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Muscular dystrophies

Medicine > Neurosciences

Background knowledge 🧠

Definition

  • Muscular dystrophies are a group of inherited disorders characterised by progressive weakness and degeneration of skeletal muscles.
  • They primarily affect the muscles responsible for voluntary movement.
  • Various types exist, each with different clinical features and progression rates.

Epidemiology

  • Muscular dystrophies are rare, with incidence varying by type.
  • Duchenne muscular dystrophy (DMD) affects approximately 1 in 3,500 male births worldwide.
  • Other types, like Becker muscular dystrophy (BMD), have lower prevalence rates.
  • Most types manifest in childhood or adolescence.

Pathophysiology

  • Genetic mutations affect proteins essential for muscle integrity and function.
  • Defective proteins lead to muscle fibre damage and necrosis.
  • Progressive muscle weakness and fibrosis occur as muscles try to repair themselves.
  • Chronic inflammation contributes to further muscle damage.

Aetiology/Risk factors

  • Primarily caused by inherited genetic mutations.
  • Duchenne and Becker muscular dystrophies are X-linked recessive disorders.
  • Other forms may be autosomal dominant or recessive.
  • Family history of muscular dystrophy is a significant risk factor.
  • Spontaneous mutations can also occur.

Types

  • Duchenne Muscular Dystrophy (DMD): Most common and severe form, affecting young boys.
  • Becker Muscular Dystrophy (BMD): Similar to DMD but with a later onset and slower progression.
  • Myotonic Dystrophy: Characterised by prolonged muscle contractions and affects multiple systems.
  • Facioscapulohumeral Dystrophy: Affects muscles of the face, shoulder blades, and upper arms.
  • Limb-Girdle Muscular Dystrophy: Affects hip and shoulder muscles.
  • Congenital Muscular Dystrophy: Present at birth, varying in severity.

Clinical Features 🌡️

Symptoms

  • Early symptoms include difficulty walking, frequent falls, and difficulty rising from the floor.
  • Progressive muscle weakness, starting in the legs and pelvis.
  • Difficulty running, jumping, and climbing stairs.
  • Calf muscle hypertrophy is common in DMD and BMD.
  • Later stages may involve respiratory and cardiac muscle involvement.
  • Delayed motor milestones in children.

Signs

  • Gower’s sign – using hands to push off the thighs when rising from the floor.
  • Waddling gait due to pelvic muscle weakness.
  • Muscle pseudohypertrophy, particularly in the calves.
  • Contractures and scoliosis in advanced stages.
  • Cardiomyopathy and respiratory insufficiency in later stages.
  • Facial muscle weakness in facioscapulohumeral dystrophy.
  • Myotonia in myotonic dystrophy.

Investigations 🧪

Initial tests

  • Serum creatine kinase (CK) levels – often markedly elevated in DMD and BMD.
  • Electromyography (EMG) to assess electrical activity in muscles.
  • Nerve conduction studies to rule out neuropathies.
  • Pulmonary function tests in later stages to assess respiratory involvement.
  • Electrocardiogram (ECG) and echocardiogram to monitor cardiac function.

Diagnostic tests

  • Genetic testing to confirm the specific type of muscular dystrophy and identify mutations.
  • Muscle biopsy to examine muscle tissue under a microscope.
  • Immunohistochemistry and Western blot to detect specific muscle proteins.
  • Molecular testing for dystrophin in DMD and BMD.

Management 🥼

Management

  • Multidisciplinary approach including neurologists, physiotherapists, and occupational therapists.
  • Corticosteroids to slow muscle degeneration in DMD.
  • Physical therapy to maintain mobility and prevent contractures.
  • Assistive devices like braces and wheelchairs for mobility support.
  • Surgical interventions for scoliosis and contractures.
  • Cardiac care, including ACE inhibitors and beta-blockers for cardiomyopathy.
  • Respiratory support with non-invasive ventilation as needed.
  • Regular monitoring and management of complications.

Complications

  • Respiratory failure due to weakened respiratory muscles.
  • Cardiomyopathy and heart failure.
  • Scoliosis and orthopaedic deformities.
  • Contractures and joint stiffness.
  • Nutritional deficiencies due to swallowing difficulties.
  • Psychosocial issues such as depression and anxiety.

Prognosis

  • Varies significantly depending on the type of muscular dystrophy.
  • Duchenne Muscular Dystrophy often leads to loss of ambulation by early teens and shortened life expectancy.
  • Becker Muscular Dystrophy has a slower progression with longer life expectancy.
  • Prognosis for other types varies based on severity and management.
  • Early intervention and supportive care can improve quality of life and longevity.

Key points

  • Muscular dystrophies are a diverse group of genetic disorders leading to progressive muscle weakness.
  • Diagnosis is based on clinical features, elevated CK levels, genetic testing, and muscle biopsy.
  • Management involves a multidisciplinary approach focusing on symptom relief and supportive care.
  • Early intervention and regular monitoring are essential to manage complications and improve outcomes.
  • Genetic counselling is important for affected families to understand inheritance patterns and risks.

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Neurosciences