When the ship fell prey to a Tyken's Rift and caused her Human crew to go insane and kill each other, Hagan was the sole survivor. An away team from the USS Enterprise-D found Hagan, himself either insane or rendered mute. Deanna Troi feared the same fate would befall her if the phenomenon happened aboard the Enterprise-D.
Hagan was treated in sickbay although Doctor Crusher wasn't able to help him. Troi communicated with him telepathically but received only information she wasn't able to interpret. (TNG: "Night Terrors")