Tuvan Syndrome is a neurological condition that affects Vulcanoid species such as Vulcans, Romulans and Rigelians. It is degenerative and uncurable as of 2375. In the United Federation of Planets, several studies on Vulcans who were affected by the disease were undertaken. Early symptoms of Tuvan Syndrome include displaced weakness of facial muscles, irregular and displaced eyelids. The disease can be accelerated by exposure to nadion radiation. By 2377, Koval's Tuvan Syndrome had progressed to the point that he was seriously ill. (ST - Vulcan's Soul novel: Epiphany)