What is the lesion of myasthenia gravis (MG)?
MG is an autoimmune disease of the motor endplate.
It presents with easy fatigability of voluntary muscles followed by recovery with rest. Its first manifestation is frequently diplopia. Often it is generalized and progressive. The incidence approximates 1:2,000 adults. MG is an antibody-mediated disorder. The amount of presynaptic acetylcholine released is normal or increased. Antibodies are produced to the acetylcholine receptor of the neuro-muscular junction. In 10–15% of patients with MG, the presence of antibodies cannot be demonstrated. Antibodies are of the immunoglobulin G (IgG) class and are specific for the α subunit of the endplate receptor. Myasthenics are noted to have 70–80% fewer receptors on the motor endplate and fewer folds in the synaptic cleft. Reduction in the number of receptors is brought about by functional block, increased rate of receptor degradation, or complement-mediated lysis of the postsynaptic membrane. Under normal conditions, only 25–30% of receptors are required for neuromuscular transmission. The decrease in receptors represents a reduction in the margin of safety of neuromuscular transmission. These patients are extremely sensitive to any agent that interferes with neuromuscular transmission.