The utility of a forehead-to-inion derivation in recording the subcortical far-field potential (P14) during median nerve somatosensory-evoked potential testing.
American Clinical Neurophysiology Society (ACNS) guidelines recommend recording P14 between an ipsilateral centroparietal electrode (CPi) and a noncephalic reference, typically the contralateral Erb's point (EPc) (American Clinical Neurophysiology Society. Guideline 9D: guidelines on short-latency somatosensory evoked potentials. J Clin Neurophysiol. 2006;23(2):168-179). We investigated the utility of a forehead (Fpz)-to-inion derivation for recording P14. We analyzed 74 median nerve somatosensory-evoked potential (SEP) studies (148 nerves) with bilaterally normal peripheral and central conductions. The presence of an identifiable P14 and its amplitude and latency were assessed in both the CPi-EPc and Fpz-inion derivations. In 7 of the 148 recordings, P14 was not identifiable in either derivation. The P14 was only identifiable in CPi-EPc in 9 recordings, and only identifiable in Fpz-inion in 4 recordings. In the remaining 128 recordings, the mean P14 latency was 13.2 ± 1.1 ms in both derivations. The mean P14 amplitude using CPi-EPc was 2.0 ± 0.6 µV, significantly larger than that using Fpz-inion, 1.2 ± 0.6 µV (P < .001). In conclusion, the CPi-EPc derivation and the Fpz-inion derivation both record the same P14 component, and latency norms based on either derivation are interchangeable. Although the CPi-EPc derivation typically yields a larger and more identifiable P14, occasionally Fpz-inion yields a larger P14, and rarely P14 is only identifiable using Fpz-inion. Thus, recording of the Fpz-inion derivation may be a useful adjunct during median nerve SEP testing.
Department of Neurology, Montefiore Medical Center and the Albert Einstein College of Medicine, Bronx, NY 10467, USA. sfried@montefiore.org
Stephen J
SJ
Fried
Alan D
AD
Legatt
D000328:Adult; D000344:Afferent Pathways / Q000503:physiopathology; D000368:Aged; D000369:Aged, 80 and over; D001927:Brain Diseases / Q000503:physiopathology*; D004558:Electric Stimulation; D004566:Electrodes; D004569:Electroencephalography / Q000379:methods*; D005073:Evoked Potentials, Somatosensory / Q000502:physiology*; D005260:Female; D005546:Forehead; D006801:Humans; D008297:Male; D008475:Median Nerve / Q000503:physiopathology*; D008875:Middle Aged; D017410:Practice Guidelines as Topic; D012189:Retrospective Studies