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- Origin of CSF: Most CSF is formed in the ventricular system. Possible sites of origin include the choroid plexus (~ 80%), the parenchyma (~20%), and the ependyma (negligible). It is believed that CSF is an ultrafiltrate of plasma that enters the basal side of the choroid epithelium and by active metabolism is transformed into CSF and secreted at the apical or ventricular side of the epithelium. This mechanism of CSF formation is largely speculative.
- Formation rate: Clinical studies have indicated the formation rate of CSF to be about 20 ml/hour or 500 ml/day in adults and children (3). The total CSF volume in the ventricles and subarachnoid space is age-dependent but reaches the adult volume of 150 ml by age 5 years. The rate of CSF formation is normal or only slightly reduced in children with hydrocephalus.
- Formation pressure independent: CSF production is pressure independent under normal physiological conditions. Markedly increased intraventricular pressure may reduce the quantity of ultrafiltrate from the choroidal capillary bed.
- Medication effects on CSF formation: Drugs that reduce CSF formation do so by interfering with the entire cellular metabolic process or specific transport mechanisms. Furosemide decreases CSF formation by interfering with chloride transport rather than by exerting any direct effect on carbonic anhydrase. Acetazolamide decreases CSF production by interfering with the function of carbonic anhydrase. It is not clear whether glucocorticoids alter CSF formation.
- Bulk flow: The only proven force responsible for bulk CSF absorption is that of a hydrostatic gradient.
- Arachnoid villi: The arachnoid villi appear to drain CSF from the subarachnoid space into the major dural sinuses by unidirectional flow after a critical opening pressure is exceeded (17, 18).
- Accessory routes: CSF drainage may occur through accessory routes: the mucosa of the paranasal sinuses, nasal mucosa, cranial nerve root sheaths, and cervical lymph nodes (17).
- CNS lymphatic system: CSF functions as a lymphatic system for the CNS.
- Path of circulation: CSF formed in the ventricles flows in bulk toward the subarachnoid space. The fluid that is formed in the lateral ventricles passes out through the paired interventricular foramina of Monro to reach the third ventricle. The fluid then flows caudally through the aqueduct of Sylvius and fourth ventricle, where it passes into the subarachnoid space by one of three foramina: the paired lateral foramina of Luschka and the midline foramen of Magendie. A small volume of fluid may also exit through the central canal of the spinal cord.
- Engine for circulation: A concentration gradient or “sink” is produced with the clearance of metabolic waste products to include macromolecules. Other factors affecting the circulation of CSF include the ciliary action of the ependyma and choroid plexus and the pulsations induced by the arterial tree and respiration.
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