OK, in all of Trek, there must have been more, but I can only find reference to only a few Psionic 'illnesses'.
Early Telepathic Manifestation; Betazoid childhood manifestation of Psionic abilities (was there a proper name given for this?)
Pa’Nar Syndrome; A Mind-Meld oddity if ever there was one...
Pon Farr; Also Vulcan, not quite an 'illness' but it vaguely qualifies...
Zanthi Fever; The Betazoid menopause.
Can anyone think of any more? I really could use some references...