In the normal profile for degeneration, the patient would begin experiencing a loss of motor skills in ten to fifteen years, with life expectancy somewhere in the twenty to twenty-five year range upon contracting it. It seems to affect the facial nerve, causing slight facial droop and motor weakness. It also affects the respiratory muscles, leading to respiratory failure in its late stages.
In 2375, Section 31 was under the impression that Koval, head of the Romulan Tal Shiar, had contracted Tuvan Syndrome. Or so they led Dr. Julian Bashir to believe, as they convinced him that he was needed to make a positive diagnosis as part of a ruse to expel Senator Kimara Cretak from her chair in the Romulan Senate. (DS9: "Inter Arma Enim Silent Leges")