Myasthenia Gravis
Disorder of neuromuscular transmission from antibody-mediated attack on nicotinic AChR at neuromuscular junctions. Fluctuating weakness improved by inhibitors of cholinesterase.
Etiology and Pathogenesis
Polyclonal IgG antibodies to AChR saturate 80% of AChR sites on muscle. Major damage to end-plates from actual loss of receptors due to complement-mediated lysis. Amplitude of miniature end-plate potentials reduced to 20% of normal. Increased sensitivity to nondepolarizing blocker curare.
Forms of Myasthenia Gravis (MG)
Juvenile and Adult Forms
Typical MG. Most common ages 10 to 40. Circulating AChR antibodies usually present (see below). Patients without demonstrable antibodies respond identically to treatment.
Neonatal Myasthenia
Placental transmission of AChR antibodies by affected mother. Affects 12% of infants born to myasthenic mothers. Onset in first 48 hours of impaired sucking, weak cry, limp limbs. Respiratory insufficiency rare; recovery inevitable if child supported. Antibodies demonstrable in baby and mother. Duration limited to life span of antibodies (days or weeks). Only supportive treatment needed.
Congenital Myasthenia
Due to abnormalities of motor end-plate, including abnormal AChR subunits. Usually autosomal recessive. No antibodies to AChR.
Mothers asymptomatic, lack anti-AChR antibodies. Often infantile onset with ophthalmoplegia; sometimes later onset with limb.
Anticholinesterase drugs sometimes helpful. Warn parents: sudden apneic spells may be induced by mild infections.
Drug-Induced Myasthenia
Most common cause: D-penicillamine. Clinical manifestations: AChR antibody titers similar to those of typical adult MG. Symptoms, antibodies disappear with discontinuation of drug.
Pathology
Thymus: lymphoid hyperplasia in 70%; thymoma in 10% to 15% after age 40.
Incidence
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