Trigeminal root recording in normal trigeminal function.

Despite many investigations on the detection of trigeminal evoked potentials (TEP), there is still no consensus on a technique or interpretation of its components. In order to obtain clear TEP waveforms and analyze the origin of early components, we recorded TEP by the conventional far-field technique at the ipsilateral auricula as well as by near-field technique with a bipolar electrode at the trigeminal root just distal to the entry zone of its root. Trigeminal recordings were performed in nine patients with intrameatal vestibular schwannomas and in three with trigeminal neuralgia without trigeminal nerve deficits. In near-field recording at the trigeminal root, a triphasic response was recorded, with the first component occurring at 3.16 ms, 2.98 ms, and 4.16 ms following supraorbital, infraorbital, and mandibular nerve stimulation, respectively. Using conventional farfield TEP, the first component recorded at the trigeminal root occurred later than the first component in all other recordings. This suggests that the N2.0 component of far-field TEP is of extra-axial origin and the N5 component is of brainstem origin. The N13.0 component may be of cortical origin. We conclude that the N5.0 component and N5.0-N13.0 interpeak latency may be used as electrophysiological parameters of brainstem function during posterior fossa surgery.