Myasthenia gravis

Myasthenia gravis is an autoimmune disorder resulting in insufficient functioning acetylcholine receptors. Antibodies to acetylcholine receptors are seen in 85-90% of cases*. Myasthenia is more common in women (2:1)

The key feature is muscle fatigability - muscles become progressively weaker during periods of activity and slowly improve after periods of rest:

• extraocular muscle weakness: diplopia
• proximal muscle weakness: face, neck, limb girdle
• ptosis
• dysphagia

Associations

• thymomas in 15%
• autoimmune disorders: pernicious anaemia, autoimmune thyroid disorders, rheumatoid, SLE
• thymic hyperplasia in 50-70%

Investigations

• single fibre electromyography: high sensitivity (92-100%)
• CT thorax to exclude thymoma
• CK normal
• antibodies to acetylcholine receptors
  • positive in around 85-90% of patients
  • In the remaining patients, about about 40% are positive for anti-muscle-specific tyrosine kinase antibodies
• Tensilon test: IV edrophonium reduces muscle weakness temporarily - not commonly used any more due to the risk of cardiac arrhythmia

Management

• long-acting acetylcholinesterase inhibitors
  • pyridostigmine is first-line
• immunosuppression is usually not started at diagnosis, but the majority of patients eventually require it in addition to long-acting acetylcholinesterase inhibitors:
  • prednisolone initially
  • azathioprine, cyclosporine, mycophenolate mofetil may also be used