The finding of dura-associated lymphatic vessels is contrary to long-held beliefs about the absence of meningeal lymphatics. These include perivascular pathways within the CNS parenchyma that support the clearance of solutes from the brain to the CSF and extra-axial meningeal lymphatic vessels associated with the dural sinuses that facilitate the movement of solutes in the CSF into the systemic vascular system. In addition to these well-described transport mechanisms, newer studies have documented the existence of other pathways involved in the movement of CSF and solutes throughout the central nervous system (CNS). Drug entry also may be altered in patients with meningitis by the accompanying inflammation, and this may subsequently rapidly change with regression of this inflammation with therapy. In contrast, ionically charged molecules generally require active transport for entry into the CSF. Lipid-soluble molecules or drugs readily diffuse across the vascular endothelium and epithelium of the choroid plexus into the interstitial fluid and CSF. These vesicles may become obstructed by bacteria or cells as a result of an inflammatory process or by red blood cells during subarachnoid hemorrhage. Movement of CSF and cellular components across arachnoid villi occurs via transport within giant vesicles. Arachnoid villi normally allow the passage of particles less than 7.5 micron in diameter from the CSF into the blood. Arachnoid villi and venous sinuses are separated by endothelial cells connected by tight junctions ( figure 1). Each arachnoid villus functions as a one-way valve permitting unidirectional flow of CSF into the blood. The CSF is propelled along the neuroaxis by a cranio-caudal pulsatile wave induced by flow in the cerebral arteries and by the associated expansions of the vascular compartment in the cranial vault.ĬSF is reabsorbed in the arachnoid villi, located along the superior sagittal and intracranial venous sinuses and around the spinal nerve roots. Thereafter, CSF passes through median (foramen of Magendie) and lateral (foramina of Luschka) apertures in the fourth ventricle into the subarachnoid space at the base of the brain and then flows over the convexities of the brain and down the length of the spinal cord. The normal rate of CSF production is approximately 20 mL per hour.ĬSF circulates from the lateral ventricles though the interventricular foramina of Monro into the third ventricle and then the fourth ventricle via the cerebral aqueduct. In normal adults, the CSF volume is 90 to 200 mL approximately 20 percent of the CSF is contained in the ventricles the rest is contained in the subarachnoid space in the cranium and spinal cord. A specialized layer of ependymal cells called the choroidal epithelium overlies these villi ( figure 1).ĬSF is formed in the choroid plexus by both filtration and active transport. The choroid plexus consists of projections of vessels and pia mater that protrude into the ventricular cavities as frond-like villi containing capillaries in loose connective stroma. PHYSIOLOGY OF CSF FORMATION AND FLOW - Cerebrospinal fluid (CSF) is produced by the choroid plexus in the lateral, third, and fourth ventricles and circulates through the subarachnoid space between the arachnoid mater and the pia mater. (See "Bacterial meningitis in children older than one month: Clinical features and diagnosis", section on 'Interpretation'.) (See "Lumbar puncture: Technique, indications, contraindications, and complications in adults".)ĬSF analysis in children is presented elsewhere. The technique for obtaining CSF via lumbar puncture and the complications and contraindications to this test are discussed separately. This topic will review the normal physiology and composition of CSF. Knowledge of the normal physiology and pathophysiology of CSF production and flow is useful in interpreting such test results. INTRODUCTION - Examination of the cerebrospinal fluid (CSF) may provide critically important diagnostic information in a number of infectious and noninfectious medical conditions.
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