Becker Muscular Dystrophy

Overview

Plain-Language Overview

Becker Muscular Dystrophy is a genetic muscle disorder that causes progressive weakness and wasting of the muscles. It usually begins in childhood or adolescence and primarily affects the muscles of the hips, pelvic area, thighs, and shoulders. People with this condition may have difficulty walking, running, and climbing stairs. The disease progresses more slowly than similar conditions, allowing many individuals to maintain mobility into adulthood. It is caused by mutations in the gene responsible for producing dystrophin, a protein important for muscle strength and stability.

Clinical Definition

Becker Muscular Dystrophy (BMD) is an X-linked recessive myopathy characterized by mutations in the DMD gene leading to reduced or abnormal production of the dystrophin protein. It presents with progressive, symmetric proximal muscle weakness typically manifesting in late childhood to adolescence. Unlike Duchenne Muscular Dystrophy, BMD has a later onset and slower progression. Histopathology shows muscle fiber degeneration with variable fibrosis and fatty infiltration. Serum creatine kinase levels are elevated but generally lower than in Duchenne. Cardiac involvement, including dilated cardiomyopathy, is common and may be a significant cause of morbidity. Diagnosis is confirmed by genetic testing identifying in-frame mutations in the DMD gene and by muscle biopsy demonstrating reduced dystrophin expression. Electromyography may show myopathic changes. Respiratory function is usually preserved until late stages. Management focuses on monitoring and treating cardiac and skeletal muscle complications.

Inciting Event

No specific inciting event; disease is caused by inherited genetic mutations.

Latency Period

none

Diagnostic Delay

Milder and later onset symptoms compared to Duchenne muscular dystrophy can lead to delayed diagnosis.
Lack of early muscle weakness recognition and variable clinical presentation contribute to delay.

Clinical Presentation

Signs & Symptoms

Progressive muscle weakness beginning in adolescence or early adulthood.
Difficulty with activities requiring proximal muscle strength, such as climbing stairs or rising from a chair.
Muscle cramps and fatigue.
Delayed motor milestones in some cases.
Preserved intellectual function.

History of Present Illness

Progressive muscle weakness beginning in proximal lower limbs, often noticed as difficulty running or climbing stairs.
Patients may report fatigue and muscle cramps.
Symptoms typically progress slowly over years with eventual involvement of upper limbs.

Past Medical History

History of delayed motor milestones may be present but less severe than Duchenne muscular dystrophy.
No significant history of cardiac or respiratory symptoms early in disease.

Family History

Positive family history of X-linked muscular dystrophy or related muscle disorders.
Affected male relatives with similar muscle weakness symptoms.
Carrier females may have mild symptoms or be asymptomatic.

Physical Exam Findings

Presence of proximal muscle weakness, especially in the pelvic and shoulder girdle muscles.
Pseudohypertrophy of the calf muscles due to fat and connective tissue replacement.
Decreased deep tendon reflexes in affected muscles.
Mild to moderate gait abnormalities, including waddling gait.

Diagnostic Workup

Diagnostic Criteria

Diagnosis of Becker Muscular Dystrophy requires identification of clinical features including progressive proximal muscle weakness with onset typically in adolescence, elevated serum creatine kinase levels, and confirmation by genetic testing showing in-frame mutations in the DMD gene. Muscle biopsy demonstrating reduced or abnormal dystrophin expression supports the diagnosis. Cardiac evaluation is essential due to frequent cardiomyopathy. Electromyography may reveal myopathic changes but is not diagnostic alone.

Pathophysiology

Key Mechanisms

Becker Muscular Dystrophy results from mutations in the dystrophin gene leading to partially functional dystrophin protein.
The defective dystrophin causes instability of the muscle cell membrane, making muscle fibers susceptible to damage.
Progressive muscle fiber degeneration and inadequate repair lead to gradual muscle weakness and wasting.

Involvement	Details
Organs	Skeletal muscles show weakness and wasting due to dystrophin deficiency.
	Heart is commonly involved with dilated cardiomyopathy and arrhythmias.
	Lungs may be compromised due to respiratory muscle weakness.
Tissues	Skeletal muscle tissue undergoes progressive fibrosis and fatty replacement.
	Cardiac muscle tissue is affected leading to dilated cardiomyopathy.
	Connective tissue increases as muscle fibers are replaced by fibrofatty tissue.
Cells	Skeletal muscle fibers are the primary cells affected, showing progressive degeneration in Becker muscular dystrophy.
	Cardiomyocytes are involved due to dystrophin deficiency leading to cardiomyopathy.
	Satellite cells participate in muscle regeneration but are insufficient to prevent disease progression.
Chemical Mediators	Creatine kinase is elevated in serum due to muscle membrane damage.
	Inflammatory cytokines such as TNF-alpha contribute to muscle degeneration.
	Calcium ions accumulate intracellularly causing muscle fiber necrosis.

Treatment

Pharmacological Treatments

Corticosteroids
- Mechanism: Reduce inflammation and slow muscle degeneration by modulating immune response
- Side effects: Weight gain, osteoporosis, hypertension, glucose intolerance
ACE inhibitors
- Mechanism: Decrease cardiac afterload and delay progression of cardiomyopathy
- Side effects: Cough, hyperkalemia, hypotension
Beta-blockers
- Mechanism: Reduce cardiac workload and arrhythmia risk in cardiomyopathy
- Side effects: Bradycardia, fatigue, hypotension

Non-pharmacological Treatments

Physical therapy helps maintain muscle strength and prevent contractures.
Occupational therapy assists with daily living activities and adaptive device use.
Cardiac monitoring is essential to detect and manage cardiomyopathy early.
Respiratory support may be required as respiratory muscles weaken.

Prevention

Pharmacological Prevention

Use of corticosteroids (e.g., prednisone) to slow muscle degeneration.
Medications for cardiac management, such as ACE inhibitors or beta-blockers, to prevent cardiomyopathy progression.

Non-pharmacological Prevention

Regular physical therapy to maintain muscle strength and prevent contractures.
Use of orthotic devices to support mobility and prevent deformities.
Respiratory support including non-invasive ventilation as needed.
Cardiac monitoring with regular echocardiograms and ECGs.

Outcome & Complications

Complications

Progressive respiratory failure due to respiratory muscle weakness.
Severe cardiomyopathy leading to heart failure.
Loss of ambulation and severe disability.
Orthopedic deformities such as contractures and scoliosis.

Short-term Sequelae	Long-term Sequelae
Increased fatigue and muscle weakness during physical activity. Transient muscle cramps and discomfort. Mild respiratory difficulties during infections.	Chronic respiratory insufficiency requiring ventilatory support. Progressive cardiac dysfunction with risk of heart failure. Permanent loss of ambulation and dependence on mobility aids. Severe musculoskeletal deformities including contractures and scoliosis.

Differential Diagnoses

Becker Muscular Dystrophy versus Duchenne Muscular Dystrophy

Becker Muscular Dystrophy	Duchenne Muscular Dystrophy
Onset usually in adolescence or early adulthood with slower progression.	Onset typically before 5 years of age with rapid progression.
Serum creatine kinase (CK) levels are elevated but generally lower than in Duchenne, often 5-20 times normal.	Serum creatine kinase (CK) levels are markedly elevated, often 10-100 times normal.
Genetic testing reveals in-frame mutations in the dystrophin gene resulting in partially functional dystrophin.	Genetic testing shows out-of-frame mutations in the dystrophin gene leading to absence of functional dystrophin.

Becker Muscular Dystrophy versus Limb-Girdle Muscular Dystrophy

Becker Muscular Dystrophy	Limb-Girdle Muscular Dystrophy
Inheritance pattern is X-linked recessive, affecting mostly males.	Inheritance pattern is often autosomal recessive, unlike Becker's X-linked pattern.
Muscle biopsy shows reduced or abnormal dystrophin staining.	Muscle biopsy shows absence or deficiency of proteins other than dystrophin (e.g., sarcoglycans).
Serum CK levels are moderately elevated, typically higher than in limb-girdle muscular dystrophy.	Serum CK levels can be normal or mildly elevated, less consistently elevated than in Becker muscular dystrophy.

Becker Muscular Dystrophy versus Myotonic Dystrophy

Becker Muscular Dystrophy	Myotonic Dystrophy
Clinical features primarily involve proximal muscle weakness without myotonia or cataracts.	Clinical features include myotonia, cataracts, and cardiac conduction defects.
Genetic testing shows mutations in the dystrophin gene, not trinucleotide repeat expansions.	Genetic testing reveals CTG trinucleotide repeat expansion in the DMPK gene.
Muscle weakness predominantly affects proximal muscles such as pelvic and shoulder girdles.	Muscle weakness is distal and facial rather than proximal.