Spectral modulation of cortical connections measured by EEG coherence in humans.

OBJECTIVE: Description of coherence patterns of cortical EEG. METHODS: EEG recordings were collected from 9 subdural electrode grids implanted in 6 patients undergoing EEG monitoring for refractory epilepsy. RESULTS: Coherence decreased with increasing inter-electrode distance and exhibited considerable variability at the same inter-electrode distances. Analysis of variance demonstrated that both spatial and temporal factors contributed significantly (P < 0.01) to this variability. The spatial factor contributed the largest portion (up to 90%) of the variability, and was modulated by frequency and inter-electrode distance. CONCLUSION: The finding that the mean frequency modulation was consistent over time for the same pair of electrodes and was different between different pairs of electrodes indicated a spatial-spectral pattern of cortical synchrony. The connections (pairs of electrodes) could be accordingly predicted from their spatial-spectral pattern, which suggested that the spatial heterogeneity of neuronal synchrony was expressed not only by the degree of synchrony, but also by distinct spectral channels of synchrony. A model based on neuronal connection and activation is proposed to account for the observations.

Epilepsy and consciousness.

Seizure activity has long been associated with alterations in consciousness. Philosophical and neurologic debates concerning the definition of consciousness have led to confusion regarding an adequate third person assessment of a subjective experience. In order to avoid these controversies, neurologic evaluation of consciousness has focused on operational definitions that permit an objective assessment of behavioral responses that are constituent functions of consciousness. Clinical experience has demonstrated that ictal and post-ictal alterations in consciousness may be associated with loss of selected behavioral responses depending upon the focus and spread of seizure activity. Ictal electrophysiologic studies and brain stimulation has assisted in determining the anatomic structures involved in specific behavioral alterations. Consciousness-dependent mental activity can be modeled as a series of interactive parallel information channels that can be selectively disrupted at any stage of processing giving rise to ictal behavioral patterns. While such modeling falls to grasp the subjective nature of consciousness, it offers the clinical community an objective measure of those responses believed to be dependent on consciousness.

Quantitative electroencephalography: a method to assess cerebral injury after hypothermic circulatory arrest.

Although hypothermic circulatory arrest and low-flow cardiopulmonary bypass are routinely used for surgical correction of congenital cardiac anomalies, use of long durations of arrest, often required for more complex repairs, raises serious concerns about cerebral safety. Searching for an intraoperative assessment that can reliably predict cerebral injury, we have found an excellent correlation between changes in quantitative electroencephalography intraoperatively and immediately postoperatively after prolonged hypothermic arrest, and neurologic and behavioral evidence of cerebral injury. After epidural placement of four recording electroencephalographic electrodes and baseline neurologic/behavioral and electroencephalographic assessment, 32 puppies were randomly assigned to one of four groups: hypothermic controls in which cooling to 18 degrees C was followed immediately by rewarming, 30 minutes of hypothermic circulatory arrest at 18 degrees C, 90 minutes of arrest at 18 degrees C, and 90 minutes of low-flow cardiopulmonary bypass at 25 ml/kg per minute at 18 degrees C. An electroencephalogram was recorded at baseline, after cooling, during rewarming, and at 2, 4, and 8 hours after the start of rewarming, as well as before the operation and 1 week after the operation. Postoperative neurologic and behavioral outcome was assessed 24 hours after cardiopulmonary bypass and daily for 1 week by means of a graded scale in which 0 is normal and 12 and 13 indicate severe neurologic injury (coma and death). Thirty animals survived the experimental protocol: two animals in the 90-minute hypothermic arrest group died before neurologic evaluation could be completed, and the remainder exhibited various degrees of neurologic and behavioral impairment, more severe on day 1 than on day 6. No animal in the remaining groups had a significant neurologic deficit. Quantitative electroencephalographic analysis shows marked differences between the 90-minute arrest group and the controls in the percent electroencephalographic silence during rewarming and at 2 hours, and in the percent recovery of baseline power at 2, 4, and 8 hours. At 2 hours after the start of rewarming, a correlation between electroencephalographic amplitude and neurologic/behavioral score on day 1 was carried out, which predicts with great certainty (p < 0.00001) that if electroencephalographic power at this time is less than 500 microV2, overt neurologic injury will subsequently become apparent. In addition, a significant shift from higher to lower frequency in the day 6 postoperative electroencephalogram compared with baseline occurs only in the 90-minute arrest group.(ABSTRACT TRUNCATED AT 400 WORDS)

Pathogenesis of spinal cord injury during simulated aneurysm repair in a chronic animal model.

The pathogenesis of paraplegia after repair of thoracic aortic aneurysms is controversial. Using direct spinal cord evoked potential monitoring, critical intercostal arteries (CICA) were identified to evaluate the impact of backbleeding and ligation versus that of preservation during simulated aneurysm repair. Thirty pigs (40 kg) were randomly assigned to one of five groups. In groups 1 through 4, a thoracic segment containing CICA was cross-clamped for 60 minutes and distal aortic perfusion was provided by a centrifugal pump. In groups 1 and 2, the thoracic segment was vented, maintaining segment pressure at 0 mm Hg; CICA were ligated in group 1 and preserved in group 2. Thoracic segment was perfused at 70 mm Hg in groups 3 and 4; CICA were ligated in group 3 and preserved in group 4. Critical intercostal artery ligations were performed at the end of the cross-clamp period. In group 5 simple cross-clamping at the left subclavian artery was performed as a control. The combination of venting and ligation of CICA correlated with impaired neurologic outcome according to Tarlov's score (median, 1.5 in group 1 versus 3 in group 2; p = 0.015), indicated by a significant difference in median values of direct spinal cord evoked potential amplitude (expressed as a fraction of baseline values) at 120 minutes after cross-clamping (0.76 in group 1 versus 0.98 in group 2; p = 0.0082).(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of cerebral metabolism and quantitative electroencephalography after hypothermic circulatory arrest and low-flow cardiopulmonary bypass at different temperatures.

Although widely used for repair of complex cardiovascular pathologic conditions, long intervals of hypothermic circulatory arrest and low flow cardiopulmonary bypass may both result in cerebral injury. This study examines cerebral hemodynamics, metabolism, and electrical activity to evaluate the risks of cerebral injury after 60 minutes of hypothermic circulatory arrest at 8 degrees C, 13 degrees C, and 18 degrees C, compared with 60 minutes of low flow cardiopulmonary bypass at 18 degrees C. Thirty-two puppies were randomly assigned to one of four experimental groups and centrally cooled to the appropriate temperature. Serial evaluations of quantitative electroencephalography, radioactive microsphere determinations of cerebral blood flow, calculations of cerebral oxygen consumption, cerebral glucose consumption, cerebral vascular resistance, cerebral oxygen extraction, systemic oxygen metabolism, and systemic vascular resistance were done. Measurements were obtained at baseline (37 degrees C), at the end of cooling, at 30 degrees C during rewarming, and at 2, 4, and 8 hours after hypothermic circulatory arrest or low flow cardiopulmonary bypass. At the end of cooling, cerebral vascular resistance remained at baseline levels in all groups, but systemic vascular resistance was increased in all groups. Cerebral oxygen consumption became progressively lower as temperature was reduced: it was only 5% of baseline at 8 degrees C; 20% at 13 degrees C; and 34% and 39% at 18 degrees C. Quantitative electroencephalography was silent in the 8 degrees C and 13 degrees C groups, but significant slow wave activity was present at 18 degrees C. Systemic vascular resistance and cerebral oxygen consumption returned to baseline values in all groups by 2 hours after hypothermic circulatory arrest or low flow cardiopulmonary bypass, but cerebral vascular resistance remained elevated at 2 and 4 hours, not returning to baseline until 8 hours after hypothermic circulatory arrest or low flow cardiopulmonary bypass. All but two of the long-term survivors (27 of 32) appeared neurologically normal; after hypothermic circulatory arrest at 8 degrees and 18 degrees C two animals had an unsteady gait. Comparison of quantitative electroencephalography before operation and 6 days after operation showed a significant increase in slow wave activity (delta activity) after hypothermic circulatory arrest and low flow cardiopulmonary bypass at 18 degrees C, a change that suggests possible cerebral injury. Although undetected after operation by simple behavioral and neurologic assessment, significant differences in cerebral metabolism, vasomotor responses, and quantitative electroencephalography do exist during and after hypothermic circulatory arrest and low flow cardiopulmonary bypass at various temperatures and may be implicated in the occurrence of cerebral injury.(ABSTRACT TRUNCATED AT 400 WORDS)

Cerebral effects of low-flow cardiopulmonary bypass and hypothermic circulatory arrest.

Although both hypothermic circulatory arrest (HCA) and low-flow cardiopulmonary bypass (CPB) are accepted techniques for the operative management of complex cardiovascular pathology, the potential for neurologic sequelae is still a concern. To assess the relative safety of these techniques, we compared cerebral hemodynamics and clinical outcome in two groups of puppies. Sixteen puppies underwent 45 minutes of either HCA or low-flow CPB (25 mL.kg-1.min-1) after cooling to 13 degrees C. Methodology included radioactive microsphere determination of cerebral blood flow; calculation of cerebral oxygen extraction (arteriovenous oxygen content difference) and consumption; measurement of glucose consumption, and determination of cerebrovascular resistance. Measurements were obtained at baseline (37 degrees C), 13 degrees C, and 30 degrees C and at 2, 4, and 8 hours after HCA or low-flow CPB. No neurologic deficits were observed in any of the survivors (15/16). In both groups, cerebral metabolic rate of oxygen was maintained at baseline or greater levels postoperatively. Cerebrovascular resistance rose slightly in the low-flow CPB group postoperatively in contrast to a marked elevation in the HCA group. During the period of high cerebrovascular resistance after HCA, cerebral metabolic rate of oxygen was maintained by increased oxygen extraction. After low-flow CPB, oxygen extraction was not significantly different from baseline, presumably because of less severe changes in cerebrovascular resistance. Glucose metabolism followed the same trends as oxygen metabolism in both groups. These data suggest that after HCA there is a vulnerable interval, lasting as late as 8 hours postoperatively, in which cerebrovascular resistance remains high and cerebral metabolism is maintained primarily by high oxygen and glucose extraction. Any additional stress during this interval (a decrease in arterial oxygen content or perfusion pressure) could result in cerebral injury.

Effect of blood-brain barrier and blood-tumor barrier modification on central nervous system liposomal uptake.

In this study of 25 central nervous system (CNS) tumor-bearing rats, the CNS biodistribution of intravenously administered, indium-labeled liposomes was investigated. In 16 animals, the blood-brain barrier and blood-tumor barrier were modified using intracarotid administration of etoposide. In control animals, analysis by autoradiography and well-counting experiments demonstrated uptake of liposomes in the tumor-bearing hemisphere (% injected dose/g tissue = 0.135) with minimal uptake in the non-tumor-bearing hemisphere (% injected dose/g tissue = 0.007), p < 0.01. Unilateral intracarotid etoposide administration enhanced liposome uptake in both hemispheres-0.215 and 0.023 (tumor-bearing and nontumor-bearing), respectively. The presence of meningeal tumor involvement in nontumor-implanted hemispheres increased liposomal uptake 10-fold. These findings may have clinical applicability in designing therapeutic protocols for the treatment of CNS tumors.

The effect of graded spinal cord injury on the extrapyramidal and pyramidal motor evoked potentials of the rat.

This study investigated the differential effects of graded spinal cord injury on the rat extrapyramidal motor evoked potential (exp-MEP) and pyramidal motor evoked potential (pyr-MEP) and the prognostic value of these effects in predicting postinjury motor performance in the rat model. In 20 rats subjected to graded spinal injury (10-100 g-cm), there was a differential injury threshold for ablation of exp-MEP and pyr-MEP. All peaks of the pyr-MEP were extinguished in the animals subjected to impact forces of 50 g-cm and above (n = 12). In contrast, the exp-MEP was completely abolished in only two animals at injuries of 80 g-cm or above. A residual exp-MEP response persisted in the remaining 18 animals. Motor performance was monitored in 16 additional animals for up to 1 week after spinal injury. The pyr-MEP was abolished in 100% of the rats subjected to a 50-g cm injury (n = 7), whereas the exp-MEP persisted up to the highest impact forces (80 g-cm). Hind leg paralysis was present in the five rats where the pyr-MEP was extinguished but with persistence of the exp-MEP. An 80% reduction in the amplitude of the pyr-MEP in four animals resulted in mild ataxia with motor improvement at the end of a week. An increase or a 70% loss in pyr-MEP peak amplitude resulted in no clinical motor deficits (n = 5).(ABSTRACT TRUNCATED AT 250 WORDS)

EEG coherence as a predictor of spike propagation.

The relationship between resting EEG coherence and the propagation of spike activity from an experimental cortical focus was investigated in 6 rats. EEG was collected from an array of 6 epicortical electrodes positioned over the posterior hind limb sensorimotor (HL) and frontal (Fr1, Fr2) cortices. Coherence decreased non-linearly for all frequency bands with increasing interelectrode distance. The greatest decrement in coherence occurred in the region corresponding to the junction between hind limb (HL) and frontal (Fr1) cortices. The decrease in coherence was greatest for the highest frequency band for all interelectrode pairs. A spike focus was induced in all 6 animals following the application of bicuculline at the most posterior electrode of the cortical array. In 5 of the 6 animals spike propagation was delayed at the junction of HL and Fr1 cortices, where the greatest decrease in coherence was observed. In one animal spike activity never advanced across this region. These results demonstrate an association between intracortical coherence and the spatial propagation of spike activity. The function of short and long cortico-cortical pathways as mediators of both EEG coherence and cortical spike propagation are discussed.

The effects of spinal gray activation by strychnine on the motor evoked potential in the rat.

Spinal motor evoked potentials were elicited by electrical stimulation of the motor cortex in 14 rats before and after the application of strychnine to the surface of the spinal cord. Strychnine applied to the high cervical cord resulted in the emergence of additional peaks at the site of application and in electrodes positioned distally on the middle and lower thoracic cord. The strychnine-induced peaks occurred earlier and were larger in amplitude (P less than 0.01) in the distal spinal cord. Strychnine applied to the lower thoracic cord resulted in similar peak generation in the lower thoracic cord and in the spinal cord proximal to the application of strychnine. These findings demonstrate that strychnine-induced motor evoked potential changes arise from spinal gray activity induced by efferent pathways activated during transcortical stimulation. The role of the propriospinal tract in mediating the distal effects of strychnine is discussed. In conclusion, the strychnine-induced peaks of the motor evoked potential may be used as a measure of spinal gray integrity in experimental spinal cord injury models.

Brain-stem auditory evoked responses in 56 patients with acoustic neurinoma.

The brain-stem auditory evoked responses (BAER's) recorded from 56 patients with acoustic neurinomas were analyzed. Ten of the patients had intracanalicular tumors and 46 had extracanalicular tumors. It was possible to obtain BAER's following stimulation of the affected side in 28 patients and after stimulation of the unaffected side in all 56. Five patients (11%) had normal BAER's following stimulation of both sides; three of these patients had intracanalicular tumors. Among BAER's obtained following stimulation of the affected ear, the mean interpeak latency (IPL) for peaks I to III associated with extracanalicular tumors was significantly prolonged relative to controls (p less than 0.001), and linear regression analysis revealed a significant positive correlation between tumor size and IPL of peaks I to III (p less than 0.05). Analysis of the 56 BAER's recorded after stimulation of the unaffected side revealed a significant positive correlation between the IPL's of peaks III to V and tumor size (p less than 0.001). This correlation was not strengthened when accounting for the degree of brain-stem compression. Finally, evidence of preserved function within the auditory pathway, even in the presence of partial hearing loss, is presented. This finding suggests that more patients might benefit from surgical procedures that spare the eighth cranial nerve.

Motor evoked potentials elicited from pyramidal stimulation and recorded from the spinal cord in the rat.

This study investigated the spinal evoked response to focal electrical stimulation of the sensorimotor cortex in 32 rats. The results demonstrate a long-latency response (beginning at 8 milliseconds) elicited by electrical stimulation, which is distinct from the short-latency motor evoked potential previously reported. The conduction velocity of this later response is similar to that reported for the pyramidal tract in the rat. Experiments confirm that the longer latency response depends upon the integrity of the pyramidal system. Focal stimulation outside the sensorimotor cortex failed to elicit a response. Experimental lesions of the pyramidal tract or ablating the sensorimotor cortex eliminated the spinal cord evoked response. The results demonstrate that focal stimulation of the sensorimotor cortex results in a spinal cord evoked response that represents activity within the pyramidal system. The utility of this response in the rat model for assessing experimental cord injury is discussed.

CNS activation patterns underlying motor evoked potentials as demonstrated by c-fos immunoreactivity.

Monopolar (n = 5) and bipolar (n = 4) electrical stimulation of the motor cortex associated with spinal motor evoked potentials (MEPs) in rats resulted in central nervous system (CNS) staining for c-fos protein. Staining was demonstrated in the cortex, hippocampus, and caudate-putamen, irrespective of the type of stimulation. C-fos protein was demonstrated in the ventral horn cells in 3 animals and in the superficial layer of the posterior horn in 8 animals. Brainstem staining was more frequent and intense in rats stimulated with monopolar electrodes. In contrast to the diffuse and bilateral cortical staining observed with electrical stimulation, motor cortex stimulation by the local application of bicuculline resulted in c-fos protein staining restricted to the stimulated motor cortex. The cerebellum failed to demonstrate c-fos protein following motor cortex stimulation or direct electrical cerebellum stimulation. The present study demonstrates that c-fos protein can be used as a marker of direct or synaptically activated neurons during electrical and chemical stimulation of the motor cortex. Activation of neural structures outside the motor pathway may reflect physiologic activation or stimulus spread. Increased c-fos protein staining of the brainstem with monopolar stimulation supports previous studies that suggest that components of the MEP following monopolar motor cortex stimulation in the rat may arise from brainstem structures.

Pheochromocytoma presenting as a skull metastasis with massive extracranial and intracranial extension.

A case of metastatic pheochromocytoma is reported in which the diagnosis was determined on the basis of a painless scalp mass. Subsequent to biopsy and histological diagnosis, further investigations revealed a large skull lesion with intracranial extension, an adrenal mass, and various catecholamine abnormalities. The radiological characteristics and operative findings of this case are described, and the literature regarding pheochromocytoma with intracranial metastatic extension is reviewed.

Effects of selective spinal cord lesions on the spinal motor evoked potential (MEP) in the rat.

The effects of selective spinal cord lesions on the motor evoked potential (MEP) in 21 rats were investigated. No significant change in peak amplitude was observed following lesions of the pyramidal tract. There was a significant decrease in peaks 1 and 2 with ventral funiculi lesions. All 4 peaks of the MEP were significantly reduced following lesions of the lateral funiculus. The most marked decrease in peak amplitude followed lateral funiculi lesions that involved the lateral grey of the spinal cord. In one animal where the lesion was confined to the grey matter in the cord there was a marked decrease in all peaks of the MEP. In 3 additional animals interruption of the descending tracts of the spinal cord via bilateral hemisections of the spinal cord failed to completely abolish the MEP. Increases in peak latency were also noted following spinal lesions. In some animals the increase in latency occurred in the absence of significant peak amplitude changes. The findings in this study refute the previously held position that the MEP in the rat arises from pyramidal tract activation. The role of the reticulospinal and propriospinal tracts in the generation and propagation of the MEP are discussed.

Systemic biodistribution of radioiodinated interleukin-2 in the rat.

Interleukin-2 (IL-2) is a lymphokine capable of modulating a variety of immune functions. In vitro and in vivo studies have shown promising cytotoxic potential. Despite numerous ongoing clinical trials, however, little is known about the biodistribution of this lymphokine after in vivo administration. In this study using a rat model, the fate of radioiodinated human recombinant IL-2 (RIL-2) was analyzed by camera imaging, autoradiography, and well counting experiments. Camera imaging demonstrated the liver and kidney to be the organs of greatest radioactivity accumulation with peak liver uptake noted at approximately 10 min from onset of infusion, and peak kidney uptake at approximately 20 min. Autoradiographic assessment of selected organs (kidney, adrenal, liver, lung, and brain) revealed marked heterogeneity of uptake in the kidney and adrenal gland with preponderance of RIL-2 in the cortex of these organs. A more homogeneous distribution of RIL-2 uptake was noted in liver, lung, and brain parenchyma. Well counting confirmed the liver and kidney as the organs of greatest RIL-2 accumulation. Knowledge of the biodistribution of IL-2 may be of benefit both in studying mechanisms of toxicity and in designing novel therapeutic approaches.

Three-dimensional data visualization and biomedical applications.

Three-dimensional data visualization is an important tool in several medical, scientific, and engineering areas. Visualization methods are based on a primitive representational element: contour, surface, or volume. Methods often incorporate options to cut open, see around, or see through structures, and form images in multiple windows or with animation. To visualize and interpret two or three related 3D data sets, composite imaging methods are required. The appropriate method depends on the user needs, application area, and available hardware. Visualization of 3D medical data is described for cranium/face, musculoskeletal systems, spine, intracranial structures, cardiovascular system, and radiation therapy.