"Left a coma"

Why it is impossible to lie for years, and then to rise and start talking. What is the coma and than it differs from clinical death? What reasons at a coma and how doctors on appearance of pupils and skin can establish these reasons happen, without conducting any researches? Whether happens so, what the person leaves a coma after several years of stay in it and at once starts going and speaking? How often regained consciousness do not recognize the relatives? The neurologist told Institutes to the Russian Academy of Sciences about it in interview of "Gazete.Ru" a brain of the person of Bekhtereva Khegay Olga. — What is the coma and than it differs from clinical death? — ...