Walsh & Hoyt: Symptoms and Signs

The hallmark clinical feature of MG is variability in the strength of the affected muscles. Weakness varies from day to day and from hour to hour, typically increasing toward evening. Transient weakness is often associated with physical exertion. Affected muscles fatigue if contraction is maintained or repeated. This must be differentiated from the neurasthenic ""fatigue"" of tiredness or depression. The most commonly affected muscles, in descending frequency, are the levator palpebrae superioris, the extraocular muscles, and orbicularis oculi; the proximal limb muscles, especially the triceps brachii, deltoids, and iliopsoas; the muscles of facial expression, mastication, and speech; and the neck extensors.