This review integrates eight aspects of cerebrospinal fluid (CSF) circulatory dynamics: formation rate, pressure, flow, volume, turnover rate, composition, recycling and reabsorption. Novel ways to modulate CSF formation emanate from recent analyses of choroid plexus transcription factors (E2F5), ion transporters (NaHCO3 cotransport), transport enzymes (isoforms of carbonic anhydrase), aquaporin 1 regulation, and plasticity of receptors for fluid-regulating neuropeptides. A greater appreciation of CSF pressure (CSFP) is being generated by fresh insights on peptidergic regulatory servomechanisms, the role of dysfunctional ependyma and circumventricular organs in causing congenital hydrocephalus, and the clinical use of algorithms to delineate CSFP waveforms for diagnostic and prognostic utility. Increasing attention focuses on CSF flow: how it impacts cerebral metabolism and hemodynamics, neural stem cell progression in the subventricular zone, and catabolite/peptide clearance from the CNS. The pathophysiological significance of changes in CSF volume is assessed from the respective viewpoints of hemodynamics (choroid plexus blood flow and pulsatility), hydrodynamics (choroidal hypo- and hypersecretion) and neuroendocrine factors (i.e., coordinated regulation by atrial natriuretic peptide, arginine vasopressin and basic fibroblast growth factor). In aging, normal pressure hydrocephalus and Alzheimer's disease, the expanding CSF space reduces the CSF turnover rate, thus compromising the CSF sink action to clear harmful metabolites (e.g., amyloid) from the CNS. Dwindling CSF dynamics greatly harms the interstitial environment of neurons. Accordingly the altered CSF composition in neurodegenerative diseases and senescence, because of adverse effects on neural processes and cognition, needs more effective clinical management. CSF recycling between subarachnoid space, brain and ventricles promotes interstitial fluid (ISF) convection with both trophic and excretory benefits. Finally, CSF reabsorption via multiple pathways (olfactory and spinal arachnoidal bulk flow) is likely complemented by fluid clearance across capillary walls (aquaporin 4) and arachnoid villi when CSFP and fluid retention are markedly elevated. A model is presented that links CSF and ISF homeostasis to coordinated fluxes of water and solutes at both the blood-CSF and blood-brain transport interfaces.

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Multiplicity of cerebrospinal fluid functions: New challenges in health and disease

✓ Measurements of intracranial pressure (ICP) were begun within hours of injury in 160 patients with severe brain trauma, and continued in the intensive care unit. Some degree of increased ICP (> 10 mm Hg) was present on admission in most cases (82%), and in all but two of the 62 patients with intracranial mass lesions requiring surgical decompression; ICP was over 20 mm Hg on admission in 44% of cases, and over 40 mm Hg in 10%. In patients with mass lesions only very high ICP (> 40 mm Hg) on admission was significantly associated with a poor neurological picture and outcome from injury, while in patients with diffuse brain injury any increase in ICP above 10 mm Hg was associated with a poorer neurological status and a worse outcome. Despite intensive measures aimed at prevention of intracranial hypertension, ICP rose over 20 mm Hg during the monitoring period in 64 of the 160 patients (40%). Postoperative increases in ICP over 20 mm Hg (mean) were seen in 52% of the patients who had had intracranial mass...

Significance of intracranial hypertension in severe head injury.

A mathematical model of the cerebrospinal fluid (CSF) system was developed to help clarify the kinetics of the intracranial pressure (ICP). A general equation predicting the time course of pressure was derived in terms of four parameters: the intracranial compliance, dural sinus pressure, resistance to absorption, and CSF formation. These parameters were measured in the adult cat, and the equation was tested by comparing experimental and calculated values of the time course of pressure in response to volume changes. The theoretical and experimental results were in close agreement, and the role of each parameter in governing the dynamic equilibrium of the ICP was determined. From this analysis, dynamic tests were developed for rapid measurement of CSF formation, absorption resistance, and the bulk intracranial compliance. These techniques are applicable to clinical settings, providing data that are useful in characterizing the physiological mechanisms responsible for raised ICP and assessing changes induced by therapy.

A nonlinear analysis of the cerebrospinal fluid system and intracranial pressure dynamics.

Intracranial Pressure IV

Assessment of cerebral autoregulation is an important adjunct to measurement of cerebral blood flow for diagnosis, monitoring or prognosis of cerebrovascular disease. The most common approach tests the effects of changes in mean arterial blood pressure on cerebral blood flow, known as pressure autoregulation. A 'gold standard' for this purpose is not available and the literature shows considerable disparity of methods and criteria. This is understandable because cerebral autoregulation is more a concept rather than a physically measurable entity. Static methods utilize steady-state values to test for changes in cerebral blood flow (or velocity) when mean arterial pressure is changed significantly. This is usually achieved with the use of drugs, shifts in blood volume or by observing spontaneous changes. The long time interval between measurements is a particular concern in many of the studies reviewed. Parallel changes in other critical variables, such as pCO2, haematocrit, brain activation and sympathetic tone, are rarely controlled for. Proposed indices of static autoregulation are based on changes in cerebrovascular resistance, on parameters of the linear regression of flow/velocity versus pressure changes, or only on the absolute changes in flow. The limitations of studies which assess patient groups rather than individual cases are highlighted. Newer methods of dynamic assessment are based on transient changes in cerebral blood flow (or velocity) induced by the deflation of thigh cuffs, Valsalva manoeuvres, tilting and induced or spontaneous oscillations in mean arterial blood pressure. Dynamic testing overcomes several limitations of static methods but it is not clear whether the two approaches are interchangeable. Classification of autoregulation performance using dynamic methods has been based on mathematical modelling, coherent averaging, transfer function analysis, crosscorrelation function or impulse response analysis. More research on reproducibility and inter-method comparisons is urgently needed, particularly involving the assessment of pressure autoregulation in individuals rather than patient groups.

https://iopscience.iop.org/article/10.1088/0967-3334/19/3/001/pdf

Assessment of cerebral pressure autoregulation in humans - a review of measurement methods

✓ The distribution of compliance and outflow resistance between cerebral and spinal compartments was measured in anesthetized, ventilated cats by analysis of the cerebrospinal fluid (CSF) pressure response to changes in CSF volume. Cerebral and spinal compartments were isolated by inflating a balloon positioned epidurally at the level of C-6. The change of CSF volume per unit change in pressure (compliance) and change of CSF volume per unit of time (absorption) were evaluated by inserting pressure data from the experimental responses into a series of equations developed from a mathematical model. It was found that 68% of total compliance is contributed by the cerebral compartment while the remaining 32% is contained within the spinal axis. The cerebral compartment accounted for 84% of total CSF absorption. The mechanism for spinal absorption appears to be similar in that no differences were obvious on the basis of pressure dynamics.

Compartmental analysis of compliance and outflow resistance of the cerebrospinal fluid system

✓ A patient with acute subdural hematoma was successfully treated with hemicraniectomy. He developed contralateral weakness 4 months after surgery which was reversed by cranioplasty. The presumptive mechanism is a gradient between atmospheric and intracranial pressure. Early cranioplasty is suggested to prevent this phenomenon.

Complication of a large cranial defect: Case report

✓ Transsphenoidal microdissection has been proposed as a preferred means of treating Cushing's disease. This procedure allows the surgeon to remove a pituitary microadenoma and at the same time to preserve normal tissue. Two cases described here were treated by this method. An interesting and important observation was that neither patient appeared to be cured for 2 to 6 weeks after surgery, as assessed by dexamethasone suppression. Later, normal suppressibility occurred and the course of each patient was compatible with cure. Patients treated by this method should not be automatically retreated because of adrenocorticotropic hormone (ACTH) non-suppressibility in the early postoperative period.

Delayed cure of Cushing's disease after transsphenoidal surgery of pituitary microadenomas. Report of two cases.

Our studies of intracranial pressure dynamics have shown that the volume extensibility, or compliance, in combination with the resistance to drainage of CSF are major parameters which affect the rate of change and resting level of the ICP. The objective of this investigation was to examine how these parameters were distributed between cranial and spinal compartments. We hypothesized that in the cranial compartment, a shift of cerebral blood volume offers the only means of rapid compensation to sudden changes in CSF volume. A second objective, then, was to examine to what extent the compliance which would be affected by the degree of this vascular compensation, is influenced by changes in mean systemic blood pressure.

A Compartmental Analysis of Compliance and Outflow Resistance and the Effects of Elevated Blood Pressure

~" Transsphenoidal microdissection has been proposed as a preferred means of treating Cushing's disease. This procedure allows the surgeon to remove a pituitary microadenoma and at the same time to preserve normal tissue. Two cases described here were treated by this method. An interesting and important observation was that neither patient appeared to be cured for 2 to 6 weeks after surgery, as assessed by dexamethasone suppression. Later, normal suppressibility occurred and the course of each patient was compatible with cure. Patients treated by this method should not be automatically retreated because of adrenocorticotropic hormone (ACTH) nonsuppressibility in the early postoperative period.

James LaMorgese

Papers

Compartmental analysis of compliance and outflow resistance of the cerebrospinal fluid system

Complication of a large cranial defect: Case report

Delayed cure of Cushing's disease after transsphenoidal surgery of pituitary microadenomas. Report of two cases.

A Compartmental Analysis of Compliance and Outflow Resistance and the Effects of Elevated Blood Pressure

Delayed cure of Cushing's transsphenoidal surgery of microadenomas disease after pituitary