Vector short-latency somatosensory-evoked potentials after median nerve stimulation

Maturation of near-field and far-field somatosensory evoked potentials after median nerve stimulation in children under 4 years of age

OBJECTIVES

The maturation of subcortical SEPs in young children.

METHODS

Median nerve SEPs were recorded during sleep in 42 subjects aged 0-48 months. Active electrodes were at the ipsilateral Erb's point, the lower and upper dorsal neck, and the frontal and contralateral centroparietal scalp; reference electrodes were at the contralateral Erb's point, the ipsilateral earlobe and the frontal scalp; bandpass was 10-3000 Hz. The peaks were labelled by their latencies in adults.

RESULTS

The peak latencies of N9 (brachial plexus potential) decreased exponentially with age during the first year, but increased with height thereafter. The interpeak latencies (IPLs) N9-N11, which measure conduction between brachial plexus and dorsal column, decreased with age (linear regression). The IPLs N11-P13 and N11-N13b, which measure conduction between the dorsal column and approximately the cervico-medullary junction, did not change across this age range. The IPLs N13a-N20, N13b-N20 and P13-N20, which measure central conduction, showed negative exponential regressions with rapidly decreasing latencies during the first year of life and slowly decreasing latencies thereafter.

CONCLUSIONS

Maturation of the peripheral segments of the somatosensory pathway progresses more rapidly than that of the central segments. The maturation of central conduction is not completed within the first 4 years of age. Our maturational data may serve as a reference source for subsequent developmental and clinical studies.

Anatomic origin of the cervical N13 potential evoked by upper extremity stimulation

There is a large consensus, based on converging evidence, that N13 recorded at lower cervical levels has a segmental postsynaptic origin in the gray matter of the cervical cord and that because of the orientation of its dipole field, the Cv6-anterior cervical derivation should be used whenever the diagnostic problem requires that this potential be assessed selectively in terms of latency and amplitude. The diagnostic utility of the lower cervical N13 recording in dorsal horn deafferentation and in lesions at the Cv6-Cv8 metameric levels has been validated in all types of cervical cord lesions. Unfortunately, such clear-cut conclusions do not apply to the N13 potential recorded at upper cervical levels. Currently, this component is not considered to provide enough reliable information, in addition to P13-P14 scalp recordings, to be used routinely in the diagnosis of cervicomedullary lesions.

Correlation between spinal cord compression and abnormal patterns of median nerve somatosensory evoked potentials in compressive cervical myelopathy: comparison of surface and epidurally recorded responses

To investigate the correlation between the level of spinal cord lesion and the abnormal pattern of median nerve somatosensory evoked potentials (SSEPs), evoked spinal cord potentials (ESCPs) were also recorded from the posterior epidural space intraoperatively in 18 patients with compressive cervical myelopathy. Levels of symptomatic spinal cord compression were determined by ESCP findings. Spinal N13 potential of the SSEPs was recorded from the surface of the posterior neck with anterior neck reference. Brainstem P14 and cortical N20 potential were recorded from the parietal scalp contralateral to the stimulated side. Spinal N13, P14, and N20 potentials were all normal when the ESCPs were abnormal at localized segmental region (C4-5 or C5-6 level alone). Spinal N13 potential was significantly attenuated in all of patients with abnormal ESCP findings at widespread segmental area of the median nerve territory. In four of these seven patients, brainstem P14 potential was also prolonged or diminished, but three patients showed normal P14 and N20 potentials. Isolated P14 abnormality with normal spinal N13 potential was characteristic in patients with abnormal ESCP at the C3-4 lesion. Although sensitivity of abnormal ESCP was higher than that of the SSEPs, abnormal patterns of spinal N13, P14 and N20 potentials following median nerve stimulation were useful in detecting not only the pathology (posterior horn and/or posterior column) but also symptomatic spinal compression level in compressive cervical myelopathy.

Dissociation between upper and lower neck N13 potentials following paired median nerve stimuli

Cervical N13 potential in response to the median nerve stimulation can be recorded either from upper (Cv2) or lower (Cv6) neck with almost equal amplitudes and latencies. It has long been debated whether they represent the same or different generator sources. Using a conditioning-test paired stimuli paradigm, we examined the differences of recovery function of Cv2- and Cv6-N13, anterior neck (AN)-P13, and scalp recorded P13/P14 in 6 healthy subjects. All cervical electrodes were referenced to the non-cephalic site. Scalp response was recorded with linked ear reference. The inter-stimulus intervals ranged from 4 to 20 ms with 2 ms increments. Throughout 4 to 18 ms ISI, Cv6-N13, AN-P13 and scalp P13/P14 were suppressed, whereas Cv2-N13 was facilitated. All but scalp P13/P14 returned close to the control at 20 ms ISI. The findings indicate that Cv2-N13, Cv6-N13 and scalp P13/P14 are independent each other and arise from different generator sources. The suppression of Cv6-N13 is consistent with a postsynaptic nature of this potential and may indeed be mediated through dorsal horn interneurons creating a current field orientation in the posterior-anterior direction. The facilitation of Cv2-N13 suggests that this is a presynaptic potential and may travel through the dorsal column with vertical orientation. The longer period of suppression of scalp P13/P14 suggests it to be of polysynaptic origin and to arise at least rostral to the cuneate nucleus.

Neural generators of tibial nerve P30 somatosensory evoked potential studied in patients with a focal lesion of the cervicomedullary junction

The tibial nerve P30 potential was studied in 6 patients with focal lesions located in the vicinity of the cervicomedullary junction. P30 potential was unaffected while cortical P39 was abnormal in the patients with a supramedullary lesion affecting the somatosensory pathway just above its decussation. Conversely, P30 was abnormal in the presence of a lesion situated caudally to the cervicomedullary junction affecting the lower limb sensory fibers just below their decussation. Median nerve P14 behaved similarly to the P30 potential in these cases. These clinical observations suggest that P30 potential, as P14 of median nerve somatosensory evoked potentials, is generated in the lower brain stem probably before the decussation of the sensory fibers; nucleus gracilis and medial lemniscus fibers in the lower brain stem are probably the anatomical structures generating P30 potential. This suggests that P30 potential may be used to study intraspinal and intracranial conduction times separately in the afferent somatosensory pathways.

Origin and distribution of P13 and P14 far-field potentials after median nerve stimulation. Scalp, nasopharyngeal and neck recording in healthy subjects and in patients with cervical and cervico-medullary lesions

We studied median nerve SEPs in 10 healthy subjects, by means of simultaneous recording over the scalp, around the neck and near the ventral surface of the medulla using a nasopharyngeal (NP) electrode. This recording technique enabled us to clearly differentiate P13 and P14 potentials. The former was always found in NP records, while the latter was more evident in scalp traces. The same technique was used to study 9 patients with various lesions of the cervical cord or cervico-medullary junction. Patients with high cervical lesions demonstrated abnormalities of both P13 and P14 potentials, while patients with lesions of the cervico-medullary junction demonstrated a clear dissociation between normal P13 in scalp and NP traces, and abnormal scalp P14. Patients with lower cervical lesions, selectively involving the central grey matter, showed normal P13 and P14 potentials, in spite of abnormal N13 cervical responses. Our findings strongly suggest that both scalp and NP P13 have the same generators in higher segments of the cervical cord, and that NP more than scalp records are effective in analyzing the P13 response. We suggest that the selective recording of the P13 potential could be useful in the assessment of focal lesions of the higher cervical cord or of the cervico-medullary junction.

Two distinct cervical N13 potentials are evoked by ulnar nerve stimulation

To investigate the dual nature of the posterior neck N13 potential, we attempted to establish the presence of a latency dissociation between caudal (cN13) and rostral (rN13) potentials on stimulating the ulnar nerve, in view of its lower radicular entry compared to the median nerve. SEPs were evaluated in 24 normal subjects after both median and ulnar nerve stimulation. cN13 was prominent in the lower cervical segments, and rN13 was localized mainly in the upper ones using anteroposterior and longitudinal bipolar montage, respectively. The N9-cN13 interpeak latency did not differ significantly from N9-rN13 when stimulating the median nerve. On the other hand, the N9-rN13 interpeak was significantly longer than the N9-cN13 interpeak when the ulnar nerve was stimulated. The rN13 presented the same latency as P13-P14 far-field potentials in 17 out of 24 ulnar nerves tested. Therefore, the ulnar nerve stimulation evokes two distinct posterior neck N13 potentials. It is widely accepted that the caudal N13 is a postsynaptic potential reflecting the activity of the dorsal horn interneurons in the lower cervical cord. We suggest that the rostral N13 is probably generated close to the cuneate nucleus, which partly contributes to the genesis of P13-P14 far-field potentials.

[Evoked potentials by median nerve stimulation (SSEP): subcortical components]

Este estudo constitui uma revisão de literatura realizada com a finalidade de se relacionar a designação, as características dos campos de potencial e os geradores implicados, para os componentes subcorticais do potencial evocado somatossensorial por estimulação do nervo mediano no punho.