Dementia was a broad syndrome that could affect several cognitive functions, including memory, attention, and problem solving abilities.
Some people who suffered from dementia spoke their native language again. (ENT: "Strange New World")
The Vidiian phage could also cause hallucinatory dementia. (VOY: "Coda")
When Jean-Luc Picard discussed returning to space to seek out the twin sister of Dahj Asha, Laris exclaimed: "Have you gone mad!? Is it dementia?" (PIC: "Maps and Legends")