90% of patients have autoantibodies to nicotinic acetylcholine receptors.
If these are negative consider looking for MUSK antibodies (= muscle specific kinase)
The key feature of MG is repetitive stimulation leading to decreased evoked response.
The order in which muscle groups are affected in MG is:
- extraoccular (--> diplopia)
- bulbar
- face
- neck
- lumb
- trunk
Reflexes are normal in MG.
Test of choice: tensilon test = edrophonium
Treatment of MG is:
- cholinesterase inhibitors eg pyridostigmine
- prednisolone if pyridostigmine unsuccessful (but be aware that paradoxically steroids can worsen MG)
- plasmaphoresis/IV IG if respiratory muscle involvement.
If there is hyperplasia of thymus/thyroma, up to 60% of cases will remit after surgery.
Up to 10% of patients with MG will have hyperthyroidism.
Drugs which exacerbate MG include:
- penicillamine
- quinidine
- beta blockers
- lithium
- phenytoin
- gentamycin
Now on to the 'similar but different' Lambert-Eaton syndrome...
