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Neuromuscular weakness

Specialties > Peri-op medicine & anaesthesia

Background knowledge 🧠

Definition

  • Neuromuscular weakness refers to a decrease in muscle strength due to dysfunction of any component of the motor unit, including the motor neuron, neuromuscular junction, or muscle fibers.
  • Results from disorders affecting the peripheral nervous system, neuromuscular junctions, or muscles themselves.
  • Commonly seen in conditions such as myasthenia gravis, Guillain-Barré syndrome, and muscular dystrophies.

Epidemiology

  • Varies widely depending on the underlying condition.
  • Myasthenia gravis: incidence of 15-20 per million per year.
  • Guillain-Barré syndrome: incidence of 1-2 per 100,000 per year.
  • Muscular dystrophies are generally rare but vary by type (e.g., Duchenne muscular dystrophy: 1 in 3,500 male births).

Aetiology and pathophysiology

  • Autoimmune: e.g., myasthenia gravis (autoantibodies against acetylcholine receptors).
  • Genetic: e.g., muscular dystrophies (mutations in genes encoding muscle proteins).
  • Infectious: e.g., Guillain-Barré syndrome (immune-mediated post-infection).
  • Toxic: e.g., botulism (neurotoxin blocking acetylcholine release).
  • Metabolic: e.g., mitochondrial myopathies (defects in energy production).
  • Degenerative: e.g., motor neuron disease (degeneration of motor neurons).

Types

  • Motor neuron diseases: e.g., amyotrophic lateral sclerosis (ALS).
  • Peripheral neuropathies: e.g., Charcot-Marie-Tooth disease.
  • Neuromuscular junction disorders: e.g., myasthenia gravis.
  • Muscle diseases: e.g., muscular dystrophies, myopathies.

Clinical Features 🌡️

Symptoms

  • Progressive muscle weakness, often symmetrical.
  • Fatigue, worsened by activity (e.g., myasthenia gravis).
  • Difficulty in performing tasks requiring muscle strength (e.g., climbing stairs, lifting objects).
  • Muscle cramps, fasciculations (e.g., in motor neuron disease).
  • Respiratory difficulties in advanced cases.
  • Dysphagia or dysarthria in bulbar involvement.

Signs

  • Reduced muscle bulk and tone.
  • Muscle fasciculations or atrophy.
  • Hyporeflexia or areflexia.
  • Weakness often in a proximal-to-distal gradient (e.g., in myopathies).
  • Ptosis, ophthalmoplegia in myasthenia gravis.
  • Gait abnormalities (e.g., foot drop in peripheral neuropathy).

Investigations 🧪

Tests

  • Electromyography (EMG) to assess muscle and nerve function.
  • Nerve conduction studies to evaluate the speed and strength of electrical impulses.
  • Serum creatine kinase (CK) levels, often elevated in muscle damage.
  • Autoantibody tests (e.g., anti-AChR antibodies in myasthenia gravis).
  • Genetic testing for hereditary neuromuscular disorders.
  • Muscle biopsy in cases of suspected myopathies.

Management 🥼

Management

  • Depends on underlying cause (e.g., immunosuppressants for myasthenia gravis).
  • Symptomatic treatment (e.g., physiotherapy to maintain muscle function).
  • Plasmapheresis or IV immunoglobulin for severe cases of autoimmune disorders.
  • Supportive care, including respiratory support if needed.
  • Multidisciplinary approach involving neurology, physiotherapy, and occupational therapy.
  • Patient education and genetic counseling where appropriate.

Complications

  • Respiratory failure in advanced neuromuscular diseases.
  • Aspiration pneumonia due to bulbar weakness.
  • Contractures and pressure sores due to immobility.
  • Cardiac complications in muscular dystrophies.

Prognosis

  • Varies widely based on the specific condition and response to treatment.
  • Some conditions, like Guillain-Barré syndrome, have a good prognosis with treatment.
  • Others, like ALS, have a poor prognosis with progressive decline.
  • Long-term management often required, focusing on symptom control and quality of life.

Key points

  • Early recognition and treatment are crucial for better outcomes.
  • Multidisciplinary management improves quality of life.
  • Regular follow-up is essential to monitor disease progression.
  • Patient education and support are vital for managing chronic neuromuscular conditions.

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Peri-op medicine & anaesthesia