The somatosensory evoked potential (SEP) measured in response to median nerve stimulation was correlated with cortical and white matter cerebral blood flow (CBF), adenosine triphosphate (ATP), and lactate levels in 14 cats subjected to graded hemorrhagic hypotension following bilateral carotid artery ligation. Three additional cats served as controls. Regional CBF was determined by the hydrogen clearance method, and the time for conduction of the sensory stimulus from the thalamus to the cortex (the thalamocortical conduction time), was used to assess SEP latency changes. A reproducible sequence of changes occurred in the SEP as ischemia developed. There was an early conduction delay that correlated well with mild white matter ischemia. Amplitude reductions in the SEP began as significant cortical ischemia occurred. The cortical SEP was abolished when white matter CBF and ATP fell to critical levels.

Vascular impingement of the affected nerve was seen in the majority of patients with trigeminal neuralgia. Overall pain relief following GKRS was comparable in those with and without evidence of vascular compression on MR imaging. In subgroup analysis of those with MR imaging evidence of vessel impingement of the affected trigeminal nerve, pain relief correlated with a higher dose to the point of contact between the impinging vessel and the trigeminal nerve. Such a finding may point to vascular changes affording at least some degree of relief following GKRS for trigeminal neuralgia.

SUMMARY Acute focal ischemia was created in 10 cats by unilateral retro-orbital middle cerebral artery (MCA) occlusion. Regional cerebral blood flow (CBF) was determined utilizing the hydrogen clearance technique from electrode recordings within the gray matter and white matter of the ectosylvian gyrus of both hemispheres. The somatosensory evoked potential (SSEP) was obtained during contralateral median nerve stimulation. When the MCA was clipped the white and gray matter blood flows in the ipsilateral ectosylvian gyrus were reduced to 14.8 ± 19.6% and 19.3 ± 23.7% of control, and the cortical component of the SSEP was abolished. In the contralateral hemisphere an average increase of 3.5% above the control latency and a 10% mean depression in the amplitude of the cortical component of the SSEP were observed following occlusion. CBF in the contralateral hemisphere was unaffected by the MCA clip. Infusion of saline or dextran to lower the hematocrit by approximately 45% did not significantly improve blood flow or restore the SSEP in the hemisphere ipsilateral to the MCA clip. However, significant increases in the contralateral hemisphere gray matter CBF occurred following hemodilution while the latency of the cortical component of the SSEP in this same hemisphere was significantly extended. Elevations in gray and white matter blood flows were achieved in the experimental hemisphere of 3 of 10 cats suggesting a wide range of variation in the collateral circulation. Stroke Vol 17, No 1, 1986OCCLUSION OF THE MIDDLE CEREBRAL artery (MCA) can result in partial or complete interruption of the blood supply of the lateral area of the cerebral hemisphere leading to focal or spreading ischemia. |JtIn the baboon MCA occlusion results in ischemia of the Sylvian gyri and nuclei of the basal ganglia. Infarction of the lateral aspect of the cerebral hemisphere only occurs providing the collateral circulation is nonfunctional. 6In the cat various reductions in the cortical blood flow of the ectosylvian, suprasylvian, and marginal gyri have been reported following MCA occlusion. 6 -7 The competency of the collateral circulation in resupplying Sylvian regions of the cat brain has been implicated in reversing ischemic abnormalities following MCA occlusion. 8 It has also been reported in the cat that the variability noted during focal ischemia was not related to any specific change in the electroencephalogram (EEG), which was recorded from depth electrodes within individual gyri. 9A significant reduction in the EEG amplitude was noted in the suprasylvian and marginal gyri following a reduction in blood flow, but there were no appreciable effects on the amplitude of the EEG in the ectosylvian gyrus which sustained the greatest decrease in flow. Thus, little correlation was made between the changes in the EEG and cerebral blood flow (CBF) using the Received October 23, 1984; revision #1 accepted March 25, 1985. focal ischemia model as demonstrated by Strong et al. 9In a global model of cerebral ischemia, reduction of white matte...

SUMMARY Eight cats were subjected to graded hemorrhaglc hypotension following bilateral carotid ligation to produce incomplete global cerebral ischemia. Three additional cats served as controls. The somatosensory evoked potential (SEP) and direct cortical response (DCR) were monitored in all animals and in each case, the cortical component of the SEP was abolished during progressive ischemia while the morphology of the DCR was well-preserved but with reduced amplitude. Determinations of adenosine triphosphate (ATP), phosphocreatine (PCr), and lactate levels in cerebral cortex and white matter were made in five experimental cats and the three controls. At the time of failure of the cortical SEP, PCr was dramatically reduced and lactate moderately elevated in the white matter while ATP remained unchanged. Cortical lactate was only mildly elevated and PCr and ATP were unchanged accounting for preservation of the DCR. In this model of global ischemia, abolition of the cortical SEP is due to a block of stimulus conduction in white matter projection pathways. A hypothesis to explain the observed metabolic changes is presented and correlation is made to clinical situations. Stroke Vol 17, No 6, 1986AS NEURONAL FUNCTION is affected by developing ischemia, cerebral electrical activity is necessarily altered. The ease with which this electrical activity can be monitored in the clinical setting on a continuous basis in comparison to measures of cerebral blood flow has prompted many investigators to study the electroencephalogram (EEG), the somatosensory evoked potential (SEP) and the direct cortical response (DCR) as potential indices of cerebral flow. The large, negative wave of the DCR is a post-synaptic potential generated by apical dendrites excited by axons in the molecular layer of the cortex. 1 It is not attenuated by isolation from underlying subcortical structures, 2 and therefore specifically reflects cortical function. The amplitude of the DCR has been found to be progressively attenuated with advancing ischemia. 3 The SEP may reflect blood flow alterations at any point along the sensory pathway manifested by reduction in the amplitude of the primary cortical wave 4 " 7 and by a lengthening of the conduction time of the stimulus to the cortex.7 " 9 The EEG is a widespread intraoperative monitor of cerebral ischemia during carotid endarterectomy but is the least specific. It has been suggested that the SEP may more accurately reflect the adequacy of cerebral flow. ml/100 gm/min and white matter ATP is below 40% of control stores. Cortical ATP, however, remained at 84% of control reflecting continued perfusion of cortex and we hypothesized that cortical energy metabolism would be adequate to generate a response should a stimulus volley be able to penetrate the white matter projections. This has important implications when interpreting the cause of failure of the cortical wave of the SEP in conditions of global ischemia. This study was undertaken in the cat subjected to graded hemorrhagic hypotension following caro...

The middle cerebral artery was occluded in 18 cats to evaluate the physiological consequences of cerebral blood flow reductions on the somatosensory evoked potential, spontaneous neuronal activity, and oxygen availability in the ipsilateral and contralateral hemispheres. In the ipsilateral ectosylvian i high-grade ischemia was produced as blood flow in the gray matter was reduced from 52.1 ± 8.6 '± SE) to 13.3 ± 9.0 ml/100 g/min and in the white matter from 33.8 ± 5.6 to 6.1 ± 6.4 ml/100 g/min. This significant reduction (p<0.05) was associated with abolition of the cortical component of the somatosensory evoked potentials. In all animals occlusion resulted in a predictable extended latency change and a variable amplitude response of the cortical component of the contralaterally recorded somatosensory evoked potentials. In 5 animals, oxygen availability was measured and spontaneous neuronal activity in the contralateral hemisphere was recorded. Volume expansion and hemodilution with either dextran or saline infusions elevated cerebral blood flow in the contralateral gray matter significantly (p < 0.05) compared with the control and clip values. Ipsilateral spontaneous activity stopped within 4-12 minutes of occlusion, while contralateral spike activity persisted at rates at least equal to those recorded immediately following occlusion. (Stroke 1987;18:77-84) P HYSIOLOGICAL monitoring of the events following middle cerebral artery (MCA) occlusion in cats has been used to describe ischemic and nonischemic brain areas. '~3 Measurements of cerebral blood flow (CBF) and the metabolic rate for glucose (CMR-glu) vary over the ischemic and nonischemic brain regions as determined by the nature of the blood supply to these territories.1 On restoration of blood flow through the MCA, measurements of CBF and CMR-glu are less likely to predict the animal's neurological outcome than are biochemical determinations of such factors as adenosine triphosphate (ATP) and the somatosensory evoked potential (SEP) during reperfusion.3 " 5 Electrophysiological measurements of extracellular K + , the EEG, and the SEP can be used to characterize an area of near-threshold CBF for electrical failure. This borderline ischemic zone is the penumbra. 6 Within this region, extracellular K + accumulates as CBF is reduced below 15 ml/100 g/min 7 and the SEP undergoes changes in its latency and amplitude^1 0 while the EEG is only slightly diminished in amplitude. In models of focal ischemia (MCA occlusion), the ischemic penumbra of the cat brain includes the supra- Philadelphia, PA 19107. Received February 6, 1986; accepted July 29, 1986. sylvian and marginal gyri 7 " and the ischemic core area involves the anterior edge of the ectosylvian gyms. 12This area generates a secondary cortical representation of the SEP, 1314 as we have shown by electrocauterization. 12 This configuration is sensitive to high-grade ischemia following MCA occlusion. In our attempts to elevate subthreshold CBF in the ipsilateral ectosylvian gyrus with saline or dextran ...