Overview
Definition
An autoimmune disorder of the neuromuscular junction characterized by fluctuating muscle weakness and fatigability.
Epidemiology
Bimodal: young women (20-30s) and older men (60-70s). Strongly associated with thymic abnormalities (thymoma or thymic hyperplasia).
Etiology & Risk Factors
- Autoantibodies directed against the Nicotinic Acetylcholine Receptors (AChR) on the post-synaptic muscle membrane, leading to receptor destruction and impaired neuromuscular transmission.
Clinical Symptoms
- Fatigable muscle weakness (worsens with use, improves with rest)
- Ocular: Ptosis (drooping eyelids) and Diplopia (double vision) - almost always the presenting symptom
- Bulbar: Dysarthria, dysphagia, fatigable chewing
- Proximal limb weakness
Clinical Approach
Diagnosis
- Serology: Anti-AChR antibodies (highly specific). If negative, check Anti-MuSK antibodies.
- EMG: Decremental response on repetitive nerve stimulation
- Ice pack test (improves ptosis due to decreased acetylcholinesterase activity in cold)
- CT Chest (essential to check for Thymoma)
Management
- Acetylcholinesterase inhibitors (Pyridostigmine) for symptomatic relief
- Immunosuppression (Corticosteroids, Azathioprine, Mycophenolate) for disease modification
- Thymectomy (improves outcomes, even without a thymoma)
- Myasthenic Crisis (respiratory failure): IVIG or Plasmapheresis + intubation
Complications
- Myasthenic Crisis (life-threatening respiratory muscle weakness, often triggered by infection or certain drugs like aminoglycosides or fluoroquinolones)