Anesthesia influences the outcome from experimental spinal cord injury

Severity of locomotor and cardiovascular derangements after experimental high-thoracic spinal cord injury is anesthesia dependent in rats

Anesthetics affect outcomes from central nervous system (CNS) injuries differently. This is the first study to show how two commonly used anesthetics affect continuously recorded hemodynamic parameters and locomotor recovery during a 2-week period after two levels of contusion spinal cord injury (SCI) in rats. We hypothesized that the level of cardiovascular depression and recovery of locomotor function would be dependent upon the anesthetic used during SCI. Thirty-two adult female rats were subjected to a sham, 25-mm or 50-mm SCI at T3-4 under pentobarbital or isoflurane anesthesia. Mean arterial pressure (MAP) and heart rate (HR) were telemetrically recorded before, during, and after SCI. Locomotor function recovered best in the 25-mm-injured isoflurane-anesthetized animals. There was no significant difference in locomotor recovery between the 25-mm-injured pentobarbital-anesthetized animals and the 50-mm-injured isoflurane-anesthetized animals. White matter sparing and extent of intermediolateral cell column loss appeared larger in animals anesthetized with pentobarbital, but this was not significant. There were no differential effects of anesthetics on HR and MAP before SCI, but recovery from anesthesia was significantly slower in pentobarbital-anesthetized animals. At the time of SCI, MAP was acutely elevated in the pentobarbital-anesthetized animals, whereas MAP decreased in the isoflurane-anesthetized animals. Hypotension occurred in the pentobarbital-anesthetized groups and in the 50-mm-injured isoflurane-anesthetized group. In pentobarbital-anesthetized animals, SCI resulted in acute elevation of HR, although HR remained low. Return of HR to baseline was much slower in the pentobarbital-anesthetized animals. Severe SCI at T3 produced significant chronic tachycardia that was injury severity dependent. Although some laboratories monitor blood pressure, HR, and other physiological variables during surgery for SCI, inherently few have monitored cardiovascular function during recovery. This study shows that anesthetics affect hemodynamic parameters differently, which in turn can affect functional outcome measures. This supports the need for a careful evaluation of cardiovascular and other physiological measures in experimental models of SCI. Choice of anesthetic should be an important consideration in experimental designs and data analyses.

Loss and spontaneous recovery of forelimb evoked potentials in both the adult rat cuneate nucleus and somatosensory cortex following contusive cervical spinal cord injury

Varying degrees of neurologic function spontaneously recovers in humans and animals during the days and months after spinal cord injury (SCI). For example, abolished upper limb somatosensory potentials (SSEPs) and cutaneous sensations can recover in persons post-contusive cervical SCI. To maximize recovery and the development/evaluation of repair strategies, a better understanding of the anatomical locations and physiological processes underlying spontaneous recovery after SCI is needed. As an initial step, the present study examined whether recovery of upper limb SSEPs after contusive cervical SCI was due to the integrity of some spared dorsal column primary afferents that terminate within the cuneate nucleus and not one of several alternate routes. C5-6 contusions were performed on male adult rats. Electrophysiological techniques were used in the same rat to determine forelimb evoked neuronal responses in both cortex (SSEPs) and the cuneate nucleus (terminal extracellular recordings). SSEPs were not evoked 2 days post-SCI but were found at 7 days and beyond, with an observed change in latencies between 7 and 14 days (suggestive of spared axon remyelination). Forelimb evoked activity in the cuneate nucleus at 15 but not 3 days post-injury occurred despite dorsal column damage throughout the cervical injury (as seen histologically). Neuroanatomical tracing (using 1% unconjugated cholera toxin B subunit) confirmed that upper limb primary afferent terminals remained within the cuneate nuclei. Taken together, these results indicate that neural transmission between dorsal column primary afferents and cuneate nuclei neurons is likely involved in the recovery of upper limb SSEPs after contusive cervical SCI.

A model of experimental spinal cord trauma based on computer-controlled intervertebral distraction: characterization of graded injury

STUDY DESIGN

: A new model of experimental spinal cord injury is detailed based on the application of tensile (distraction) force to the vertebral column of the rat.

OBJECTIVES

: To develop an experimental model of graded spinal cord injury by application of tensile forces to the vertebral column.

SUMMARY OF BACKGROUND DATA

: Distraction is frequently an integral component of human spinal cord injury, but the acute application of tensile forces to the spinal cord has not been modeled rigorously.

METHODS

: A computer-controlled, motorized outrigger device was used to apply a longitudinal stretching force to sublaminar hooks oriented proximally at T9 and distally at T11. Distraction force was applied using a program that varied the length, speed, and duration of its distraction. A modified 14-point Tarlov score was used to establish the presence of hindlimb dysfunction. This score was correlated with acute changes in somatosensory-evoked potential amplitude, the comprehensive open-field test of locomotor function at 4 weeks, and postmortem measurements of serotonin content and metabolism in spinal cord rostral and distal to the site of injury.

RESULTS

: Of distraction parameters, only length of distraction correlated significantly with each outcome measure. For outcome measures, open-field test inventory and distal/proximal ratio of the spinal content of serotonin were correlated most closely with final Tarlov scores. Acute somatosensory-evoked potential amplitudes proved to be an excellent index of the acute injury but were poor measures of long-term outcome.

CONCLUSIONS

: Distraction-induced spinal cord injury was uniformly mild in rats with intact facet capsular ligaments, regardless of distraction parameters. Cutting the facet joint ligaments consistently generated outcome measures associated with mild, moderate, and severe spinal cord injury at 3-, 5-, and 7-mm distraction lengths, respectively.

Experimental modeling of spinal cord injury: characterization of a force-defined injury device

We examined the ability of a novel spinal cord injury (SCI) device to produce graded morphological and behavioral changes in the adult rat following an injury at thoracic level 10 (T10). The injury device uses force applied to the tissue as the control variable rather than tissue displacement. This has the advantage of eliminating errors that may arise from tissue movement prior to injury. Three different injury severities, defined by the amount of force applied to the exposed spinal cord at T10 (100, 150, and 200 kdyn), were evaluated at two different survival times (7 and 42 d). Unbiased stereology was employed to evaluate morphological differences following the injury. Quantitative behavioral assessment employed the Basso, Beattie, and Bresnahan locomotive rating scale. There was a significant force-related decline in locomotive ability following the injury. Animals subjected to a 200-kdyn injury performed significantly worse than animals subjected to a 100- and 150-kdyn injury. The locomotor ability at different days post injury significantly correlated with the amount of force applied to the spinal cord. Statistical analysis revealed several significant force-related morphological differences following the injury. The greatest loss of white and gray matter occurred at the site of injury impact and extended in both a rostral and caudal direction. Animals subjected to the greatest force (200 kdyn) displayed the least amount of spared tissue at both survival times indicative of the most severe injury. The amount of spared tissue significantly correlated with the locomotor ability. This novel rodent model of SCI provides a significant improvement over existing devices for SCI by reducing variability with a constant preset force to define the injury.

Experience-associated structural events, subependymal cellular proliferative activity, and functional recovery after injury to the central nervous system

Considerable structural plasticity is possible in the damaged neocortex and connected brain areas, and the potential for significant functional recovery remains even during the chronic phases of the recovery process. In this article, the authors review the literature on use-dependent morphologic events, focusing on the direct interaction of behavioral experience and structural changes associated with plasticity and degeneration. Experience-associated neural changes have the potential to either hinder or enhance functional recovery; therefore, issues concerning the nature, timing, and intensity of behavior-based intervention strategies are addressed.

Focal brain injury, FGF-2 and the adverse effects of excessive motor demand on cortical and nigral degeneration: marked protection by delayed intermittent exposure to halothane

The neuroprotective potential of halothane anesthesia was investigated following unilateral electrolytic lesions to the forelimb representation area of the sensorimotor cortex (FL-SMC). Previously, it was found that the FL-SMC lesion increases substantially in size when the intact forelimb is immobilized with a plaster of paris cast for the first 7 days postlesion, which forces extreme overuse of the impaired forelimb during a time when nonlethally damaged tissue is vulnerable to behavioral demand. Initially, the purpose of this study was to investigate whether intracisternal infusion of basic fibroblast growth factor (bFGF or FGF-2), a potent neurotrophic factor that has been shown to have neuroprotective and plasticity promoting properties in focal stroke and other injury models, could prevent this use-dependent exaggeration of injury. Although intracisternal bFGF (starting 24 h after surgery, twice per week) was not found to produce significant neuroprotective or behavioral effects, the brief exposure to halothane anesthesia (15-20 min) during bFGF or vehicle administration was found to prevent expansion of the lesion size, and to reduce delayed loss of neurons in the substantia nigra pars reticulata (SNr). The data have implications for investigations of the effects of neurotrophic factor in vivo, and other investigations requiring brief, intermittent halothane anesthesia.

An imposed oscillating electrical field improves the recovery of function in neurologically complete paraplegic dogs

We show that an applied electric field in which the polarity is reversed every 15 minutes can improve the outcome from severe, acute spinal cord injury in dogs. This study utilized naturally injured, neurologically complete paraplegic dogs as a model for human spinal cord injury. The recovery of paraplegic dogs treated with oscillating electric field stimulation (OFS) (approximately 500 to 600 microV/mm; n = 20) was compared with that of sham-treated animals (n = 14). Active and sham stimulators were fabricated in West Lafayette, Indiana. They were coded, randomized, sterilized, and packaged in Warsaw, Indiana, and returned to Purdue University for blinded surgical implantation. The stimulators were of a previously unpublished design and meet the requirements for phase I human clinical testing. All dogs were treated within 18 days of the onset of paraplegia. During the experimental applications, all received the highest standard of conventional management, including surgical decompression, spinal stabilization (if required), and acute administration of methylprednisolone sodium succinate. A radiologic and neurologic examination was performed on every dog entering the study, the latter consisting of standard reflex testing, urologic tests, urodynamic testing, tests for deep and superficial pain appreciation, proprioceptive placing of the hind limbs, ambulation, and evoked potential testing. Dogs were evaluated before and after surgery and at 6 weeks and 6 months after surgery. A greater proportion of experimentally treated dogs than of sham-treated animals showed improvement in every category of functional evaluation at both the 6-week and 6-month recheck, with no reverse trend. Statistical significance was not reached in comparisons of some individual categories of functional evaluation between sham-treated and OFS-treated dogs (ambulation, proprioceptive placing); an early trend towards significance was shown in others (deep pain), and significance was reached in evaluations of superficial pain appreciation. An average of all individual scores for all categories of blinded behavioral evaluation (combined neurologic score) was used to compare group outcomes. At the 6-month recheck period, the combined neurologic score of OFS-treated dogs was significantly better than that of control dogs (p = 0.047; Mann-Whitney, two-tailed).

Spinal cord and nerve root blood flow in acute double level spinal stenosis

STUDY DESIGN

Twenty-four pigs were randomized into three groups of eight pigs; a control group with 0% stenosis, a 25% stenosis group, and a 50% stenosis group. A fourth 75% stenosis group was added when results of the randomized experiment had been analyzed. Blood flow of the spinal cord and nerve roots and spinal evoked potentials were determined before and 1 hour after induction of the spinal stenoses.

OBJECTIVES

To study the acute effects of different degrees of spinal stenosis on neural tissue blood flow and spinal evoked potentials.

SUMMARY OF BACKGROUND DATA

Spinal cord dysfunction may be caused by vascular impairment or mechanical injury to neural tissue. Experimental double level compression of the cauda equina causes reversible nerve root edema, stasis, blood flow decrease, and compromised neural function. The vascular pathophysiology after spinal cord trauma was studied previously, and both increased and decreased neural tissue blood flow have been reported.

METHODS

Two level spinal stenosis was introduced by placement of stenosing bands around the dural sac at L4 and L6. Neurologic function was monitored by sensory and motor evoked potentials. Regional blood flow (RBF) was measured in the stenotic segments between the bands and other regions of neural tissue by radioactive microspheres before and after induction of stenosis.

RESULTS

Regional blood flow increased in the stenotic segments after 0% sham stenosis. Analysis of variance revealed no differences in RBF between the three randomized groups under comparable conditions of 0% stenosis. However, the RBF level of the added 75% group was lower than that of the other three groups. By comparison of RBF within groups before and after stenosis, no decrease in RBF was found between the stenosing bands in any of the groups. Fifty percent stenosis changed the amplitude of evoked potentials in half of the animals. Seventy-five percent stenosis caused severe changes in evoked potentials in 7 of 8 animals.

CONCLUSIONS

Blood supply of the spinal cord and nerve roots in the segments between two central stenoses is preserved immediately after stenosis introduction by way of the segmental nerve pathway, even if nerve conduction is impaired.