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.