癲癇 stands for Japanese tenkan ‘epilepsy; epileptic fit’. It also conveys Chinese for epilepsy: diānxián.
As with lots of other Chinese compounds, I wonder how the two parts got to be combined. Initially I guessed that 癲 may have been modifying 癇, giving something like: crazy convulsions → epilepsy. Unfortunately, according to Kroll’s dictionary, both the words that 癲 and 癇 respectively pointed to, originally already had a meaning of ‘epilepsy’.
Initially I also hoped that the smooth alliteration of diānxián might be significant. But if Middle Chinese really was ten hean that seems less convincing as well.