Bermans Iskandar, M. Iskandar, M. Deopujari, M. Muzumdar, M. Badami, B. Authors Andrew Jea, M. Section Editor Shlomi Constantini, M. Editor in Chief Rick Abbott, M. 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. This can happen when there is a tumor on the choroid plexus, for example.
CSF flows from the lateral ventricles through two narrow passageways into the third ventricle. From the third ventricle, it flows down another long passageway known as the aqueduct of Sylvius into the fourth ventricle.
From the fourth ventricle, it passes through three small openings called foramina and into the subarachnoid space surrounding the brain and the spinal cord. If the flow of CSF at any of these points is blocked, hydrocephalus can develop. This is often referred to as non-communicating hydrocephalus. It has traditionally been thought that CSF is absorbed through tiny, specialized cell clusters called arachnoid villi near the top and midline of the brain.
The CSF then passes through the arachnoid villi into the superior sagittal sinus, a large vein, and is absorbed into the bloodstream. Once in the bloodstream, it is carried away and filtered by the kidneys and liver in the same way as other bodily fluids. However, more recent research has shown that CSF is also absorbed through other pathways as well. When CSF absorption is blocked or reduced, hydrocephalus can develop.
The interstitial space of the brain is separated from the ventricular CSF by the ependymal lining and from the subarachnoid CSF by the glia limitans. The glia limitans is a thick layer of interdigitating astrocytic processes with an overlying basement membrane.
This layer seals the surface of the CNS and dips into brain tissue along the perivascular space see below. External to it is the pia matter, a thin layer of connective tissue cells with a small amount of collagen.
The ependymal barrier is far more permeable than the BBB. The major cerebral arteries and veins traverse the subarachnoid space and penetrate into the brain, where they branch into smaller vessels and eventually capillaries.
Capillaries are in contact with astrocytic processes. Vessels larger than capillaries are separated from the surrounding brain tissue by a space the perivascular or Virchow-Robin space , which is an extension of the subarachnoid space. The glymphatic system helps rid the brain of waste products.
Such products are filtered through the arachnoid villi and removed by the venous circulation. Additionally, it has become apparent in recent years that there is a system of lymphatic vessels closely associated with the dural sinuses. This system may also be important for clearing waste products and for immune surveillance.
The outer surface of this perivascular space PVS is formed by the glia limitans. The inner surface is the vascular basement membrane. Postcapillary venules are also surrounded by a PVS. The PVS that surrounds postcapillary venules is the portal of entry of leukocytes into the brain in the normal state and during inflammation. Circulating monocytes and lymphocytes normally traverse postcapillary venules and enter the PVS.
In the course of inflammation, such as MS, this entry is increased because of leukocyte interactions with inflamed endothelial cells. Furthermore, leukocytes penetrate the glia limitans and enter into the CNS. The latter move is facilitated by matrix metalloproteinases MMPs produced by macrophages, which loosen the glia limitans.
Blood: Blood may be spilled into the CSF by accidental puncture of a leptomeningeal vein during entry of the LP needle. Such blood stains the fluid that is drawn initially and clears gradually. If it does not clear, blood indicates subarachnoid hemorrhage. Erythrocytes from subarachnoid hemorrhage are cleared in 3 to 7 days. A few neutrophils and mononuclear cells may also be present as a result of meningeal irritation. Xanthochromia blonde color of the CSF following subarachnoid hemorrhage is due to oxyhemoglobin which appears in 4 to 6 hours and bilirubin which appears in two days.
Xanthochromia may also be seen with hemorrhagic infarcts, brain tumors, and jaundice. Increased inflammatory cells pleocytosis may be caused by infectious and noninfectious processes.
Polymorphonuclear pleocytosis indicates acute suppurative meningitis. Mononuclear cells are seen in viral infections meningoencephalitis, aseptic meningitis , syphilis, neuroborreliosis, tuberculous meningitis, multiple sclerosis, brain abscess and brain tumors. Tumor cells indicate dissemination of metastatic or primary brain tumors in the subarachnoid space. The most common among the latter is medulloblastoma.
They can be best detected by cytological examination. A mononuclear inflammatory reaction is often seen in addition to the tumor cells. Oligoclonal bands are also seen occasionally in some chronic CNS infections. The type of oligoclonal bands is constant for each MS patient throughout the course of the disease. Oligoclonal bands occur in the CSF only not in the serum. These quantitative and qualitative CSF changes indicate that in MS, there is intrathecal immunoglobulin production.
MBP can be detected by radioimmunoassay. MBP is not specific for MS. It can appear in any condition causing brain necrosis, including infarcts. Low glucose in CSF is seen in suppurative, tuberculous and fungal infections, sarcoidosis, and meningeal dissemination of tumors. Glucose is consumed by leukocytes and tumor cells. Alzheimer's disease AD.
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