Twenty-four patients of comparable age, blood pressure, and degree of dementia were classified by an "Ischemic Score" based on clinical features into "multi-infarct" and "primary degenerative" dementia. Regional cerebral blood flow (CBF) was measured by the intracarotid xenon 133 method. Both groups showed a decreased proportion of rapidly clearing brain tissue (largely gray matter). Cerebral blood flow per 100 gm brain per minute was normal in the primary degenerative group but low in the multi-infarct group. This suggests the blood flow is adequate for metabolic needs of the brain in patients with primary degenerative dementia but inadequate for those with multi-infarct dementia. There was no correlation between degree of dementia and CBF in the primary degenerative group but an inverse relationship existed in the multi-infarct group. Reactivity of blood vessels to reduction of arterial carbon dioxide pressure was normal in both groups.

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As shown previously, the electrical function of the brain is critically dependent on cerebral blood flow in the sense that reduction beyond an ischemic threshold of approximately 15 ml/100 gm per minute (approximately 35% of control) in the baboon leads to complete failure of the somatosensory evoked response. This study tests the hypothesis that electrical failure in ischemia may be directly associated with a massive release of intracellular K+ or with a critical degree of extracellular acidosis. By microelectrode techniques, measurements of blood flow, extracellular activity of K+ and H+ as well as evoked potential were made in the baboon neocortex. Reductions in blood flow were obtained by occlusion of the middle cerebral artery and depression beyond the ischemic threshold of electrical function achieved by a reduction of systemic blood pressure which, in the ischemic zones, changed local cerebral blood flow proportionally. Abolition of evoked response could not be explained by depolarization by release of intracellular K+, nor was it critically dependent on cortical pH. However, the massive release of intracellular K+ was by itself critically dependent on cortical blood flow and occurred at 18 greater than 6 greater than 2 ml/100 gm per minute (median with 5% confidence limits). Thus a dual threshold in ischemia for neuronal function is described, the threshold for release of K+ being clearly lower than the threshold for complete electrical failure. Further, the findings support the concept of an ischemic penumbra during which the neurons remain structurally intact but functionally inactive. That neurons can survive for some time in this state of lethargy is evidenced by the observations that an increase in rCBF, if sufficient, can restore evoked potential and normalize extracellular K+ activity as well as pH.

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The clinical features, diagnosis, and management of 23 posterior fossa epidermoid cysts and 9 petrous apex lesions presenting to one unit over a period of 20 years are summarized. Of the epidermoid cysts, 13 were entirely infratentorial, but the other 10 had an additional supratentorial component. Presenting symptoms and signs were usually long-standing and at onset had often been vague and nonspecific. With time, however, a variety of neurological deficits that depended on the site of the lesion developed. These were generally combinations of cerebellopontine (CP) angle and jugular foramen syndromes, deafness, facial palsy, and motor weakness. Diagnostic procedures have changed greatly over the review period. Computed tomography and magnetic resonance imaging have replaced air encephalography and contrast ventriculography. The better preoperative localization of these lesions allows rational planning of the surgical approach required for optimal tumor exposure, which is essential for any attempt at total excision, considering the large size of the majority of these tumors when diagnosed. We favor operation through a posterior fossa craniectomy for those tumors restricted to the CP angle or 4th ventricle, but routinely use a combined supra- and infratentorial approach if the lesion has a more rostral component. The infiltrating nature of epidermoid cysts within the cranium compromises the extent of excision if neurological deficit is not to be increased, but we attempt as complete an excision of tumor and capsule as possible in the hope that many years will pass before symptoms recur. Cholesterol granulomas seem to respond well to simple cavity drainage and have shown no tendency to recur.

Summary: Changes in extracellular ion activities were measured during par tial ischaemia of the cerebral cortex of primates anaesthetised with a chloralose. Triple-barrelled, double-ion-sensitive microelectrodes were used to measure the extracellular potassium (K,.) and calcium (Ca,.) activity at the same point simultaneously. The ion changes were related to local cerebral blood flow, and it was shown that at a blood flow of approximately 10 ml 100 g-l min-I, there is a threshold below which ion homeostasis is disturbed. This is associated with a dramatic rise in Kl' and fall in Cae. Cae falls from a normal value of 1.31 ± 0.1 mM to approximately 0.28 mM in densely ischaemic tissue. In ischaemia, Kl' reaches 13.4 ± 3.8 mM before C� begins to fall. The faIl in Cae, although related to reduced blood flow, is closely associated with and follows the rise in Ke. The change in Cal' is probably due to an increase in membrane permeability, as a result of either depolarisation or a critical lower ing of cellular energy reserves.

The technique of hydrogen clearance by an inhalation method is discussed. The electronic instrumentation necessary to secure stability and reproducibility from the recordings is described. Clearance rates in gray matter of about 80 ml/100 gm per minute in the cortex and putamen have been obtained, and of about 20 ml/100 gm per minute in white matter.Clearance curves have invariably been monoexponential in character in white matter, and in over half the cases in the putamen. In the remainder of the putamen curves and in 60% of the cortical clearance curves, the curves could be resolved into only two exponentials. Using bicompartmental analysis, the fast-clearing components of biexponential curves in both cortex and deep nuclei gave the same figures as clearance curves of an entirely monoexponential character from these two tissues. The importance of recirculation time, concentration of hydrogen inhalation, and verification of the tissue placement by subsequent dissection are discussed. The capacity of the method to detect sudden changes in flow during clearance is described.