INTRODUCTION TO CEREBRAL SPINAL FLUID DYNAMICS
Cerebral spinal fluid (CSF) is found within the brain’s ventricular system and around the brain and spinal cord. An adult produces approximately 500 mL of CSF per day at a rate of 0.5 mL/min, and it is replenished every 4 to 6 hours. An infant has a total CSF volume of approximately 50 mL. CSF is produced by the ependymal cells of the choroid plexus in the lateral ventricles. It flows from the lateral ventricles through the foramen of Monroe to the third ventricle, and from there flows through the cerebral or Sylvian aqueduct to the fourth ventricle. Then the CSF exits via the cisterna magna and the lateral cisterns to the subarachnoid space and the central canal of the spinal cord. From here, it travels cranially to surround the sulci of the cerebral cortex. CSF is resorbed by entering the dural venous sinuses via the arachnoid granulations or villi.
Intracranial pressure is the pressure of the brain parenchyma and the CSF and, thus, the pressure within the subarachnoid space. The measurement of intracranial pressure can be done in brain parenchyma, within the ventricles, or by lumbar puncture opening pressure. Clinical signs of increased intracranial pressure will be discussed in this chapter as we review the clinical pathophysiology of hydrocephalus and pseudotumor cerebri.
Hydrocephalus is a disorder whereby excess CSF accumulates in extracerebral spaces, leading to an increase in intracranial pressure. Hydrocephalus may be caused by an obstruction of CSF flow, excess production of CSF, or decreased absorption.
PATHOGENESIS AND EPIDEMIOLOGY
Obstructive, also known as noncommunicating, hydrocephalus occurs when the CSF flow is blocked along the ventricles or along a passage connecting the ventricles, causing ventricular dilation proximal to the point of blockage. Obstruction may be congenital or acquired. Congenital hydrocephalus is estimated to occur at a rate of 1 to 2 per every 1000 live births. Aqueductal stenosis, due to stenosis of the Sylvain aqueduct connecting the third and fourth ventricles, is the most frequent cause of congenital hydrocephalus. Other etiologies include complications of myelomeningoceles and Chiari malformations that obstruct CSF outflow from the fourth ventricle. Acquired causes of obstructive hydrocephalus frequently are posterior fossa tumors, including medulloblastomas, astrocytomas, or ependymomas.
Nonobstructive, or communicating, hydrocephalus may be the result of excess CSF production or decreased absorption, which causes the dilation of the entire ventricular system. Increased production of CSF may occur in the case of a choroid plexus papilloma. Decreased absorption may result from central nervous system hemorrhage, infection, inflammation, or increased venous pressure.
In preterm infants, intraventricular hemorrhages (IVHs) commonly lead to hydrocephalus. As many as 35% of preterm infants have IVH as a complication. Other sources of hemorrhage, including ruptured arteriovenous malformations, ruptured aneurysms, trauma, or bleeding disorders, may cause hydrocephalus by decreasing reabsorption at ...