Myasthenia gravis (MG) is a chronic autoimmune disorder of the post-synaptic membrane at the neuromuscular junction in skeletal muscle.

Epidemiology - Affects men and women at different ages. Typical patients are either a women under 40 or a man over 60

Associations:

• Thymoma - 10-20% of patients with myasthenia gravis have a thymoma. 20-40% of patients with a thymoma develop myasthenia gravis.

Pathophysiology

In 85% of of patients with MG:

• acetylcholine receptor antibodies are produced, they bind to the postsynaptic NMJ receptors, preventing the binding and stimulation from acetylcholine for muscular contraction
• During muscle activity - more receptors being used up and blocked up = less effective stimulation with increased activity and more weakness the more the muscles are used. Improves with rest as receptors are freed up
• Antibodies also activate complement - damaging cells at post synaptic membrane - further worsens Sx.

The other 15% of cases:

• Antibodies against Muscle-specific kinase (MuSK)
• Antibodies against low-density lipoprotein receptor-related protein 4 (LRP4)

These proteins are important for the creation and organisation of acetylcholine receptors, destruction = inadequate acetylcholine receptors = Myastenia Gravis

Most affected muscle groups: Proximal muscles and small muscles of the head and neck

Drugs that can worsen MG:

• Beta blockers
• several antibiotics