Write a short note on myasthenia gravis.

Definition

Aetiology

• Autoantibodies (IgG) against nicotinic ACh receptors on the postsynaptic membrane — present in 85%
• Anti-MuSK antibodies in a further 5–10%
• Associated with thymic hyperplasia (70%) or thymoma (10%)
• Commoner in young women and older men

Pathophysiology

Antibody binds ACh receptor → Receptor blocked, cross-linked and destroyed → ↓ Number of functional receptors → ↓ End plate potential → EPP fails to reach threshold in some fibres → Weakness, worse on repetition

• The safety factor of transmission is lost

Clinical Features

• Fatigability — weakness worsens with use, improves with rest; worst in the evening
• Ocular — ptosis and diplopia; the commonest presenting feature
• Bulbar — difficulty in chewing, swallowing, nasal speech
• Proximal limb weakness; neck muscle weakness
• Reflexes, sensation and pupils are normal
• Myasthenic crisis — respiratory muscle failure; a medical emergency

Diagnosis

• Neostigmine or edrophonium (Tensilon) test → dramatic improvement
• Repetitive nerve stimulation → decremental response (> 10% fall)
• Anti-AChR antibody assay
• CT chest for thymoma

Treatment and Contrast

• Anticholinesterases — pyridostigmine, neostigmine
• Immunosuppression — steroids, azathioprine
• Thymectomy; plasmapheresis and IVIG in crisis