Myasthenia Gravis

Basics

Autoimmune antibody–mediated condition that results in painless, fatigable skeletal muscle weakness
Ocular or generalized:
- Ocular (eyelids and extraocular) muscle weakness:
  - Most common initial symptom (60%)
  - ∼80% of myasthenia gravis (MG) patients who present with ocular weakness initially will progress to general weakness within 2 yr
- Generalized:
  - Usually affects proximal limbs, axial muscle groups such as neck, face, bulbar muscles
Acute or subacute, with relapses and remissions
Associated with thymoma in 10–15% and thymic hyperplasia in 60–70%
Myasthenic crisis:
- Respiratory failure or inability to protect airway due to weakness
- Triggers:
  - Infection
  - Surgery
  - Trauma
  - Pregnancy
  - Medication changes (e.g., rapid tapering of steroids)
- Difficult to distinguish from cholinergic crisis resulting from excessive doses of acetylcholinesterase (AChE) inhibitors:
  - Cholinergic crisis may also include muscarinic effects such as sweating, lacrimation, salivation, and GI hyperactivity in addition to weakness
  - Cholinergic crisis due to therapeutic AChE inhibitor use is rare, typically at very high doses

Pediatric MG is rare and distinct:
- Congenital MG: Genetic defect
- Juvenile MG: Autoimmune disorder
- Transient neonatal MG: Postdelivery complication from placental transfer of maternal antibodies
Adult MG has bimodal distribution:
- First peak in second and third decades affecting mostly women
- Second peak in sixth and seventh decades affecting men
Patients with MG may have other autoimmune conditions (e.g., lupus, rheumatoid arthritis, Graves)

Antibody-mediated attack on nicotinic acetylcholine receptors on the postsynaptic membrane of the neuromuscular junction
Up to 20% of patients may be acetylcholine receptor antibody (AChR Ab) negative:
- Antibodies against other postsynaptic proteins (e.g., muscle-specific receptor tyrosine kinase, MuSK) may be involved
Many medications may worsen myasthenic weakness:
- Aminoglycosides, macrolides, quinolones, antimalarials
- Local anesthetics
- Antidysrhythmics (propafenone, quinidine, procainamide)
- β-blockers, calcium-channel blockers
- Anticonvulsants (phenytoin, carbamazepine)
- Antipsychotics (phenothiazine, atypicals)
- Neuromuscular blocking agents
- Statins
- Iodine-containing radiocontrast
Penicillamine can induce anti-AChR antibody production

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