Spinally elicited peripheral nerve responses are sensory rather than motor.

OBJECTIVES: Spinally elicited peripheral nerve responses, commonly called neurogenic motor evoked potentials (NMEPs), are widely used to monitor spinal cord motor function during surgery. However, numerous evidence suggests that these responses are primarily sensory rather than motor. The collision technique was utilized to address this issue. METHODS: Collision studies were performed in 7 patients during surgery. An ascending volley of sensory (AS) and motor activity (AM) was elicited by posterior tibial nerve stimulation at the popliteal fossa. After a short time delay, high cervical spinal stimulation produced a descending volley of sensory (DS) and motor (DM) activity. The AM volley ascended only to the anterior horn cells whereas the AS and DS volleys collided in the spinal cord. The inter-stimulus delays were varied so as to affect the degree of spinal cord collision. The DS and DM activity which remained after collision was recorded from the posterior tibial nerves at the ankle. RESULTS: Inter-stimulus delays of 18 ms or less resulted in no apparent peripheral descending volleys. These findings were consistent for all the patients studied. CONCLUSIONS: Spinally elicited peripheral nerve responses are primarily sensory rather than motor and are mediated by the same neural pathways as SEPs.

Detection of sacral sparing in acute spinal cord injury.

Sacral dermatomal-evoked responses (DEP) and posterior tibial nerve (PTN) somatosensory-evoked potentials (SSEP) were recorded 28 hours after gunshot injury to the 12th thoracic vertebra of a 17-year-old male. Repeatable responses were obtained from stimulation of the fourth sacral dermatome despite absence of repeatable PTN responses. This suggestion of "sacral sparing" was accompanied by clinical improvement of neural function during the hospital course and 10 months after discharge. This case suggests DEP can make valuable contributions in the diagnosis of sacral sparing in acute spinal cord injury. Considerations regarding the unusual ipsilateral scalp recording location for sacral DEP are discussed.

The use of dermatomal evoked responses during surgical procedures that use intrapedicular fixation of the lumbosacral spine.

Monitoring of electrophysiologic function during intrapedicular fixation of the lumbosacral spine can be useful because this fixation technique has been associated with a significant number of postoperative radicular complications. Somatosensory evoked potentials (SEPs) traditionally have been used to monitor neurologic function during spinal instrumentation procedures. A case is presented of an intrapedicular fixation procedure that was monitored with SEPs and that resulted in false-negative SEP findings. This result suggests that SEPs may not be a sensitive enough monitoring tool for detecting compromise of single root function, and as a result, other monitoring techniques should be used. Dermatomal somatosensory evoked potentials (DSEPs) have been reported to be useful in this regard. To test their usefulness, 81 lumbosacral intrapedicular fixation procedures were monitored with DSEPs. Repeatable responses were obtained from all but one of the patients. The responses were sensitive to the compromise of root function. Predictions of postoperative outcome were dependent only on the responses at closing and not on changes that occurred during surgery.

Effects of L-glutamate and GABA on the response of expiratory neurons to mechanical loads.

Expiratory neurons in the area of the nucleus retroambigualis were studied in anesthetized cats to determine their responsiveness to the iontophoretic application of the putative neurotransmitters, glutamate and gamma-aminobutyric acid (GABA). Previous studies with glutamate and GABA revealed that these two substances were very effective in modulating the spontaneous activity of phasic medullary respiratory neurons. Mechanical loading of expiration, both resistive and elastic, was employed to test whether the presence of these transmitter substances altered the sensitivity of the expiratory cell to its volume related vagal input. Expiratory unit activity analysis included: spikes/burst, burst duration, and average firing rate. Addition of mechanical loads on expiration caused consistent increases in all parameters monitored. Iontophoretically applied glutamate (means = 65 nA) resulted in modest increases in all parameters. When mechanical loads were applied in the presence of a sustained level of glutamate the effects were additive. The general shape of the firing profile observed with loading remained essentially unchanged. Application of GABA (means = 46 nA) resulted in a significant decrease in the parameters monitored. However, as long as phasic activity remained, loads applied in the presence of GABA produced approximately the same absolute change as they did during control. Some cells exposed to high concentrations of GABA lost their phasic activity. This study suggests that either the synaptically activated receptors are not affected by glutamate or that these particular sites are not accessible via iontophoretic application. GABA depressed the activity of the cells in a graded fashion, but in modest concentrations did not interfere with the overall effectiveness of the vagally mediated input.

Optimal transcranial magnetic stimulation sites for the assessment of motor function.

The optimal placement sites for eliciting motor evoked potentials from the abductor digiti minimi and abductor hallucis muscles by means of transcranial magnetic stimulation were determined using a commercially available circular coli. Fifty volunteers were used for the study. The ability to elicit responses was found to be strongly dependent upon the scalp placement of the stimulator coil. The effects of altering the direction of current flow in the coil were tested on two different sets of volunteers: clockwise in one and counterclockwise in the other. It influenced only the locations of the sites which were optimal for eliciting responses from the abductor hallucis muscles and not those which were optimal for eliciting responses from the abductor digiti minimi muscles. Response latencies were found to be significantly dependent only upon volunteers' heights and not on their sex, age, or weight or the stimulus intensities used to elicit responses. No previous studies have defined the optimal scalp placements for eliciting responses from the lower extremities. This information may have clinical importance for making reliable assessments of patients with significantly impaired motor function.

Effects of excitatory and inhibitory amino acids on phasic respiratory neurons.

The responsiveness of phasically active brainstem respiratory neurons to several amino acids was investigated in cats under Dial anesthesia. Four-barreled microelectrodes were used to extrude iontophoretically the putative neurotransmitters L-glutamate, L-asparatate, glycine, and gamma-aminobutyric acid (GABA), L-glutamate and L-aspartate caused increased activity when applied to either inspiratory or expiratory neurons and appeared to be equal in efficacy. Likewise, GABA and glycine depressed ongoing phasic neural activity of both inspiratory and expiratory units. In this case, however, the dosage of GABA required to produce a given depression was significantly less than the required dosage of glycine. These findings support the hypothesis that L-glutamate and/or L-aspartate may act as excitatory neurotransmitter agents at the synapses of brainstem respiratory neurons and conversely, GABA may act as the natural inhibitory neurotransmitter.

Effects of midazolam on median nerve somatosensory evoked potentials.

We have studied the effect of i.v. midazolam on median nerve somatosensory evoked potentials (SSEP) in 10 unpremedicated adults. Anaesthesia was induced with midazolam by bolus administration (0.2 mg kg-1) followed by infusion (5 mg h-1). The latency and amplitudes of the SSEP responses over the second cervical vertebrae (SC2) and sensory cortex (P17, N20, P25) were recorded before and for 10 min after induction. Data were analysed over that period for time-related alterations. Small but statistically significant increases in latency of the cortical N20 (P less than 0.005) and P25 (P less than 0.001) waves and interwave conduction times of SC2 to P25 (P less than 0.005) and N20 to P25 (P less than 0.021) were observed. Cortical amplitude (N20-P25) decreased significantly (P less than 0.012), to approximately 60% of baseline. These results demonstrated that midazolam produced a depression of cortical SSEP amplitude without clinically significant alterations in latency.

Somatosensory evoked potential quantification of ulnar nerve blockade.

We investigated the appropriateness of the somatosensory evoked potentials (SSEPs) as an objective monitor of sensory blockade of the ulnar nerve by comparing the effects of saline and lidocaine infiltration of the ulnar nerve at the elbow, with and without epinephrine. The four treatments were administered in random order to each of eight volunteers who were asked to assess the intensity of sensory blockade on a 10-cm visual analogue scale (SVAS). Concomitantly, ulnar nerve SSEPs were recorded at the Erb's point, over the posterior spine at or rostral to C-7, and over the sensory cortex. All 16 of the saline and saline with epinephrine infiltrations resulted in SVAS scores of 1 or less. In contrast, all the lidocaine and lidocaine with epinephrine injections were associated with SVAS scores in excess of 8. With lidocaine this lasted 83 +/- 22 min, and the duration of lidocaine with epinephrine was 248 +/- 82 min. SSEP responses were considered abnormal when there were 15 min or more of a decrease in amplitude by 50% or more and an increase in latency by 15% or more. After plain lidocaine injection, all the amplitude measurements showed depression over 50%, with the exception of cortical measurements in one subject. Similarly, there were increased latencies at Erb's point and C-7 with lidocaine blocks. In contrast, change in cortical latency was seldom seen. Variable selection by multiple linear regression analysis of the six SSEP measurements gave a statistical model that sensory block duration (SVAS greater than or equal to 8) could be specified by the duration of the reduced C-7 amplitude (r = 0.96).(ABSTRACT TRUNCATED AT 250 WORDS)

Onset, intensity of blockade and somatosensory evoked potential changes of the lumbosacral dermatomes after epidural anesthesia with alkalinized lidocaine.

The onset and intensity of blockade of the lumbosacral dermatomes after epidural anesthesia with alkalinized lidocaine were investigated in a randomized, double-blind study in 26 patients. Control subjects (n = 13) received 20 mL of 1.37% lidocaine (1.5% lidocaine plus 1 mL saline per 10 mL lidocaine) with added 1:200,000 epinephrine; the solution pH was 6.20 +/- 0.08. Patients in the alkalinized lidocaine group (n = 13) were given 20 mL of 1.37% lidocaine plus added bicarbonate (1 mL sodium bicarbonate per 10 mL 1.5% lidocaine) and 1:200,000 epinephrine; the solution pH was 7.18 +/- 0.10. Posterior tibial nerve (PTN) somatosensory evoked potentials (SSEPs) and L5 and S1 dermatomal SSEPs of both lower extremities were done before and after the epidural. Alkalinization of lidocaine resulted in a significantly shorter time to block the L2, L4, L5, and S1 dermatomes. Motor blockade was significantly more profound in the alkalinized lidocaine group. Thirteen of 78 PTN and L5 and S1 dermatomal SSEPs were abolished in the alkalinized lidocaine group compared to 4 of 78 SSEPs in the nonalkalinized group. Alkalinization of lidocaine is recommended to shorten the time to block the L5-S1 dermatomes when epidural anesthesia is planned for lower extremity surgery.

Glossopharyngeal evoked potentials in normal subjects following mechanical stimulation of the anterior faucial pillar.

The anterior faucial pillar, which is innervated by the glossopharyngeal nerve, is thought to be important in eliciting the pharyngeal swallow in awake humans. Glossopharyngeal evoked potentials (GPEP), elicited by mechanically stimulating this structure, were recorded from 30 normal adults using standard averaging techniques and a recording montage of 16 scalp electrodes. Ten of the subjects experienced a desire to swallow in response to stimulation. Repeatable responses were recorded from all 30 subjects. The GPEPs recorded from the posterior scalp were W-shaped and consisted of P1, N1, P2, N2 and P3 peaks. Mean latencies of P1, N1 and P2 were 11, 16 and 22 msec, respectively, for both left and right pillar stimulation. In contrast, latencies of N2 and P3 varied significantly between left and right pillar stimulation. Mean latencies of N2 and P3 were 27 and 34 msec for left, and 29 and 35 msec for right pillar stimulation. Topographical maps acquired at peak latencies for P1, N1 and P2 revealed consistent asymmetrical voltage distributions between the two hemispheres; the largest responses were recorded from the hemisphere ipsilateral to the side of stimulation. The scalp topography of N2 and P3 varied between male and female subjects as well as between left and right pillar stimulation. These findings support the hypothesis that mechanical stimulation to the anterior faucial pillar alone can elicit repeatable responses from the central nervous system. The integration of this subcortical/cortical activity with that of the medullary swallowing center may play an important role in eliciting the pharyngeal swallow.

Dermatomal somatosensory evoked potentials in children with myelomeningocele.

Evoked responses were obtained from electrical stimulation of dermatomes representing individual lumbar and sacral roots. Normal ranges of latency have been established for adults, but have not been extended to children at this time. DSSEPs can be utilized, however, to sequentially follow lumbo-sacral cord function in individual children at risk of progressive myelopathy.

Changes in venous blood lactate, venous blood gases, and somatosensory evoked potentials after tourniquet application.

The effects of inflation of a 7-cm tourniquet applied to the upper arm of eight volunteers on venous lactate, venous blood gases, and ulnar nerve somatosensory evoked potentials (SSEPs) were investigated. The inflation pressure was 100 mmHg over the systolic pressure. Venous blood samples for lactate and blood gas determinations were withdrawn before tourniquet inflation; immediately and at 2, 5, 10, 15 min after tourniquet deflation; and additionally at 30, 45, and 60 min after deflation in the last four volunteers. SSEP stimulating surface electrodes were placed over the ulnar nerve at the wrist. Recording electrodes were placed over the ipsilateral ulnar groove of the elbow, Erb's point, and on the contralateral cortex. Averaged responses were acquired before inflation of the tourniquet, every 5-10 min during tourniquet inflation, and every 5-10 min for 45-60 min after tourniquet deflation. The tourniquet was inflated for 36 +/- 11 min. After deflation of the tourniquet, postdeflation pain and paresthesias were felt by five volunteers; these occurred at 30-120 s after deflation and lasted for 75-120 s. The postdeflation pains were characterized as burning, cramping, paresthesias, buzzing, or severe expansion of the hand. The venous blood lactate levels were significantly elevated for 10 min, and the time course of its change did not correlate with reperfusion pain. The PO2 and O2Hb saturation in venous blood were significantly elevated for 10-15 min after deflation. The elevated lactate and PO2 levels in the presence of a restored blood flow probably result from continued anaerobic muscle metabolism secondary to capillary closure from the tourniquet-induced ischemia.(ABSTRACT TRUNCATED AT 250 WORDS)

The usefulness of electrical stimulation for assessing pedicle screw placements.

The purpose of this study was to further establish the efficacy of pedicle screw stimulation as a monitoring technique to avoid nerve root injury during screw placement. The study population consisted of 662 patients in whom 3,409 pedicle screws were placed and tested by electrical stimulation. If stimulation resulted in a myogenic response at a stimulation intensity of 10 mA or less, the placement of the screw was inspected. Inspection was necessary for 3.9% of the screw placements in 15.4% of the study population. None of the patients in the study experienced any new postoperative neurologic deficits. These findings provide guidelines for the interpretation of stimulation data and support the use of this technique as an easy, inexpensive, and quick method to reliably assess screw placements and protecting neurological function.