Three patients who had excessive diphenylhydantoin blood levels (29μg, 30μg, and 43μg per milliliter) developed choreoathetosis. The involuntary movements rapidly disappeared with a reduction of the diphenylhydantoin dose. One of the three patients did not have any of the common neurologic signs of diphenylhydantoin toxicity. The mechanism for the production of this phenomenon with excessive diphenylhydantoin is unknown, but it appears that the reaction is most likely to occur in epileptic patients with decreased intellect or persistent neurologic signs.
(JAMA 230:1170-1171, 1974)
Shuttleworth E, Wise G, Paulson G. Choreoathetosis and Diphenylhydantoin Intoxication. JAMA. 1974;230(8):1170-1171. doi:10.1001/jama.1974.03240080052031