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In many chapters to follow, reference is made to the ways in which changes in the cerebrospinal fluid (CSF) reflect the basic pathologic processes in a wide variety of inflammatory and infectious, metabolic, neoplastic, demyelinative, and degenerative diseases. The CSF alterations in these varied circumstances raise so many important problems that we consider it worthwhile to discuss in one place the mechanisms involved in the formation, circulation, and absorption of the CSF, particularly as they pertain to alterations of intracranial pressure (ICP). Considered in this chapter are hydrocephalus, pseudotumor cerebri, and syndromes produced by reduced pressure in the CSF compartment. In all of these conditions, the primary abnormalities are those of the CSF and its circulation. Examination of the CSF as a diagnostic aid in neurology was discussed in Chap. 2; the primary infectious and noninfectious inflammatory reactions of the pia-arachnoid (leptomeninges) and ependyma of the ventricles are considered in Chap. 32.

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A few historical points call to mind that our knowledge of the physiology, chemistry, and cytology of the CSF is relatively recent. Although the lumbar puncture was introduced by Quincke in 1891, it was not until 1912 that Mestrezat made correlations between disease processes and the cellular and chemical changes in the CSF. In 1937, Merritt and Fremont-Smith published their monograph on the CSF changes in a broad variety of disease. Our knowledge of CSF cytology has accumulated since the late 1950s, when membrane filtration techniques (particularly the cellulose ester or Millipore filter) were introduced. The studies of Dandy (1919) and of Weed (1935) provided the basis of our knowledge of CSF formation, circulation, and absorption. Then followed the important studies of Pappenheimer and of Ames and their colleagues, and the monographs of Fishman and of Davson and coworkers, which are important modern contributions. (See Chap. 2 for references.)

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The primary function of the CSF appears to be a mechanical one; it serves as a kind of water jacket for the spinal cord and brain, protecting them from potentially injurious blows to the spinal column and skull and acute changes in venous pressure. Also, it provides the brain with buoyancy. As pointed out by Fishman, the 1,500-g brain, which has a water content of approximately 80 percent, weighs only 50 g when suspended in CSF, so the brain virtually floats in its CSF jacket. Many of the physiologic and chemical mechanisms described below are committed to maintaining the relatively constant volume–pressure relationships of the CSF. In addition, because the brain and spinal cord have no lymphatic channels, the CSF, through its “sink action” (see below), serves to remove the waste products of cerebral metabolism, the main ones being CO2, lactate, and hydrogen ions. The composition of the CSF is maintained within narrow limits, despite major alterations in the blood; thus the CSF, along with the intercellular fluid of the brain, helps to preserve a stable chemical environment for neurons and their myelinated fibers. There is ...

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