Subcortical somatosensory evoked potentials after posterior tibial nerve stimulation in children

We report our normative data of somatosensory evoked potentials (SEP) after posterior tibial nerve (PTN) stimulation from a group of 89 children and 18 adults, 0.4-29.2 years of age. We recorded near-field potentials from the peripheral nerve, the cauda equina, the lumbar spinal cord and the somatosensory cortex. Far-field potentials were recorded from the scalp electrodes with a reference at the ipsilateral ear. N8 (peripheral nerve) and P40 (cortex) were present in all children but one. N20 (cauda equina) and N22 (lumbar spinal cord) were recorded in 94 and 106 subjects, respectively. P30 and N33 (both waveforms probably generated in the brainstem) were recorded in 103 and 101 subjects, respectively. Latencies increased with age, while central conduction times including the cortical component, decreased with age (up to about age 10 years). The amplitudes of all components were very variable in each age group. We report our normative data of the interpeak latencies N8-N22 (peripheral conduction time), N22-P30 (spinal conduction time), N22-P40 (central conduction time) and P30-P40 (intracranial conduction time). These interpeak latencies should be useful to assess particular parts of the pathway. The subcortical PTN-SEPs might be of particular interest in young or retarded children and during intraoperative monitoring, when the cortical peaks are influenced by sedation and sleep, or by anesthesia.     

Scalp recorded somatosensory evoked potentials to posterior tibial nerve stimulation in humans

The somatosensory evoked potentials (SEPs) produced by stimulation of the right and left posterior tibial nerves individually and also by their simultaneous stimulation were recorded in 84 adult normal subjects up to 150 msec after the stimulus by electrodes placed on the cranial vertex and by rows of electrodes over the sagittal and coronal lines using references on the ear or in the nasopharynx. The statistical distribution of the latencies of their different peaks was established. The effect of simultaneous stimulation of right and left posterior tibial nerves on the early SEP components was described. Some details of the anatomy of the rolandic sulcus were inferred from the amplitude distribution of these potentials.     

Cortical somatosensory evoked potentials to posterior tibial nerve stimulation in newborn infants

Successful cortical recordings of somatosensory-evoked potentials (SEPs) to posterior tibial nerve (PTN) stimulation were obtained in 21 (87.5%) for P1 and 22 (91.7%) for N1 of 24 infants who were followed up for at least 3 years and had a normal outcome. There were linear decreases with increasing post menstrual age in both P1 and N1 peak latency. Of the four cases with diplegia later, three showed definite abnormalities, no responses and delayed latency in PTN SEPs respectively, however, the other case showed normal responses. Of the three cases with mental retardation, two showed relatively long latency and borderline responses respectively, and the other case showed normal responses. As the pathway of PTN SEPs traverses the periventricular area of the brain likely to be affected by ischemic lesions in premature infants, abnormalities in the responses might indicate a later motor disorder.     

Influence of concurrent tactile stimulation on somatosensory evoked potentials following posterior tibial nerve stimulation in man

A topographical study was made of SEPs following stimulation of the right posterior tibial nerve at the ankle, with and without concurrent tactile stimulation of the soles of either foot or the palm of the right hand. Effects of the interfering stimulus were best demonstrated by subtracting the wave forms to derive "difference' potentials. The majority of SEP components were significantly attenuated by tactile stimulation of the ipsilateral foot, and the difference wave form was of similar morphology to the control response. Components of opposite polarity peaking at 39 msec were consistent with the field of a cortical generator with dipolar properties, situated in the contralateral hemisphere just posterior to the vertex with the positive poles oriented towards the ipsilateral side. By analogy with median SEP findings, these potentials were believed to originate in the foot region of area 3b where neurones are mainly concerned with cutaneous sensory processing. When the tactile stimulus was applied to the contralateral foot, difference potentials maximally recorded just posterior to the vertex were of smaller amplitude but similar morphology to ipsilateral foot difference components. This suggested the possibility that input from the two lower extremities may converge at cortical or subcortical level, the effect being manifested in the response of certain neurones in area 3b. With both contralateral foot and ipsilateral hand stimulation, other difference potentials were present which suggested that there may be cortical regions responding to combinations of sensory stimuli applied to various parts of the body surface.     

Subcortical and cortical somatosensory potentials evoked by posterior tibial nerve stimulation: normative values

Cortical somatosensory evoked potentials to posterior tibial nerve stimulation were obtained in 29 normal controls varying in age and body height. In obtaining these potentials we varied recording derivations and frequency settings. Our recordings demonstrated the following points: N20 (dorsal cord potential) and the early cortical components (P2, N2) were the only potentials that were consistently recorded. All other subcortical components (N18, N24, P27, N30) were of relatively low amplitude and not infrequently absent even in normals. All absolute latencies other than N2 were correlated with body height. However, interpeak latency differences were independent of body height. Below the age of 20, subcortical but not cortical peak latencies correlated with age, but this appeared to be due to changes in body height in this age group. Absolute amplitudes and amplitude ratios (left/right and uni/bilateral) showed marked interindividual variability and have very limited value in defining abnormality. The use of restricted filter windows facilitated the selective recording of postsynaptic potentials (30-250 Hz) and action potentials (150-1500 Hz).     

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.     

Cervical synapse-dependent somatosensory evoked potential following posterior tibial nerve stimulation

We have demonstrated the presence of a localized, synapse-dependent negativity (N29) recorded over the upper cervical spine after bilateral stimulation of the posterior tibial nerves at the ankle. The amplitude of N29 is maximal at the level of the second cervical spine and decreases at more rostral and caudal levels. The peak latency of N29 remains constant at all levels. N29 has a long refractory period when compared with the refractory period of the afferent volley recorded at either the sacral or thoracic level. N29 is most likely generated by activation of the nucleus gracilis by the afferent volley. The cervical N13 after median nerve stimulation probably has multiple generator sites, including the nucleus cuneatus.     

Anatomic and physiologic bases of posterior tibial nerve somatosensory evoked potentials

Following stimulation of the posterior tibial nerve, lumbar electrodes record a response that is the composite of two signals, one (PV) corresponding to the afferent volley in the cauda equina and gracile tract, and another (N22) generated postsynaptically in the gray matter of the lumbar cord. Subcortical structures generate two distinct, widely distributed signals, recordable from scalp electrodes using a noncephalic reference, P31 and N34. P31 is most likely generated by the afferent volley in the caudal medial lemnicus. N34 probably reflects subcortical postsynaptic activity in brain stem and/or thalamus, respectively. The "primary" cortical response, P38/N38, has a complex scalp distribution reflecting the location of the leg area on the mesial aspect of the postcentral gyrus, within the interhemispheric fissure. It is most likely a composite waveform with multiple cortical generators.     

Middle-latency somatosensory evoked potentials following median and posterior tibial nerve stimulation in Down's syndrome.

Middle-latency somatosensory evoked potentials (SEPs) following median and posterior tibial nerve stimulation were studied in 40 patients with Down's syndrome and in age- and gender-matched healthy controls as well as in middle-aged and aged healthy subjects. In median nerve SEPs, latencies of the initial cortical potentials, N18 and P18, showed no significant difference, but the following potentials N22, P25, N32, P41 and P46 were relatively or significantly shorter in latency in Down's patients than in the controls. Amplitudes of all components in Down's patients were significantly larger than those of age- and gender-matched controls as well as of those of middle-aged healthy subjects, but there was only a small difference in their amplitudes from aged healthy subjects. Results of posterior tibial nerve SEPs were generally consistent with those of median nerve SEPs. Therefore, 'short latency with large amplitude' is the main characteristic of middle-latency SEPs in Down's syndrome, possibly related to accelerated physiological aging of the central nervous system.     

Effect of stimulus intensity on subcortical and cortical somatosensory evoked potentials by posterior tibial nerve stimulation

The effects of stimulus intensity on subcortical and cortical somatosensory evoked potentials (SEPs) to posterior tibial nerve (PTN) stimulation were studied in 16 normal controls. Stimulus intensity was evaluated as a function of sensory threshold (S). Motor threshold (M) varied between 1 S and 2 S. The amplitude of N18 (afferent volley immediately before it enters the spinal canal) increased approximately linearly up to at least 4.5 S. N20 (dorsal cord potential) also demonstrated a linear increase up to at least 4 S but the rate of increase was significantly smaller. All central components (subcortical brain-stem components P27 and N30, and cortical components N1 and P2) showed an even smaller rate of increase which was non-linear and reached a plateau at 3 S. The relatively higher rate of increase of N18 as compared with N20 was most probably due to the recording of sensory impulses plus antidromic impulses in motor fibers. The smaller rate of increase and early saturation of all the central components compared with N20 suggests that of all the afferent fibers generating N20 only the low threshold fibers participate in the generation of more central components. Stimulus intensities of 3 S are recommended for clinical studies of the central SEPs to PTN stimulation.     

Maturational study of short-latency somatosensory evoked potentials after posterior tibial nerve stimulation in infants and children

SSEPs produced in response to PTN stimulation were studied in 41 normal infants and children from 4 months to 16 years in age. SSEPs were recorded on the scalp with reference electrodes attached to the contralateral knee, shoulder and earlobe. Four positive SSEPs, PI, PII, PIII and PIV, named in order of appearance, and one negative SSEP, N0, were recorded as FFPs on the scalp with the cKn reference. Following these FFPs, the cortical component P1 which corresponded to P37 in adults was recorded. Preceding P1, another negative wave, N1, could be recognized solely at Cz' mainly at the onset of P1. P1 and N1 could be identified in all children with derivations with noncephalic references, although they could not be identified in 5 of 41 children with a Cz' - Fpz derivation. PI, bilobed in configuration, was considered to originate at the sacral plexus or entry to the spinal canal. PII was the least reproducible potential and was considered to originate at the dorsal root, dorsal horn or conus medullaris. PIII, PIV and N0 were considered to originate at the cervical cord, brain stem and thalamus, respectively. With the peak latencies of PI, PII, PIII, PIV, N0, N1 and P1, the RV was calculated in order to eliminate the influence of body height. The RV of the later appearing components leveled off in the older age categories. The RV of P1 reached a steady level at 3 years of age. RVs of PII and PIII appeared to level off by the age of 6 years. The RV of PIV leveled off by the age of 9 years. RVs of N0 and N1 leveled off by the age of 12 years, and that of P1 decreased until over 12 years of age. Furthermore, to eliminate the influence of naturation in the peripheral nerves, the RV was obtained from PI-PIV, PI-P1, PIV-P1 and N1-P1 interpeak latencies. The RVs of these 4 interpeak latencies all decreased until over 12 years of age. Accordingly, the maturation of afferent conduction in the central nervous system after PTN stimulation appeared to be complete after 12 years of age.     

Scalp topography of the short latency somatosensory evoked potentials following posterior tibial nerve stimulation in man

The scalp topography of the short latency somatosensory evoked potentials (SEPs) to unilateral posterior tibial nerve stimulation at the ankle was studied by using a non-cephalic reference in 22 normal young adults. At least 3 components (P28, N31 and N32) were identified preceding the major positive peak (P36). The first 2 components had similar peak latency at all scalp electrodes, and were considered to be generated in deep structures. However, N32 was localized to the hemisphere contralateral to the side of stimulation. P36 was maximal at the midline foot sensory area, or at the contralateral parasagittal area, and its amplitude decreased more steeply anteriorly than posteriorly. The peak latency of P36 progressively increased from ipsilateral to the side of stimulation in the coronal plane. P36 occurred earlier in the somatosensory area, and increased in peak latency anteriorly. Generator source of scalp-recorded far-field potentials (P28 and N31) remains to be elucidated. N32 might reflect activities of the thalamo-cortical pathway or an initial cortical response. P36 appeared to be generated in the somatosensory foot area.     

Somatosensory evoked potentials to posterior tibial nerve stimulation in children

The somatosensory evoked potentials (SEPs) to stimulation of the tibial nerve were studied in 88 children ranging in age from 1 day to 16 years. SEPs were not evidenced in 10 out of 44 infants less than 1 year old. In others it was a major positive wave (P) with a variable topographic distribution on the midline. The onset and peak latencies of this P were highly variable in different subjects of the same age or body-size, and in the same subject with the active electrode placed in different locations. The lowest values for latency were in subjects about 3 years old. The ascending time of P was the only parameter strictly correlated with age. The results are compared with SEPs to upper limb stimulation, which are constant and more reliable. These results indicate: that the maturation of the peripheral somatosensory pathway proceeds at a faster rate than that of the central somatosensory pathway; that the maturation of the somatosensory pathway of the upper limb precedes that of the lower limb; and that the ascending time of P is a good index of thalamo-cortical maturation. The clinical utility of these SEPs in pediatrics is discussed.     

Ontogenesis of lumbar spinal somatosensory evoked potentials after posterior tibial nerve stimulation in the rat.

Although data are available regarding lumbar spinal somatosensory evoked potentials (SSEPs) after posterior tibial nerve stimulation in the mature rat, age-related changes spanning the period of early development have not been defined. We obtained lumbar spinal SEPs after posterior tibial nerve stimulation in 6 cohorts of animals (N = 36) ranging in age from weanling (15 days) to early adulthood (110 days) by recording from needle electrodes placed in the L1-2 and L5-6 interspinous ligaments. Absolute latencies of the major wave form components at the two recording sites declined rapidly until the mid-juvenile period (36 days) and more slowly thereafter. Mean peripheral conduction velocities (+/- S.D.) increased from 11.06 (+/- 0.02) to 33.22 (+/- 3.55) m/sec and mean central conduction velocities (+/- S.D.) increased from 6.27 (+/- 1.07) to 23.64 (+/- 3.84) m/sec from 15 to 110 days respectively. The linear relationships of central and peripheral conduction velocities to both age and weight as defined by standard regression were highly significant. No sex differences were noted for peripheral velocities at all ages studied. Central velocities revealed significant sex differences at 110 days but not earlier. This study demonstrates that lumbar spinal SEPs after posterior tibial nerve stimulation undergo a predictable evolution which can be represented by a simple cable model of a lengthening myelinated pathway.     

Effects of halothane on intraoperative scalp-recorded somatosensory evoked potentials to posterior tibial nerve stimulation in man

Somatosensory evoked potentials (SEPs) were monitored in 116 patients receiving halothane anesthesia during spinal fusion surgery. Whereas it has been generally assumed that the use of halogenated inhalational anesthetics should be avoided with SEP monitoring because of their purported deleterious effects on scalp-recorded sensory responses, we found that reproducible SEPs were obtained throughout the surgical procedure in 91% of the cases we monitored while using halothane at concentrations of 0.25-2.0%. In those cases in which halothane was delivered continuously at 0.5%, reproducible evoked responses were recorded in 96% (75 of 78) of the patients. Our data demonstrated 3 major effects of halothane on the SEP: (a) a small but significant decrease in the average amplitude of the first two components (N25 and P30), (b) a significant increase in the average latency of the late positive component (P53) of the wave form, and (c) occasional obliteration of components N25, N40, P53, and N71, but never of P30. These effects did not, in most cases, interfere with our ability to obtain clinically useful recordings. Our results suggest that in many instances the use of halothane anesthesia can be combined successfully with the recording of intraoperative SEPs.     

Effects of stimulus intensity on posterior tibial nerve somatosensory evoked potentials.

Relationships between stimulus intensity and peak latencies and amplitudes in posterior tibial nerve somatosensory evoked potential patterns were evaluated in ten healthy subjects. Eight intermediate latency peaks between 30 and 125 milliseconds (ms) after stimulus onset and seven amplitudes were analyzed. In general, there was a decrease in latency with each increase in stimulus intensity over a five step intensity range between 5 and 19 milliamps (mA) for most peaks. Similarly, increases in amplitudes generally occurred with increases in stimulus intensity for most peaks. Later peaks N105 and P115 as well as amplitudes P90-N105 and N105-P115 were least sensitive to stimulus intensity changes. The greatest changes in peak latency and amplitude occurred as stimulus intensity was increased from 7 to 11 mA. Beyond 11 mA relatively little change was observed in either peak latencies or amplitudes. Under anesthesia, by contrast, there was no stimulus intensity-peak latency interaction and beyond 11 mA there were decreases in amplitudes. Possible reasons for these findings are discussed.     

The effect of aging on somatosensory evoked potentials following stimulation of the posterior tibial nerve in man

Spinal- and scalp-recorded somatosensory evoked potentials following stimulation of the posterior tibial nerve were obtained in 20 normal young subjects, and 45 aged subjects who were classified into group A (61-74 years) and group B (75-88 years). The results may be summarized as follows: (1) Spinal potentials, N19 at the twelfth thoracic vertebra and N28 at the second cervical vertebra, were significantly prolonged in latency in aged subjects. The interpeak latency, N19-N28, which represents the conduction time through the spinal cord, was also prolonged in aged subjects. (2) The interpeak latencies, P28-N31 and P28-P35, which represent the conduction time from the medial lemniscus to the thalamus and to the sensory cortex, respectively, were prolonged in aged subjects, particularly in group B. (3) The interpeak latencies of cortical potentials following P35, which represent the intracortical transit times, did not show any significant difference between young and aged subjects. (4) Amplitudes of the spinal, and short- and middle-latency cortical potentials were smaller in the aged subjects than those of young subjects, particularly the far-field N31 potential at Cz' electrode. In contrast, the long-latency cortical potentials were larger in aged subjects, although not significantly so.     

Subcortical somatosensory evoked potentials after median nerve and posterior tibial nerve stimulation in high cervical cord compression of achondroplasia

Children with achondroplasia may have high cervical myelopathy from stenosis of the cranio-cervical junction resulting in neurological disability and an increased rate of sudden death. To detect myelopathy we recorded somatosensory evoked potentials after median nerve (MN) and posterior tibial nerve (PTN) stimulation in 77 patients with achondroplasia aged 0.3–17.8 years (mean 2.7 years). In addition to the conventional technique of recording the cortical components and the central conduction time (CCT) we employed non-cephalic and mastoid reference electrodes to record the subcortical waveforms N13b and P13 (MN-SEP) as well as P30 (PTN-SEP), respectively, which are generated near the cranio-cervical junction. The findings were related to the MRI results. Thirty-four patients had abnormal MRI findings including spinal cord compression (n = 28) and/or myelomalacia (n = 24) at or below the cranio-cervical junction. The sensitivity of the MN-SEPs was 0.74 including all abnormal upper cervical cord MRI findings (specificity 0.98), and the sensitivity was 0.79 (specificity 0.92) for cervical cord compression, respectively. The sensitivity of the PTN-SEPs was 0.52 (specificity 0.93) for all abnormal MRI findings and 0.59 (specificity 0.92) for cervical cord compression. The subcortical SEPs N13b and P13 as well as P30 were more sensitive than the conventional recordings. The MN-SEPs, notably the subcortical tracings, are useful for the detection of cervical myelopathy in children with achondroplasia. The PTN-SEPs are less sensitive. However, the tibial nerve SEPs might contribute additional information from the lumbar or thoracic spinal cord, which was, however, not tested in this study.     

Intraoperative recordings of spinal somatosensory evoked potentials to tibial nerve and sural nerve stimulation

Somatosensory evoked potentials (SSEPs) to stimulation of the tibial nerve at the knee (TN-K) and ankle (TN-A), and the sural nerve at the ankle (SN-A), were recorded from 3 or 4 spinal levels during surgery for scoliosis in 11 neurologically normal subjects. With stimulation of all 3 nerves, the propagation velocity along the spine was nonlinear: it was faster over cauda equina and midthoracic cord than over caudal spinal cord. Over the mid-thoracic cord, TN-K SSEP propagation was faster than that of TN-A and SN-A SSEPs, whereas over the caudal spinal cord these values were similar on stimulation of all 3 nerves. These data suggest that fast conducting second order afferent fiber systems contribute to spinal cord SSEPs evoked by stimulating both mixed and cutaneous peripheral nerves.     

Lumbar-spinal somatosensory evoked potentials in the rat after stimulation of the tibial nerve

Spinal somatosensory evoked potentials after stimulation of the tibial nerve were recorded from the lumbospinal cord of rats. Their components and the respective latencies recorded over L5/6 and L1/2 in 40 normal animals are described. Using this method exact statements concerning lesions particularly of the proximal segment of the peripheral nerves and their roots can be made.