Yes: the prince has a nervous illness that the narrator calls epilepsy (in Russian, paduchaya, from "to fall"), and which Myshkin himself describes to his neighbors in the railway car as something in the nature of epilepsy. Dostoevsky gave the character his own illness.
The fits shown in the book are two: on the hotel staircase, when he cries "Parfen, I don't believe it!" on seeing Rogozhin's hand raised with something that glints, and at the solemn gathering, after knocking over the Chinese vase in the middle of a speech. In both, the narrator describes the instant of light that comes before them.
That instant is the most interesting part. The prince knows the light comes from the illness, and that the dullness and idiocy come after it, but he cannot deny what he felt: if in that second it happened that he told himself that for that moment one could give a whole life, then "that moment alone was worth a whole life." He calls it beauty and prayer.
The illness has three functions in the novel: it explains the title, because in childhood the fits left him almost an idiot; it saves the prince from Rogozhin's knife; and it raises the question the book does not resolve, whether his goodness is a choice or a symptom. The author himself had a crisis diagnosed as epilepsy on the way back from his 1857 wedding, and there are two readings of what that gives the character: a way of knowing the limits of a man, or an excuse the book always keeps within reach.