affected than the upper limbs. There may be joint contractures, especially of the tendo-Achilles. The facial, ophthalmic and bulbar muscles are not involved. The condition is slowly progressive and about 40% of affected patients will eventually become wheelchair-dependent. In wheelchair dependent patients, restrictive respiratory insufficiency occurs due to weakness of the intercostals and diaphragm. Cardiac involvement leads to a dilated cardiomyopathy, which can be disproportionate to the extent of skeletal muscle involvement.
BMD is caused by dystrophin deficiency due to in-frame deletions, mutations or duplications in the DMD gene (Xp21.2).
Diagnosis is suspected on the basis of the clinical picture, family history and laboratory findings (raised serum creatine kinase to 10-100 times the normal level). The diagnosis is confirmed by muscle biopsy (which shows dystrophic features and reduced dystrophin staining) and/or DNA testing for DMD anomalies.
Differential diagnosis includes the limb girdle muscular dystrophies, Duchenne muscular dystrophy, malignant hyperthermia and metabolic muscle diseases (see these terms).
If the diagnosis has been confirmed by genetic testing of carriers, then antenatal diagnosis is possible.
BMD is an X-linked recessive disease. Genetic counseling is recommended, all of the daughters of an affected male will be carriers. Carriers will have a 50% risk of male fetuses being affected.
Management and treatment
Management includes multidisciplinary care with physiotherapy to reduce joint contractures and prolong walking. Night time ankle-foot orthoses are prescribed for children to reduce tendo-Achilles contractures. Cardiac surveillance and monitoring of respiratory function are important. Early treatment of cardiomyopathy with ACE inhibitors is recommended and referral for cardiac transplantation is appropriate for severe cases. Patients with respiratory insufficiency should have pneumococcal and flu vaccines. The respiratory insufficiency responds well to nocturnal BIPAP, which corrects nocturnal hypoventilation and may prolong survival.
BMD is slowly progressive with wide phenotypic variability. Despite childhood onset, independent walking is never lost before 16 years of age. Life-expectancy for patients can be normal or significantly shortened by dilated cardiomyopathy or respiratory failure.