In the chapters to follow, reference is made to the ways in which changes in the cerebrospinal fluid (CSF) reflect the basic pathologic processes in a variety of inflammatory and infectious, neoplastic, demyelinative, and degenerative diseases. The CSF alterations in these circumstances raise so many important problems that we consider it worthwhile to discuss in one chapter the mechanisms of the formation, circulation, and absorption of the CSF, particularly as they pertain to alterations of intracranial pressure (ICP). It follows that the diseases that result from perturbations in these mechanisms, namely hydrocephalus, pseudotumor cerebri, and syndromes produced by reduced pressure in the CSF compartment, are also presented. Further information on the management of raised ICP, particularly as it pertains to traumatic brain injury, can be found in Chap. 34. Examination of the CSF as a diagnostic aid in neurology was discussed in Chap. 2, and the primary infectious and noninfectious inflammatory reactions of the pia-arachnoid (leptomeninges) and ependyma of the ventricles are considered in Chap. 31.
A few historical points call to mind that our understanding of the physiology, chemistry, and cytology of the CSF is the result of a technical innovation that was introduced a century ago. 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 a monograph on 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. The important studies of Pappenheimer and colleagues and of Ames and colleagues followed, and then the monographs of Fishman and of Davson and coworkers, which are important modern contributions. (See Chap. 2 for references.) More recent inceptions in analyzing the lymphocytic cells and protein fractions in the CSF for diagnostic purposes in cancer, immune disorders such as multiple sclerosis and a variety of infections, have expanded on modern technology that was developed in hematology.
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. Many of the physiologic 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 a “sink action,” serves to remove 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 other main function of CSF, by its contiguity with the extracellular compartment of the brain, is to preserve a stable chemical environment for neurons, astrocytes, and nerve fibers. There is no reason, however, to believe that the CSF is actively involved in the metabolism of the cells of the brain and spinal cord.