Myasthenia Gravis

Definition

Myasthenia Gravis (MG) is a chronic autoimmune neuromuscular disorder characterized by fluctuating skeletal muscle weakness, often worsening with activity and improving with rest, due to autoantibodies against acetylcholine receptors or associated proteins at the neuromuscular junction.

Epidemiology

• Prevalence: ~20 per 100,000 globally.
• Bimodal age distribution: women in 20–40 years, men in 50–70 years.
• All ethnic groups are affected, but incidence varies slightly by region.
• Thymic abnormalities (hyperplasia or thymoma) present in 10–15% of patients.

Etiology

• Autoantibodies against acetylcholine receptor (AChR) in ~85% of patients.
• Anti-MuSK (muscle-specific kinase) antibodies in a subset of patients.
• Thymic abnormalities: hyperplasia, thymoma.
• Genetic predisposition and environmental triggers may contribute.

Pathophysiology

• Autoantibodies bind to AChRs or associated proteins at the neuromuscular junction.
• Reduced number of functional AChRs and impaired neuromuscular transmission.
• Complement-mediated damage to the postsynaptic membrane.
• Fatigue and fluctuating weakness result from impaired neuromuscular signaling.
• Muscles with high activity demand (ocular, bulbar, limb) are preferentially affected.