Percutaneous flexible bipolar epidural neuroelectrode for spinal cord stimulation. Technical note

The Relationship between Trans-Lesional Conduction, Motor Neuron Pool Excitability, and Motor Function in Dogs with Incomplete Recovery from Severe Spinal Cord Injury

Spontaneous, acute, complete thoracolumbar spinal cord injury (TL-SCI) in dogs frequently results in permanent deficits modeling chronic paralysis in people. Recovery of walking without recovery of sensation has been interpreted in dogs as reflexive spinal walking. To evaluate this assumption, this study characterized the electrophysiological status of motor and sensory long tracts and local reflex circuitry in dogs with absent recovery of sensation after acute TL-SCI and correlated findings to gait scores. Twenty dogs with permanent deficits after acute, clinically complete TL-SCI and 6 normal dogs were prospectively enrolled. Transcranial magnetic motor evoked potentials (MEPs), somatosensory evoked potentials (SSEPs), H-reflex, and F-waves were evaluated. Gait was quantified using an ordinal, open field scale (OFS) and treadmill-based stepping and coordination scores (SS, RI). MEP latency and H-reflex variables were compared between cases and controls. Associations between presence of MEPs, SSEPs, F-waves or H-reflex variables, and gait scores were determined. Pelvic limb MEPs were detected in 4 cases; no case had trans-lesional sensory conduction. Latency was longer and conduction velocity slower in cases than controls (p_a = 0.0064, 0.0023, respectively). Three of 4 cases with pelvic limb MEPs were ambulatory, and gait scores (OFS, SS, RI) were each associated with presence of trans-lesional conduction (p_a = 0.006, 0.006, 0.003, respectively). H threshold in cases (mean, 3.2mA ±2.5) was lower than controls (mean, 7.9mA ±3.1; p_a = 0.011) and was inversely associated with treadmill-based scores, SS, and RI (p_a = 0.042, 0.043, respectively). The association between pelvic limb MEPs and gait scores supports the importance of descending influence on regaining walking after severe TL-SCI in dogs rather than just activation of spinal walking. The inverse association between H-reflex threshold and gait scores implies that increases in motor neuron pool excitability might also contribute to motor recovery.

Monitoring of evoked potentials during spinal cord ischaemia: experimental evaluation in a rabbit model

OBJECTIVES

Somatosensory evoked potentials (SEPs), spinal evoked potentials (Spinal-EPs), and motor-evoked potentials (MEPs) were monitored in a rabbit model of spinal cord ischaemia to evaluate their accuracy and relationship to clinical status.

METHODS

A modified rabbit spinal cord ischaemia model of infrarenal aortic occlusion for 21 min was employed (30 rabbits). After baseline SEPs, Spinal-EPs, and MEPs were obtained, evoked potentials were recorded continuously during and after clamping of the aorta (30 min). Neurological outcome at 24 h was correlated with evoked potentials, and histopathological findings.

RESULTS

Fifteen animals became paraplegic. MEPs were always abolished after clamping of the aorta while Spinal-EPs and SEPs remained. The sensory evoked potentials (SEPs and Spinal-EPs) were the least sensitive to spinal cord ischaemia, and their presence had no correlation with the final clinical status (50% of false negatives). This was consistent with histopathological examination that showed damage almost entirely confined to the anterior horn, while the dorsal columns were generally well preserved. High spine MEPs evoked by twitch stimulation was the best predictor of clinical outcome (0% of false negatives, 0% of false positives).

CONCLUSIONS

SEPs and Spinal-EPs cannot be used as safe monitors of ischaemia of the spinal cord. High spine MEPs evoked by twitch stimulation was the most useful for real-time evaluation of spinal cord ischaemia, and the best predictor of neurologic outcome during reperfusion.

Prevention of spinal cord ischaemic complications after thoracoabdominal aortic surgery

Since the publication of prior reviews on this topic, substantial clinical experience with a variety of operative strategies to prevent ischaemic cord complications has been reported. The available data on angiographic localisation of critical intercostal vessels, and, in particular, the evoked potential response to cross-clamping in patients indicates that risk of paraplegia varies considerably even among patients with equivalent TAA extent. Factors such as individual development of the ASA, patent critical intercostals, and the particulars of collateral circulation when intercostal aortic ostia are already occluded likely account for this variability. Information available from SSEP monitoring relative to the dynamic course of cord ischaemia with cross-clamping, and the parallel, if not, frustrating experience with angiographic localisation and intercostal vessel reconstruction indicates that a narrow temporal threshold of cord ischaemia with clamping is present in many patients. This reinforces the importance of both expeditious clamp intervals, critical intercostal re-anastomoses, and the desirability of neuroprotective manoeuvres during cross-clamp induced cord ischemia. As suggested in compelling experimental work our contemporary clinical experience, and predicted by prior reviewers, regional cord hypothermia provides significant promise for limiting or eliminating, in particular, immediate perioperative deficits. Avoidance of postoperative hypotension, spinal cord oedema, and preservation of critical intercostal vessels are additional strategies necessary to impact the development of delayed deficits favourably.

The risk of ischemic spinal cord injury in patients undergoing graft replacement for thoracoabdominal aortic aneurysms

PURPOSE

We developed a monitoring system to detect spinal cord ischemia during aortic cross-clamping (AXC). This system was used to prospectively determine in which patients ischemia occurs, in which patients reimplantation of intercostal arteries is unnecessary or mandatory, and when reperfusion of intercostal arteries (ICAs) is urgent.

METHODS

Two hundred sixty patients underwent thoracoabdominal aortic aneurysm (TAA) repair with simple AXC. In 167 patients, two electrocatheters were placed before the onset of anaesthesia at level L1/L2 (stimulation) and level T5/T6 (recording) within the epidural space. During surgery, spinal cord function was monitored by recording spinal somatosensory evoked potentials (sSSEP). According to the extent of aortic replacement, most patients were expected to have a high risk of paraplegia.

RESULTS

In group A (59 patients), sSSEP remained normal throughout surgery, and in 54 of these patients ICAs were not reattached outside the proximal aortic anastomosis. In the other five patients ICAs were reimplanted separately because of possible anatomic relation to spinal cord blood supply. No patient in group A had postoperative neurologic deficit. In group B (54 patients) sSSEP remained normal until 15 minutes after AXC but were impaired thereafter. Nineteen patients had early reimplantation of ICAs. Of the 19, three had paraparesis and two had paraplegia. Neurologic deficit developed in the patients without early reimplantation of ICAs. In four patients separate reimplantation of ICAs was performed late in the procedure because of incomplete sSSEP recovery. Subsequently, the sSSEP returned to normal and only one of the four patients had mild paraparesis. The total rate of neurologic deficits in this group was 13% (paraplegia, 3.5%; paraparesis, 9.5%). All 54 patients in group C showed rapid loss of sSSEP within 15 minutes of AXC. In 28 patients ICAs were reimplanted only within the proximal anastomosis. Twenty-one of these patients showed prompt signal recovery after blood-flow release into the reimplanted ICAs, and none had neurologic deficit. Seven patients had no or very late and incomplete sSSEP recovery. Of the seven, three had paraplegia and four had paraparesis. In 26 patients ICAs were reimplanted separately to the proximal anastomosis. This was done early during the procedure in 17 patients, of whom 13 had full recovery of sSSEP and normal neurologic status. Four patients had incomplete or no recurrence of sSSEP, followed by paraplegia in one and paraparesis in three. In nine patients ICAs were reimplanted after the aortic replacement had been completed because of sSSEP recovery was not satisfactory. In all patients in this subgroup sSSEP returned to normal. Six patients had a normal neurologic status and three had mild paraparesis. The total neurologic complication rate in group C was 26% (paraplegia, 7.5%; paraparesis, 18.5%).

CONCLUSION

The risk of ischemic spinal cord injury during replacement for TAA can be assessed continuously by monitoring the sSSEP directly from the spinal cord. Patients without sSSEP changes during aortic reconstruction do not require ICA reattachment and will not have neurologic deficit. Patients who lose sSSEP after AXC are at risk for paraplegia. Patients with impairment or loss of sSSEP >15 minutes after AXC have some collateral vessels, and must have ICAs reimplanted only if sSSEP do not return within normal recovery time after blood-flow release into the proximal anastomosis. Loss of sSSEP within 15 minutes of AXC shows poor collateralization and mandates early restoration of spinal cord blood supply. If the surgeon can achieve the return of sSSEP to normal by subsequent separate reimplantation of ICAS, paraplegia will not occur and paraparesis will be rare and mild. Spinal cord monitoring is a valuable guide to detect whether the spinal cord is at risk and to take measures against par

Somatosensory evoked potential, a prognostic tool for the recovery of motor function following malperfusion of the spinal cord: studies in dogs

The potential usefulness of somatosensory evoked potential monitoring during aortic cross-clamping is slowly being realized. In addition, the protection of endangered spinal nervous tissue during aortic cross-clamping has not been sufficiently evaluated. To test the pharmacologic protective efficacy of various agents, we recorded spinal evoked somatosensory potentials (bipolar epidural catheter) in dogs under controlled conditions (N2O/O2-enflurane anesthesia) following clamping of the aorta for 1 hour. There were 5 groups of animals: those treated with different medications, such as prostaglandin E1 (PGE1), prostacyclin (PGI2), superoxide dismutase (SOD), and PGE1 plus SOD for pharmacologic protection during ischemia, and the controls. The time to recovery of evoked potentials during the reperfusion period was 36 minutes in the controls, 15.9 minutes in the SOD group (p < 0.01), 12.5 minutes in the PGE1 group (p < 0.001), 10.8 minutes in the PGI2 group (p < 0.001), and 3.8 minutes in the combination group (p < 0.001). In addition, treatment resulted in a better neurologic outcome on the seventh postoperative day when compared with the control group. While in the control group only 1 animal could walk (9%), 7 of 12 in the PGE1 group (58%), 4 of 12 in the SOD group (33.8%), 8 of 12 in the PGI2 group (66.7%), and all animals in the combination group (100%) could walk. We computed an exponential correlation that related the mean time of potential recovery during reperfusion with Tarlov scoring (grade 0 = paraplegia; grade 1 = paraplegia with little movements; grade 2 = paraparesis; grade 3 = paraparesis with some problems; grade 4 = normal motor function) in the various groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Does prostaglandin E1 and superoxide dismutase prevent ischaemic spinal cord injury after thoracic aortic cross-clamping?

The beneficial use of prostaglandin E1 (PGE1) and superoxide dismutase (SOD) on the tolerance to ischaemia of the spinal cord was evaluated following thoracic aortic cross-clamping in dogs. Aside from spinally evoked somatosensory potential (SEP) by means of a bipolar epidural catheter, postoperative evaluation of motor deficits was used to determine the efficiency of pharmacological protection when compared with controls. The animals were divided into four groups. Group I (n = 12) served as controls. The dogs of Group II (n = 12) were treated with PGE1 (100 ng/kg/min) during clamping and the first hour after declamping. In the third group (n = 12) SOD was given as an intra-arterial bolus (1 mg/kg) prior to declamping which was followed by a continuous perfusion (0.4 mg/kg/min) into the carotid artery for 25 min. In Group IV (n = 12) the dogs were treated with a combination of PGE1 and SOD in the same manner as in Groups 3 and 4. Results after pharmacological protection were significantly better than controls. In Group I all animals but one (92%) were paraplegic, as were five in Group II (42%) and eight in Group III (67%). In contrast no dog in Group IV developed paraplegia. There was a close correlation of SEP and postoperative recovery. The group with combination therapy (PGE1 plus SOD) was characterised by a loss of the evoked potential for a mean of 15 min, the PGE1 group for 45.8 min and the SOD group for 58.5 min. While the control group was characterised by a loss of 72.7 min.(ABSTRACT TRUNCATED AT 250 WORDS)

Epidural-evoked potentials: a more specific indicator of spinal cord ischemia

The purpose of this experimental study was to examine the differences between peripheral nerve stimulation and direct spinal stimulation in generating cortical somatosensory-evoked potential (SEP) responses for monitoring spinal cord ischemia during thoracic aorta cross-clamping. Adult mongrel dogs (n = 6) were placed under general anesthesia and a left thoracotomy was performed. A conventional stimulating electrode was placed over the posterior tibial nerve (PN-SEP), and a special bipolar electrode was placed epidurally over the spinal cord at L1-2 (SC-SEP). The aorta was cross-clamped proximal to the left subclavian artery. Stimulations were alternately performed through both electrodes, and SEP responses were continuously monitored. The cross-clamp was released after one hour and the animal was observed for another hour prior to sacrifice. Excellent SEPs were obtained with six stimuli over 3 sec via the SC-SEP stimulus in contrast to the 200 stimulations over 90 sec required for the PN-SEP stimulus. Aortic cross-clamping resulted in a significantly longer mean time to loss of SEPs for SC-SEP (mean +/- SEM, 13.7 +/- 1.0 min for SC-SEP vs 11.3 +/- 0.7 min for PN-SEP, P less than 0.05). Likewise, unclamping of the aorta consistently resulted in a shorter mean time to return of SEPs for SC-SEP compared with PN-SEP. These data indicate that direct epidural stimulation for evoked cortical responses is a more sensitive means of determining the adequacy of posterior spinal cord blood flow as reflected by posterior spinal cord function.(ABSTRACT TRUNCATED AT 250 WORDS)

Recording of spinal somatosensory evoked potentials for intraoperative spinal cord monitoring

The authors' experience with intradural and epidural recording of spinal somatosensory evoked potentials (SSEP's) during 26 cases of spinal surgery is described. The techniques of monitoring spinal cord function provided good quality SSEP waveforms in patients both with and without neurological deficits. The SSEP configuration and peak latencies remained stable for up to 5 hours during anesthesia with nitrous oxide, halothane, and fentanyl. Patterns of baseline SSEP's were characteristic of different spinal segments. Distortion and asymmetry of these baseline patterns were seen in several patients with spinal neoplasms. Loss of waveform components during surgery occurred with profound hypotension, overdistraction of the vertebral axis, dorsal midline myelotomy, and removal of intramedullary tumors. Persistent loss of waveform components was associated with an acquired neurological deficit. Fluctuations in the amplitude of the SSEP's were common but were not associated with postoperative neurological deficits. Spinal cord monitoring by means of SSEP recording would appear to be useful during extradural spinal surgery, but there are limitations associated with this technique during some types of intradural surgery.