Myasthenia gravis (MG)
Chronic disease characterised by muscle weakness and fatigue. Defect is in the acetylcholine receptor (post-synaptic) immune mediated destruction. Characterised by a fluctuating weakness and fatigue that can be improved with cholinesterase inhibitors. Can be associated with other autoimmune disorders, especially thyrotoxicosis (15% have a thyoma). Affects 1/20000.
5-7% familial. HLA-A1, -B8 and -DR3 are associated with it in younger Caucasian women; in older men - -A3, B7, Drw2
F 1.5-2x>M. Highest incidence between ages 20-40. Maximum symptoms normally occur within 3 years of onset. 10% go into remission.
Variable amounts of fluctuating weakness and fatigue associated with muscle groups – most commonly external ocular, facial and bulbar muscles – proximal limb muscles can also be involved. Symptoms worse at end of day – varies from day to day.
Limb muscle weakness after repetitive weakness – dramatic improvement on rest
EMG – considered positive if >10% decrease in amplitude of evoked compound action potential
Myasthenic crisis – acute exacerbation with respiratory and bulbar dysfunction need hospitalisation for respiratory support
Anticholinesterase drugs (neostigmine, pyridostigmine) often gives good symptomatic relief (positive response is considered diagnostic).