Spinal somatosensory evoked potentials: maturational and clinical studies

4-aminopyridine improves evoked potentials and ambulation in the taiep rat: A model of hypomyelination with atrophy of basal ganglia and cerebellum

The taiep rat is a tubulin mutant with an early hypomyelination followed by progressive demyelination of the central nervous system due to a point mutation in the Tubb4a gene. It shows clinical, radiological, and pathological signs like those of the human leukodystrophy hypomyelination with atrophy of the basal ganglia and cerebellum (H-ABC). Taiep rats had tremor, ataxia, immobility episodes, epilepsy, and paralysis; the acronym of these signs given the name to this autosomal recessive trait. The aim of this study was to analyze the characteristics of somatosensory evoked potentials (SSEPs) and motor evoked potentials (MEPs) in adult taiep rats and in a patient suffering from H-ABC. Additionally, we evaluated the effects of 4-aminopyridine (4-AP) on sensory responses and locomotion and finally, we compared myelin loss in the spinal cord of adult taiep and wild type (WT) rats using immunostaining. Our results showed delayed SSEPs in the upper and the absence of them in the lower extremities in a human patient. In taiep rats SSEPs had a delayed second negative evoked responses and were more susceptible to delayed responses with iterative stimulation with respect to WT. MEPs were produced by bipolar stimulation of the primary motor cortex generating a direct wave in WT rats followed by several indirect waves, but taiep rats had fused MEPs. Importantly, taiep SSEPs improved after systemic administration of 4-AP, a potassium channel blocker, and this drug induced an increase in the horizontal displacement measured in a novelty-induced locomotor test. In taiep subjects have a significant decrease in the immunostaining of myelin in the anterior and ventral funiculi of the lumbar spinal cord with respect to WT rats. In conclusion, evoked potentials are useful to evaluate myelin alterations in a leukodystrophy, which improved after systemic administration of 4-AP. Our results have a translational value because our findings have implications in future medical trials for H-ABC patients or with other leukodystrophies.

Transcranial Motor-Evoked Potentials Are More Readily Acquired Than Somatosensory-Evoked Potentials in Children Younger Than 6 Years

BACKGROUND

There is a general belief that somatosensory-evoked potentials (SSEPs) are more easily obtained than transcranial motor-evoked potentials (TcMEPs) in children younger than 6 years. We tested this assumption and the assumption that motor-evoked potentials are rarely obtained in children younger than 2 years.

METHODS

The records of all patients who were monitored during surgical procedures between April 1, 2010, and June 30, 2013, were reviewed and those who were younger than 72 months at the time of surgery were identified and analyzed for the rate of obtaining clinically useful SSEPs and motor-evoked potentials. Subgroup analysis was performed by age.

RESULTS

A total of 146 patients were identified, 9 had SSEPs without TcMEPs monitored, 117 had both TcMEPs and SSEPs monitored, and the remainder had only electromyographic monitoring. All patients who were to have TcMEPs recorded received a total IV anesthetic. Among the 117 patients who had both SSEPs and TcMEPs monitored, clinically relevant TcMEPs were obtained more frequently than SSEPs (110/117 vs 89/117; χ = 14.82; P = 0.00012). There were significant differences between the rates of obtaining SSEPs and TcMEPs in the 0- to 23-month (P = 0.0038) and 24- to 47-month (P = 0.0056) age groups. Utilization of a double-train stimulation technique facilitated obtaining TcMEPs in the youngest patients.

CONCLUSIONS

TcMEPs can be obtained more easily than SSEPs in patients younger than 72 months if a permissive anesthetic technique is used. The success rate for obtaining TcMEPs can be further enhanced by the use of a temporal facilitation (double-train) stimulation technique.

The use of somatosensory evoked potentials in infants and children

Somatosensory evoked potentials (SSEPs) are a useful, reliable means of assessing function of the somatosensory system. Complex maturational changes of the CNS such as synaptogenesis and myelination, as well as body growth, complicate interpretation of SSEPs. An understanding of these factors enhances clinical interpretation in infants and children.

Brain-stem auditory evoked responses as indicators of early brain insult

The relationship between cranial ultrasonograms (SONOs) and brain-stem auditory evoked responses (BAERs) was evaluated in 2 independent samples of newborn infants at risk for brain injury (n = 113 and 203). Features of the BAER wave forms subjected to stepwise linear discriminant analysis formed the basis of an algorithm used to detect and follow early brain injury. Using this algorithm, information derived from BAERs reliably predicted SONO abnormalities at least 82.3% of the time in the initial study which was replicated with the second sample (77.3%). The wave I component latency (CL) and the wave III-V inter-peak latency interval (IPL) were independent of each other, and both contributed to a prediction of SONO abnormality. Possible mechanisms for these BAER results include compromise to the cochlear membrane or to the auditory nerve itself as well as prolongation of transmission in the brain-stem due to brain-stem hemorrhage, edema, or compression. Normative BAER values and non-linear regression functions for the wave I, III and V CLs, and the I-III, III-V, and I-V IPLs were calculated across age using data from 109 infants who demonstrated normal BAER patterns and had no history of SONO abnormalities. Our analyses indicate BAER techniques, where a single higher intensity is used to produce the BAER wave form, are both valid and efficient for use in the evaluation of early brain injury.

Development of lumbar spinal cord and cortical evoked potentials after tibial nerve stimulation in the pre-term newborns: effects of gestational age and other factors

Pre-term neonates are at increased risk for neurological dysfunction. Several investigators have found scalp recorded somatosensory evoked potential studies (SSEPs) after median nerve stimulation useful in the evaluation of newborn infants with asphyxiation and the effects of other adverse prenatal and perinatal factors. In order to evaluate the entire developing neuraxis, we undertook SSEPs after tibial nerve stimulation (PTN-SSEP) in pre-term neonates. Using bilateral simultaneous stimulation, potentials were recorded from the following sites: PF-spT6 (N5), spL1-spT6 (N16), spC7-Fpz (N27), Cz' (1 cm behind the vertex)-Fpz (P55). In all newborns studied, the N5 and N16 were reliably recorded. The N5 appeared relatively independent of the length of the newborns. The N16 correlated inversely with length. The N27 and P55 were recorded in 52% and 65% of the newborns, respectively. N27 inversely correlated modestly with length. The P55 was independent of most factors and probably reflects variable rates of cerebral myelination, neuronogenesis, varying states of alertness, and possibly subclinical encephalopathies. These results demonstrate the feasibility of obtaining such data in pre-term newborns.

Evaluation of children and young adults with tethered spinal cord syndrome. Utility of spinal and scalp recorded somatosensory evoked potentials

The diagnosis of tethered spinal cord syndrome should be considered in young patients with progressive orthopedic deformities, lower extremity weakness, urinary and fecal incontinence, low back pain, or combinations of these symptoms. Myelographic, computed tomographic, and urodynamic studies are useful for establishing a diagnosis, but contribute little to the evaluation of lower extremity sensory function or to the assessment of electrophysiologic impairment of the spinal cord itself. To determine the diagnostic usefulness of the somatosensory evoked potential after posterior tibial nerve stimulation (posterior tibial nerve somatosensory evoked potential) in tethered spinal cord syndrome, 22 consecutive patients with symptoms of tethered spinal cord syndrome (aged 18 months to 22 years) underwent recording of posterior tibial nerve somatosensory evoked potential; results were correlated with clinical, myelographic, and operative findings. In patients with clinical symptoms but no myelographically demonstrable lesions, posterior tibial nerve somatosensory evoked potentials were within normal limits, suggesting normal physiologic function. In patients with myelographically and operatively confirmed tethering dysraphic lesions, posterior tibial nerve somatosensory evoked potential was predictive of the level and laterality of the lesion. Similarly, ranking the severity of neurological impairment and extent of dysraphism at operation, as well as the extent of abnormality of posterior tibial nerve somatosensory evoked potential, revealed a significant (r = 0.81, p less than 0.001) correlation between clinical severity and posterior tibial nerve somatosensory evoked potential abnormalities. Postoperatively, in 8 patients, posterior tibial nerve somatosensory evoked potential also reflected improved function in relation to the level and type of dysraphic lesion present. These findings indicate that posterior tibial nerve somatosensory evoked potential is a sensitive indicator of neurophysiologic status in patients with tethered spinal cord, and is useful for determining the level of the conus medullaris, degree of spinal cord displacement, and severity of neurological impairment associated with this congenital disturbance of neuraxis formation. Recording of posterior tibial nerve somatosensory evoked potential is noninvasive and offers a more sensitive diagnostic tool than the clinical testing of sensation for detection of the development of neurologic deficits in patients with tethered cord syndrome.

Noninvasive measurement of spinal cord conduction: review of presently available methods

The ability to measure spinal cord conduction velocity noninvasively is limited by available methodology. Surface recording of small spinal potentials, although feasible in infants and young children, is problematical in adults, especially when recording over the cervical spine. On the other hand, indirect methods designed to improve the signal-to-noise ratio, which include recording of somatosensory cortical evoked potentials, F-waves, or other muscle responses, are limited by the unproven assumptions necessary in calculating spinal conduction. Additionally, each method has its own particular limitations. The majority of presently available noninvasive methods take a restricted, or no, account of conduction through the motor pathways. Despite these often serious limitations, each of the reviewed methods does play a useful clinical role in the electrophysiologic investigation of cord disease that is not visible radiologically. Knowledge of them allows for sufficient diversity to tackle most relevant problems until an ideal physiologic method is developed.

Developmental assessment of spinal cord and cortical evoked potentials after tibial nerve stimulation: effects of age and stature on normative data during childhood

Somesthetic information from lower extremities is processed by cerebral cortex after traversing the sensory pathways of peripheral nerve, spinal cord, brain-stem and thalamus. Clinical utility of somatosensory evoked potentials (SSEPs) during human development requires systematic analysis of normative data acquired during various stages of body growth and nervous system maturation. Accordingly, SSEPs after tibial nerve stimulation were studied in 32 normal awake children (1-8 years old) and compared with values obtained in young adults (18-40 years old). Potentials were recorded from the tibial nerve (N5), first lumbar spinous process (N14), seventh cervical spinous process (N20) and from the scalp, 2 cm behind the vertex (P28). In all children studied, the N5, N14 and N20 latencies were positively correlated with age and height yielding a predictive nomogram. An extremely variable electropositive cortical SSEP was recorded from Cz' which did not show a highly predictable linear relationship in association with a relatively poor correlation coefficient for height and age. It may be concluded that between 1 and 8 years of normal postnatal development, latencies reflecting peripheral nerve and lumbar spinal cord vary directly with height and age and can be represented by a simple cable model of a lengthening myelinated pathway. In contrast, the latency of the cortical SSEP reflects asynchronous maturation of elongating polysynaptic pathways and apparently requires a more complex model for prediction in order to enhance its clinical utility.

Nerve impulse propagation along central and peripheral fast conducting motor and sensory pathways in man

Forty-four limbs from 11 healthy volunteers were examined. Spinal and scalp somatosensory evoked potentials to median and peroneal nerve stimulation were recorded and the peripheral (wrist-Erb, Erb-cervical, knee-thoracic spine) and central (cervical-scalp, thoracic-cervical spine, spine-scalp) conduction times and velocities (CTs, CVs) were calculated. Sensory and mixed trunks of median and peroneal nerves were also stimulated and their motor and sensory CVs in mid-distal districts were measured. Motor responses to scalp (motor areas for hand and leg muscles) and spinal cord stimulation (cervical and lower thoracic levels) were carried out through skin rectangular plate electrodes delivering high voltage (880-1870 V) brief anodal pulses. The intracranial (scalp-cervical) and intraspinal (cervical-thoracic spine) CTs and CVs of motor pathways were measured. The elbow-cervical and knee-thoracic spines CTs of motor fibres were also calculated through the F wave method, which gave values almost superimposable on those obtained through direct spine stimulation. Nerve propagation was faster in sensory than in motor fibres in peripheral nerve mid-distal districts, while this difference was reduced or reversed in more proximal segments, including nerve roots. The scalp-cervical CT was slightly shorter in motor than in sensory fibres after subtraction of synaptic delays (6.12 vs. 6.18 msec). The scalp-lower thoracic spine, as well as the intraspinal, CVs were 7-12% faster in sensory than in motor pathways (45.3 vs. 38.7 m/sec for the former; 62.65 vs. 55.4 m/sec for the latter). The reported method allows the evaluation of fast conducting motor and sensory pathways along 'central' and 'peripheral' nerve structures of the entire body. Preliminary findings on scalp stimulation of brain motor areas with low voltage pulses are also included.

Short-latency scalp somatosensory evoked potentials and central spine to scalp propagation characteristics during peroneal and median nerve stimulation in multiple sclerosis

Peripheral (cauda-lumbar, wrist-Erb, Erb-cervical) and central (cauda-vertex, cervical-scalp) nervous impulse propagation velocities and times to peroneal and median nerve stimulation were investigated in 34 patients suffering from definite (17 cases), probable (6 cases) and possible (11 cases) forms of multiple sclerosis (MS). In 6 cases short- and intermediate-latency scalp somatosensory evoked potentials to peroneal nerve stimulation were recorded with 'open' (1-5000 Hz, -6 dB) bandpass filters and subsequently digitally filtered through a 'narrow' bandpass (200-5000 Hz, -6 dB). The lumbar response was abnormal in 2.95% of legs, while the Erb response was always within normal limits. The cauda-vertex conduction was altered in 75% of the examined limbs (86.2% definite, 58.3% probable, 63.6% possible MS). Absent scalp responses to peroneal stimulation were often encountered during narrow bandpass recording (54.9%), while a slowed central conduction was less frequent (33.3%). Scalp responses when recorded with open bandpass were always identifiable, being delayed in 3 out of 6 cases. In 5 of these the short-latency wavelets were either absent or showed a prolonged interpeak time even when open filter records were normal. Median nerve SEPs were altered in 60.3% of cases, more frequently because of a delayed scalp response or of a prolonged cervical-scalp conduction time than because of an absent cervical or scalp response. When peroneal and median nerve data were considered together, the rate of abnormality rose to 88.2% of patients. Due to their length, afferent pathways from the lower limb might suffer from a loss of high frequency impulse coding as an early sign of defective impulse propagation.

Spine and scalp somatosensory evoked potentials in normal subjects and patients with spinal cord disease: evaluation of afferent transmission

Spine and scalp somatosensory evoked potentials (SEPs) to peroneal nerve stimulation were recorded from 20 normal subjects using 1 restricted and 3 open frequency filter bandpasses. Spine to spine and spine to scalp propagation velocities were calculated. Of those recording parameters investigated, optimal recordings were obtained using an open bandpass (5-1500 or 30-1500 Hz) and recording from 3 surface spine bipolar channels and 1 scalp bipolar channel. This method was then investigated in 40 patients with disease of the spinal cord and peripheral nervous system. Focal spinal cord compressive lesions generally resulted in slowing of spine to spine and spine to scalp propagation velocities. Diffuse or multifocal lesions of the spinal cord generally resulted in the absence of scalp responses. Although there was no consistent correlation of the SEP findings with the sensory exam, there was a correlation of the SEP findings with the clinical prognosis.

Pathways of ascending evoked spinal cord potentials of dogs

Pathways of the ascending evoked potentials (AEPs) of the spinal cord of the dog were investigated by stimulating rear leg nerves and recording at thoracic levels before and after making selective partial or complete transections of topographic regions of the spinal cord in the L-1 segment: hemisection; ventral quadrant (VQ); dorsal quadrant (DQ); dorsolateral fasciculus (DLF); dorsal columns (DCs). The AEPs were found to be propagated in all topographic areas of the spinal cord. The contribution by the DQ ipsilateral to the stimulated nerve appeared to be the largest. Within the DQ, both the DLF and the DC participated in the AEP but the DLF contribution tended to predominate in the earliest parts while the DC contribution tended to lag somewhat behind that of DLF. The predominance of DLF activity in early phases of the AEP probably reflected the influence of high conduction velocities in muscle afferents and in postsynaptic DLF axons having connections with muscle afferents or with cutaneous afferents. The VQ contribution to the AEP appeared to arise from both crossed and uncrossed pathways but was not otherwise defined. Based on the results and on data in the literature, important considerations in interpretation of AEPs are: the relative numbers of muscle or cutaneous afferent fibers in the stimulated nerves; temporal dispersion of action potentials associated with differences in conduction velocities; conduction distances in the stimulated primary afferent axons and in postsynaptic spinal cord axons.

Saphenous nerve evoked potentials and the assessment of intraabdominal lesions of the femoral nerve

A technique for recording somatosensory evoked potentials after stimulation of the patellar branch of the saphenous nerve is described and the results from 11 normal subjects and from two patients with intraabdominal femoral nerve injuries are presented. One patients developed a nerve lesion after an abdominal surgical procedure and the second suffered a gunshot wound causing femoral nerve paralysis. Both patients recovered spontaneously. Evoked potentials from saphenous nerves showed a significant discrepancy between faster motor recovery and delay in reinstating sensory axonal conduction.