Myasthenia Gravis

Definition | Aetiology | Pathophysiology | Risk Factors | Signs and Symptoms | Investigations | Management

Definition

Myasthenia gravis (MG) is a chronic autoimmune neuromuscular disorder that causes weakness in skeletal muscles due to impaired neuromuscular transmission.

Aetiology

MG is caused by autoantibodies that disrupt communication between nerves and muscles. It can be classified as:

1. Autoimmune Myasthenia Gravis:

  • Most common form.
  • Caused by antibodies against the acetylcholine receptor (AChR) or muscle-specific kinase (MuSK).

2. Congenital Myasthenic Syndromes:

  • Rare genetic disorders affecting neuromuscular transmission.

3. Thymoma-Associated MG:

  • About 10-15% of MG cases are associated with a thymic tumour.

Pathophysiology

  • Autoantibodies block acetylcholine receptors at the neuromuscular junction.
  • Leads to reduced transmission of nerve signals to muscles.
  • Results in progressive muscle weakness and fatigue.

Risk factors

  • Family history of autoimmune diseases.
  • Thymoma or thymic hyperplasia.
  • Other autoimmune conditions (e.g., rheumatoid arthritis, lupus).
  • Female sex (more common in younger women and older men).

Signs and symptoms

MG presents with fluctuating muscle weakness that worsens with exertion and improves with rest.

Common Symptoms:

  • Ocular weakness: ptosis (drooping eyelids), diplopia (double vision).
  • Bulbar weakness: dysphagia (difficulty swallowing), dysarthria (slurred speech).
  • Generalised muscle weakness: weakness in arms, legs, and neck.
  • Respiratory involvement: risk of myasthenic crisis with respiratory failure.

Investigations

  • Serum autoantibodies:
    • Acetylcholine receptor (AChR) antibodies (most common).
    • Muscle-specific kinase (MuSK) antibodies (in some cases).
  • Electromyography (EMG): shows abnormal neuromuscular transmission.
  • Edrophonium test (Tensilon test): temporary improvement in muscle strength after acetylcholinesterase inhibition.
  • CT/MRI of the chest: to check for thymoma.
  • Forced vital capacity (FVC): assesses respiratory function.

Management

1. Symptomatic Treatment:

  • Acetylcholinesterase inhibitors (e.g., pyridostigmine): improve neuromuscular transmission.

2. Immunosuppressive Therapy:

  • Corticosteroids (e.g., prednisolone): used for moderate to severe cases.
  • Azathioprine or mycophenolate mofetil: for steroid-sparing immunosuppression.
  • Rituximab: used in refractory cases (especially MuSK-positive MG).

3. Thymectomy:

  • Indicated in patients with thymoma or generalised MG.
  • May improve symptoms in some cases.

4. Myasthenic Crisis Management:

  • Monitor respiratory function (FVC, blood gases).
  • Plasmapheresis or intravenous immunoglobulin (IVIG) for rapid symptom control.
  • Consider ICU admission for ventilatory support if needed.

5. Supportive Care:

  • Speech and swallowing therapy for bulbar symptoms.
  • Physiotherapy to maintain muscle function.
  • Patient education on medication adherence and trigger avoidance.
NeurologymypanotesMyasthenia Gravis