myasthenia gravis

Question 1

definition

Answer 1

MG is an autoimmune disease mediated by antibodies to nicotinic acetylcholine receptors (AChR), interfering with the neuromuscular transmission via depletion of working post-synaptic receptor sites (fig 1). Both B and T cells are implicated.

Patients present with muscle weakness, which typically worsens with continued activity (fatigue) and improves on rest. Severity varies from isolated eye muscle weakness to generalised muscle weakness and respiratory failure requiring mechanical ventilation.

Question 2

symptoms

Answer 2

• muscle fatigability
• ptsosis
• diplopia
• dysphagia
• dysarthria
• facial paresis
• proximal limb weakness
  Muscle groups affected, in order: extra- ocular; bulbar (swallowing, chewing); face; neck; limb girdle; trunk.

TENDON REFLEXES ARE NORMAL!

weakness is worsened by:
pregnancy, hypokalemia, infection, over-treatment, change of climate, emotion, exercise, gentamicin, opiates, tetracycline, quinine, procainamide, beta-blockers.

Question 3

investigations

Answer 3

1st investigations:

• serum AChR antibody analysis
• serum muscle-specific tyrosine kinase antibodies
• serial pulmonary function tests
• arrange CT upper thorax (to check for thymic hyperplasia)

Question 4

tensilon test

Answer 4

used to differentiate MG from other conditions. Tensilon injection given, and muscle strength assessed after. tensilon prevents the breakdown of ACh.