Cerebrospinal fluid (CSF) surrounds the brain and spinal cord in the subarachnoid space. It primarily protects these structures as a cushion and mechanical barrier. Although composed of 99% water, CSF contains glucose, proteins, and lipids to provide nutrition to the central nervous system. Moreover, as part of the blood–CSF barrier, it serves as an excretory pathway to remove waste products by tightly regulating the brain’s extracellular ionic milieu.
Production of CSF occurs in the lateral cerebral ventricles by the choroid plexus. About 20 mL/h (500 mL/day) is produced, but absorption at arachnoid villi in cerebral venous sinuses maintains total CSF volume at 100-150 mL. The entire CSF volume is replaced about 3-4 times daily. Cerebrospinal fluid flow proceeds from lateral ventricles to the third ventricle through the intraventricular foramina, and then enters the fourth ventricle via the cerebral aqueduct. From the fourth ventricle, CSF reaches the subarachnoid space to surround the brain and the spinal cord (Figure 120-1).
The flow of CSF in the central nervous system. (Reproduced with permission from Waxman SG, Clinical Neuroanatomy, 27th ed. McGraw-Hill Companies, Inc. 2013. All rights reserved.)
Excess CSF results in increased intracranial pressure and hydrocephalus, most commonly through obstructed CSF circulation and noncommunicating hydrocephalus. Overproduction or underabsorption, communicating hydrocephalus, rarely occurs as well. A shunt or drain can surgically displace CSF. Cerebrospinal fluid production can be decreased pharmacologically as well, using osmotic (ie, mannitol) or loop diuretics (ie, furosemide). The least invasive means of lowering CSF are patient positioning (ie, elevated head of bed by 30 degrees) and short-term hyperventilation, either via encouragement of spontaneous respirations or positive pressure ventilation.
Certain medications and anesthetics interfere with CSF production. Carbonic anhydrase inhibitors (ie, acetazolamide), furosemide, and thiopental decrease CSF production, whereas desflurane, halothane, and ketamine increase CSF production.