Nystagmus, anisocoria, and disturbances in accommodation have been recognized as features of intoxication from excessive doses of both diphenylhydantoin and primidone; diplopia and defective convergence have been observed after diphenylhydantoin alone. In four cases, partial or total external ophthalmoplegia resulted from excessive ingestion of these antiepileptic drugs. In three of the cases it is not possible to state which of the agents was primarily responsible for the ophthalmoplegia. In one (case 3), however, it appears that diphenylhydantoin alone was involved.
Orth DN, Almeida H, Walsh FB, Honda M. Ophthalmoplegia Resulting From Diphenylhydantoin and Primidone IntoxicationReport of Four Cases. JAMA. 1967;201(6):485–487. doi:10.1001/jama.1967.03130060159024