Chronic brain ischemia in the monkey assessed by somatosensory evoked potentials and local blood flow measurements

Brain retraction injury

This paper reviews the literature of the brain retraction injury during the last century. The review focused on the instrument characteristic as well as the physiopathological and histopathological damage of the brain induced by brain retraction. It was found that lesions were induced by cerebral ischemia. We conclude that a better monitoring system needs to be developed to avoid brain injury.

Impaired neurovascular coupling by transhemispheric diaschisis in rat cerebral cortex

In acute brain disorders, elimination of the excitatory output from an injured brain region reduces activity in connecting brain regions remote from the lesion site (i.e., diaschisis). The authors examined the effect of functional ablation of the left cerebral cortex by cortical spreading depression (CSD) or topical application of tetrodotoxin on single cell spiking activity, baseline CBF, and neurovascular coupling in the right rat sensory cortex. CSD or tetrodotoxin in left cortex reduced the right cortical spontaneous spike rate by 36% and 45%, respectively. Baseline CBF in the right cortex was unaffected by a left-sided CSD, but decreased by 12% for left cortical application of tetrodotoxin. This suggested dissociation between spontaneous spiking activity and basal CBF. Left in-fraorbital nerve stimulation evoked local field potentials in right cerebral cortex that were reduced in amplitude by 19% for left CSD and by 23% for left tetrodotoxin application. The corresponding declines in the evoked CBF responses were 42% for CSD and 23% for tetrodotoxin. Vascular reactivity to adenosine remained unchanged in right cortex. Thus, transhemispheric diaschisis produced a pronounced decrease in the spontaneous spike rate accompanied by no reduction or a small reduction in basal CBF, and an attenuation in amplitudes of evoked synaptic responses and corresponding rises in CBF. The findings suggest that disturbed neurovascular coupling may contribute to the disturbance in brain function in acute transhemispheric diaschisis.

Vulnerability of the thalamic somatosensory pathway after prolonged global hypoxic-ischemic injury

The aim of this study was to test the hypothesis that under prolonged global ischemic injury, the somatosensory thalamus and the cortex would manifest differential susceptibility leading to varying degrees of thalamo-cortical dissociation. The thalamic electrical responses displayed increasing suppression with longer durations of ischemia leading to a significant thalamo-cortical electrical dissociation. The data also point to a selective vulnerability of the network oscillations involving the thalamic relay and reticular thalamic neurons. An adult rat model of asphyxial cardiac arrest involving three cohorts with 3 min (G1, n=5), 5 min (G2, n=5) and 7 min (G3, n=5) of asphyxia respectively was used. The cortical evoked response, as quantified by the peak amplitude at 20 ms in the cortical evoked potential, recovers to more than 60% of baseline in all the cases. The multi-unit responses to the somatosensory stimuli recorded from the thalamic ventral posterior lateral (VPL) nuclei consists typically of three components: (1). the ON response (<30 ms after stimulus), (2). the OFF response (period of inhibition, from 30 ms to 100 ms after stimulus) and (3). rhythmic spindles (beyond 100 ms after stimulus). Asphyxia has a significant effect on the VPL ON response at 30 min (P<0.025), 60 min (P<0.05) and 90 min (P<0.05) after asphyxia. Only animals in G3 show a significant suppression (P<0.05) of the VPL ON response when compared to the sham group at 30 min, 60 min and 90 min after asphyxia. There was no significant reduction in somatosensory cortical N20 (negative peak in the cortical response at 20 ms after stimulus) amplitude in any of the three groups with asphyxia indicating a thalamo-cortical dissociation in G3. Further, rhythmic spindle oscillations in the thalamic VPL nuclei that normally accompany the ON response recover either slowly after the recovery of ON response (in the case of G1 and G2) or do not recover at all (in the case of G3).We conclude that there is strong evidence for selective vulnerability of thalamic relay neurons and its network interactions with the inhibitory interneurons in the somatosensory pathway leading to a thalamo-cortical dissociation after prolonged durations of global ischemia.

Neuronal deactivation explains decreased cerebellar blood flow in response to focal cerebral ischemia or suppressed neocortical function

Functional neuroimaging in humans with acute brain damage often reveals decreases in blood flow and metabolism in areas unaffected by the lesion. This phenomenon, termed diaschisis, is presumably caused by disruption of afferent excitatory input from the lesioned area to other brain regions. By characterizing its neurophysiological basis, we used cerebellar diaschisis to study the relationship between electrical activity and blood flow during decreased neuronal activity. Here we show that focal cerebral ischemia in rats causes diaschisis in the cerebellar cortex characterized by pronounced decreases in Purkinje cell spiking activity and small decreases in cerebellar blood flow. The findings were explained by decreased excitatory input to the cerebellar cortex, i.e., deactivation, as cerebellar neuronal excitability and vascular reactivity were preserved. Functional ablation of the cerebral cortex by either spreading depression or tetrodotoxin reproduced the changes in cerebellar function with complete recovery of Purkinje cell activity and cerebellar blood flow concomitant with recovery of neocortical function. Decreases of activity involving the contralateral frontal cortex produced the largest decrease in cerebellar electrical activity and blood flow. Our data suggest that deactivation explains the decreases in blood flow and metabolism in cerebellar diaschisis observed in human neuroimaging studies. Decreases in spiking activity were 3-7 times larger than the respective decreases in flow. Therefore, under pathological conditions, neuroimaging methods based on hemodynamic signals may only show small changes, although the underlying decrease in neuronal activity is much larger.

Quantitative analysis of synaptophysin immunoreactivity in human neocortex after cardiac arrest: confocal laser scanning microscopy study

Transient global ischaemia caused by cardiac arrest results in lesions that involve all brain structures. The aim of this study was to investigate the condition of synapses in patients surviving, but remaining in a persistent vegetative state, following resuscitation after cardiac arrest. We performed a quantitative analysis of the distribution and density of elements containing a synaptic vesicle protein--synaptophysin (p38)--in human neocortex in cases which survived for 1 week, 2 months, and 1 year after the cardiac arrest. Neurologically healthy cases that died following an accident served as control. Dual-channel confocal laser scanning microscopy (CLSM) was used to image p38-immunoreactivity (IR) and lipofuscin autofluorescence. In control cases no statistically significant differences were found for p38-IR between layers II-III and V-VII. However, the temporal cortex had a higher density of p38-immunoreactive structures than the motor cortex. In postischaemic cases a reduction in the density of p38-IR elements was apparent, mainly in the frontal and motor cortices and less pronounced in the temporal cortex. The least decrease compared with controls was observed in the visual cortex. In the 1 week survival case, a maximal decrease in p38-IR (35% below control) was found. In this case, the number of p38-IR elements per visual field was decreased, and big aggregates of p38-IR structures were observed. In general, the amounts of p38-IR structures were higher in all of the control cases compared with the postischaemic cases.

Hyperbaric oxygen after global cerebral ischemia in rabbits reduces brain vascular permeability and blood flow

BACKGROUND AND PURPOSE

Hyperbaric oxygen (HBO) has been advocated as a therapy to improve neurological recovery after ischemia, since HBO may improve tissue oxygen delivery. We examined the effect of HBO treatment after global cerebral ischemia on early brain injury.

METHODS

Rabbits were subjected to 10 minutes of global cerebral ischemia by cerebrospinal fluid compression. After 30 minutes of reperfusion, rabbits either were subjected to HBO for 125 minutes and then breathed 100% O2 at ambient pressure for 90 minutes or breathed 100% O2 for 215 minutes. At the end of reperfusion and 90 minutes after exposure, brain vascular permeability and cerebral blood flow were measured. Somatosensory evoked potentials were monitored throughout the experiment.

RESULTS

HBO treatment reduced (P < .05) brain vascular permeability by 16% in gray matter and by 20% in white matter. Cerebral blood flow was lower (P < .05) in the HBO group (40.9 +/- 1.9 mL/min per 100 g, mean +/- SEM) compared with controls (50.8 +/- 2.0 mL/min per 100 g). Somatosensory evoked potential recovery was similar in the two groups (P > .05).

CONCLUSIONS

HBO administered after global cerebral ischemia promoted blood-brain barrier integrity. HBO treatment also reduced cerebral blood flow; this effect was not associated with a reduction in evoked potential recovery. Since neurological outcome after global cerebral ischemia is generally poor and treatment options are limited, HBO should be further investigated as a potential therapy.

Hyperbaric oxygen after global cerebral ischemia in rabbits does not promote brain lipid peroxidation

OBJECTIVE

To determine whether hyperbaric oxygen administered immediately after global cerebral ischemia increases free radical generation and lipid peroxidation in the brain or alters neurophysiologic recovery.

DESIGN

Prospective, randomized, controlled trial.

SETTING

Animal research laboratory.

SUBJECTS

Adult male New Zealand white rabbits.

INTERVENTIONS

Anesthetized rabbits were subjected to 10 mins of global cerebral ischemia by infusing a mock cerebrospinal fluid into the subarachnoid space and increasing intracranial pressure equal to mean arterial pressure. Immediately upon reperfusion, one group of rabbits (n = 9) was treated with hyperbaric oxygen at 2.8 atmospheres absolute for 75 mins while the control group (n = 9) breathed room air for an equivalent period of time. At the end of the reperfusion period, oxyradical brain damage was determined by measuring brain levels of oxidized and total glutathione and free malondialdehyde. Neurophysiologic brain injury was assessed with cortical somatosensory evoked potentials.

MEASUREMENTS AND MAIN RESULTS

Both oxidized glutathione and the ratio of oxidized glutathione to reduced glutathione (total minus oxidized) were higher (p < .05) in the hyperbaric oxygen group, indicating that hyperbaric oxygen increased free radical generation. Nonetheless, brain malondialdehyde content, an index of lipid peroxidation, was similar (p > .05) in the two groups. Cortical somatosensory evoked potential recovery at the end of reperfusion was 50% higher (p < .05) in the hyperbaric oxygen-treated animals compared with controls.

CONCLUSIONS

Treatment with hyperbaric oxygen after ischemia increased the amount of oxygen free radicals in the brain. However, this increase in free radical generation was not associated with an increase in lipid peroxidation or a reduction in neurophysiologic recovery when measured after 75 mins of recirculation. These results suggest that hyperbaric oxygen administered immediately after global ischemia does not promote early brain injury.

[Neuro-anesthetic contribution to the prevention of complications caused by mechanical cerebral retraction: concept of a chemical brain retractor]

During most intracranial procedures, the microscope is used to allow the surgeon to work on structures which are deeply located in the brain. Under these circumstances, brain retraction is required for adequate exposure. It was rapidly suspected and later confirmed that brain retraction causes secondary brain damage. This is due not only to direct effect of the retractor on the cortical surface, but also because a pressure is generated under the retractor, on the brain tissue, which compromises local cerebral blood flow and local cerebral perfusion pressure, thus causing cerebral ischaemia. The need for retraction is increased if the lesion is located deeply and/or if the brain is tensed; thus the risk to generate ischaemic conditions is enhanced. These secondary surgical lesions are promoted and worsened by associated systemic conditions such as hypotension, hypoxaemia, hypercapnia. As an attempt to respond to the problem generated by surgical retraction, the "chemical brain retractor" concept is proposed. By compulsively rendering the brain as relaxed and compliant as possible, the chemical brain retractor should allow the surgeon to operate on without the use of a surgical brain retractor and, if such a retractor is still needed, to reduce the pressure under it. These goals are achieved with an osmotic agent like mannitol to improve brain compliance, and intravenous anaesthetic agents, moderate hypocarbia and a normal or elevated blood pressure, to minimize cerebral blood volume. In conjunction with the chemical brain retractor, two other manoeuvres should be used to enhance cerebral compliance: CSF drainage and moderate head up position during the procedure.

A review of brain retraction and recommendations for minimizing intraoperative brain injury

Brain retraction is required for adequate exposure during many intracranial procedures. The incidence of contusion or infarction from overzealous brain retraction is probably 10% in cranial base procedures and 5% in intracranial aneurysm procedures. The literature on brain retraction injury is reviewed, with particular attention to the use of intermittent retraction. Intraoperative monitoring techniques--brain electrical activity, cerebral blood flow, and brain retraction pressure--are evaluated. Various intraoperative interventions--anesthetic agents, positioning, cerebrospinal fluid drainage, operative approaches involving bone resection or osteotomy, hyperventilation, induced hypotension, induced hypertension, mannitol, and nimodipine--are assessed with regard to their effects on brain retraction. Because brain retraction injury, like other forms of focal cerebral ischemia, is multifactorial in its origins, a multifaceted approach probably will be most advantageous in minimizing retraction injury. Recommendations for operative management of cases involving significant brain retraction are made. These recommendations optimize the following goals: anesthesia and metabolic depression, improvement in cerebral blood flow and calcium channel blockade, intraoperative monitoring, and operative exposure and retraction efficacy. Through a combination of judicious retraction, appropriate anesthetic and pharmacological management, and aggressive intraoperative monitoring, brain retraction should become a much less common source of morbidity in the future.

Effect of lidocaine on somatosensory evoked response and cerebral blood flow after canine cerebral air embolism

BACKGROUND AND PURPOSE

Victims of air embolism often recover rapidly on hyperbaric treatment then deteriorate again, even if hyperbaric treatment is continued. In previous animal experiments, lidocaine has been shown to improve recovery of somatosensory evoked response amplitude after air embolism. However, animals in these experiments rarely deteriorated. We have shown that the induction of air embolism and transient hypertension in canines produces deterioration despite hyperbaric treatment, and we decided to test the effect of lidocaine on somatosensory evoked potential recovery and cerebral blood flow in this model.

METHODS

Dogs were treated with repeated doses of lidocaine or equivalent volumes of saline during hyperbaric therapy after internal carotid air embolism and transient hypertension. The investigators were unaware of treatment group assignment during the experiments. The amplitude of the median nerve somatosensory evoked potential and cerebral blood flow measured with carbon-14-labeled iodoantipyrine autoradiography were used to assess effect of therapy.

RESULTS

Lidocaine-treated dogs recovered 60 +/- 10% (mean +/- 95% confidence limits) of the baseline somatosensory evoked potential amplitude 220 minutes after air embolism; saline-treated dogs recovered 32 +/- 10% (a significant difference at p less than 0.01). Lidocaine-treated dogs also had higher cerebral blood flow values than saline-treated dogs 220 minutes after air embolism.

CONCLUSIONS

Lidocaine ameliorated the delayed deterioration of evoked potential associated with air embolism and hypertension in this canine model. The improved cerebral blood flow may be a mechanism of action of lidocaine or an associated effect of improved neuronal survival.

Transhemispheric diaschisis. A review and comment

We review here the literature in both animal models and humans concerning electrical activity, blood flow, and metabolism in the hemisphere contralateral to unilateral cerebral ischemia. We analyze the data by periods based on the time from initial injury to emphasize the time course of transhemispheric diaschisis. Contralateral electrical activity, such as evoked potential amplitude, is increased in the late stages after unilateral infarction, with the data from the more acute periods being inconclusive. Contralateral blood flow changes probably depend on the magnitude of the ischemic injury, with a larger insult resulting in a decrease not seen with smaller insults. Some studies have shown a decrease in contralateral blood flow over the first week followed by a gradual return toward baseline. Most measures of contralateral metabolism show a time course similar to blood flow, that is, a decrease followed by gradual recovery. The effects of corpus callosum section on transhemispheric diaschisis are not yet established. We provide examples to show that under certain conditions, diaschisis may represent a loss of remote inhibition rather than a loss of remote facilitation, as von Monakow originally suggested. By following the contralateral changes over time, particularly during the first minutes and hours of ischemia, insight will be gained into the brain's responses remote from the focus of ischemic injury. These responses should bear a relation to the brain's defense mechanisms ipsilaterally to the region of ischemia.

An intravascular technique to occlude the middle cerebral artery in baboons

A technique is described for occlusion of the middle cerebral artery in the baboon by an intravascular approach. A torque catheter is introduced under fluroscopic control into the internal carotid artery by transfemoral catheterization. In conjunction with a guide wire an infusion microcatheter with increasing stiffness from the distal tip to the proximal shaft is positioned in the proximal part of the middle cerebral artery via the introducer system. N-Butyl-2-cyanoacrylate-monomers are injected into the microcatheter for permanent occlusion of the middle cerebral artery. The procedure was successfully completed in 21 out of 24 baboons. In 3 baboons the occlusion could not be achieved since the torque catheter could not pass proximal extreme tortuosities of possibly arterisclerotic internal carotid arteries. Infarcts in the 21 animals were confirmed by computerized tomography and/or autopsies in all animals.

The relationships among the severity of spinal cord injury, motor and somatosensory evoked potentials and spinal cord blood flow

To characterize the changes in axonal function in the motor and somatosensory tracts of the cord after spinal cord injury (SCI) and to correlate these changes with spinal cord blood flow (SCBF), the relationships among the severity of SCI, motor and somatosensory evoked potentials (MEPs and SSEPs) and SCBF were examined. Fifteen rats received a 1.5 g (n = 5), 20 g (n = 5) or 56 g (n = 5) clip compression injury of the cord at C8. SCBF at the injury site was measured by the hydrogen clearance technique 35 min before and 30 min after SCI. Concomitantly MEPs from the cord at T10 (MEP-C) and from the sciatic nerve (MEP-N) and SSEPs were recorded. A linear relationship (r = -0.89, P less than 0.002) was found between the severity of SCI and the reduction in SCBF at the injury site. Linear discriminant analysis revealed that both the MEP (P less than 0.0001) and SSEP (P less than 0.003) were significantly related to the severity of SCI. Furthermore, the amplitude of the MEP (r = 0.65, P less than 0.0001) and SSEP (r = 0.58, P less than 0.001) was significantly correlated with the posttraumatic SCBF. Multiple regression revealed that both the severity of cord injury and the degree of posttraumatic ischemia were significantly related to axonal dysfunction after SCI. While the MEP was more sensitive to injury than the SSEP, the SSEP more accurately distinguished between mild and moderate severities of cord injury. Axonal conduction in the motor and somatosensory tracts of the cord was significantly correlated with the reduction in posttraumatic SCBF and, therefore, these data provide quantitative evidence linking posttraumatic ischemia to axonal dysfunction following acute cord injury. Furthermore, this study validates the hypothesis that the combined recording of MEPs and SSEPs is an accurate technique to assess the physiological integrity of the cord after injury.

Effects of preexisting bypass graft on rCBF and SSEP's following acute stroke in dogs

Regional cerebral blood flow (rCBF) was measured with radiolabeled microspheres in a canine model of superficial temporal artery-middle cerebral artery (STA-MCA) bypass and acute ischemia. Ischemic zone flows in seven dogs with the bypass first closed and then open showed no significant contribution of bypass flow in the intact vascular system. Following acute proximal occlusion, rCBF was preserved by bypass flow. A significant flow decrease ensued when the bypass was then clipped, confirming the adequacy of the lesion and the protective effect of the bypass. Reopening the bypass after 15 minutes of ischemia restored 76% of the previous flow. This was a significant increase from the global ischemia values, and was not statistically different from preocclusive values. Preocclusion somatosensory evoked potentials (SSEP's) in these animals showed a consistent biphasic wave at 8 to 10 msec after stimulation. This wave, with some decrease in amplitude, was preserved by bypass flow following creation of the arterial lesion. Bypass clipping abolished these ipsilateral SSEP's. Variable return of SSEP's occurred following reopening of the graft, but the recordings never reached preischemic amplitudes. This experimental study shows that, in this model, a prophylactic bypass subjected to immediate demand (with no time for "maturation") can adequately augment cortical rCBF and is superior to delayed revascularization. The data lend theoretical support to placement of a prophylactic STA-MCA bypass prior to elective carotid artery sacrifice or in surgery where the risk of acute vascular injury is high.

Differentiation between cortical and subcortical lesions following focal ischemia in cats by multimodality evoked potentials

Regional ischemia was induced in cats by occluding the middle cerebral artery. Evoked and spontaneous electrical activity as well as regional cerebral blood flow (rCBF) were recorded with platinum depth macroelectrodes in three primary cortical areas: the auditory cortex (A, middle ectosylvian gyrus) and the front and hind limb somatosensory cortex (SF and SH, lateral and medial posterior sigmoid gyrus). To distinguish among the various evoked potentials after click, median or tibial nerve stimulation, electrical field interactions had to be eliminated using a multiplex stimulation and analysis system. Spontaneous electrocortical activity was evaluated by power spectral analysis. In all areas, evoked potentials were abolished 10 min after arterial occlusion. However, rCBF behaved differently in these regions: it was severely reduced in A, decreased moderately in SF and remained unchanged in SH. The graded reduction of rCBF in the three cortical areas was related to changes in electrophysiological activity during the first minutes of ischemia. In A, auditory potentials were abolished within 3 min after occlusion, whereas in SH, the decrease of somatosensory responses started after about 5 min. In SF, two components of the EP changes were found: an early decrease immediately and a later decrease about 5 min after occlusion. The different rates of EP impairment possibly correspond to two types of ischemia. The fast EP abolishment seems to be caused by local cortical damage whereas the delayed EP decrease probably reflects impairment of subcortical white matter structures. Thus, this method may be useful for distinguishing between gray and white matter ischemia.

Topography and cortical generators of somatosensory potentials evoked by median nerve stimulation in the cat

Epidural and cortical mapping of somatosensory evoked activity after median nerve stimulation was performed under barbiturate anaesthesia in 6 cats. Depth profiles were made to confirm the site of the cortical generators. The area studied revealed two cortical generators in SI and one in SII. In all cases polarity changes in intracortical tracks were demonstrated. The peak latency of all these generators was 12 msec. In SI a P8 was also a consistent finding in epidural, epicortical and intracortical measurements. No evidence, however, could be obtained for a cortical origin of the P8. The most rostral generator of the P12 was localized in the posterior sigmoid gyrus (area 3a). The N12 originates from the dorso-medial bank of the coronal sulcus in SI (area 2). Histological evidence for these projections was obtained with use of electrode markation. Within the second somatosensory area one source was active; the P12 originated just lateral to the anterior aspect of the suprasylvian sulcus in the anterior ectosylvian gyrus.