Influence of hypoxia on the composition of isolated edema fluid in cold-induced brain edema

Analytic Reviews : Fluid Resuscitation in Head Injury

Head injury, either alone or in combination with hy povolemic shock, is the leading cause of traumatic death in this country. Factors contributing to mortality in clude the primary impact injury as well as subsequent ischemia and hypoperfusion. Intravenous fluid therapy is required in all of these patients. However, fluid ther apy may increase brain swelling and cerebral edema formation which could lead to an increase in intracra nial pressure and a reduction in cerebral perfusion pres sure. The use of standard fluid therapy has been ques tioned, and novel therapies involving hyperosmolar and hypertonic solutions are now being investigated. This review covers recent advances in the understanding of the effects of fluid resuscitation on the brain. It also includes a brief summary of the determinants of trans- capillary fluid exchange and a review of relevant cere bral circulatory physiology and the physiological aberra tions produced by brain injury.

High altitude cerebral edema and acute mountain sickness. A pathophysiology update

The diagnosis, treatment and prevention of high altitude cerebral edema (HACE) are fairly well established. The major unresolved issues are 1) the pathophysiology, 2) the individual susceptibility, and 3) the relationship of HACE to acute mountain sickness (AMS) and to high altitude pulmonary edema (HAPE). In the context of the two types of cerebral edema, cytotoxic (intracellular) and vasogenic, a leaking of proteins and water through the blood-brain barrier (BBB), a recent MRI study in persons ill with HACE (16) suggested a predominantly vasogenic mechanism. Causes of increased BBB permeability might include mechanical factors (loss of autoregulation and increased capillary pressure), ischemia, neurogenic influences (adrenergic and cholinergic activation), and a host of permeability mediators. Once vasogenic edema develops, cytotoxic edema generally follows, and although likely in HACE, this is still unproven. Symptoms of HACE are related to increased intracranial pressure (ICP), and death is from brain herniation. Treatment is directed both to lowering ICP by reducing the volume of intracranial contents, and to stopping the vasogenic leak. Evidence is accumulating that established moderate to severe AMS is due to cerebral edema, but whether this is true for early AMS (headache) is unclear. New work suggests that the brain swells on ascent to altitude, but that this is unrelated to AMS. Preliminary data showing that those with less cerebrospinal fluid volume (a tighter fit of the brain in the cranium) were more likely to develop AMS supports the hypothesis of Ross that those with less ability to accommodate the increased brain volume are the ones that suffer AMS. The blood-brain barrier and intracranial hemodynamics are the two key elements in the pathophysiology of HACE and AMS.

The cerebral etiology of high-altitude cerebral edema and acute mountain sickness

Despite normal cerebral oxygenation and normal global cerebral metabolism, vasogenic edema develops in humans (and sheep) who become moderately ill with AMS/ HACE during 24 hr or more of hypoxic exposure. Hypoxic cerebral vasodilatation appears to be a necessary ingredient but does not per se explain the development of brain edema. In addition to mechanical factors, a number of biochemical mediators might play a role in altering the blood-brain barrier. Brain cell swelling (cytotoxic edema) likely has a role only in the later stages of HACE and not in AMS. The hypothesis that AMS is due to cerebral edema appears to be true for moderate-to-severe illness, but whether early AMS, especially the headache, is caused by edema is not known. Other mechanisms for the headache, perhaps similar to those of migraine, need to be explored. New data suggest that the brain swells on ascent to high altitudes regardless of AMS. Whether this is due to edema or engorgement with blood is not yet clear. The "tight fit" hypothesis proposes that individual anatomy of the craniospinal axis determines tolerance to mild brain edema and might help explain individual susceptibility; preliminary studies support this notion. Therapy for AMS and HACE is directed to reducing brain volume and stopping the BBB leak (i.e., oxygenation, diuretics, and steroids) before secondary ischemia develops. New therapies directed specifically toward the defect in BBB permeability are likely to be successful.

The effect of intraventricular albumin in experimental brain oedema

Therapy for vasogenic brain oedema (VBE) is still an unsolved problem. Experimental work with the aim of establishing an oncotherapeutic option is presented. VBE is performed by focal freeze injury in rats. Using a stereotactic head holder hypo- or hyperosmolar human serum albumin is administered via the intraventricular route. The goal is to enhance the migration of oedema fluid with the aid of oncotic pressures. Early and late results are obtained for each group respectively four and twenty-four hours after the infliction of cold injury. The efficacy of therapy is evaluated by cerebrospinal fluid (CSF) osmolality, cerebral water content, tissue specific gravity, and blood-brain barrier (BBB) permeability. Posttherapeutic values for CSF osmolality are obtained by cisterna magna puncture. Hyperosmolar CSF after performance of cold injury (p < 0.05) is thought to be a result of fluid accumulation in the traumatized region partially from the intraventricular space. Posttherapeutic values after hyperosmolar albumin administration have revealed iso-osmolar CSF, increase in specific gravity (p < 0.001), and decrease in BBB permeability (p < 0.05). These results are in accordance with withdrawal of oedema fluid into the ventricles which can be interpreted as a positive therapeutic effect. Late results in hyperosmolar group have disclosed a hypo-iso-osmolar CSF, persistent increase in specific gravity, and no regression. These values have shown that hyperosmolar albumin administration does not interfere with CSF circulation. Early results of hypoosmolar albumin application are discouraging. This preliminary work of a therapeutic trial on VBE may be a basis for future investigations with different dosages and time modalities.

The normal and pathological physiology of brain water

The physicochemical properties of water enable it to act as a solvent for electrolytes, and to influence the molecular configuration and hence the function--enzymatic in particular--of polypeptide chains in biological systems. The association of water with electrolytes determines the osmotic regulation of cell volume and allows the establishment of the transmembrane ion concentration gradients that underlie nerve excitation and impulse conduction. Fluid in the central nervous system is distributed in the intracellular and extracellular spaces (ICS, ECS) of the brain parenchyma, the cerebrospinal fluid, and the vascular compartment--the brain capillaries and small arteries and veins. Regulated exchange of fluid between these various compartments occurs at the blood-brain barrier (BBB), and at the ventricular ependyma and choroid plexus, and, on the brain surface, at the pia mater. The normal BBB is relatively permeable to water, but considerably less so to ions, including the principal electrolytes Brain fluid regulation takes place within the context of systemic fluid volume control, which depends on the mutual interaction of osmo-, volume-, and pressure-receptors in the hypothalamus, heart and kidney, hormones such as vasopressin, renin-angiotensin, aldosterone, atriopeptins, and digitalis-like immunoreactive substance, and their respective sites of action. Evidence for specific transport capabilities of the cerebral capillary endothelium, for example high Na+K(+)-ATPase activity and the presence at the abluminal surface of a Na(+)--H+ antiporter, suggests that cerebral microvessels play a more active part in brain volume regulation and ion homoeostasis than do capillaries in other vascular beds. The normal brain ECS amounts to 12-19% of brain volume, and is markedly reduced in anoxia, ischaemia, metabolic poisoning, spreading depression, and conventional procedures for histological fixation. The asymmetrical distributions of Na+ K+ and Ca2+ between ICS and ECS underlie the roles of these cations in nerve excitation and conduction, and in signal transduction. The relatively large volume of the CSF, and extensive diffusional exchange of many substances between brain ECS and CSF, augment the ion-homeostasing capacity of the ECS. The choroid plexus, in addition to secreting CSF principally by biochemical mechanisms (there is an additional small component from the extracellular fluid), actively transports some substances from the blood (e.g. nucleotides and ascorbic acid), and actively removes others from the CSF. In contrast with CSF secretion, CSF reabsorption is principally a biomechanical process, passively dependent on the CSF-dural sinus pressure gradient. Pathological increases in intracranial water content imply development of an intracranial mass lesion. The additional water may be distributed diffusely within the brain parenchyma as brain oedema, as a cyst, or as increase in ventricular volume due to hydrocephalus. Brain oedema is classified on the basis of pathophysiology into four categories, vasogenic, cytotoxic, osmotic and hydrostatic. The clinical conditions in which brain oedema presents the greatest problems are tumour, ischaemia, and head injury. Peritumoural oedema is predominantly vasogenic and related to BBB dysfunction. Ischaemic oedema is initially cytotoxic, with a shift of Na+ and CI- ions from ECS to ICS, followed by osmotically obliged water, this shift can be detected by diffusion-weighted MRI. Later in the evolution of an ischaemic lesion the oedema becomes vasogenic, with disruption of the BBB. Recent imaging studies in patients with head injury suggest that the development of traumatic brain oedema may follow a biphasic time course similar to that of ischaemic oedema. Hydrocephalus is associated in the great majority of cases with an obstruction to the circulation or drainage of CSF, or, occasionally, with overproduction of CSF by a choroid plexus papilloma. In either case, the consequence is a ris

Classification of the cerebral edemas with reference to hydrocephalus and pseudotumor cerebri

Cerebral edema is a common clinical disorder that results from an abnormal increase in water content within the extracellular (EC) compartment of the brain. It is distinguished from two other types of brain bulk enlargement: (1) vascular swelling, caused by arterial dilatation or venous obstruction; and (2) cellular swelling, caused by cytotoxic injuries or metabolic storage. Under normal conditions, the EC compartment has two fluids, the interstitial fluid (ISF) and the cerebrospinal fluid (CSF), and extends from the blood brain barrier (BBB) through a series of 100 to 150-A-wide intercellular spaces that are anatomically continuous with the CSF spaces. There are four primary types of EC edema: (1) vasogenic edema, which results from an increase in brain capillary permeability, the most common type, in which leakage of plasma constituents into the brain follows the pathways of ISF bulk flow and is governed by the interaction of systemic arterial pressure and tissue resistance; (2) osmotic edema, which results from an unfavorable osmotic gradient between the plasma and ISF across an intact BBB; (3) compressive edema, which results from obstruction of ISF bulk flow pathways; and (4) hydrocephalic edema, which results from obstruction of CSF bulk flow pathways. In this latter type of edema, distension of the collecting channels proximal to the block leads to retrograde flooding of the EC compartment with the formation of periventricular edema. The syndrome of pseudotumor cerebri includes several different types of brain bulk enlargement.

In vitro models differentiating between direct and indirect effects of ischemia on astrocytes

Mouse astrocytes in primary cultures were subjected to an in vitro model of ischemia (hypoxia combined with substrate deprivation, excess potassium, or elevated glutamate) and examined with the light (phase) and electron microscope. Three hours of hypoxia alone or in combination with the other insults had little effect upon the morphology of astrocytes but did cause disaggregation of polyribosomes. With reoxygenation, polyribosomes reformed and many mitochondria changed from the orthodox to the condensed configuration. Notably, there was little swelling. Excess (50 mM) potassium, added (as KCl) to a normal isotonic medium, also caused no swelling. However, when 50 mM potassium was substituted for a similar amount of sodium, marked astrocyte swelling did occur. A morphologically similar swelling was seen when glutamate (50 microM to 1 mM) was added to the culture medium, both with or without hypoxia with or without substrate deprivation. Potassium or glutamate-induced swelling was reversible with 1 h of recovery in normal medium. These results show that alterations in postischemic astrocytic morphology in vivo to a large extent can be reproduced in astrocytes in primary cultures. In addition, they suggest that postischemic astrocyte swelling is related to alterations in extracellular milieu, including accumulation of glutamate and/or alterations in the potassium/sodium ratios with increased potassium and decreased sodium. In contrast, morphologic alterations in polyribosomes and in mitochondria appear to be a direct response to ischemia itself.

The kallikrein-kinin system as mediator in vasogenic brain edema. Part 3: Inhibition of the kallikrein-kinin system in traumatic brain swelling

Evidence has previously been provided that administration of kinins to the cerebrum causes edema and opening of the blood-brain barrier. It has further been shown that these highly active compounds are formed in the brain under pathophysiological conditions. Their formation was enhanced when cerebral blood flow became compromised by an increase in intracranial pressure. Final evidence, however, was not available as to whether specific inhibition of the kallikrein-kinin (KK) system has a therapeutic function in acute head injury. The authors have demonstrated in rabbits that inhibition of the activating enzyme kallikrein by aprotinin or by aprotinin plus soybean trypsin inhibitor (SBTI), which interfere with plasma and tissue kallikrein, is associated with a decrease in formation of posttraumatic swelling after a standardized cold lesion to the brain. Saline-treated control animals with cerebral cold-induced injury had an increase in hemispheric weight 24 hours later of 13.0% +/- 0.8% (standard error of the mean) in the damaged hemisphere compared to the contralateral nondamaged hemisphere. Administration of aprotinin or aprotinin plus SBTI led to a significant reduction of hemispheric swelling of 10.1% +/- 0.7% or 10.4% +/- 0.7%, respectively. In animals receiving SBTI only, hemispheric swelling evolving from cold injury was not significantly reduced. Therapeutic reduction of brain edema by aprotinin cannot be attributed to a nonspecific effect on the blood pressure, which in the experimental groups remained almost normal as compared to the control animals. Failure of SBTI to influence posttraumatic brain swelling may have resulted from disturbances in intravascular coagulation. Measurements of aprotinin in plasma and tissue demonstrate that the inhibitor doses employed are within an effective therapeutic range. Attenuation of brain edema by specific inhibition of the KK system provides evidence for a mediator role of kinins in vasogenic edema. Clinical trials with inhibitors of the KK system in acute forms of traumatic lesions associated with vasogenic edema appear worthwhile.

Enzymatic changes in serum and cerebrospinal fluid in neurological injury

✓ Enzymatic determinations in serum and cerebrospinal fluid (CSF) of creatine phosphokinase (CPK) and its isoenzymes, lactic dehydrogenase (LDH) and its isoenzymes, and glutamic oxaloacetic transaminase (GOT) were performed on patients with closed head injury with Glasgow Coma Scale (GCS) scores of 3 to 15. The purpose of the work was to study the usefulness of these determinations as a biochemical index of brain injury. Detailed analysis of serum determinations on 139 patients demonstrated a prognostic correlation for only the CPK1 isoenzyme. The presence of the CPK1 isoenzyme correlated with the degree of head injury (as indexed by the GCS) and with the ultimate outcome. Although the presence of the CPK1 isoenzyme was a foreboding sign, it was not consistently present even with severe head injury, and its presence was not invariably associated with poor outcome. Therefore, serum enzymatic determinations have an inadequate sensitivity and specificity for use as an index of neurological trauma.

Fifty-seven patients had CSF enzymatic determinations, and each of the enzymes studied was correlated directly with GCS and with the ultimate outcome. Within the subgroup of severely head-injured patients with a GCS score of 3 to 7, only the CPK1 and LDH1 isoenzymes correlated with the degree of head trauma and outcome. The CPK1 isoenzymes were not detectable in CSF from control patients, but were invariably present following head trauma. These CPK1 isoenzymes in the CSF were particularly useful in that they appeared in the acute course and were subsequently absent unless secondary injury to the brain occurred causing additional neurological damage. Secondary injuries due to delayed hemorrhage, infarction, hypoxia, or pathological evaluations of intracranial pressure were readily detected. The LDH1 isoenzyme is present in the CSF from normal patients and does elevate with neurological trauma; these LDH1 isoenzymes appear to be elevated for a period of weeks to months following injury and thus are less useful in detecting secondary injuries. An attempt was made to investigate the effect of Decadron (dexamethasone) on these enzymatic changes, but no significant effect was identified. Also noted in this study was the presence of CPK1 isoenzymes in the CSF of patients with gunshot wounds to the head, spinal cord injuries, and herniation syndromes. It is concluded, therefore, that CPK1 isoenzymes in the CSF appear to be a specific marker for neurological trauma, and may be of value both in clinical practice and in clinical investigations.

Early proliferative changes in astrocytes in postischemic noninfarcted rat brain

Transient cerebral ischemia in rats was produced by permanent occlusion of the vertebral arteries and 30-minute occlusion of the common carotid arteries. This model produces ischemic necrosis of neurons in the corpus striatum, cerebral cortex, and hippocampus; infarcts, with necrosis of neuropil, astrocytes, and blood vessels, are rare. Changes in striatal astrocytes at 40 minutes and 3 hours of reperfusion were evaluated by electron microscopy, and quantitative estimates of increases in cytoplasmic and mitochondrial area were performed. In areas of corpus striatum with moderate ischemic cell change, the percentage of astrocytic nuclei increased from 10.79% in controls to 17.76% at 40 minutes after ischemia (p less than 0.01) and 19.86% at 3 hours (p less than 0.01). Astrocytic cytoplasm was expanded and contained increased numbers of mitochondria, many of which were pleomorphic and had dilated intracristal spaces and condensed matrix. Rough endoplasmic reticulum was increased. Total mitochondrial area and number of mitochondrial profiles rose significantly in the astrocytic perikarya and foot processes at 3 hours postischemia. The greater number of astrocytes, the increases in mitochondria and rough endoplasmic reticulum and the configurational changes in the mitochondria suggest increased metabolic activity of astrocytes in postischemic, noninfarcted brain.