As the population ages, the average age at onset has increased correspondingly, and now males are more often affected than females, and the onset of symptoms is usually after age 50. Initial weakness is rarely limited to single muscle groups such as neck or finger extensors or hip flexors. The severity of weakness fluctuates during the day, usually being least severe in the morning and worse as the day progresses, especially after prolonged use of affected muscles. The course of disease is variable but usually progressive. The rest have progressive weakness during the first 2 years that involves oropharyngeal and limb muscles.
Maximum weakness occurs during the first year in two-thirds of patients.
Spontaneous improvement frequently occurred early in the course. The active stage is followed by an inactive state in which fluctuations in strength still occurred but are attributable to fatigue, intercurrent illness, or other identifiable factors. The ACh quanta diffuse across the synaptic cleft and bind to receptors on the folded muscle end-plate membrane. In acquired myasthenia gravis, the post-synaptic muscle membrane is distorted and simplified, having lost its normal folded shape. The concentration of ACh receptors on the muscle end-plate membrane is reduced, and antibodies are attached to the membrane.