This is actually an autoimmune condition. Antibodies are bring formed that are blocking acetylcholine receptors.
As with many autoimmune disorders there appears to be a microbial involvement. In this case several viruses, including herpes simplex, is suspected as a trigger for many cases.
I always recommend going with full range antimicrobials first in cases of autoimmunity. Especially chaparral, pau d' arco and andrographis combined and taken 3 times daily on an empty stomach. Then focus on the adrenals. The adrenals produce corticosteroids that help to modulate the immune response allowing the body to produce the proper high affinity (specific) antibody as opposed to the autoimmune low affinity (nonspecific) antibodies.
Here is a link to my write up on autoimmunity that goes in to a lot more detail: