Dynamic aspects of cerebrospinal fluid

Mechanisms of reduced cerebral blood flow in cerebral edema and elevated intracranial pressure

A mechanism of elevated intracranial pressure (ICP) in cerebral edema and its effects on cerebral blood flow (CBF) are presented in this paper. To study and demonstrate these effects, a mathematical model of intracranial hydrodynamics was developed. The model simulates the intracranial hydrodynamics and the changes that occur when cerebral edema predominates. To account for an edema pathology, the model includes resistances to cerebrospinal fluid (CSF) and interstitial fluid (ISF) flows within the parenchyma. The resistances change as the intercellular space becomes smaller due to swelling of brain cells. The model demonstrates the effect of changes in these resistances on ICP and venous resistance to blood flow by accounting for the key interactions between pressure, volume, and flow in the intracranial compartments in pathophysiological conditions. The model represents normal intracranial physiology as well as pathological conditions. Simulating cerebral edema with increased resistance to cerebral ISF flow resulted in elevated ICP, increased brain volume, markedly reduced ventricular volume, and decreased CBF as observed in the neurointensive care patients. The model indicates that in high ICP values, alternation of the arterial-arteriolar resistance to flow minimally affects CBF, whereas at low ICP they have a much greater effect on CBF. The model demonstrates and elucidates intracranial mechanisms related to elevated ICP.NEW & NOTEWORTHY Study goal was to elucidate the role of "bulk flow" of ISF through brain parenchyma. A model was developed to simulate fluid shifts in brain edema, ICP elevation, and their effect on CBF. Bulk flow resistance affected by edema elevates ICP and reduces CBF. Bulk flow affects transmural pressure and volume distribution in brain compartments. Changes in bulk flow resistance result in increase of venous resistance to flow and decrease in CBF.

Mechanisms of fluid movement into, through and out of the brain: evaluation of the evidence

Interstitial fluid (ISF) surrounds the parenchymal cells of the brain and spinal cord while cerebrospinal fluid (CSF) fills the larger spaces within and around the CNS. Regulation of the composition and volume of these fluids is important for effective functioning of brain cells and is achieved by barriers that prevent free exchange between CNS and blood and by mechanisms that secrete fluid of controlled composition into the brain and distribute and reabsorb it. Structures associated with this regular fluid turnover include the choroid plexuses, brain capillaries comprising the blood-brain barrier, arachnoid villi and perineural spaces penetrating the cribriform plate. ISF flow, estimated from rates of removal of markers from the brain, has been thought to reflect rates of fluid secretion across the blood-brain barrier, although this has been questioned because measurements were made under barbiturate anaesthesia possibly affecting secretion and flow and because CSF influx to the parenchyma via perivascular routes may deliver fluid independently of blood-brain barrier secretion. Fluid secretion at the blood-brain barrier is provided by specific transporters that generate solute fluxes so creating osmotic gradients that force water to follow. Any flow due to hydrostatic pressures driving water across the barrier soon ceases unless accompanied by solute transport because water movements modify solute concentrations. CSF is thought to be derived primarily from secretion by the choroid plexuses. Flow rates measured using phase contrast magnetic resonance imaging reveal CSF movements to be more rapid and variable than previously supposed, even implying that under some circumstances net flow through the cerebral aqueduct may be reversed with net flow into the third and lateral ventricles. Such reversed flow requires there to be alternative sites for both generation and removal of CSF. Fluorescent tracer analysis has shown that fluid flow can occur from CSF into parenchyma along periarterial spaces. Whether this represents net fluid flow and whether there is subsequent flow through the interstitium and net flow out of the cortex via perivenous routes, described as glymphatic circulation, remains to be established. Modern techniques have revealed complex fluid movements within the brain. This review provides a critical evaluation of the data.

Compartmental analysis of cerebrospinal fluid transfer and absorption in simulated hydrocephalus

A mathematical model of the cerebrospinal fluid (CSF) flow dynamics found in hydrocephalic infants with myelomeningocele lesions was constructed using criteria obtained from analogous clinical situations where 125I-labelled and 131I-labelled ortho-iodobenzoyl-amino-acetic acid (hippuran) had been employed to measure CSF flow dynamics. The quantitative results from this study allowed clinical data to be assessed and the importance of various CSF transfer mechanisms to be discussed. Our mathematical model indicates that the majority of radiopharmaceutical passes from the cerebral reservoir (the ventricles) into the blood. Experimental evidence indicates that the principal mechanism responsible for this movement is the bulk flow of CSF between its sites of production in the choroid plexus and absorption by the arachnoid villi.

The transport of sugars across the perfused choroid plexus of the sheep

The blood-perfused choroid plexuses from the lateral ventricles of the sheep were used to determine the nature of sugar exchanges between blood and cerebrospinal fluid (c.s.f.). There was a net entry of sugars from blood to c.s.f. at all concentrations of sugars which were used and this net entry was seen when the sugars were measured either directly by enzymic analysis or by the use of isotopically labelled sugars. From competition experiments the order of affinity of the transporting system from both blood to c.s.f. and c.s.f. to blood was the same, i.e. 2-deoxy-D-glucose much greater than D-glucose greater than 3-O-methyl-D-glucose much greater than D-galactose. The transport of sugars from c.s.f. to blood and blood to c.s.f. consists in both cases of a non-saturable and a saturable component. However, the affinity of the two systems is markedly different, the blood to c.s.f. being a system of low affinity and high capacity while that of the c.s.f. to blood has a high affinity and a low capacity. The concentration of glucose in the newly formed c.s.f. was estimated from the rate of c.s.f. secretion and the net flux of glucose across the choroid plexus. The concentration of glucose in this fluid was some 45-60% of that in plasma and so the low glucose concentration observed in bulk c.s.f. would appear to be a result of the entry process and not that of cerebral metabolism.

Electrolyte levels in the CSF of children with nontumoral hydrocephalus. Relation to clinical parameters

The levels of calcium and magnesium (by atomic absorption spectrophotometry), sodium and potassium (by flame photometry), inorganic phosphate and proteins (by spectrophotometry) were measured in the CSF of 27 children with hydrocephalus and compared with the CSF data on 10 controls, so as to study the effect of the blood-brain and blood-CSF barrier alterations observed in hydrocephalus, on the CSF electrolytes. Ca (P less than 0.001), Mg (P less than 0.01) and phosphate (P less than 0.05) were found to be increased, K (P less than 0.01) decreased, and Na levels not significantly altered. Linear regression analysis of hydrocephalic patients showed a linear correlation (r = +0.62) between phosphate and proteins (P less than 0.01) and (r = +0.66) between phosphate and Ca (P less than 0.001). Comparison of control CSF electrolyte levels with those found in the CSF of hydrocephalic patients were grouped according to clinical variables: sex (F, M), age (less than or equal to 1.5 months, greater than 1.5 months), etiological variants (congenital, acquired), treated infection (no, yes), surgical treatment (no, yes), time of onset (less than or equal to 1 month, greater than 1 month), ventricular dilation (mild, severe) and intracranial hypertension (mild or severe). These comparisons showed specific increases of Mg for males (P less than 0.05), acquired hydrocephalus (P less than 0.05) and severe intracranial hypertension (ICH) (P less than 0.001), moderate decrease in the Na levels in acquired hydrocephalus (P less than 0.05) and mild ICH (P less than 0.05), specific increases of phosphate in females (P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

The morphological correlates of primate cerebrospinal fluid absorption

Arachnoid villi from cynomolgus monkeys subjected to various states of cerebrospinal fluid (CSF) absorption have been examined with scanning (SEM) and transmission (TEM) electron microscopy. Pressures within the superior sagittal sinus and the subarachnoid space were rigidly controlled, both prior to and during perfusion fixation and, as such, we created and studied conditions of normal, absent and increased cerebrospinal fluid absorption. Under normal conditions, the most prominent feature of the CSF/blood interface was the presence of endothelial intracytoplasmic vacuoles. The presence of these vacuoles was suggested with SEM and readily confirmed with TEM. Occasionally, these vacuoles coalesced with both the CSF and sagittal sinus fronts, thereby creating transcellular channels as identified by TEM or surface pores as seen with SEM. Villi perfused during conditions of no CSF flow exhibited none of the previously described vacuoles, channels, or pores, whereas increased CSF flows were associated with increased numbers of these structures. The significance of these findings was discussed in relation to CSF absorption and to previously reported ultrastructural studies.

Vacuolar transcellular channels as a drainage pathway for cerebrospinal fluid

1. Based on our ultrastructural investigations in monkeys, we report here a new concept as to the physiological mechanism of drainage of cerebrospinal fluid (c.s.f.) which would seem to bridge the gap between the two apparently opposing views of ;closed' and ;open' system.2. Our studies reveal the presence of an intact mesothelial lining of the arachnoid mater, including its villus-like projections and herniations into the dural sinus and its lacunae, adjacent cells being joined by tight junctions; in addition we have observed for the first time that many lining cells in the region of the superior sagittal sinus are characterized by unit membrane-bound, electron-optically empty giant vacuoles of several micrometres diameter. In one monkey with a fresh subarachnoid haemorrhage, many vacuoles were filled with plasma proteins and some contained intact blood corpuscles.3. Serial section analysis showed that the vacuoles were in fact invaginations from the basal aspect of the cell surface and were evidently in direct communication with the c.s.f. in the subarachnoid space. Some vacuoles in addition showed openings on their apical surface thus constituting transcellular channels or pores. Basal openings up to 3.5 mum and apical openings up to 2.3 mum were seen.4. It is postulated that vacuoles are stages in the formation of a dynamic system of transcellular pores which allow the bulk outflow of c.s.f. down a pressure gradient, and that the mesothelial vacuolation cycle, in providing the requisite number of transcellular pores across the mesothelial barrier at any time, is a controlling factor in the outflow of c.s.f. and in the maintenance of its fluid-pressure within the subarachnoid space.5. The basic similarity between the bulk flow of the aqueous humour and c.s.f. from the anatomically closed cavities of the anterior chamber and the subarachnoid space, respectively, is underlined.6. The present study provides further support for our hypothesis that the bulk outflow of fluid, via a dynamic system of transcellular pores formed by gradually enlarging membranous surface infoldings in a single cell, termed as giant vacuoles, is a fundamental biological process not hitherto described.