Bendii syndrome is a neurological condition which affects a minority of Vulcans in old age, typically over 200 years old.
Symptoms include loss of emotional control, which can result in emotional outbursts and quite dangerously to others, telepathic projections of the sufferer's emotions. (TNG episode: "Sarek") These psychic disturbances that were broadcast by the sufferer of the syndrome did not occur until they were of a more advanced stage. (TNG novel: The Devil's Heart)
In 2377, a group of Vulcans lead by the Master N'Kal attempted to develop a cure for the syndrome using Borg nanoprobes. Unfortunately, they lost control and N'Kal became the leader of a rogue Borg Collective known as the Wardens. Fortunately, the crew of the USS Incursion were able to curtail the Wardens activities and free N'Kal from the nanoprobe control, though a cure for Bendii syndrome remains elusive. (TNG video game: Away Team)