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:
Associations
Investigations
Management
Management of myasthenic crisis
*antibodies are less commonly seen in disease limited to the ocular muscles
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
- autoantibodies: around 85-90% of patients have antibodies to acetylcholine receptors. 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 anymore due to the risk of cardiac arrhythmia
Management
- long-acting anticholinesterase e.g. pyridostigmine
- immunosuppression: prednisolone initially
- thymectomy
Management of myasthenic crisis
- plasmapheresis
- intravenous immunoglobulins
*antibodies are less commonly seen in disease limited to the ocular muscles