Monday, 20 September 2010

MRCP revision battle 16.3: Myasthenia Gravis

Myasthenia gravis is an autoimmune condition in which muscles become easily tired and weak.  It affects 1 in 20 000.

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...