Conjugate lateral gaze nystagmus and free phenytoin concentrations in plasma: lack of correlation.

Electrooculographic recordings during left and right conjugate lateral gaze fixation from 10 degrees to 50 degrees were made in 29 patients being treated with phenytoin. At the time of electrooculographic recordings, venous blood samples were drawn for analysis of total and free phenytoin plasma concentrations. Rhythmic horizontal nystagmus occurred in 7 patients, and only at extreme (40 degrees, 50 degrees) later gaze in 6 of these. Measurable free phenytoin levels in the 29 patients ranged from 0.2 to 3.2 mug/ml, and in the 7 patients with nystagmus were 0.6, 0.8, 1.0, 1.0, 1.7, 1.8, and 3.2 mug/ml. Neither the occurrence of nystagmus nor the degree of lateral gaze at onset could be correlated with free phenytoin concentrations in plasma.