It is learned during "Endgame", the final episode of the series, that Tuvok suffers from a degenerative neurological disease, which can only be cured by undergoing a mind meld with a family member. In 2377, he was still well enough to perform his duties. However, in his elderly years, in an alternate timeline, the disease finally ravaged Tuvok's mind as he was not able to return to the Alpha Quadrant in time for a cure. This left him confined to a mental hospital. However, to save Tuvok among others, Captain Janeway's future self changed history, by finding a way to return the "Voyager" crew to the Alpha Quadrant back in 2377, the time before his disease worsened. (In the novelization of "Endgame", Tuvok's eldest son Sek arrives aboard "Voyager" and performs the required mind meld, curing Tuvok.)