Effects of induced hypothermia on somatosensory evoked potentials in patients with chronic spinal cord injury

Therapeutic Hypothermia in Spinal Cord Injury: The Status of Its Use and Open Questions

Spinal cord injury (SCI) is a major health problem and is associated with a diversity of neurological symptoms. Pathophysiologically, dysfunction after SCI results from the culmination of tissue damage produced both by the primary insult and a range of secondary injury mechanisms. The application of hypothermia has been demonstrated to be neuroprotective after SCI in both experimental and human studies. The myriad of protective mechanisms of hypothermia include the slowing down of metabolism, decreasing free radical generation, inhibiting excitotoxicity and apoptosis, ameliorating inflammation, preserving the blood spinal cord barrier, inhibiting astrogliosis, promoting angiogenesis, as well as decreasing axonal damage and encouraging neurogenesis. Hypothermia has also been combined with other interventions, such as antioxidants, anesthetics, alkalinization and cell transplantation for additional benefit. Although a large body of work has reported on the effectiveness of hypothermia as a neuroprotective approach after SCI and its application has been translated to the clinic, a number of questions still remain regarding its use, including the identification of hypothermia's therapeutic window, optimal duration and the most appropriate rewarming rate. In addition, it is necessary to investigate the neuroprotective effect of combining therapeutic hypothermia with other treatment strategies for putative synergies, particularly those involving neurorepair.

Novel Potassium Channel Blocker, 4-AP-3-MeOH, Inhibits Fast Potassium Channels and Restores Axonal Conduction in Injured Guinea Pig Spinal Cord White Matter

We have demonstrated that 4-aminopyridine-3-methanol (4-AP-3-MeOH), a 4-aminopyridine derivative, significantly restores axonal conduction in stretched spinal cord white-matter strips and shows no preference in restoring large and small axons. This compound is 10 times more potent when compared with 4-AP and other derivatives in restoring axonal conduction. Unlike 4-AP, 4-AP-3-MeOH can restore axonal conduction without changing axonal electrophysiological properties. In addition, we also have confirmed that 4-AP-3-MeOH is indeed an effective blocker of IA based on patch-clamp studies using guinea pig dorsal root ganglia cells. Furthermore, we have also provided the critical evidence to confirm the unmasking of potassium channels following mechanical injury. Taken together, our data further supports and implicates the role of potassium channels in conduction loss and its therapeutic value as an effective target for intervention to restore function in spinal cord trauma. Furthermore, due to its high potency and possible low side effect of impacting electrophysiological properties, 4-AP-3-MeOH is perhaps the optimal choice in reversing conduction block in spinal cord injury compared with other derivatives previously reported from this group.

Recent advances in pathophysiology and treatment of spinal cord injury

Thirty years ago, patients with spinal cord injury (SCI) and their families were told "nothing can be done" to improve function. Since the SCI patient population is reaching normal life expectancy through better health care, it has become an obviously worthwhile enterprise to devote considerable research effort to SCI. Targets for intervention in SCI toward improved function have been identified using basic research approaches and can be simplified into a list: (1) reduction of edema and free-radical production, (2) rescue of neural tissue at risk of dying in secondary processes such as abnormally high extracellular glutamate concentrations, (3) control of inflammation, (4) rescue of neuronal/glial populations at risk of continued apoptosis, (5) repair of demyelination and conduction deficits, (6) promotion of neurite growth through improved extracellular environment, (7) cell replacement therapies, (8) efforts to bridge the gap with transplantation approaches, (9) efforts to retrain and relearn motor tasks, (10) restoration of lost function by electrical stimulation, and (11) relief of chronic pain syndromes. Currently, over 70 clinical trials are in progress worldwide. Consequently, in this millennium, unlike in the last, no SCI patient will have to hear "nothing can be done."

Clinical and electrophysiologic correlates of quantitative sensory testing in patients with incomplete spinal cord injury

OBJECTIVE

To determine the degree of association among indices of preserved sensation derived from quantitative sensory testing (QST), somatosensory evoked potentials (SEPs), and the clinical characteristics of patients with spinal cord injury (SCI).

DESIGN

A controlled correlational study of diverse measures of preserved sensory function.

SETTING

Regional SCI rehabilitation center in Ontario, Canada.

PARTICIPANTS

Thirty-three patients with incomplete SCI and 14 able-bodied controls.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

QST measures of perceptual threshold for temperature and vibration, American Spinal Injury Association sensory scores (light touch, pinprick), and tibial nerve SEPs.

RESULTS

There was a low degree of association (kappa) between QST results and sensory scores (|kappa|=.05-.44). QST measures yielded greater numbers of patients with SCI being classified as impaired, suggesting a greater sensitivity of QST to detect more subtle sensory deficits. QST measures of vibration threshold generally corresponded to the patients' SEP recordings. QST measures of modalities conveyed within the same tract were significantly (P<.05) correlated (|r|=.46-.84) in patients with SCI, but not in controls, whereas those modalities mediated by different pathways had lower and generally nonsignificant correlations (|r|=.05-.44) in both patients and controls.

CONCLUSIONS

The low degree of association between QST measures and sensory scores is likely attributable to measurement limitations of both assessments, as well as various neuroanatomic and neuropathologic factors. QST provides more sensitive detection of preserved sensory function than does standard clinical examination in patients with incomplete SCI.

The role of directly applied hypothermia in spinal cord injury

STUDY DESIGN

The effect of intense local hypothermia was evaluated in a precision model of spinal canal narrowing and spinal cord injury in rats. The spinal cord injury was cooled with a custom cooling well used over the epidural surface. Basso, Beattie, and Bresnahan (BBB) motor scores and transcranial magnetic motor-evoked potential (tcMMEP) responses were used after injury to accurately evaluate neurologic recovery.

OBJECTIVE

This study was undertaken to determine whether the prognosis for neurologic recovery in a standardized rat spinal cord injury model is altered by the direct application of precisely controlled hypothermia to the area of injury.

SUMMARY OF BACKGROUND DATA

The role of hypothermia in the treatment of spinal cord injuries with neurologic deficits remains undefined. Hypothermia may decrease an area of spinal cord injury and limit secondary damage, therefore improving neurologic recovery. However, it has been difficult to consistently apply localized cooling to an area of spinal cord injury, and the use of systemic hypothermia is fraught with complications. This fact, along with the unavailability of a precise spinal cord injury model, has resulted in inconsistent results, both clinically and in the laboratory. In a rat model of spinal cord injury, 37 C and 19 C temperatures were used to study the role of hypothermia on neurologic recovery.

METHODS

Male Spraque-Dawley rats (n = 52; weight, 277.7 g) were anesthetized with pentobarbital and subjected to laminectomy at T10. The rats were divided into three groups: 1) placement of a 50% spacer in the epidural space (16 rats), 2) severe (25 g/cm) spinal cord injury (16 rats), 3) 50% spacer in combination with spinal cord injury (16 rats). Eight rats in each group were tested at two temperatures: normothermic (37 C) and hypothermic (19 C). With the use of a specially designed hypothermic pool placed directly over the spinal cord for 2 hours, epidural heating to 37 C, and epidural cooling to 19 C was accomplished. Simultaneous measurements of spinal cord and body temperatures were performed. The rats underwent behavior testing using the BBB motor scores and serial tcMMEPs for 5 weeks. Statistical methods consisted of Student's t tests, one-way analysis of variance, Tukey post hoc t tests and chi2 tests.

RESULTS

There was a significant improvement in motor scores in rats subjected to hypothermia compared with those that were normothermic after insertion of a 50% spacer. This improvement was observed during the 5-week duration of follow-up. In the severe spinal cord injury group and the spinal cord injury-spacer groups, no significant improvement in motor scores were obtained when the spinal cord was exposed to hypothermia.

CONCLUSION

The results demonstrate that there is a statistically significant (P < 0.05) improvement in neurologic function in rats subjected to hypothermia (19 C) after insertion of a spacer that induced an ischemic spinal cord injury. This indicates that directly applied hypothermia may be beneficial in preventing injury secondary to ischemic cellular damage. The data demonstrated minimal therapeutic benefit of hypothermia (19 C) after a severe spinal cord injury.

Temperature dependence of membrane sealing following transection in mammalian spinal cord axons

Using an in vitro sucrose-gap recording chamber, sealing of cut axons in isolated strips of white matter from guinea pig spinal cord was measured by recording the "compound membrane potential". This functional sealing was found to correlate well with anatomical resealing, measured by a horseradish peroxidase uptake assay. Near-complete functional and anatomical recovery of the axonal membrane occurred routinely within 60 min following transection at 37 degrees C in regular Krebs' solution. The rate of membrane potential recovery is exponential, with a time-constant of 20+/-5 min. The sealing process at 31 degrees C was similar to that at 37 degrees C, and was effectively blocked at 25 degrees C, under which condition most axons continued to take up horseradish peroxidase for more than 1h, and failed to substantially recover their membrane potential. Seventy-five percent of the cords transected at 40 degrees C had similar sealing behavior to those at 37 degrees C and 31 degrees C. The balance failed to seal the cut end. Two-dimensional morphometric analysis has shown that raising the temperature from 25 degrees C to above 31 degrees C significantly decreases axonal permeabilization to horseradish peroxidase (increases the sealing of transected ends) across all areas of a transverse section of spinal cord. Moreover, this enhancement of sealing exists across all axon calibers. Since severe cooling compromises membrane resealing, caution needs to be taken when hypothermic treatment (below 25 degrees C) is applied within the first 60 min following mechanical injury. In summary, we have found that at normal temperature (37 degrees C), nerve fibers repair their damaged membrane following physical injury with an hour. This is similar at mildly lower (31 degrees C) and relatively higher (40 degrees C) temperature, although some fibers tend to collapse under this febrile temperature. Moreover, severely low temperature (25 degrees C) hindered the repair of damaged membranes. Based on our study, caution is needed in treating spinal cord injury with low temperatures.

Effects of core body temperature on changes in spinal somatosensory-evoked potential in acute spinal cord compression injury: an experimental study in the rat

STUDY DESIGN

Acute spinal cord injury was induced by a clip compression model in rats to approximate spinal cord injury encountered in spinal surgery. Spinal somatosensory-evoked potential neuromonitoring was used to study the electrophysiologic change.

OBJECTIVES

To compare and correlate changes in evoked potential after acute compression at different core temperatures with postoperative neurologic function and histologic change, to evaluate current intraoperative neuromonitoring warning criteria for neural damage, and to confirm the protective effect of hypothermia in acute spinal cord compression injury by electrophysiologic, histologic, and clinical observation.

SUMMARY OF BACKGROUND DATA

With the increase in aggressive correction of spinal deformities, and the invasiveness of surgical instruments, the incidence of neurologic complication appears to have increased despite the availability of sensitive intraoperative neuromonitoring techniques designed to alert surgeons to impending neural damage. Many reasons have been given for the frequent failures of neuromonitoring, but the influence of temperature-a very important and frequently encountered factor-on evoked potential has not been well documented. Specifically, decrease in amplitude and elongation of latency seem not to have been sufficiently taken into account when intraoperative neuromonitoring levels were interpreted and when acceptable intraoperative warning criteria were determined.

METHODS

Experimental acute spinal cord injury was induced in rats by clip compression for two different intervals and at three different core temperatures. Spinal somatosensory-evoked potential, elicited by stimulating the median nerve and recorded from the cervical interspinous C2-C3, was monitored immediately before and after compression, and at 15-minute intervals for 1 hour.

RESULTS

Spinal somatosensory-evoked potential change is almost parallel to temperature-based amplitude reduction and latency elongation. Significant neurologic damage induced by acute compression of the cervical spinal cord produced a degree of effect on the amplitude of spinal somatosensory-evoked potential in normothermic conditions that differed from the effect in moderately hypothermic conditions. Using the same electromonitoring criteria,moderately hypothermic groups showed a significantly higher false-negative rate statistically (35%) than normothermic groups (10%).

CONCLUSIONS

Systemic cooling may protect against the detrimental effects of aggressive spinal surgical procedures. There is still not enough published information available to establish statistically and ethically acceptable intraoperative neuromonitoring warning and intervention criteria conclusively. Therefore, an urgent need exists for further investigation. Although a reduction of more than 50% in evoked potential still seems acceptable as an indicator of impending neural function loss, maintenance of more than 50% of baseline evoked potential is no guarantee of normal postoperative neural function, especially at lower than normal temperatures.

Conduction block in acute and chronic spinal cord injury: different dose-response characteristics for reversal by 4-aminopyridine

The effect of the potassium channel blocker, 4-aminopyridine (4-AP), on conduction of action potentials in injured guinea pig spinal cord axons was measured using isolated tracts in oxygenated Krebs' solution at 37 degrees C. The dose-response characteristics of acutely and chronically injured axons were compared. The maximal improvement of conduction occurred in acutely injured axons at a concentration of 100 microM 4-AP, but in chronically injured spinal cord at 10 microM. The threshold for significant response to 4-AP was between 0.5 and 1 microM in chronically injured cords, and between 1 and 10 microM following acute compression injury. The difference in susceptibility to potassium channel blockade may be related to underlying differences in the mechanism of conduction block at the two stages of injury. Initially, junctions between axons and myelin are acutely disrupted, altering primarily the leakage resistance of the myelin sheath and periaxonal space. In chronically injured cords, there is widespread but incomplete process of repair in the lesion site, which leaves many axons partially myelinated. The difference in sensitivity to 4-AP suggests there is also some modification of the accessibility of axonal potassium channel or a change in their affinity for the drug.

[Mild hypothermia and cerebral protection]

To define the part played by mild-to-moderate hypothermia in neuroprotection, it is necessary to take into account the thermoregulatory responses that occur in the normal human as the change in central temperature exceeds 0.2 degrees C. The mechanisms induced by cold are cutaneous vasoconstriction and shivering. They must be suppressed before starting controlled hypothermia. In these conditions, controlled moderate hypothermia between 32 and 35 degrees C does not seem to have deleterious side-effects, especially on coagulation. Caution is needed with the analysis of the numerous papers reporting experiments concerning the effects of moderate hypothermia in animals with induced cerebral ischaemia because of significant differences in the study designs. These differences concern mainly the time of onset of hypothermia, viz before or after ischaemia, the fact that the ischaemia is either global or focal, that it is caused by vascular occlusion posttraumatic or initiated by hypo or hyperglycemia. Some differences are also existing in the criteria used to appreciate the neuronal damage, as well as in the level of temperature and the site where it is measured. The mechanism of neuroprotection from moderate hypothermia seems to be not only a decrease in cerebral metabolism, but also involves a specific action on some intra-cellular events such as the blocking of the release of glutamate and of lipid peroxydation in brain tissue. An indirect proof of the neuroprotective effect of moderate hypothermia is the increase in the neuronal damage induced by moderate hyperthermia. It is conceivable that moderate hypothermia could exert a better neuroprotective effect than the drugs having this reputation, such as barbiturates, isoflurane and propofol.(ABSTRACT TRUNCATED AT 250 WORDS)