Angioarchitecture in experimental hydrocephalus

The pulsating brain: A review of experimental and clinical studies of intracranial pulsatility

The maintenance of adequate blood flow to the brain is critical for normal brain function; cerebral blood flow, its regulation and the effect of alteration in this flow with disease have been studied extensively and are very well understood. This flow is not steady, however; the systolic increase in blood pressure over the cardiac cycle causes regular variations in blood flow into and throughout the brain that are synchronous with the heart beat. Because the brain is contained within the fixed skull, these pulsations in flow and pressure are in turn transferred into brain tissue and all of the fluids contained therein including cerebrospinal fluid. While intracranial pulsatility has not been a primary focus of the clinical community, considerable data have accrued over the last sixty years and new applications are emerging to this day. Investigators have found it a useful marker in certain diseases, particularly in hydrocephalus and traumatic brain injury where large changes in intracranial pressure and in the biomechanical properties of the brain can lead to significant changes in pressure and flow pulsatility. In this work, we review the history of intracranial pulsatility beginning with its discovery and early characterization, consider the specific technologies such as transcranial Doppler and phase contrast MRI used to assess various aspects of brain pulsations, and examine the experimental and clinical studies which have used pulsatility to better understand brain function in health and with disease.

Brain macrophages and microglia in human fetal hydrocephalus

Whereas several studies have addressed the activation of microglia (the resident mononuclear phagocytes of the brain) and macrophages within the nervous system in experimental animal models of congenital and induced hydrocephalus, little is known of their state of activation or regional distribution in human fetal hydrocephalus. This investigation aimed to address such questions. Ten human fetal cases [20-36 gestational weeks (GW) at postmortem] previously diagnosed with hydrocephalus on ultrasound examination in utero, and 10 non-hydrocephalic controls (22-38 GW at postmortem) were assessed immufcnohistochemically with antibodies directed against MHC class II and CD68 antigens, and lectin histochemistry with Lycopersicon esculentum (tomato lectin). Adjacent sections were also immunoreacted with an antiserum to laminin to detect cerebral blood vessels. Eight out of the 10 hydrocephalus cases showed numerous CD68 and tomato lectin-positive macrophages located at focal regions along the ependymal lining of the lateral ventricles (particularly within the occipital horn). However, only five of these cases demonstrated MHC class II positive macrophages associated with the ventricular lining. Microglial reactivity within periventricular regions could also be identified using the lectin in four cases, two of which were also immunoreactive with CD68 (but not with MHC class II). By comparison, in control cases five out of 10 fetal brains (aged between 20 and 24 GW) showed few or no ependymal or supraependymal macrophages. One case at 28 GW, and cases at 32 and 38 GW (two of which were diagnosed with intrauterine hypoxic-ischemia) did, however, show some MHC class II (CD68 negative) cells located at the ependymal surface. Nevertheless, these were not as numerous or intensely immunoreactive as in the hydrocephalus cases. Microglia interspersed throughout the intermediate zone and circumscribing the basal ganglia were within normal confines in all cases examined. Hydrocephalic cases additionally showed focal regions of hypovascularization or alterations in the structure and orientation of capillaries within periventricular areas, compared to controls. The macrophage response detected at the ependymal lining of the ventricles and within the periventricular area in hydrocephalus may be related both to the severity of hydrocephalus and the age of the fetus.

Neuron tolerance during hydrocephalus

Whether or not neuron death plays a major role in pathophysiology during hydrocephalus is not well known. The goals of this study were to determine if neural degeneration occurred during hydrocephalus, and to determine if neuron tolerance developed during this pathophysiologic procedure.Neural damage as visualized by a sensitive staining technique, silver impregnation, was observed in three experimental groups: (1) adult hydrocephalic rats induced by kaolin injection into the cisterna magna, (2) adult rats with chronic hydrocephalus for 10 weeks subjected to acute forebrain ischemia induced by four-vessel occlusion, and (3) adult rats without hydrocephalus subjected to acute forebrain ischemia. The magnitude of hydrocephalus was also evaluated during this time. In mild or moderate hydrocephalus, little cell death was found. In severe hydrocephalus, axon and neuropil degeneration was extensively distributed, but cell death was still rarely observed. Although some neuron degeneration was found after acute forebrain ischemia in hydrocephalic rats, the extensive cell death in cortical layers III and V, and in hippocampal areas CA1 and CA4 that is commonly observed in the ischemic brain without hydrocephalus, was not seen. This study suggests that neuron death was not a major pathological change in the brain during hydrocephalus, with cerebral ventricles being enlarged during the development of hydrocephalus. Less neuron death in hydrocephalic rats after acute forebrain ischemia suggests that neuronal tolerance to ischemia occurs during hydrocephalus.

Effects of ventriculoperitoneal shunt removal on cerebral oxygenation and brain compliance in chronic obstructive hydrocephalus

OBJECT

The pathophysiology of shunt malfunction has not been fully examined, probably because of the paucity of appropriate animal models. Using a canine model of chronic obstructive hydrocephalus, the effects of shunt placement and removal on physiological parameters were evaluated.

METHODS

Fifteen dogs, nine in which chronic hydrocephalus was induced and six controls, were used in the experiment. Thirteen weeks after the induction of hydrocephalus, intracranial pressure (ICP), tissue and cerebrospinal fluid O2 saturation, response to hyperventilation, and brain compliance at low (5-15 mm Hg) and high (15-25 mm Hg) pressures were measured (untreated stage). Following this procedure, ventriculoperitoneal shunts were implanted in the dogs suffering from hydrocephalus. Two weeks later, the same series of measurements were repeated (shunted stage), following which the shunt systems were removed. One week after shunt removal, the last measurements were obtained (shunt-removed stage). All dogs underwent magnetic resonance imaging four times: before induction of hydrocephalus and before each measurement. All dogs with hydrocephalus also had ventriculomegaly (1.42 +/- 0.89 ml before induction of hydrocephalus compared with 3.4 +/- 1.64 ml 13 weeks after induction, p = 0.0064). In dogs in the untreated hydrocephalus stage, ICP remained within the normal range (8.33 +/- 2.60 mm Hg)--although it was significantly higher than that in the control group (5 +/- 1.41 mm Hg, p = 0.014). Tissue O2 saturation in the dogs in the hydrocephalus group (26.1 +/- 5.33 mm Hg) was lower than that in the dogs in the control group (48.7 +/- 4.27 mm Hg, p < 0.0001). After the dogs underwent shunt placement, significant improvement was observed in their ICP (5.22 +/- 2.17 mm Hg, p = 0.012) and tissue O2 saturation (35.2 +/- 6.80 mm Hg, p = 0.0084). However, removal of the shunt reversed these improvements back to the preshunt status. Hyperventilation induced significant decreases in ICP and O2 saturation at every measurement time and induced a significant decrease in tissue O2 saturation during the shunted stage, but not during the untreated and shunt-removed stages. Brain compliance measured at high pressure demonstrated a significant gradual decrease at every measurement.

CONCLUSIONS

In chronic obstructive hydrocephalus, shunt placement improves ICP and cerebral oxygenation as well as the response to hyperventilation in the tissue. Shunt removal reverses these improvements back to levels present during the untreated stage. The decrease in brain compliance may be one of the factors responsible for symptoms in shunt malfunction.

Cerebrovascular adaptation in chronic hydrocephalus

This study characterizes the regional changes in vascularity, which accompanies chronic progressive hydrocephalus. Fifteen dogs underwent surgical induction of hydrocephalus and were used for histologic studies. Animals were divided into 4 groups: surgical control, short term (< or = 5 weeks), intermediate term (8 weeks), and long term (10 to 12 weeks). Vessel diameter, density, and luminal area were calculated by imaging quantification after manual vessel identification in the cortical gray, white matter, and caudate nucleus. Capillary vessel diameter decreased 23.5% to 30.2% (P < 0.01) in the caudate, but then returned to normal at 12 weeks. Capillary vessel density decreased 53.5% (P < 0.05) in the cortical gray, but then increased to 234.8% (P < 0.01) over surgical controls at 12 weeks. There was no initial decrease in capillary density in the caudate; however, the long-term group capillary density was significantly greater (172.8% to 210.5%, P < 0.01) than surgical controls. Overall, there was a short-term decrease in lumen area, with recovery in the longer term. Glial fibrillary acidic protein (GFAP) immunohistochemistry demonstrated the pattern of GFAP staining and reactive astrocytes differed in the caudate compared with the occipital cortex. This data suggest that an increase in capillary density and diameter may be an adaptive process allowing maintenance of adequate cerebral perfusion and metabolic support in the hypoxic environment of chronic hydrocephalus.

Posthemorrhagic hydrocephalus and brain injury in the preterm infant: dilemmas in diagnosis and management

Advances in neonatal critical care have reduced the incidence of intraventricular hemorrhage (IVH) in the newborn. Paradoxically, however, the prevalence of the complications of IVH including posthemorrhagic hydrocephalus (PHHC) has increased. By virtue of its association with long-term neurodevelopmental disability, posthemorrhagic hydrocephalus is an ominous diagnosis in the premature infant. Animal models have demonstrated that ventricular distention may cause direct cerebral parenchymal injury. Evidence for secondary parenchymal injury in the premature infant with PHHC is by necessity indirect. The precise impact of secondary parenchymal injury on the overall neurological outcome of premature infants with PHHC remains unclear in large part because of the vulnerability of the immature brain to other forms of injury (e.g., periventricular leukomalacia) that may be difficult to distinguish from injury due to distention. Furthermore, parenchymal injury due to PVL may cause ventricular enlargement that does not benefit from CSF diversion. Because these primary and secondary mechanisms of injury may operate concurrently, the precise or dominant cause of ventricular enlargement is often difficult to establish with certainty in the neonatal period. These diagnostic dilemmas have in turn impeded the development and evaluation of therapies specifically aimed at reversing ventricular distention and preventing secondary parenchymal injury. This article focuses on the current dilemmas in diagnosis and management of this potentially reversible form of injury as well as on potential future strategies for its prevention.

Reduced local cerebral blood flow in periventricular white matter in experimental neonatal hydrocephalus-restoration with CSF shunting

The extent to which the reduction in CBF occurring in hydrocephalus is a primary or secondary event in the pathogenesis of the brain injury that ensues has not been clearly established. This is particularly true in neonatal hydrocephalus, where the disorder is most common, and where timing of the treatment of the developing nervous system is so important. We investigated the changes in local CBF (lCBF) in an animal model of severe progressive neonatal hydrocephalus before and after CSF shunting. Hydrocephalus was induced in 27 1-week-old kittens by percutaneous injection of 0.05 ml of 25% kaolin into the cisterna magna. Fourteen littermates acted as controls. The lCBF was measured by 14C-iodoantipyrine quantitative autoradiography after 1 week in 15 animals (8 hydrocephalic, 7 controls) and after 3 weeks in 26 animals (19 hydrocephalic, 7 controls) following induction of hydrocephalus. Twelve of the 3-week hydrocephalic group received a ventriculoperitoneal shunt 10 days following kaolin injection. At 1 week following induction of hydrocephalus, lCBF was globally reduced in cortical gray matter and white matter as well as deep subcortical structures. The maximum reduction was in the parietal white matter, to 37% of control levels. At 3 weeks a significant reduction in lCBF persisted only in the white matter (parietal, occipital, and corpus callosum; average, 42% of control levels), whereas cortical gray and deep subcortical structures had returned to normal levels spontaneously.(ABSTRACT TRUNCATED AT 250 WORDS)

Change in regional cerebral blood flow following glycerol administration predicts. Clinical result from shunting in normal pressure hydrocephalus

Cerebral haemodynamics were measured in 22 adult patients with secondary normal pressure hydrocephalus (NPH) before and after glycerol administration to determine which patients might benefit from a shunt procedure. Of these 22 patients, 14 were found to be shunt-responsive (group 1) and 8 were shunt-unresponsive (group 2). Measurement of regional cerebral blood flow (rCBF) was performed by xenon-enhanced computerized tomography (XeCT). Clinical factors such as the Evans' index and the presence or absence of brain atrophy, periventricular lucency (PVL), ventricular reflux, stagnation of cerebrospinal fluid on cisternography, and increased intracranial pressure were not statistically significant predictors of shunt responsiveness. Preoperative rCBF values did not differ between groups 1 and 2. The rCBF value in every cerebral region of group 1 patients increased significantly after shunting except for the basal ganglia. On preoperative rCBF measurement, all rCBF values in group 1 significantly increased after glycerol administration except for the periventricular lucency (PVL). Patients in group 2, however, lacked such an increase in rCBF. We therefore propose that, in patients with secondary NPH, shunt surgery will be likely to be effective in those with a demonstrated rise in rCBF after glycerol administration.

Local cerebral blood flow in rats with congenital hydrocephalus

Local cerebral blood flow (LCBF) has been measured by [14C]iodoantipyrine quantitative autoradiography in H-Tx rats with inherited congenital hydrocephalus at 10, 21, and 30 days after birth. LCBF at 10 days was uniformly low in all ten brain regions studied and not significantly different between hydrocephalic and age-matched control rats. By 21 days LCBF had increased significantly in control rats. LCBF was significantly lower (< 69%) in the hydrocephalics in all cortical regions and in the inferior colliculus LCBF at both 21 and 30 days. The cerebellar cortex, pons, and caudate were not significantly affected. At 30 days LCBF ranged from 55 to 115 ml 100 g-1 min-1 for hydrocephalics and from 100 to 183 ml 100 g-1 min-1 for controls.

Lipid peroxide level increase in experimental hydrocephalus

The aetiology of incomplete adrenergic denervation and reduction in the number and caliber of the cerebral vessels in hydrocephalus is still obscure. Stretching of the blood vessels alone is far from explaining these major vascular changes. Previous studies have shown that increased lipid peroxidation produces toxic effects on vessels. This experimental study was designed to investigate the possible aetiology of vascular changes in hydrocephalic rats with special reference to lipid peroxidation. Hydrocephalus was induced by injecting 50 mg/Kg sterilized kaolin suspension into the cisterna magna in 10 rats (Group A). A sham operation was performed for Group B. After three weeks the rats were anaesthetized and perfused transcardially. The brains were dissected, and cut to visualize the degree of hydrocephalus. The arteries of the circle of Willis were removed for light microscopic examination and the brains were kept for the measurement of lipid peroxidation levels. Light microscopic studies of cerebral arteries in hydrocephalic rats revealed spastic vessels with folding and corrugation of the lamina elastica. The level of lipid peroxidation in group A (260 +/- 9.129 nmol TBAR/gr wet tissue) was significantly higher than that of group B (106 +/- 3.59 nmol TBAR/gr wet tissue). It is suggested that vascular changes observed in hydrocephalic rats may be due to the high level of lipid peroxidation, which in turn may be the consequence of ischaemia caused by the hydrocephalus related stretching of cerebral vessels.

Improvement of cortical morphology in infantile hydrocephalic animals after ventriculoperitoneal shunt placement

As a sequel to our previous descriptions of the pathological changes induced by hydrocephalus in the infantile cerebral cortex, the study presented here has evaluated the effects of surgical decompression on cortical cytology and cytoarchitecture. Hydrocephalus was induced in 14 kittens by the intracisternal injection of kaolin at 4 to 11 days of age. Nine of these hydrocephalic animals received low-pressure ventriculoperitoneal shunts at 9 to 15 days after kaolin injection; these animals were monitored preoperatively and postoperatively by ultrasound and were killed at various postshunt intervals up to 30 days. Five normal or saline-injected animals served as age-matched controls. At the time of shunt placement, the ventricular index confirmed that all recipient animals had attained moderate or severe degrees of ventriculomegaly. Within 3 days after shunt placement, the size of the lateral ventricles had decreased to control levels and was accompanied by rapid and dramatic improvements in behavior and skull ossification. When the animals were killed, gross inspection revealed that about half of the animals exhibited mild to moderate ventriculomegaly, with cortical mantles 50 to 80% their normal thickness. Tissue from frontal (primary motor), parietal (association), and occipital (primary visual) cortical areas was processed for light microscopic analysis. Pyknotic or dark shrunken neurons, which are found typically in hydrocephalic brains, were observed only occasionally in the cortex of shunted animals. Gliosis and mild edema were prevalent, however, in the periventricular white matter. The laminae of the cerebral cortex could be identified in all shunted animals. In those animals with mild residual ventriculomegaly, the entire cortical mantle was somewhat compressed, as evidenced by an increased packing density of neurons. Furthermore, the somata of some neurons were disoriented. Overall, these results indicate that most of the morphological characteristics of the cerebral cortex are preserved after surgical decompression and suggest that ventriculoperitoneal shunts may prevent neuronal damage and/or promote neuronal repair.

Changes in the cerebral vascular bed in experimental hydrocephalus: an angio-architectural and histological study

The angio-architectural and histological changes of small cerebral blood vessels in experimental hydrocephalus were studied to assess the changes of the vascular bed in the cerebral mantle. Change of the microvasculature assessed from microcorrosion casts by scanning electron microscopy (SEM) and the histological changes shown by light and electron microscopy were compared before and after shunting for hydrocephalus. The changes of the rCBF were also evaluated by the hydrogen clearance method. In hydrocephalus, a reduction in the number and caliber of the capillaries was noted in both the white and gray matter in the SEM study, but the capillaries were preserved and changes were mild and nonspecific in the electron microscopic examination. Shunting resulted in the reversal of all these changes to normal along with recovery of the rCBF, which had decreased in hydrocephalus. These observations suggest that changes of the vascular bed participate in the alteration of cerebral mantle width in the hydrocephalic process, and that the changes of the microvasculature result not only from damage to the capillaries themselves but also from changes of the perivascular structures.

The cerebral cortex in congenital hydrocephalus in the H-Tx rat: a quantitative light microscopy study

Hydrocephalus in the H-Tx rat first develops in late gestation and causes death at 4-7 weeks. The effect of hydrocephalus on overall cortical dimensions and on five specific regions (frontal, sensory-motor, parietal, auditory and visual) has been studied by quantitative light microscopy at 10 and 30 days after birth. The lateral ventricle volumes in hydrocephalic rats were about 40 x larger than controls and increased fourfold between 10 and 30 days. Cortical volume was reduced by a small amount at 10 days but was larger in hydrocephalics at 30 days. Thinning of the cortical mantle was severe with disruption of the laminar structure, particularly in the auditory and visual regions, where it was already present at 10 days. The density of cortical cells (neurones and glia) was not altered in hydrocephalics at 10 days but was reduced in all regions at 30 days. Estimates of total cell number suggest that the lower density was not associated with an overall loss of cells. Capillary numerical density was not affected by the hydrocephalus at 10 days after birth but by 30 days it was significantly lower, particularly in the worst-affected posterior regions. The results show that the cerebral cortex is severely distorted and that in advanced hydrocephalus, although overall cell number is not affected, both cell density and capillary density are lower by up to 30%.

Changes in periventricular vasculature of rabbit brain following induction of hydrocephalus and after shunting

Hydrocephalus was induced in rabbits by injection of silicone oil into the cisterna magna. At 1 and 8 weeks postinjection the rabbits were either sacrificed or treated by cerebrospinal fluid shunting for 1 week. Blood vessel profiles in the periventricular neuropil were examined by light microscopy. In the caudate nucleus, septal area, and corpus callosum, hydrocephalus caused a reduction in the number of capillaries but no changes were observed in the number of larger blood vessels. Shunting reduced the size of the ventricles to normal and the number of capillaries increased if hydrocephalus was present for 1 week prior to shunting. If hydrocephalus was present for 8 weeks prior to shunting, the number of capillaries did not increase. These observations support the concept that collapse of capillaries may account for the decreased cerebral blood flow that has been measured in hydrocephalic brains.