I am absolutely convinced that Flaubert was a writer of rare merit, and had he not been attacked by his terrible nervous illness he would have been a writer of genius.Sartre and Vargas Llosa both considered Flaubert's seizures to be hysterical or affective, rather than entirely organic, in origin. More recent opinions have disputed this judgment.
Here are some print sources on Flaubert's epilepsy:
Gastault, Henri, Gastault, Yvette, and Broughton R. "Gustave Flaubert's Illness: A Case Report in Evidence against the Erroneous Notion of Psychogenic Epilepsy," Epilepsia 25(5)(October 1984): 622-637.
Jallon, P., and Jallon H. "Gustave Flaubert's Hidden Sickness," in Bogousslavsky and Boller, eds., Neurological Disorders in Famous Artists (Basel: Karger 2005): 46-56.
Wall, Geoffrey. "The Invisible Man: An Essay on Flaubert and Celebrity," The Cambridge Quarterly 35(2)(2006): 133-150.