[Recording of subcortical somatosensory evoked potentials using nasopharyngeal electrodes--a study in sedated patients].

Nasopharyngeal recording of evoked potentials is only seldom used, and a systematic description of subcortical SEP derivations using this method is essentially lacking. In twelve patients, sedated after neurosurgical operations and without signs of brainstem or cervical spinal cord dysfunction, subcortical SEP were recorded from nasopharynx (Pgz) and neck after stimulation of median and tibial nerves. Generally, a midfrontal reference (Fz) was used; in four cases, evoked potentials were additionally recorded against a non-cephalic reference. The main feature in Pgz-Fz recordings was a negative wave corresponding to the N13 (N30) component in neck derivations after stimulation of median (tibial) nerve. This wave is interpreted as reflection of medial lemniscal activity; so Pgz-Fz derivations make possible the isolated recording of the subcomponent N13b (medial lemniscus, or cuneate nucleus), without overlap with N13a (dorsal horn) - as do neck-to-scalp recordings-; and without the considerable artifacts known from farfield recordings. This interpretation is confirmed by SEP recordings in a brain death patient, showing a clear N13 component in neck-to-scalp derivations while lacking any negative wave at Pgz-Fz. In Pgz recordings against a non-cephalic reference, the P9 and P11 components (corresponding to the well known scalp farfield potentials) were clearly seen, whereas the P14 component was of low amplitude in three cases and lacking at all in one case. In conclusion, nasopharyngeal electrodes provide a useful tool in subcortical SEP recording; due to their position medio-ventrally to the lower brainstem, they complete standard electrode montages giving further informations about generator dipole location.