Motor speed is a contaminating factor in evaluating the "cognitive" effects of phenytoin.

Many studies suggestive of adverse effects of phenytoin (PHT) on mental abilities have used testing procedures which have timed or motor speed elements. Therefore, to what degree the motor speed element alone may have resulted in attributing adverse higher level intellectual or cognitive effects to PHT instead of the identified construct to be measured (e.g., memory, abstraction, decision making) is not clear. To help distinguish "motor" effects from these more complex "cognitive" effects, neuropsychological data on 70 adult PHT monotherapy patients were reanalyzed. Initially, a series of statistically significant differences favored the low serum level group over the high serum level group in neuropsychologic performance. However, when a simple measure of motor speed (Finger Tapping Test) was covaried out, all statistically significant differences between the groups disappeared. Thus, losses in cognitive abilities could not be associated with PHT even though markedly elevated blood levels had been achieved.