Unsure, There was a link posted by HeatIntolerant that described some statistics of one sided vs two sided symptoms of ptosis in an MG study. The link is here:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1772854/
About 50% of the people in the study had one sided ptosis and about 20% had two sided ptosis. MG can produce either symptom.