Aetiology

Caused by an error in the transmission of nerve impulses to muscles. It occurs when normal communication between the nerve and muscle is interrupted at the NMJ where nerve cells connect with the muscles they control

• Disorder usually becomes apparent during adulthood, however, symptom onset may occur at any age
• Disease occurs sporadically for unknown reasons
• 5% of patient may have family members with MG or other AI disorders
• Increased frequency of certain HLA genes may suggest genetic predisposition

Other AI diseases also appear to occur with increased frequency in individuals with MG, including thyroid disorders and SLE à ?genetic predisposition to MG which requires a trigger from the environment to cause disease

Pathophysiology

• Abnormal immune reaction (antibody-mediated autoimmune response) in which the body’s immune defences (antibodies) inappropriately attack certain proteins in muscles that receive nerve impulses.
• Autoantibodies are inappropriately directed against acetyl-choline receptors on the surface of certain muscles at the neuromuscular junction. These autoantibodies are known as ‘anti-acetylcholine receptor antibodies.
• The abnormal immune response results in a decreased number of acetylcholine receptors results in failed nerve transmission at certain NMJ’s and associated deficiency or weakness of muscle contractions
• In five to eight percent of patients an antibody in the blood cannot be identified but patients have other tests consistent with myasthenia gravis.

Risk Factors

• Personal or family history of autoimmune disease
• Men >60, women <40

Clinical Features