Effect of cervical spinal cord lesions on early components of the median nerve somatosensory evoked potential

The contribution of neurophysiology in the diagnosis and management of cervical spondylotic myelopathy: a review

STUDY DESIGN

Topical review of the literature.

OBJECTIVE

The objective of this review article was to assess indications and usefulness of various neurophysiological techniques in diagnosis and management of cervical spondylogenic myelopathy (CSM).

METHODS

The MEDLINE, accessed by Pubmed and EMBASE electronic databases, was searched using the medical subject headings: 'compressive myelopathy', 'cervical spondylotic myelopathy (CSM)', 'cervical spondylogenic myelopathy', 'motor evoked potentials (MEPs)', 'transcranial magnetic stimulation', 'somatosensory evoked potentials (SEPs)', 'electromyography (EMG)', 'nerve conduction studies (NCS)' and 'cutaneous silent period (CSP)'.

RESULTS

SEPs and MEPs recording can usefully supplement clinical examination and neuroimaging findings in assessing the spinal cord injury level and severity. Segmental cervical cord dysfunction can be revealed by an abnormal spinal N13 response, whereas the P14 potential is a reliable marker of dorsal column impairment. MEPs may also help in the differential diagnosis between spinal cord compression and neurodegenerative disorders. SEPs and MEPs are also useful in follow-up evaluation of sensory and motor function during surgical treatment and rehabilitation. EMG and NCS improve the sensitivity of cervical radiculopathy detection and may help rule out peripheral nerve problems that can cause symptoms that are similar to those of CSM. CSP also shows a high sensitivity for detecting CSM.

CONCLUSION

Neuroimaging, especially magnetic resonance imaging, represents the procedure of choice for the diagnosis of CSM, but a correct interpretation of morphological findings can be achieved only if they are correlated with functional data. The studies reported in this review highlight the crucial role of the electrophysiological studies in diagnosis and management of CSM.

Steady-state activation in somatosensory cortex after changes in stimulus rate during median nerve stimulation

Passive electrical stimulation activates various human somatosensory cortical systems including the contralateral primary somatosensory area (SI), bilateral secondary somatosensory area (SII) and bilateral insula. The effect of stimulation frequency on blood oxygenation level-dependent (BOLD) activity remains unclear. We acquired 3-T functional magnetic resonance imaging (fMRI) in eight healthy volunteers during electrical median nerve stimulation at frequencies of 1, 3 and 10 Hz. During stimulation BOLD signal changes showed activation in the contralateral SI, bilateral SII and bilateral insula. Results of fMRI analysis showed that these areas were progressively active with the increase of rate of stimulation. As a major finding, the contralateral SI showed an increase of peak of BOLD activation from 1 to 3 Hz but reached a plateau during 10-Hz stimulation. Our finding is of interest for basic research and for clinical applications in subjects unable to perform cognitive tasks in the fMRI scanner.

Somatosensory evoked potentials after multisegmental lower limb stimulation in focal lesions of the lumbosacral spinal cord

OBJECTIVES

Recording techniques permit the separate analysis of the response from cauda equina roots and the spinal potential that is probably generated by the activation of dorsal horn cells. To improve the functional assessment of focal lesions of the lumbosacral cord, lower limb somatosensory evoked potentials (SEPs) were measured by multisegmental stimulation.

METHODS

Common peroneal and tibial nerves SEPs were recorded in 14 patients in whom MRI demonstrated compressive cord damage ranging from T9 to L1 levels. SEPs were recorded in each patient at the lumbar level (cauda equina response), lower thoracic level (spinal response), and from the scalp (cortical response).

RESULTS

Abnormalities in spinal response occurred in 50% and 70% of tibial and common peroneal nerve SEPs respectively; these findings were well explained by the radiological compression level, involving in most of the patients lumbar rather than sacral myelomeres. The SEPs were often more effective than the clinical examination in showing the actual extension of damage.

CONCLUSIONS

The recording of spinal SEPs after multisegmental lower limb stimulation proved useful in assessing cord dysfunction and determining the cord levels mainly involved by the compression.

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.

Open-door laminoplasty for cervical stenotic myelopathy: surgical technique and neurophysiological monitoring

OBJECT

A modified technique of open-door laminoplasty for cervical stenotic myelopathy (CSM) is described, and the role of evoked potential monitoring in selecting patients for surgery and evaluating results is discussed.

METHODS

Between October 1992 and October 1996, 33 patients with CSM underwent open-door laminoplasty. After surgery, in 27 patients (81.8%) different levels of clinical improvement were demonstrated, and in five of them (15%) full recovery was observed. The Japanese Orthopaedic Association score increased from 5 to 12 (mean 9.8) preoperatively to 8 to 14 (mean 11.6) postoperatively. At 1-year follow up, the N13 cervical response was restored in nine (75%) of 12 patients with isolated presurgical abnormality and in 57.1% of those with combined abnormalities of both N13 and P14 response. Although significant clinical improvement was observed in 82% of the cases, in 24 of 33 patients motor evoked potential abnormalities persisted at least at one explored level.

CONCLUSIONS

Of several laminoplasty techniques, the one described here offers some advantages: preservation of biomechanical function of posterior muscular-ligamentous complex, prevention of laminar collapse, smaller degrees in reduction of range of cervical motion, stabilization of the spine with no postoperative malalignment, and maintenance of decompressive effect that avoids recurrent stenosis. Neurophysiological studies sometimes clarified neurological disorders that were only suspected on the basis of history and/or clinical examination, leading to early diagnosis.

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.

Selective abnormality of the N13 spinal SEP to dermatomal stimulation in patients with cervical monoradiculopathy

Scalp somatosensory evoked potentials (SEP) to dermatomal stimulation have so far proved to be only partially useful in the diagnosis of monoradiculopathy, mostly in cases without motor impairment. The aim of our study was to test the sensitivity of the spinal N13 potential in uncovering lesions of single cervical roots. We studied five patients suffering from cervical monoradiculopathy, using a recording technique allowing specific recording of the genuine N13 potential which is probably generated by dorsal horn cells. No patient showed signs of muscle impairment and needle EMG was always normal. In four patients, the N13 SEP was absent following stimulation of the dermatome corresponding to the damaged root, while both the lemniscal P14 and the cortical N20 components were normal. SEP recorded after stimulation of upper limb troncular nerves showed no abnormality in all patients. Our findings suggest that the N13 potential, the loss of which after dermatomal stimulation could be due to deafferentation of dorsal horn neurones, is particularly sensitive to initial root compression. Therefore, our montage allowing analysis of the genuine N13 SEP can improve the sensitivity of dermatomal SEP recording in patients with cervical monoradiculopathies.

Generators of short latency human somatosensory-evoked potentials recorded over the spine and scalp

Somatosensory evoked potentials (SEPs) are most commonly obtained after stimulation of the median nerve and the posterior tibial nerve. SEPs reflect conduction of the afferent volley along the peripheral nerve, dorsal columns, and medial lemniscal pathways to the primary somatosensory cortex. Short-latency SEPs are recorded over the spine and scalp. After posterior tibial nerve stimulation, the following waveforms are recorded: N22, W3, the dorsal column volley, N29, P31, N34, and P37. After median nerve stimulation, the brachial plexus volley, dorsal column volley (N11), N13, P14, N18, N20, and P22 potentials are recorded. We discuss the current state of knowledge about the generators of these SEPs. Such information is crucial for proper interpretation of SEP abnormalities.

Long sensory tracts (cuneate fascicle) in cervical somatosensory evoked potential after median nerve stimulation

Low amplitude high frequency waves (LHW) were investigated in normal and patient cervical somatosensory evoked potentials after median nerve stimulation (CSEP) in parallel to normal and patient conducted somatosensory evoked potentials (SEP) after tibial nerve stimulation. Normal recordings were obtained in five subjects undergoing dorsal root entry zone (DREZ) coagulation for pain relief. Patient recordings were obtained in 11 subjects suffering from either syringomyelia, spinal cord tumour, or both. All recordings were made intraoperatively from the dorsal spinal cord surface using the subpial recording technique. Normal CSEP showed typical triphasic potential starting with an initial P9, followed by N13 and a final positivity, P1. Numerous LHW were superimposed on slow triphasic potential. To improve the visibility of LHW, slow triphasic potential was removed from the original CSEP. Potentials thus obtained contained only high frequency components of CSEP, i.e. LHW. They were compared with conducted SEP after tibial nerve stimulation. Comparison revealed similarities in high frequency, low amplitude and general wave form, LHW thus showing characteristics of conducted potential. Duration was found to be significantly shorter than normal duration in both patient LHW (Student's t-test, P < 0.0005) and patient conducted SEP (Student's t-test, P = 0.064). A shorter duration was associated with worsening of configuration in patient LHW and patient conducted SEP. These changes of LHW could not be connected with distortion of N13 seen in patient CSEP. A shorter duration and worsening of configuration in patient LHW were most prominent in cases with a loss of vibration and posture senses, but were also observed in cases where only pain and temperature senses were affected. We therefore concluded that cuneate fascicle is the most likely generator of LHW, although the participation of other cervical long sensory tracts, e.g. spinothalamic tract, cannot be ruled out.

Diagnostic utility of somatosensory evoked potentials (SEPs) in presurgical assessment of cervical spondylotic myelopathy

Median and tibial nerve SEPs were recorded in 27 patients with spondylotic myelopathy. SEP data were compared with clinical and MRI data. SEPs were abnormal in all but five patients. Segmental dysfuction of the cervical cord was observed in 12 patients (45%), whereas abnormality of dorsal column conduction was observed in 10 (37%) and 16 (59.3%) patients after median and tibial nerve stimulation, respectively. No clear correlation was found between the severity of MRI abnormalities and that of clinical presentation or SEP abnormalities. However, there was no patient with normal SEPs and severe MRI abnormalities, including narrowing of cervical cord diameter or impingement of the cervical cord or intramedullary T2 signal hyper intensity. Conversely, 8 of the 13 patients with no evidence of cord narrowing or T2 signal abnormality showed abnormal SEPs. This divergence between SEP and MRI data suggests that SEP recording should be included in presurgical assessment of these patients, especially those without clear MRI evidence of cervical cord compression.

Median nerve somatosensory evoked potentials in cervical syringomyelia: correlation of preoperative versus postoperative findings with upper limb clinical somatosensory function

Median nerve somatosensory evoked potentials (SEPs) were recorded in 30 patients with cervical syringomyelia before and after surgery. The different SEP components were compared with clinical somatosensory findings. The N13 potential (generated in the dorsal horn at C5-C6) was pathological in 85% of the upper extremities, or 90% of the patients, and correlated with pain/temperature as well as vibration/joint position sense; it was of higher sensitivity in syringomyelia than any other clinical symptom or SEP component. P14 (brain stem) and N20 (postcentral cortex) were less often affected and correlated with only vibration/joint position sense. Short-term postoperative clinical or SEP changes were most often seen after syringoendoscopy and less often after syringostomy, resection of cerebellar tonsils, or tumor extirpation. Alterations of SEPs after surgery occurred in more patients (60%) than did changes in clinical condition (approximately 27%); there was, however, no general correlation between these findings. We conclude that median nerve SEP testing with a proper recording technique identifying the different subcortical components is a valuable supplement in the pre- and postoperative diagnostic evaluation of syringomyelia and is of higher sensitivity than clinical somatosensory examination alone.

The role of upper limb somatosensory evoked potentials in the management of cervical spondylotic myelopathy: preliminary data

We studied upper limb somatosensory evoked potentials (SEPs) in 11 patients with MRI and clinical evidence of cervical spondylotic myelopathy (CSM), before and after cervical open-door laminoplasty. SEP studies before surgery revealed two main types of abnormality, the first characterized by the isolated loss of the spinal N13 response, reflecting the dysfunction of dorsal horn cervical cells in 4 patients. The second combined abnormalities of both spinal N13 and scalp far-field P14 potential, suggesting the involvement of both dorsal horn cells and dorsal columns at the cervical level in 7 patients. After surgery, N13 recovered in 9 patients, while P14 abnormalities remained unchanged. Clinical recovery, evaluated by means of the Japanese Orthopaedic Association (JOA) disability scale, was accompanied by SEP improvement. Moreover, this improvement was more pronounced in patients with isolated loss of the N13 than in patients with combined abnormalities of the N13 and scalp P14 response. Our data strongly suggest that upper limb SEPs can be useful in monitoring the effectiveness of surgery, as well as in selecting before surgery patients who are likely to have a better postsurgical outcome.

Spinal somatosensory evoked potentials in patients with tethered cord syndrome

We studied the electrophysiological changes occurring in six patients with tethered cord syndrome. Evidence of spinal malformations was provided by magnetic resonance imaging. The functional assessment of the spinal cord was performed by analysing both spinal and cortical somatosensory evoked potentials. The evoked electrospinogram was recorded from the thoracic and lumbosacral spinous processes. The N22 lumbosacral potential was selectively affected, being rostrocaudally displaced and reduced in amplitude or even absent in patients with neurological signs indicating a segmental lower cord lesion. Inter-peak somatosensory evoked potentials latency was normal in all cases, suggesting that ascending axonal potentials in the dorsal column fibres may be synchronized. Segmental potentials of the lumbosacral response, originating from the post-synaptic activity of dorsal horn interneurons, are selectively affected in this syndrome resulting from the rostrocaudal displacement of the spinal cord due to tethering. Our findings in the clinical field are consistent with previous experimental evidence indicating a different sensitivity of the gray vs. white matter to progressive stretching.

Somatosensory evoked potentials after multisegmental upper limb stimulation in diagnosis of cervical spondylotic myelopathy

Radial, median, and ulnar nerve somatosensory evoked potentials (SEPs) were recorded, with non-cephalic reference montage, in 38 patients with clinical signs of cervical myelopathy and MRI evidence of spondylotic compression of the cervical cord. Upper limb SEPs are useful in spondylotic myelopathy because SEPs were abnormal in all patients for at least one of the stimulated nerves and SEP abnormalities were bilateral in all patients but one. Reduction of the amplitude of the N13 potential indicating a segmental dysfunction of the cervical cord was the most frequent abnormality; it occurred in 93.4%, 84.2%, and 64.5% of radial, median, and ulnar nerve SEPs respectively. A second finding was that the P14 far-field potential was more sensitive than the cortical N20 potential to slowing of conduction in the dorsal column fibres. The high percentage of N13 abnormalities in the radial and median rather than in the ulnar nerve SEPs correlated well with the radiological compression level, mainly involving the C5-C6 vertebral segments. Therefore the recording of the N13 response is a reliable diagnostic tool in patients with cervical spondylotic myelopathy and P14 abnormalities, though less frequent, can be useful in assessing subclinical dorsal column dysfunction.

Discrepancy between SEPs directly recorded from the dorsal column nuclei following upper and lower limb stimulation in patients with syringomyelia

Somatosensory evoked potentials (SEPs) in response to electrical stimulation of the median nerve (MN) and posterior tibial nerve (PTN) were studied in 2 patients with syringomyelia. Intraoperative recordings were made from the surface of the dorsal column nuclei as well as from the scalp. Following MN stimulation, there was a preservation of scalp-recorded P9, P11, P13, and N20, however, there was an absence of spinal N13-P13. The dorsal column SEPs to MN stimulation were normal, characterized by a major negativity (N1), preceded by a small positivity (P1) and followed by a large positivity (P2). On the other hand, there was little or no cortical response (P37) to PTN stimulation. The dorsal column SEPs to PTN stimulation showed a disappearance of the normal P1'-N1'-P2' configuration, being replaced by a series of small spiky waves. The syringomyelic cavity may have thus compressed the gracile dorsal column which courses more medially than the cuneate pathway, causing desynchronization of the dorsal column SEPs. These findings suggest that dorsal column pathway arising from the lower limb is more vulnerable than that from the upper limb when a cervical syrinx is present.

Electric and CO2 laser SEPs in a patient with asymptomatic syringomyelia

We recorded electrically stimulated somatosensory evoked potentials (electric SEPs) and pain-related SEPs following CO2 laser stimulation (CO2 laser SEPs) from a 17-year-old patient affected by myotonic dystrophy whose MRI disclosed a large syrinx extending from spinal level C2 to S3. Careful clinical and electromyographic examinations revealed no motor or sensory disturbances, apart from myotonia. The only abnormality noted in median and ulnar nerve short-latency electric SEPs (recorded with a non-cephalic reference electrode) was the absence of cervical component N13, the other SEP responses (N9, N10, N11, P14, N20) being normal. The cutaneous pain threshold and CO2 laser SEPs (both obtained by a CO2 laser beam applied to the back of the hand) were normal. Thus cervical component N13 appears to be highly sensitive to the effects of central cord lesions, even when these are asymptomatic.

SEPs and CNS magnetic stimulation in syringomyelia

Somatosensory evoked potentials (SEPs) and motor evoked potentials (MEPs) to transcranial and spinal stimulation from upper and lower limb muscles were elicited in 13 patients with syringomyelia. Seven had an associated Chiari type I anomaly. Diagnosis was confirmed by MRI. In 5 cases, SEPs and MEPs were performed before and after surgical treatment. Prolonged central motor conduction times or absent motor responses in upper or lower limbs were found in most patients. The greatest number of abnormalities was disclosed by measurement of CMCT followed by SEPs after tibial nerve stimulation. Two of 5 cases undergoing surgery improved clinically and showed reduction in CMCT after surgical treatment. Our study shows that MEPs were useful in the evaluation of neurophysiological status in syringomyelia patients, helping to estimate anterolateral spinal cord function.

Origin of the widespread N18 in median nerve SEP

The widespread N18 potential in median nerve SEP was studied in normal subjects and in patients with high cervical, brain-stem and thalamic lesions who had profound disturbances of deep sensation. N18 was well identified in the HSi-CV2 derivation in every normal subject as a broad elevation from the baseline lasting about 20 msec. The cortical N20 was absent in all patients. N18 was absent in a patient with a dorsal column lesion at C1-2 level. The amplitude and configuration of N18 were normal in all other patients with brain-stem and thalamic lesions, including a patient with a lesion at the ponto-medullary junction. The sagittal distribution of N18 was studied in a patient with a thalamic lesion and an oblique distribution with the maximum region between Cz and nasion was demonstrated. The present results indicate that at least the greater part of N18 is generated at the caudal most brain-stem or through branches from this level. Taking previous animal and intraoperative studies into consideration, we think it most probable that the main part of N18 corresponds to the ventro-rostral negative pole of the dipolar potential generated at the cuneate nucleus by the primary afferent depolarization of presynaptic terminals of dorsal column fibers.

AAEM case report #24: electrodiagnosis in posttraumatic syringomyelia

An adult male with C-7 quadriplegia developed neck pain. Axillary F central latencies were prolonged, and MRI showed a syrinx extending to C-1. After shunting, F latencies normalized. At subsequent follow-up, a rostral syrinx persisted by magnetic resonance imaging (MRI); motor evoked potential (MEP) latencies were prolonged but F latencies were normal. Later, the syrinx was less distended by MRI, MEPs normalized, and strength improved. We discuss the electrophysiologic methods available for diagnosing and monitoring posttraumatic syringomyelia.

Amplitude abnormalities in the scalp far-field N18 of SSEPs to median nerve stimulation in patients with midbrain-pontine lesion

Various amplitude ratios were measured in 20 normal controls and 36 patients with midbrain-pontine, thalamic or putaminal lesions in order to evaluate the amplitude abnormalities in scalp far-field N18 following median nerve stimulation. A study of normal controls showed that the distributions of P9/N18, P14/N18 and N18/P14 + N18 resembled a gaussian distribution and could be used as criteria for determining the decrease in N18 amplitude in each patient. There was a decrease in N18 amplitude, or the absence of N18, in patients with midbrain-pontine lesions, but not in those with thalamic or putaminal lesions. Nine amplitude ratios (P11/P9, P14/P9, N18/P9, P9/P11, P9/P14, P9/N18, N18/P14, P14/N18 and N18/P14 + N18) were compared statistically for normal controls and 3 groups of patients based on non-parametric, Wilcoxon's non-pairs and signed-rank tests. A decrease in N18 amplitude in midbrain-pontine lesion was shown by significant changes in N18/P9, P9/N18, N18/P14, P14/N18 and N18/P14 + N18, no amplitude decreases in P11 and P14 being found from the amplitude ratios of P11/P9, P9/P11, P14/P9 and P9/P14. No significant changes were seen in any of the 9 amplitude ratios when the normal controls and patients with thalamic and putaminal lesions were compared. The amplitude ratios of N18 can be used to detect a decrease in N18 amplitude in patients with midbrain-pontine lesions. The data obtained support the hypothesis that N18 originates in the midbrain-pontine region and that neither the thalamus nor thalamocortical radiation make major contributions to the formation of the N18 peak.

Somatosensory and spinal evoked potentials in patients with cervical syringomyelia

We recorded somatosensory evoked potentials (SEPs) from the skin surface and spinal evoked potentials (SpEP) from the posterior epidural space after median nerve stimulation in 11 patients with cervical syringomyelia. We compared SEPs with SpEP to assess the possible feasibility of using these techniques to localize the offending lesion. SEP abnormalities were present on one or both sides of 7 patients (9 of 22 upper limbs; 41%). The abnormal SEP pattern was the attenuation or loss of N13, which was of little value for delineating the lesion. In patients with abnormal SEPs, three types of abnormal SpEPs were noted. In the Type A abnormality (three limbs), potentials were attenuated in all cervical segments, suggesting that the syrinx itself had enlarged to involve the posterior column. In Type B (two limbs), there was reduced amplitude or absent upper cervical potentials, probably a result of the accompanying tonsilar herniation. Finally, the Type C (four limbs) abnormality was a mixture of Type A and B abnormalities in that the attenuated cervical potentials were again affected in the most upper cervical segment. We concluded that SpEP revealed various kinds of involvement of the dorsal column pathway in the syringomyelic patients, a finding not expressed with conventional SEPs.

Inadequacy of the forehead reference montage for detecting abnormalities of the spinal N13 SEP in cervical cord lesions

Cervical somatosensory evoked potentials (SEPs) recorded using forehead and anterior cervical reference montages were compared in 6 patients whose MRI showed a cervical syrinx. All patients presented with a segmental loss of pain and temperature sensation in upper limbs, but no clinical evidence of dorsal column system dysfunction. Cervical SEPs recorded using the forehead reference montage were normal in all cases, while the N13 potential recorded using an anterior cervical reference was reduced, or absent, in 11 median nerve SEPs out of 12. This discrepancy results from persisting scalp P13-P14 far-field potentials, which were picked up by the forehead, but not by the anterior cervical, reference. It is concluded that the forehead reference montage is inadequate for assessing selectively the spinal N13 potential and should be abandoned for cervical SEP recording.

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

A new method has been developed for recording short-latency somatosensory-evoked potentials after median nerve stimulation. Negative electrical forces recorded with three orthodiagonal bipolar electrodes in the neck had a direction opposite to that of impulse conduction in the proximal peripheral and cervical somatosensory pathway. Sequential tracings of vectors opposite the electrical forces were made in three-dimensional display, thus reproducing the actual time sequence of electrical events in those structures. Fixed generators such as the subcortical nuclei were also analyzed with this technique, and multiple generators of N13 potential (N13a and N13b) were visualized. This technique may be useful in the functional evaluation of the somatosensory pathway in the cervical cord.

Skin and epidural recording of spinal somatosensory evoked potentials following median nerve stimulation: correlation between the absence of spinal N13 and impaired pain sense

A clinical lesion study and intraoperative epidural recordings were made to test the origin and clinical significance of the spinal N13 and P13 of somatosensory evoked potentials (SEP) that follow median nerve stimulation. Intraoperatively, the respective peak latencies of spinal P13 and N13 coincided with those of the N1 component of the dorsal cord potential and its phase reversed positivity. On both the ventral and dorsal sides of the cervical epidural space, maximal amplitude was at the C5 vertebral level to which nerve input from the C6 dermatome is the main contributor. The modality of sensory impairment in the hand dermatome was examined in selected patients with cervical lesions, who showed such normal conventional SEP components as Erb N9, far-field P9, P11, P14, N18 and cortical N20, with or without loss of spinal N13. Statistically, the loss of spinal N13 was associated with decrease of pain sensation in the C6 dermatome. This was interpreted as being due to damage to the central grey matter of the cord, including the dorsal horn. Our results suggest the spinal N13 and P13 originate from the same source in the C6 spinal cord segment and that they are good indicators for the detection of centromedullary cervical cord damage.

Somatosensory evoked potentials and magnetic resonance imaging in syringomyelia

Somatosensory evoked potentials (SEPs) to median and posterior tibial stimulation were obtained in 22 patients with syringomyelia. All patients had magnetic resonance imaging (MR) which defined the maximum transverse diameter of the syrinx as well as its longitudinal extension. SEP was abnormal in 16 (72%) patients. Median and posterior tibial SEPs were abnormal in 11 and 15 patients respectively. Both tests were abnormal in 10 patients. Ten patients showed absence of one or more central potentials (P/N13, N20, N22) and 7 patients demonstrated increased conduction times (N9-N20, P/N13-N20, N22-P40). The mean maximum transverse diameter of the syrinx was 7.5 mm in patients with normal SEPs and 16.2 mm in patients with abnormal SEPs. Abnormal SEP was observed in all 5 patients with loss of position sense, in 9 of 13 (69%) with loss of superficial pain and temperature, and 1 of 2 patients with motor deficit only. Central SEP abnormalities were observed in 3 of 5 patients with sensory deficits indistinguishable from a peripheral neuropathy and in 2 patients in the asymptomatic extremity. Three of 4 patients with syringomyelia and Chiari malformation had a normal SEP.

Somatosensory evoked potentials and magnetic resonance imaging in intraspinal neoplasms

Median and posterior tibial somatosensory evoked potentials (SEPs) were studied on 25 patients with pathologically proven intraspinal neoplasms, and the results were compared and correlated with the details of clinical examination and the information derived from magnetic resonance imaging (MR). MR was abnormal in all cases and in 23 of 25 (92%) demonstrated an intraspinal expansile lesion. SEP was abnormal in 19 of 25 patients (76%). Abnormal SEPs were found in 18 of 19 patients (94%) with cervical or thoracic neoplasms but only in 1 of 6 patients (16%) with the tumor in the thoracolumbar or lumbar region. SEP-MR correlation was significant (P less than 0.05) for thoracic intraspinal neoplasms where all 9 had an abnormal SEP showing a similar pattern of normal median and abnormal posterior tibial study. Clinically, all 7 patients with posterior column sensory deficits had abnormal SEP (100%). Abnormal SEPs were seen in 7 of 11 (63%) patients with spinothalamic deficits and in 4 of 8 (50%) of those with normal sensory examinations. Four of 9 patients (44%) with a normal neurological examination or an examination disclosing ambiguous results indistinguishable from a peripheral pathology had an abnormal SEP strongly suggesting a central sensory disorder. Comparison of preoperative and postoperative SEPs did not disclose useful prognostic information pertaining to the functional recovery.

Posterior cervical N13 in median nerve SEP has two components

Somatosensory evoked potentials (SEPs) to median nerve stimulation were investigated in normal controls and patients with cervical lesions. Attention was paid primarily to the N13 and P13 components in the posterior and anterior cervical records with non-cephalic references. In normal subjects the CV2 and CV6 electrodes registered N13 with almost the same amplitude. Dissociation between N13 at the CV2 electrode (ucN13) and N13 at the CV6 electrode (lcN13) was observed in the patients. In 4 patients with cervical dorsal column lesions, lcN13 was preserved but ucN13 was almost completely absent. Anterior cervical P13 (acP13) was preserved. In a patient with syringomyelia, lcN13 and acP13 were greatly attenuated while ucN13 was relatively well preserved. These results suggested that the origins of ucN13 and lcN13 are different. The generator of lcN13-acP13 was assumed to be the postsynaptic potential of the dorsal horn interneurons. Upon comparison with previous animal studies and intraoperative studies, it was concluded that the generator of ucN13 is the postsynaptic potential of the cuneate nucleus.

Somatosensory evoked potentials in a patient with a cervical glioma and syrinx

Short-latency somatosensory evoked potentials (SSEPs) were obtained in a 61-year-old patient with a cervical spinal cord glioma extending from C3 to C7. SSEPs in response to tibial nerve stimulation were normal; however, all components of the median nerve responses were markedly abnormal bilaterally (except for normal brachial plexus responses). This selective abnormality of median nerve SSEPs has not been reported previously in a patient with cervical cord tumor associated with a syrinx and illustrates the use of the SSEP in the anatomical localization of the lesion.

Absence of spinal N13-P13 and normal scalp far-field P14 in a patient with syringomyelia

Short latency somatosensory evoked potentials to median or ulnar nerve stimulation were recorded in a patient with syringomyelia. Scalp-recorded far-field P14 was clearly preserved, but spinal N13-P13 components disappeared. Our findings support the hypothesis that spinal N13-P13 is generated by structures intrinsic to the cervical cord, most likely in the ventral central gray matter.