Ableitung subkortikaler somatosensibel evozierter Potentiale mit Nasopharyngealelektroden - eine Untersuchung an sedierten Patienten

Summary

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 (fig. 2, 3, 4). 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 (fig. 8) -; and without the considerable artifacts known from far-field 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 (fig. 7).

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 (fig. 5, 6).

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.

Zusammenfassung

An 12 sedierten Patienten wurden subkortikale SEP nach Medianus- und Tibialisstimulation vom Nasopharynx abgeleitet und mit den herkömmlichen Ableitungen vom oberen Nacken verglichen. Von Pgz-Fz läßt sich eine negative Welle (vor dem kortikalen Primärkomplex) ableiten, die der vom Nacken ableitbaren N13 (Medianus-SEP) bzw. N30 (Tibialis-SEP) entspricht. In der Ableitung Pgz gegen eine extrazephale Referenz kommen (nach Medianusstimulation) die Far field-Komponenten P9 und P11 deutlich, P14 dagegen mit nur niedriger Amplitude zur Darstellung.

Die Befunde werden dahingehend interpretiert, daß die von Pgz-Fz ableitbare N13-DZW. N30-Aktivität des Lemniscus medialis widerspiegelt, und daß (nach Medianusstimulation) im Gegensatz zu den üblichen Nacken-Skalp-Ableitungen isoliert die Subkomponente N13b (ohne Überlagerung durch N13a) zur Darstellung kommt.

Key words:

Somatosensory evoked potentials - Subcortical potentials - Nasopharyngeal electrodes - Generator dipoles - Humans