Hydrocephalus is a condition in which there is an accumulation
of cerebrospinal fluid (CSF) within the ventricles, leading to increased
intracranial pressure. Hydrocephalus may be caused by increased
production, decreased absorption, or obstruction to flow of CSF.
By contrast, ventricular dilatation due to loss of brain tissue
or atrophy is not considered to be hydrocephalus.
Cerebrospinal Fluid Dynamics
Typically, cerebrospinal fluid (CSF) is produced at a volume
of 20 mL/hour, or more than 500 mL per day, in the adult.
Total CSF volume in infants is approximately 50 mL, as compared to
approximately 150 mL in normal adults. Cerebrospinal fluid is produced
by active secretion and diffusion predominantly in the choroid plexus,
located in the lateral ventricles and the roof of the fourth ventricle.
Cerebrospinal fluid passes from the lateral ventricles into the
paired foramina of Monro to reach the third ventricle and then along
the cerebral aqueduct into the fourth ventricle. Once in the fourth
ventricle, CSF flows through the midline foramen of Magendie or
laterally through the paired foramina of Luschka into focally enlarged
areas of subarachnoid space known as the basal cisterns, which connect
the spinal and intracranial subarachnoid spaces (Fig.
553-1). Cerebrospinal fluid is in continual flow around the
brain and spinal cord and within the ventricles in a cephalad direction.
Production of CSF is regulated by the enzyme carbonic anhydrase,
and reabsorption occurs within the arachnoid villi in the meninges
into the venous channels of the sagittal sinus. Smaller amounts
of CSF absorption also occurs across the ependymal lining of the
ventricles. Typically, there is a delicate balance between CSF production
and reabsorption; hydrocephalus results when that balance is disrupted.
However, CSF production rarely fluctuates in normal circumstances
and even in the face of rising intracranial pressure, unless extremely
elevated pressures are reached.
Schematic diagram of a sagittal section of the brain
with arrows that trace the flow of cerebrospinal fluid. Cerebrospinal
fluid flows out of the ventricular system through the fourth ventricle
outlets into the basal cisterns, and then through the tentorium
over the cerebral hemispheres toward the arachnoid granulations
in the superior sagittal sinus. There is bidirectional flow of CSF
in the spinal canal. A, lateral ventricle; B, subarachnoid space;
C, cerebellum; D, foramen of Magendie; E, aqueduct of Sylvius; F,
third ventricle; G, foramen of Monro.
Hydrocephalus is generally classified as communicating, obstructive,
and external, depending on the underlying etiology.1 Communicating
hydrocephalus refers to a situation in which cerebrospinal fluid
(CSF) flows freely into the subarachnoid space, but the total volume
of CSF is increased secondary to either impaired reabsorption or overproduction.
Impaired reuptake of CSF may result from blood within the ventricular
system (as occurs with subarachnoid hemorrhage) increased protein content
(resulting from meningitis) or an increased cell content from malignant