" When he was emotionally agitated or under stress from the pressure of events, Peter’s face sometimes began to twitch uncontrollably. The disorder, usually troubling only the left side of his face, varied in degree of severity: Sometimes the tremor was no more than a facial tic lasting only a second or two; at other times, there would be a genuine convulsion, beginning with a contraction of the muscles on the left side of his neck, followed by a spasm involving the entire left side of his face and the rolling up of his eyes until only the whites could be seen. At its worst, when violent, disjointed motion of the left arm was also involved, the convulsion ended only when Peter had lost consciousness. "

― Robert K. Massie , Peter the Great: His Life and World