Jama 2005;293:1906

Cause: Autoimmune

Pathophys: Antibodies to neuromuscular junction acetylcholine receptors (Nejm 1977;296:125); measured antibody doesn't correlate with disease severity; but some patients have more lethal antibody function than others (Nejm 1982;307:769)

Esp in young females, and in males age 65-75 yr; female/male: 3:1 overall.

Associated with:

• Thymomas, esp in older males; 30% have; 30% of patients with thymomas have myasthenia
• Thyrotoxicosis, esp Hashimoto's
• Collagen vascular disease, especially SLE
• HLA-B8

Sx:

Precipitated by thyrotoxicosis, muscle-paralyzing anesthesia, antibiotics like streptomycin, etc, quinine (including gin and tonic), quinidine, perhaps Dilantin, procainamide, propranolol

Diplopia and other ocular sx (60%), muscle fatigue with repetitive use

Si: Increased weakness with repeated use, often in extraocular muscles 1st w positive "peek" test (lids open a little after pt asked to close and keep closed), or pharynx; reflexes intact

Prognosis much poorer if thymoma present (Neuro 1966;16:431); remissions during times of stress often, eg, pregnancy

Respiratory failure; transmission to fetus, normalizes within 6 mo as maternal IgG decreases

r/o botulism; Guillain-Barré; polio; ALS; Graves disease; curare poisoning; SNAKE BITE(Nejm 2002;347:347), acetylcholine receptor blockade, rx with edrophonium (Tensilon, etc) (Nejm 1986;315:1444) or, sheep or horse antivenom Fab fragments iv w/i 6 h (Med Let 2001;43:55); EATON-LAMBERT SYNDROME, a similar disease associated with cancer esp of lung and pancreas (Nejm 1989;321:1267) and caused by antibodies to calcium channels responsible for neurotransmitter acetylcholine release (Nejm 1995;332:467)