J Korean Neurol Assoc.  1999 Jan;17(1):112-116.

Nasopharyngeal recordings of N18 far-field potential in Somatosensory Evoked Potential ( SEP )

• Kim DH
• Cha JK
• Kim SH
• Kim JW

Abstract

BACKGROUNDS: The widespread N18 in the median nerve SEP is a broad negativity following positive far-field potentials and is widely distributed over the scalp. It is suggested that the N18 may be originated from subthalamus or upper brainstem nuclei. But there remains substantial controversy regarding its origin. Recently, it is proposed that the N18 is generated in the medulla. For the evaluation of its origin, we used nasopharyngeal electrode for recording SEP stimulated by median nerve stimulation.
METHODS
Standard methods of recording SEP were used in 19 normal adults(36 sides, 19-68 years, 9 males). 11 patients with following locations of the lesions were studied: 2 high cervical cord, 1 ponto-medullary junction, 3 pons, and 5 thalamus. We evaluated the N18 potentials at ventral medulla level, using nasopharyngeal electrode and recorded them at scalp by non-cephalic reference recording. We compared the amplitude and duration of the N18 at scalp with those at nasopharyngeal electrode.
RESULTS
The mean amplitude and duration of N18 at scalp showed no significant difference from that at nasopharyngeal electrode(1.76+/-0.4 microvolt, 16.48+/-2.47 msec and 1.75+/-0.4 microvolt, 16.56+/-2.19 msec, p>0.5). The N18 was absent in 2 patients with high cervical cord lesions. The amplitude and duration of the N18 were normal in all other patients with brainstem and thalamic lesions, including a patient with lesion at the ponto-medullary junction.
CONCLUSIONS
These results suggested that N18 is generated in the medulla and not more rostrally in the brainstem.

Keyword

Median nerve; Far-field potential; Somatosensory evoked potential

MeSH Terms

Brain Stem
Electrodes
Evoked Potentials, Somatosensory*
Humans
Median Nerve
Pons
Scalp
Subthalamus
Thalamus