Cerebrospinal Fluid Production and Absorption

Although a small proportion of CSF may be produced from the ependyma and brain parenchyma, the predominant site of CSF production is the choroid plexus, contributing 70-80% of the daily volume. Production occurs by a combination of filtration across the capillary endothelium and active secretion of sodium by the choroidal epithelium [1]. Cerebrospinal fluid production does appear to be reduced in the presence of elevated intracranial pressure and reduced cerebral perfusion pressure; however, the effect is small and production is largely independent of pressure under physiological conditions. By contrast, CSF absorption shows a linear relationship to ICP.

The mechanisms of absorption of CSF have been extensively investigated. Direct absorption by the brain parenchyma, the choroid plexus itself and by lymphatic channels in the region of the cribiform plate have been postulated. It is, however, via the arachnoid villi and granulations that most absorption is presumed to take place. Arachnoid villi are herniations of arachnoidal tissue into the dural venous sinuses. For a long time, two models of CSF absorption were debated. The "closed" concept held that the villi were blind diverticulae and absorption of CSF occurred by a process of seepage across their endothelial covering. The alternative "open" model inferred the presence of channels across the villus, opening and closing in a valve-like manner, permitting the unidirectional flow of CSF. Tripathi and Tripathi [2] attempted to reconcile these opposing views and proposed a transmembrane transport mechanism consisting of vacuoles ferrying CSF across the covering endothelial layer. Interestingly, recent work has focused on the role of the CNS microcirculation in the absorption of CSF - a concept that would have major implications for our understanding of the pathogenesis of hydrocephalus [3]. Whilst the precise mechanism of CSF absorption and the relative contributions of the various absorptive pathways currently remain unclear, our classification and management of hydrocephalus is based on traditional concepts of CSF circulation.

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The term vaginitis is one that is applied to any inflammation or infection of the vagina, and there are many different conditions that are categorized together under this ‘broad’ heading, including bacterial vaginosis, trichomoniasis and non-infectious vaginitis.

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