THE UTILITY OF A FOREHEAD-TO-INION DERIVATION IN RECORDING THE SUBCORTICAL FAR-FIELD POTENTIAL (P14) DURING MEDIAN NERVE SOMATOSENSORY-EVOKED POTENTIAL TESTING (SSEP TESTING)
Stephen J. Fried1 and Alan D. Legatt1,2
Abstract
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 Fpzinion 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 mV, significantly larger than that using Fpz-inion, 1.2 + 0.6 mV (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.
Keywords
P14, median nerve SEP, somatosensory-evoked potential testing, SSEP testing, recording montage, far-field potential
Received March 23, 2011; accepted June 30, 2011.
Introduction
Median nerve SEP recordings are indispensable in the burgeoning field of intraoperative monitoring (IOM) and are also useful as an adjunct in the diagnosis of neurologic conditions such as multiple sclerosis. Following stimulation of the median nerve at the wrist, several different components are identified, originating in elements of the somatosensory pathways from the brachial plexus (N9 component) to primary somatosensory cortex (N20 component). The P14 component, which is generated at the level of the cervicomedullary junction, is important for anatomic localization of the abnormality in clinical SEP studies of patients in whom the cortical SEP component is delayed or absent. The P14 component is also vital during IOM as it is more resistant to the anesthetic effects than are the cortical SEPs,1 thus providing a way to assess spinal cord function in patients in whom the cortical SEPs are suppressed by anesthesia. If the structures at risk are rostral to the lower medulla, the P14 component can be used to verify that the caudal somatosensory pathways are still being activated in case the more rostrally generated SEPs deteriorate.2 If the cortical SEPs are lost during surgery at the level of the cervicomedullary junction or lower medulla (eg, Arnold-Chiari decompression), examination of the P14 component can help to localize the site of the intraoperative injury.
1 Department of Neurology, Montefiore Medical Center and the Albert Einstein College of Medicine, Bronx, NY, USA
2 Department of Neuroscience, Montefiore Medical Center and the Albert Einstein College of Medicine, Bronx, NY, USA
Corresponding Author:
Stephen J. Fried, Department of Neurology, Montefiore Medical Center, 111 East 210 Street, Bronx, NY 10467, USA Email: sfried@montefiore.org
Clinical EEG and Neuroscience 43(2)
Figure 1. Multiple spatiotemporal dipole model of SEPs to left median nerve stimulation. The spatiotemporal analysis was carried out on the SEP waveforms between 10 and 60 ms poststimulus. The equivalent dipole waveforms are shown at the left, and their orientations and locations within the head are shown in the cartoons (center and right). The residual variance, as a function of poststimulus time, is shown on a logarithmic scale (bottom center). Both ‘‘R’’ and “T” are the radial and tangential components of the cortical SEP components. The P14 field at the scalp was modeled by a single dipole (‘‘B’’) located in the region of the brain stem (reproduced with permission from Buchner et al11). SEP indicates somatosensory-evoked potential.
Early work in the field of SEPs showed that the P14 component is most likely generated in the medial lemniscus,3 with subsequent studies indicating origin in the most caudal part of the medial lemniscus; lesions at the level of the cervicomedullary junction or lower medulla abolish this component,4 whereas it is preserved in lesions of the upper medulla and above.5 analysis of the equivalent dipole of the P14 component shows that the dipole is oriented within 7 degrees of the anatomical orientation of the lemniscal–thalamic pathway.6
The P14 component is a far-field potential,7 with a wide distribution over the head and body and thus can be recorded using a variety of electrode derivations. Recordings between the parietal scalp and a noncephalic reference (eg, the shoulder contralateral to the stimulation) pick up not only the P14 component generated in the lower medulla but also a P13 component that may originate in the upper cervical spinal cord.8,9 Placement of the reference electrode on the earlobe ipsilateral to the stimulation selectively records the medullary P14 component without the P13. Valeriani et al9 reported that in a series of patients with rostral spinal cord pathology, use of the earlobe reference allowed for correct localization of the lesion to the upper cervical spinal cord, rather than an erroneous localization to above the level of the foramen magnum.
Current ASEP guidelines recommend recording the P14 component between a CPi electrode to the stimulated median nerve and a noncephalic reference electrode, typically the EPc. The CPi recording electrode also picks up the subcortical N18 component without contamination from the near-field N20, which is generated contralateral to the stimulus. The CPi-EPc recording derivation is also well suited for recording the equivalent dipole of the P14 component (labeled ‘‘B’’ in Figure 1)10 because there is 1 recording electrode (Cpi) on the positive side of the equivalent dipole and 1 electrode (EPc) on its negative side.
One of the potential drawbacks of this derivation, aside from the possibility described above of misreading the P13 as a P14, is the large amount of noise (EMG, EKG, etc) picked up from the noncephalic reference electrode, possibly obscuring the SEP. A reference that introduces less noise such as the inion may be preferable, although it would be expected to decrease the amplitude of the P14 component somewhat since the inion electrode is not optimally placed to pick up the maximum negativity produced by the P14 dipole (Figure 1). However, a frontal electrode might augment the signal by picking up the positive side of the dipole at higher amplitude. Therefore, we have also recorded the P14 component between the forehead (Fpz) and the inion, and herein we compare the SEPs recorded using this derivation to SEPs recorded using the derivation recommended in the ACNS guidelines.
Methods
We retrospectively reviewed median nerve SEP studies that were performed at the Montefiore Medical Center over a 2- year period in which both CPi-EPc and Fpz-inion derivations were included in the recording montage. Studies that were interpreted as normal, with normal peripheral conduction velocities and normal total central conduction times (N9-to-N20 interpeak intervals) following stimulation of either median nerve, were included in the analysis. The P14 component is typically smaller in amplitude than the N9 and N20 components and is sometimes obscured by noise (none of the patients was sedated). Thus, it is not considered an obligated component in our laboratory, and the presence of an identifiable P14 component was not required for an SEP study to be classified as normal. Normal SEPs were present in 74 individuals, and the SEPs to stimulation of 148 median nerves were therefore included in the analysis.
The SEP data were recorded using Protektor-evoked potential recording systems (XLTek Corporation, Oakville, Ontario, Canada). The median nerve was stimulated with 200 msduration square pulses delivered at a rate of 6.2 Hz, with the stimulus intensity titrated to achieve a visible thumb twitch. Data were filtered with a band-pass of 5 Hz to 3000 Hz for the cortical SEP recording channels and 30 Hz to 3000 Hz for the other recording channels. After automatic artifact rejection (using a single-sweep reject threshold of 100 mV), SEP waveforms with 2000 sweeps per average were recorded. The left and right median nerves were tested separately, and for each nerve, at least 2 SEP runs recorded and these were superimposed in order to establish reproducibility of the evoked potential components. The P14 amplitude was measured as the voltage difference between the P14 peak and following negative peak.
The P14 component was assessed in both the CPi-EPc and the Fpz-inion channels. In each waveform, the P14 component was classified as either identifiable or not identifiable. If it was identifiable, its peak latency and its amplitude were measured. Means, standard deviations, and percentage differences were calculated for the set of values in each derivation. These data sets were compared using the 2-tailed t test.
Results
Of the 74 individuals with normal SEP studies to stimulation of either median nerve (148 total studies), 54 were female and 20 were male. The average age of the individuals was 41 + 19 years, with a minimum of 11 years and a maximum of 83 years.
The results for the presence of an identifiable P14 component are shown in Table 1. In 7 of the 148 recordings, the P14 component was not identifiable in either channel (Figure 2). It was only identifiable in the CPi-EPc channel in 9 studies (Figure 3), and only identifiable in the Fpz-inion channel in 4 studies (Figure 4). In the example shown in Figure 4, the P14 component in the CPi-EPc channel is obscured by noise that is picked up by the noncephalic reference electrode.
In the remaining 128 studies where the P14 component was identifiable using both derivations (Figure 5), its latency was not always identical in the 2 channels. However, the average latency of the P14 component in the CPi-EPc derivation, across all of these normal SEP studies in which it was identifiable, was 13.2 + 1.1 ms, and the average latency of the P14 component in the Fpz-inion derivation, across all of these normal SEP studies in which it was identifiable, was also 13.2 + 1.1 ms. In 51 studies, the latency using the Fpz-inion was shorter; and in 77 studies, the latency using the CPi-EPc derivation was shorter. The mean absolute latency difference between the 2 derivations was 0.2 ms, with a maximum value of 1.1 ms.
In the studies wherein the P14 component was identifiable using both derivations, the average amplitude of the P14 component using the CPi-EPc derivation was 2.0 + 0.6 mV. This was significantly larger than the average amplitude of the P14 component using the Fpz-inion derivation, 1.2 + 0.6 mV (P < .001, t test, 2-tailed). On average, the amplitude of the P14 component using the CPi-EPc derivation was 38% larger than that using the Fpz-inion derivation; in individual studies, it was as much as 85% larger. However, in 12 studies, the P14 amplitude using the Fpz-inion derivation was actually larger than that using the CPi-EPc derivation (as much as 77% larger). The P14 amplitudes in the 2 derivations were equal in 7 recordings.
Discussion
Due to its far-field nature, the P14 component of the median nerve SEP can be recorded using a variety of recording electrode positions and derivations. The CPi-EPc derivation and the Fpz-inion derivation both record the same component, and this study shows that latency norms based on either derivation are interchangeable. Compared to the Fpz-inion derivation, the CPi-EPc derivation is more likely to yield an identifiable P14 component, and typically it yields a larger P14 component. Thus, it is appropriate to recommend that the CPi-EPc derivation be used in all clinical median nerve SEP studies. Another advantage of this derivation is that it will record the N18 component as well.
However, the Fpz-inion derivation occasionally yields a larger P14 component. More importantly, the P14 component in the CPi-EPc derivation can be obscured by noise coming from the noncephalic reference electrode; in that case, the P14 component would only be identifiable if an Fpz-inion derivation is used. Thus, an Fpz-inion recording channel is a useful adjunct during median nerve SEP testing.
During IOM, contamination of the SEP data by noise is typically more problematic than in a diagnostic SEP study performed in the evoked potential laboratory, and recordings between scalp and noncephalic electrodes may be unacceptably noisy due to the large distance between the paired electrodes. The Fpz-inion derivation can record the P14 component with a signal-to-noise ratio that is sufficient for IOM. Additionally, as this derivation can be used to record the P31 component to posterior tibial nerve stimulation, allocation of a separate channel will often not be required. If the inion is too close to the surgical field, such as during surgery at the level of the foramen magnum, an Fpz-earlobe or Fpz-mastoid derivation can also record a P14 component that is adequate for IOM (Figure 6).
In this study, we have shown that the Fpz-inion and the CPiEPc derivations are recording the same component. Although the P14 component recorded with the ACNS-recommended CPi-EPc derivation is typically larger than that recorded with the Fpz-inion derivation, in some cases, it may be smaller. Moreover, in other cases, it is obscured by noise from the noncephalic reference so that no identifiable P14 is present if only the CPi-EPc derivation is used. Use of the Fpz-inion derivation may yield an identifiable P14 component in such cases. Recording the P14 component using the Fpz-inion derivation is therefore a useful adjunct both during IOM and in the clinical/laboratory setting.
Authors’ Note
This study was presented at the 2010 Annual Meeting of the American Clinical Neurophysiology Society, February 5-7, 2010, San Diego, California.
Declaration of Conflicting Interests
Dr Fried is enrolled in the NIH loan repayment program.
Funding
The authors received no financial support for the research, authorship, and/or publication of this article.
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