Spinal segmental somatosensory evoked potentials in lumbosacral radiculopathies

We studied 21 patients with lumbosacral radiculopathy with segmental somatosensory evoked potentials (SEPs) recorded over both spine and scalp following saphenous, superficial peroneal, and sural nerve stimulation. Spinal SEPs were abnormal in 10 patients. In 3 patients, SEPs detected abnormalities not seen on EMG examination. With 1 exception, all anatomic levels of SEP abnormalities matched that of radiographic, EMG, or clinical abnormalities. SEPs were abnormal in 41% of nerve roots shown to be involved by other techniques. SEPs added to the clinical evaluation in 4 patients, but were less accurate than a combination of EMG and radiography in indicating the extent of nerve root involvement. We conclude that spinal SEPs following segmental sensory stimulation are useful in the evaluation of lumbosacral radiculopathies and complement information provided by the EMG. In contrast, scalp-recorded segmental SEPs rarely provide additional useful clinical information.     

H-reflex to S1-root stimulation improves utility for diagnosing S1 radiculopathy

ObjectiveThe H-reflex on stimulation of the tibial nerve in the popliteal fossa is routinely used in the diagnosis of first sacral (S1) nerve-root radiculopathy. The H-reflex latency, however, is considered to lack sensitivity since a small change from the focal root pathology can be diluted in a relatively long reflex latency. We have studied the soleus H-reflex elicited by stimulation of the S1 nerve root at the S1 foramen. The normal values for the S1-foramen H-reflex have been reported in a previous study, but there are no definitive reports in patients with S1 radiculopathy. This study was undertaken to determine whether stimulating at the S1 nerve root can improve the utility of the H-reflex for detecting an S1-root lesion.MethodsA randomised paired-study design was utilised to evaluate two H-reflexes: one elicited with tibial nerve stimulation and one elicited with S1-root stimulation. Fifty-five patients with unilateral S1 radiculopathy, confirmed by clinical, electrodiagnostic and magnetic resonance imaging (MRI) evidences were studied. A high-voltage electrical stimulator was used to elicit H-reflexes bilaterally at the S1 foramen and L4/L5 spine level. Latencies were compared with previously generated normal values and similar responses from the asymptomatic leg, focussing on the interval between the peak of M- and H-waves (HMI).ResultsOn the symptomatic side, 39 of the 55 patients had abnormal tibial H-reflex latencies and 54 patients had abnormal responses on S1-foramen stimulation (absent in 18; HMI prolonged >0.4 ms in 36). On the asymptomatic side, all 55 patients had normal tibial H-reflexes, and 52 had normal responses on S1-foramen stimulation. In three patients, the HMI was abnormal on S1-foramen stimulation. In 46 patients tested with L4/L5-level stimulation, H-reflex was present in 39 and absent in seven. The latency of the M-wave to S1 stimulation was normal.ConclusionsAbnormal S1-root H-reflexes reveal lesions at the S1 root in patients with normal tibial H-reflexes; therefore, enhancing diagnostic sensitivity. The appearance of the H-reflex to L4/L5-level stimulation in patient with absent H-reflex to S1-foramen stimulation further localises the site of S1 nerve-root lesion to the L5/S1 spine level. Thus, H-reflex to S1-root stimulation significantly increases the diagnostic sensitivity for S1 radiculopathy.SignificanceIn our study, the S1-root H-reflex with high-voltage electrical stimulation has shown greater sensitivity than the tibial H-reflex in evaluating S1 compressive radiculopathies. An abnormal S1-root H-reflex helps to localise the lesion to the S1 root in patients with concurrent abnormal tibial nerve H-reflex, which may increase diagnostic specificity.     

Evoked potentials in HIV infection]

The study of the literature data on the multimodal evoked potentials in HIV infected patients shows many abnormalities as well in asymptomatic subjects without AIDS as in AIDS subjects with or without neurological signs. Visual evoked potentials (VEPs) reveal prolonged P100 wave latency in 22% of HIV asymptomatic subjects and in 26% of HIV symptomatic subjects; brainstem auditory evoked potentials (BAEPs) reveal an increase of the interpeak latency I-V in 16% of asymptomatic subjects and in 32% of symptomatic subjects; somatosensory evoked potentials (SEPs) by median nerve stimulation reveal prolonged central conduction time in 6% of asymptomatic subjects and in 11% of symptomatic subjects; somatosensory evoked potentials (SEPs) by tibial nerve stimulation reveal prolonged central conduction time in 4% of asymptomatic subjects and in 45% of symptomatic subjects; motor evoked potentials (MEPs) by magnetic stimulation reveal prolonged central motor conduction time in 46% of asymptomatic subjects.     

Recovery of sensation and somatosensory evoked potentials following toe-to-digit transplantation in man

Recovery of digital nerve function in 21 patients with toe-to-digit transplantation was evaluated by clinical sensory tests and somatosensory evoked potentials (SEPs) to median and digital nerve stimulation. The mean interval between injury and surgery was 7 months, and that between surgery and study was 31 months. The transplanted toes achieved a satisfactory but incomplete recovery in temperature (warm and cold), pinprick, touch, vibration, and two-point discrimination in that order. The overall sensory status of the transplanted toes appeared to be closer to normal toes than to normal fingers. In SEPs from the transplanted side, median N9, N13, and N20 components had normal latency but reduced amplitude, whereas digital N9 component was usually absent, but N13 and N20 components had prolonged latency and reduced amplitude. Transplantation performed within 1 month after injury prevented amplitude reduction in median SEPs and latency prolongation in digital SEPs. The SEP data suggest that timing of surgery was critical in preventing retrograde effect on the median nerve, and that recovery of digital nerve function was incomplete correlating with clinical sensory findings. © 1995 John Wiley & Sons, Inc.     

The H-reflex to magnetic stimulation of lower-limb nerves.

We elicited H-reflexes by magnetic and electrical stimulation of several different nerves in 10 healthy subjects and two patients with S-1 radiculopathy. The posterior tibial nerve at the popliteal fossa and the femoral nerve at the inguinal ligament were tested with both electrical and magnetic stimulation; the proximal sciatic nerve was tested only with magnetic stimulation. Muscle activity was recorded from the soleus muscle for posterior tibial and sciatic nerve stimulation and from the vastus medialis muscle for femoral nerve stimulation. No significant difference was found between the latency of H-reflexes evoked by magnetic or electrical stimulation. With magnetic stimulation, the mean (+/- SD) Ia sensory fiber conduction velocity in the proximal segment of the sciatic nerve was 72.4 +/- 3.3 m/s, while the motor nerve fiber conduction velocity in the same portion of the nerve was significantly slower, at 60.6 +/- 2.0 m/s. In two patients with unilateral S-1 radiculopathy, the latency of the H-reflex from the soleus muscle to both magnetic and electrical stimulation of the posterior tibial nerve was absent or prolonged on the affected side. Magnetic stimulation can be used to study the H-reflex and Ia fiber conduction velocity and is particularly advantageous when testing deeply located nerve trunks.     

Acute Contralateral Radiculopathy after Unilateral Transforaminal Lumbar Interbody Fusion

ObjectiveCases of contralateral radiculopathy after a transforaminal lumbar interbody fusion with a single cage (unilateral TLIF) had been reported, but the phenomenon has not been explained satisfactorily. The purpose of this study was to determine its incidence, causes, and risk factors.MethodsWe did retrospective study with 546 patients who underwent a unilateral TLIF, and used CT and MRI to study the causes of contralateral radicular symptoms that appeared within a week postoperatively. Clinical and radiological results were compared by dividing the patients into the symptomatic group and asymptomatic group.ResultsContralateral symptoms occurred in 32 (5.9%) of the patients underwent unilateral TLIF. The most common cause of contralateral symptoms was a contralateral foraminal stenosis in 22 (68.8%), screw malposition in 4 (12.5%), newly developed herniated nucleus pulposus in 3 (9.3%), hematoma in 1 (3.1%), and unknown origin in 2 patients (6.3%). 16 (50.0%) of the 32 patients received revision surgery. There was no difference in visual analogue scale and Oswestry disability index between the two groups at discharge. Both preoperative and postoperative contralateral foraminal areas were significantly smaller, and postoperative segmental angle was significantly greater in the symptomatic group comparing to those of the asymptomatic group (p<0.05).ConclusionThe incidence rate is not likely to be small (5.9%). If unilateral TLIF is performed for cases when preoperative contralateral foraminal stenosis already exists or when a large restoration of segmental lordosis is required, the probability of developing contralateral radiculopathy is increased and caution from the surgeon is needed.     

Dissociation of early SEP components in unilateral traumatic section of the lower medulla

Spinal and scalp early SEPs were recorded, using a noncephalic reference electrode, in a patient with a traumatic cervicomedullary lesion causing unilateral loss of position sense. Cervical N11 and N13 and scalp-recorded far-field P14 SEPs were clearly dissociated following stimulation of the affected side. The findings suggest that the P14 component is generated above the foramen magnum, whereas the cervical N13 has a spinal generator.     

Frontal SEPs and parietal SEPs following median nerve stimulation--clinical correlation and consideration of their generation sites

In order to know the characteristics of frontal and parietal SEP components following median nerve stimulation, 25 patients with unilateral cerebral lesions above the thalamus were examined, and their SSEPs were carefully compared with the clinical and radiological findings. In 10 normal subjects, there were three cortical components of the frontal SEPs (P 20-N 28-P 44) and four those components of the parietal SEPs (N 18-P 22-N 26-P 42). In patient's group, central conduction times (CCTs) between components P 13 and each cortical component were measured and the latency differences between normal side and affected side were calculated. When the latency differences increased over 3 S.D. from the mean of the control values or the some cortical components disappeared, they were regarded as abnormal. According to the combination of the abnormalities in frontal and parietal SEPs, three groups were classified as follows: group 1; frontal and parietal SEPs were normal (n = 10), group 2; frontal and parietal SEPs were both affected (n = 10), group 3; parietal SEPs were affected but frontal components were preserved in normal range (n = 5). CT scan showed that the region from internal capsule to cortex around the central sulcus remained intact in the patients of group 1, while this region was involved in various degrees in all cases of the group 2. In patients of group 3, frontal or parietal regions were variously affected. Both the motor and sensory functions were mainly intact in group 1, and disturbed in group 2.(ABSTRACT TRUNCATED AT 250 WORDS)     

Dermatomal somatosensory evoked potentials in unilateral lumbosacral radiculopathy

We examined scalp-recorded somatosensory evoked potentials (SSEPs) to electrical stimulation of the peroneal nerves and to stimulation in the L5 and S1 dermatomes in 19 patients with unilateral radiculopathies involving these segments. For the dermatomal studies at least two trials of 512 responses were recorded from the vertex with reference to both the midfrontal and contralateral parietal electrodes, using an averaging technique. Findings on the symptomatic and asymptomatic sides were compared in each patient. We found that peroneal SSEPs were normal in all patients. Dermatomal SSEPs correctly identified the lesion in 5 patients. In 1 patient dermatomal SSEPs lateralized the lesion correctly but localized it to the adjacent root. In 10 cases dermatomal SSEPs gave misleading information, indicating an abnormality on the asymptomatic side in 1 patient and no abnormality in 9. In the remaining 3 patients, both SSEPs and radiological contrast studies failed to identify any lesion, although the radiculopathy was confirmed by electromyography. These findings raise doubt about the ultimate utility of these evoked potential techniques in the evaluation of patients with suspected radiculopathies.     

Epidurally recorded cervical spinal activity evoked by electrical and mechanical stimulation in pain patients

Spinal SEPs to electrical and mechanical stimulation of the upper limb of the non-painful side in 7 pain patients were recorded from the cervical epidural space. In response to electrical stimulation of the median nerve, the longitudinal distribution of the spinal postsynaptic negativity (N13) along the cord had a distinct level of maximal amplitude at the C5 vertebral body. When recorded at increasing distances cranial or caudal to this level, the latency of N13 was successively prolonged, in agreement with a spread-out near-field generator in the dorsal horn. Similar patterns of distribution and levels of maximal amplitude were demonstrated for the N13 wave evoked by electrical stimulation of the ulnar and thumb nerves as well as by mechanical stimulation of the thumb ball. The amplitude ratios of the N13 waves evoked by electrical stimulation of the median nerve and the thumb nerves, and by mechanical stimulation of the thumb ball were 3.9 to 1.4 to 1. The slow positive wave (P18), which has been assumed to represent recurrent presynaptic activity, had a somewhat different distribution, with a lower maximal amplitude and a less marked falling off in amplitude along the cord, as compared to the N13 component. The initial presynaptic positivity (P10) appeared with an almost constant amplitude along the cord. Tactile stimuli produced responses with considerably longer latency and duration than those obtained with electrical stimulation. There seemed to be a non-linear relationship between the amplitude of the response and the depth of skin indentation. The presented data contribute a more detailed picture of epidurally recorded spinal SEPs than previous studies. They will serve as a reference for further analysis of SEPs evoked by stimulation of the affected side in pain patients, to explore whether the painful state is associated with altered SEPs before or after therapeutic intervention.     

Premovement gating of somatosensory evoked potentials after tibial nerve stimulation

Somatosensory evoked potentials (SEPs) are attenuated or gated during movement. The mechanism for this includes both centrifugal gating of afferent input and competition with other afferents caused by the movement (peripheral gating). Using a paradigm in which the signal for triggering movement is the electric stimulus for SEPs, we studied the gating of SEPs after tibial nerve stimulation prior to foot movement, and compared it with that during counting task. Significant gating was found for P40 component, which distributed centrally and ipsilaterally to the side of the stimulation, whereas the contralateral N40 component showed no changes. Dissociated gating of P40 and N40 indicates multiple generators of these components, in contrast to the previous view of a single generator dipole projecting tangentially. Together with the previous findings in median SEPs, these gating phenomena should represent a general mechanism for sensori-motor integration in preparation for limb movement.     

Scalp topography of somatosensory evoked potentials following electrical stimulation of femoral nerve

Up to 29 channels of somatosensory evoked potentials (SEPs) were recorded in 10 normal volunteers following unilateral femoral nerve (FN) and tibial nerve (TN) electrical stimulation. Typical short latency FN SEPs consisted of 6 components, P15, N19, P26, N34, P44 and N56. P15 and N19 were widely distributed on the scalp. The first localized scalp component, P26, was strictly postrolandic and distributed on the contralateral parietal scalp close to midline with a prerolandic phase reversal, N26. This scalp distribution is clearly different from the first localized potential of tibial nerve SEPs. N34 and P44 were maximal at the vertex with a distribution that spread to the ipsilateral central and parietal scalp. The amplitude of P26 increased and latency shortened with increasing stimulus intensity and both values plateaued after the stimulus intensity reached motor threshold. No correlation was found between the peak latency of P26 and body height.     

Gating of SEPs by contraction of the contralateral homologous muscle during the preparatory period of self-initiated plantar flexion

To investigate the centrifugal change in somatosensory information processing caused by contraction of the contralateral homologous muscle, we recorded the somatosensory-evoked potentials (SEPs) during the preparatory period of a self-initiated plantar flexion. The SEPs following stimulation of the right tibial nerve at the popliteal fossa were recorded in nine healthy subjects. Self-initiated plantar flexion of the left ankle was performed once every 5 to 7 s. The electrical stimulation was delivered continuously, and the subjects were instructed to concentrate on the movement and not to pay attention to the electrical stimulation. Based on the components of movement-related cortical potential, Bereitschaftspotential (BP) and Negative slope (NS), the preparatory period was divided into four sub-periods (NS, BP-1, BP-2, and Pre-BP). To obtain pre-movement SEPs, the signals following stimulation in each sub-period were averaged. SEPs were attenuated in the preparatory period, especially in the NS sub-period. The amplitude of N40 component was significantly attenuated compared with that in the stationary state and other sub-periods. The amplitude of P53 and N70 was smaller in the NS sub-period than other pre-movement sub-periods. Since there was no centripetal effect on SEPs in the preparatory period, these findings suggested that the activity of motor-related areas modulated the somatosensory information from the contralateral non-movement limb (centrifugal gating). It was assumed that an inhibition on the somatosensory inputs from contralateral limb was caused by the projection via either the corpus callosum or ipsilateral cortico-cortical projections.     

Focal capsular vascular lesions can selectively deafferent the prerolandic or the parietal cortex: somatosensory evoked potentials evidence

Four patients with a unilateral focal vascular accident involving the internal capsule (but not the cortex) were studied electrophysiologically. Averaged somatosensory evoked potentials (SEPs) to electrical stimulation of the median nerve on the left or the right side were analyzed. In the 3 patients with hemiparesis and normal somatic sensation, the precentral P22 and N30 SEP components were lost, whereas the parietal components were preserved. In another patient with clinical somatosensory loss unaccompanied by any central motor impairment, the precentral SEP components were preserved, whereas the parietal SEP components were lost. Thus, a small capsular lesion can eliminate distinct cortical SEP components by selectively involving either the axons of the thalamic VPLc nucleus going to parietal receiving cortex or the axons of thalamic VPLo going to motor area 4. These findings extend to subcortical lesions the diagnostic value of SEPs in patients with dissociated clinical motor and sensory signs.     

Detection and peripheral or central localization of sensory pathway involvement of the lower limbs by somatosensory evoked potentials

In the presence of more or less atypical sensory or sensorimotor symptoms the questions that arise most frequently concern the authenticity of the disorders and the precise level of the lesion. In this study, somatosensory evoked potentials (SEPs) to stimulation of the tibial nerve at the ankle were recorded at different levels in 35 healthy subjects and 32 patients with sensory disorders. Recording electrodes were placed at the popliteal fossa (peripheral sensory nerve conduction velocity), at the T12-L1 level (medullary potential: N21) and at the vertex (P40 wave). The spine to cortex time interval was measured. A systematic study of evoked responses to median nerve stimulation was performed. The 32 patients were divided into 4 groups: Group I (3 cases) had slowed sensory conduction velocity (SCV), similar delay in N21 latency and normal N21-P40: peripheral neuropathy. Group II (4 cases) had normal SCV, delayed N21 latency and normal N21-P40: radicular or conus medullaris injury. Group III (19 cases) had normal SCV, normal N21 latency and lengthened N21-P40 interval. A study of responses to median nerve stimulation made it possible to discriminate between spinal and cortical or subcortical impairment. Group IV (6 cases) had abnormalities from any two of the three groups defined above. In 24 out of 32 patients (75 p. 100), further investigations (myelography, MRI, EMG) confirmed the localization determined by evoked responses. In the other 8 patients (25 p. 100) whose clinical picture suggested a medullary or radicular impairment, SEPs alone clearly revealed an injury. SEPs can distinctly show a spinal impairment and determine the choice of further investigations.     

Bit-mapped imaging of somatosensory evoked potentials after stimulation of the posterior tibial nerves and dorsal nerve of the penis/clitoris.

Somatosensory evoked potentials (SEPs) to unilateral or bilateral posterior tibial nerve (PTN) stimulation and to stimulation of the dorsal nerve (DN) of the penis/clitoris were recorded on 32 channels in 10 volunteers. SEPs to unilateral PTN stimulation consisted of the classic 'W' complex P38-N45-P56-N75 maximal on the ipsilateral central and parietal leads, and two negative waves, N33 and N37, maximal on the contralateral post- and prerolandic areas, respectively. A lemniscal P30 was also recorded. Bilateral PTN stimulation caused, by algebraic summation, the disappearance of both N33 and N37; the W complex was symmetrical and the amplitude of P30 increased. The SEPs to DN stimulation were also symmetrical, and N33 and N37 were absent. These features can be explained by the bilateral character of DN stimulation. They also differed from bilateral PTN SEPs in 3 respects; the absence of P30, the small amplitude and the weaker gradients of field distribution of the 'W' complex, and the somewhat different distribution of penile from clitoral or bilateral PTN, N45 and P56. These differences can be explained both by physiological (the different fiber composition of the DN) and anatomical (the deeper localization of the DN cortical receiving area) mechanisms.     

Segmental dysfunction of the cervical cord revealed by abnormalities of the spinal N13 potential in cervical spondylotic myelopathy.

We studied somatosensory potentials (SEPs) evoked by stimulation of radial, median, and ulnar nerves in 11 patients with MRI evidence of cervical spondylosis. All patients presented with progressive spastic paraparesis that was either isolated or associated with lower motor neuron signs in the upper limbs, with preserved joint, touch, pain, and temperature sensations in the four limbs. In all patients, scalp SEPs reflecting the activity of the dorsal column system up to the parietal cortex were normal while segmental cervical cord dysfunction was manifested by an abnormal spinal N13 potential in 95% of radial, 90% of median, and 54% of ulnar nerve SEPs. These subclinical abnormalities of the spinal N13 SEP probably result from reduced blood supply due to compression of the anterior spinal artery in patients with cervical spondylotic myelopathy.     

Symptomatic and asymptomatic abnormalities in patients with lumbosacral radicular syndrome: Clinical examination compared with MRI

OBJECTIVE: To determine the frequency of symptomatic and asymptomatic herniated discs and root compression in patients with lumbosacral radicular syndrome (LRS) and to correlate clinical localization with MRI findings. METHODS: Fifty-seven patients with unilateral LRS were included in the study. Using the visual analogue scale, two physicians independently localized the most likely lumbar level of complaints. These clinical predictions of localizations were correlated with the MRI findings. RESULTS: MRI showed abnormalities on the symptomatic side in 42 of 57 patients (74%). In 30% of the patients, MRI confirmed an abnormality at the exact same level as determined after clinical examination. On the asymptomatic side, MRI showed abnormalities in 19 of 57 patients (33%), 13 (23%) of these patients had asymptomatic root compression. CONCLUSIONS: In more than two-thirds of the patients with unilateral LRS there was no exact match between the level predicted by clinical examination and MRI findings. These discrepancies complicate the decision whether or not to operate.     

Effect of transcranial DC sensorimotor cortex stimulation on somatosensory evoked potentials in humans.

OBJECTIVE: To study the after-effect of transcranial direct current stimulation (tDCS) over the sensorimotor cortex on the size of somatosensory evoked potentials (SEPs) in humans. METHODS: SEPs were elicited by electrical stimulation of right or left median nerve at the wrist before and after anodal or cathodal tDCS in 8 healthy subjects. tDCS was applied for 10 min to the left motor cortex at a current strength of 1 mA. RESULTS: Amplitudes of P25/N33, N33/P40 (parietal components) and P22/N30 (frontal component) following right median nerve stimulation were significantly increased for at least 60 min after the end of anodal tDCS, whereas P14/N20, N20/P25 (parietal components) and N18/P22 (frontal component) were unaffected. There was no effect on SEPs evoked by left median nerve stimulation. Cathodal tDCS had no effect on SEPs evoked from stimulation of either arm. CONCLUSIONS: Anodal tDCS over the sensorimotor cortex can induce a long-lasting increase in the size of ipsilateral cortical components of SEPs. SIGNIFICANCE: tDCS can modulate cortical somatosensory processing in humans and might be a useful tool to induce plasticity in cortical sensory processing.     

Influence of the ipsilateral hemisphere upon somatosensory evoked potential in cats

Somatosensory evoked potentials (SEPs) were recorded on the skull corresponding to the (left) SI area by (right) superficial radial nerve stimulation. Amongst the various components of the SEP, special attention was directed to the negative component (N15) with a latency of approximately 15 ms. Changes in this potential followed by conditioning stimulation of the ipsilateral (right) hemisphere were observed and the following results were obtained: (i) when conditioning stimuli were applied to the contralateral (left) superficial radial nerve, the ipsilateral (right) thalamic VPL nucleus and the ipsilateral (right) sensory cortex, the amplitude of N15 decreased to 65-80% of the control level at C-T intervals less than 100 ms and (ii) following functional elimination of the unilateral sensory cortex by KCl application, the amplitude of N15 recorded at the opposite side significantly increased. In this condition, the inhibitory effects of the ipsilateral thalamus and contralateral peripheral nerve disappeared. From these observations, ipsilateral homologous cortex may well have an inhibitory influence upon the near field potential (N15) of the SEP.