Biomedical subjects

R J Hurlbert

Publications and source records attributed to R J Hurlbert.

30 records · Page 2Linked to original sources

PubMed1995

Use of sensory-evoked potentials recorded from the human occiput for intraoperative physiologic monitoring of the spinal cord.

STUDY DESIGN: This is a report of a prospective case series. OBJECTIVE: The purpose of this study was to determine whether evoked potential activity recorded from occipital scalp electrodes in humans is similar to that recorded in animals, and to evaluate the independence of this activity from the classical somatosensory-evoked potential. SUMMARY OF BACKGROUND DATA: Intraoperative somatosensory-evoked potentials can be of limited usefulness in predicting spinal cord injury because they are transmitted primarily through the dorsal columns, and therefore do not reflect integrity of important ventral pathways. It recently has been shown in animal studies that a sensory-evoked potential recorded from the cerebellum is mediated via ventral tracts and is useful as an adjunct to the somatosensory-evoked potential in spinal cord monitoring. METHODS: Twenty-five patients undergoing spinal or posterior fossa surgery were consecutively entered into the study. Evoked responses were recorded transcranially from over the cerebellar hemispheres and from the vertex. Recordings were made directly from the surface of the cerebellar hemispheres in seven of these patients. RESULTS: Waveforms could be recorded and reproduced in all but one of the patients. The overall appearance of the occipital recordings was similar to the appearance of responses obtained in animals. The mean latency of the first negative peak recorded from the ipsilateral occiput was 33.0 msec (standard deviation, 3.7 msec) compared with 52.4 msec (standard deviation, 6.1 msec) for the somatosensory-evoked potential. In addition, the amplitude of this response (0.35 microV; standard deviation, 0.20 microV) was independent of the amplitude of the somatosensory-evoked potential (0.76 microV; standard deviation, 0.69 microV). In five cases, one evoked potential could be recorded in the absence of the other. Recordings from the surface of the cerebellum were of the same morphology, but of greater amplitude than the transcranial recordings. CONCLUSION: Evoked responses can be reliably recorded from over the occiput and show characteristics independent of the classical somatosensory-evoked potential. These responses are very similar to the cerebellar-evoked potential recently characterized in animals and may provide a method for assessing the physiologic integrity of the ventral tracts of the spinal cord in humans.

Animals↗

PubMed1995

"Unexplained" delayed death from fungal meningitis after meningioma resection.

BACKGROUND: Progressive deterioration and ensuing death following a neurosurgical procedure often represents a diagnostic challenge to the team responsible for patient care. Many, but not all, causes are treatable if a diagnosis is made early. METHODS: A 69-year-old woman who died 6 weeks post-operatively following a meningioma resection is reported. An initial routine post-operative course became complicated by progressive neurological deterioration 3-4 weeks later. Despite extensive investigation she died 6 weeks post-operatively without a diagnosis. RESULTS: Autopsy demonstrated extensive Candida meningitis. A review of the literature demonstrates this to be a reported complication in high risk patients, difficult to diagnose, but treatable when identified. CONCLUSIONS: Fungal meningitis should be high in the differential diagnosis in the post-operative patient with delayed, unexplained neurological deterioration, especially when associated with negative CSF cultures.

Aged↗

PubMed1994

Characterization of longitudinal field gradients from electrical stimulation in the normal and injured rodent spinal cord.

The purpose of this experiment was to characterize the longitudinal field gradients from exogenously applied electrical stimulation in the normal and injured rodent spinal cord. In addition, we compared the field gradients arising from stimulation with two different types of stimulating electrodes. Twenty normal rats underwent the surgical implantation of either extradural disc (n = 10) or cuff (n = 10) electrodes in the lower cervical and upper thoracic spinal cord. Sine waves of 1.5 to 50 microA and 0.5 to 50 Hz were used for stimulation. Field gradients were measured differentially from two extracellular glass microelectrodes, positioned stereotactically in the spinal cord at different locations between the stimulating electrodes. The effect of acute spinal cord injury on local field strength was studied in five animals from each group. The field gradients from stimulation with disc electrodes were greatest in close proximity to the discs and decreased markedly toward the point equidistant between the electrodes. In contrast, the gradients produced by cuff electrodes were much more evenly distributed along the spinal cord, increasing only slightly in proximity to the electrodes. These fields were also more evenly distributed throughout the spinal cord in cross-section but were generally weaker than those arising from disc electrodes. Acute spinal cord injury significantly increased the field gradients arising from both disc and cuff electrodes. However, the observed gradients remained substantially lower than those reported to enhance neurite growth in vitro. We conclude that the position and design of stimulating electrodes has a profound effect on longitudinal field gradients within the mammalian spinal cord, as does the presence of an acute spinal cord injury.

Animals↗

PubMed1993

An in vivo technique for 3-dimensional field mapping in the rodent spinal cord.

The purpose of this investigation was to develop a reliable method for in vivo mapping of small exogenously applied electrical fields in the mammalian spinal cord. Experiments were performed in vitro to validate the technique in addition to recordings made in vivo on normal and spinal cord injured rats. Sine waves of 1.5-50 microA and 0.5-50 Hz were used for stimulation. Field gradients were measured with a lock-in differential amplifier. Glass recording micro-electrodes were used for in vivo experiments. The accuracy of the technique was found to be highly dependent on the waveform of the current and impedance matching between the recording electrodes. Recordings made in vivo demonstrated that measured fields were linearly related to the intensity of stimulation. Field gradients were not influenced by stimulation frequencies of 5-50 Hz. Estimates were made of the influence of the surgical exposure on the measured gradients. Our results demonstrate that small electrical fields can be accurately mapped in vivo and interpolated throughout a broad range of intensities and frequencies.

Animals↗

PubMed1993

Sensory evoked potentials for selective monitoring of the rat spinal cord: a cerebellar evoked potential to assess ventral cord integrity.

The purpose of this study was to evaluate two types of ascending sensory evoked potentials (SEPs) in the rat and their capacity for selective monitoring of dorsal versus ventral spinal cord integrity. SEPs were elicited by direct sciatic nerve stimulation. A cerebellar evoked response was recorded over the paramedian lobule of the cerebellar hemisphere (CEPpml) while somatosensory evoked potentials (SSEPs) were simultaneously recorded over the sensorimotor cortex. All components of the CEPpml and SSEP except the longest latency positive waves were present in each animal. At stimulus intensities of 3 to 10 mA, no significant changes in latency of the peaks were observed, but amplitudes of the longer latency responses tended to increase throughout this stimulation range. Unilateral sciatic stimulation resulted in bilateral cortical responses, larger ipsilaterally for the CEPpml, and contralaterally for the SSEP. Selective spinal cord lesions demonstrated N9 and P14 of the CEPpml to be mediated primarily through the ventral spinal cord, while P14 and N19 of the SSEP were conducted primarily through the dorsal columns. Sectioning of the cerebellar peduncles abolished N9 and P14 of the CEPpml despite persistence of the SSEP. This study demonstrates that selective assessment of the ventral and dorsal spinal cord is possible in the rat by monitoring SEPs.

Animals↗

PubMed1993

Deep cerebral venous system thrombosis: case report.

A 38-year-old woman with thrombosis of the deep cerebral venous system, presumed to be secondary to the use of the oral contraceptive pill, is reported. The use of cerebral angiography and magnetic resonance imaging in establishing the diagnosis and management strategies is discussed. It is concluded that magnetic resonance imaging is an appropriate investigative modality for diagnosis and therapeutic decision making, and that, as with superficial cerebral dural sinus and venous thrombosis, anticoagulation is a safe and effective therapy.

Adult↗

PubMed1993

Dose-response study of the pathological effects of chronically applied direct current stimulation on the normal rat spinal cord.

Electrical stimulation of the mammalian central nervous system (CNS) can result in extensive destruction of tissue unless applied within specific stimulation parameters. Classically, unbalanced or monopolar currents have been avoided in order to minimize these harmful effects. However, direct current (DC) fields have recently been proposed for the treatment of spinal cord injury. Until now, no rigorous analysis has been made of the safety of these fields in the mammalian CNS. The purpose of this study was to determine the amount of chronically applied DC current that can be tolerated by the normal rodent spinal cord stimulated with metal disc electrodes. Thirty-five normal rats underwent implantation of DC stimulating devices and were allowed to recover for a period of 2 to 12 weeks. The stimulators delivered constant currents of 0 to 50 microA through two disc-shaped platinum/iridium electrodes positioned extradurally at the C-7 and T-3 levels. Following sacrifice of the animals, serial 8-microns cross sections of the spinal cord at the electrode sites were examined microscopically. Evidence of demyelination presumed due to the physical presence of the rostral electrode was seen in animals from most groups including control animals. Pathological changes directly attributable to the applied fields were seen with current as low as 3 microA. It was concluded that DC's of 3 microA or more are harmful to the mammalian CNS with this method of stimulation. In addition, the data suggest that the maximum current density tolerated by the rodent spinal cord is in the order of 75 microA/sq cm. These findings have important implications for the use of chronic DC stimulation in the mammalian CNS.

Animals↗

PubMed1992

Evoked potentials from direct cerebellar stimulation for monitoring of the rodent spinal cord.

Although the assessment of spinal cord function by electrophysiological techniques has become important in both clinical and research environments, current monitoring methods do not completely evaluate all tracts in the spinal cord. Somatosensory and motor evoked potentials primarily reflect dorsal column and pyramidal tract integrity, respectively, but do not directly assess the status of the ventral funiculus. The present study was undertaken to evaluate the use of evoked potentials, elicited by direct cerebellar stimulation, in monitoring the ventral component of the rodent spinal cord. Twenty-nine rats underwent epidural anodal stimulation directly over the cerebellar cortex, with recording of evoked responses from the lower thoracic spinal cord, both sciatic nerves, and/or both gastrocnemius muscles. Stimulation parameters were varied to establish normative characteristics. The pathways conducting these "posterior fossa evoked potentials" were determined after creation of various lesions of the cervical spinal cord. The evoked potential recorded from the thoracic spinal cord consisted of five positive (P1 to P5) and five negative (N1 to N5) peaks. The average conduction velocity (+/- standard deviation) of the earliest wave (P1) was 53 +/- 4 m/sec, with a latency of 1.24 +/- 0.10 msec. The other components followed within 4 msec from stimulus onset. Unilateral cerebellar stimulation resulted in bilateral sciatic nerve and gastrocnemius muscle responses; there were no significant differences (p greater than 0.05) in the thresholds, amplitudes, or latencies of these responses elicited by right- versus left-sided stimulation. Recordings performed following creation of selective lesions of the cervical cord indicated that the thoracic response was carried primarily in the ventral funiculus while the sciatic and gastrocnemius responses were mediated through the dorsal half of the spinal cord. It is concluded that the posterior fossa evoked potential has research value as a method of monitoring pathways within the ventral spinal cord of the rat, and should be useful in the study of spinal cord injury.

Animals↗

PubMed1991

The electrophysiological assessment of the pyramidal and non-pyramidal tracts of the spinal cord of rats.

The present paper summarizes our experience with the use of motor evoked potentials (MEP) and cerebellar evoked potentials (CEP) to assess the physiological integrity of the motor tracts of the spinal cord in a rat model. The MEP elicited by intracortical microstimulation of motor cortex reflected activity only in the pyramidal tracts; in contrast the MEP obtained by extradural cortical stimulation with a macroelectrode also reflected activity in a number of non-pyramidal pathways including the lateral vestibulospinal and rubrospinal tracts. The CEP was principally conducted in ventral spinal cord tracts and may reflect direct activation of brainstem nuclei. Data are presented which correlate the relationship between the MEP and axonal integrity after cord injury.

Animals↗

PubMedDate not supplied

Nonsteroidal anti-inflammatory drugs and bone metabolism in spinal fusion surgery: a pharmacological quandary.

The use of nonsteroidal anti-inflammatory drugs (NSAIDs) is ubiquitous in contemporary medical practice and these agents are efficacious in a number of clinical contexts. In particular, NSAIDs have proven to be highly effective adjuncts in the amelioration of postoperative pain in the subset of patients undergoing spinal surgery requiring fusion. NSAIDs act through inhibition of cyclooxygenase enzymes and therefore diminish prostaglandin production. However, prostaglandins are intimately involved in the modulation of bone metabolism and the balance of data, from both clinical and laboratory contexts, indicate that prostaglandins preferentially favor bone anabolism. Most recently, limited emerging evidence suggests that NSAID administration in patients undergoing spinal fusion surgery may increase nonunion rates, which in turn, has important ramifications to the patient, their family and the entire medical system. Hence, disparate views have evolved regarding the use of NSAIDs in postoperative pain control in patients undergoing spinal surgery requiring fusion. NSAIDs have proven efficacy in the management of postoperative pain in these patients, however, this must be weighed against the risk of nonunion and its associated consequences. In this review, the role of prostaglandins in bone metabolism, the pharmacology of NSAIDs and the modulation of bone metabolism by NSAIDs are discussed. Additionally, the current evidence examining the use of NSAIDs in spinal surgery is presented. As rates of spinal surgery continue to rise, it is imperative that the apparent pharmacological quandary surrounding the administration of NSAIDs in patients undergoing spinal surgery requiring fusion be addressed, both to guide present clinical practice and to outline further directions for investigation.

Animals↗

PubMedDate not supplied

Acute spinal cord injury, part I: pathophysiologic mechanisms.

Spinal cord injury (SCI) is a devastating and common neurologic disorder that has profound influences on modern society from physical, psychosocial, and socioeconomic perspectives. Accordingly, the present decade has been labeled the Decade of the Spine to emphasize the importance of SCI and other spinal disorders. Spinal cord injury may be divided into both primary and secondary mechanisms of injury. The primary injury, in large part, determines a given patient's neurologic grade on admission and thereby is the strongest prognostic indicator. However, secondary mechanisms of injury can exacerbate damage and limit restorative processes, and hence, contribute to overall morbidity and mortality. A burgeoning body of evidence has facilitated our understanding of these secondary mechanisms of injury that are amenable to pharmacological interventions, unlike the primary injury itself. Secondary mechanisms of injury encompass an array of perturbances and include neurogenic shock, vascular insults such as hemorrhage and ischemia-reperfusion, excitotoxicity, calcium-mediated secondary injury and fluid-electrolyte disturbances, immunologic injury, apoptosis, disturbances in mitochondrion function, and other miscellaneous processes. Comprehension of secondary mechanisms of injury serves as a basis for the development and application of targeted pharmacological strategies to confer neuroprotection and restoration while mitigating ongoing neural injury. The first article in this series will comprehensively review the pathophysiology of SCI while emphasizing those mechanisms for which pharmacologic therapy has been developed, and the second article reviews the pharmacologic interventions for SCI.

Acute Disease↗

PubMedDate not supplied

Acute spinal cord injury, part II: contemporary pharmacotherapy.

Spinal cord injury (SCI) remains a common and devastating problem of modern society. Through an understanding of underlying pathophysiologic mechanisms involved in the evolution of SCI, treatments aimed at ameliorating neural damage may be developed. The possible pharmacologic treatments for acute spinal cord injury are herein reviewed. Myriad treatment modalities, including corticosteroids, 21-aminosteroids, opioid receptor antagonists, gangliosides, thyrotropin-releasing hormone (TRH) and TRH analogs, antioxidants and free radical scavengers, calcium channel blockers, magnesium replacement therapy, sodium channel blockers, N -methyl-D-aspartate receptor antagonists, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid-kainate receptor antagonists, modulators of arachadonic acid metabolism, neurotrophic growth factors, serotonin antagonists, antibodies against inhibitors of axonal regeneration, potassium channel blockers (4-aminopyridine), paclitaxel, clenbuterol, progesterone, gabexate mesylate, activated protein C, caspase inhibitors, tacrolimus, antibodies against adhesion molecules, and other immunomodulatory therapy have been studied to date. Although most of these agents have shown promise, only one agent, methylprednisolone, has been shown to provide benefit in large clinical trials. Given these data, many individuals consider methylprednisolone to be the standard of care for the treatment of acute SCI. However, this has not been established definitively, and questions pertaining to methodology have emerged regarding the National Acute Spinal Cord Injury Study trials that provided these conclusions. Additionally, the clinical significance (in contrast to statistical significance) of recovery after methylprednisolone treatment is unclear and must be considered in light of the potential adverse effects of such treatment. This first decade of the new millennium, now touted as the Decade of the Spine, will hopefully witness the emergence of universal and efficacious pharmacologic therapy and ultimately a cure for SCI.

Acute Disease↗