CEREBROSPINAL FLUID PRODUCTION, FLOW, AND ABSORPTION

Hydrocephalus is a disturbance of CSF circulation. Average CSF production is 0.44 mL/minute, corresponding to a total of about 500 mL/day¹ (see Gjerris, 2000). The normal volume of CSF is approximately 150 mL. CSF is secreted by the epithelial cells of the choroid plexus, through an energy-dependent process involving ion pumps and enzyme systems.² Specifically, an adenosine triphosphate-dependent, ouabain-sensitive sodium-potassium exchange pump at the apical (CSF) surface of the choroidal cells drives secretion, with water entering via aquaporin 1 channels, after the accumulation of sodium and chloride. The (passive) import of sodium into the choroidal cells from the pericapillary space is necessarily coupled with that of chloride, itself exchanged for bicarbonate. Carbonic anhydrase inhibitors, by reducing available bicarbonate, indirectly reduce CSF formation.

CSF production is relatively independent of intracranial pressure (ICP), whereas, in patent CSF pathways and venous systems, CSF absorption is proportional to ICP. ICP is therefore determined by the pressure at which the rate of absorption rate balances the rate of production.¹ CSF is also produced through transependymal bulk flow from the brain parenchyma itself.^3,4 There is a marked diurnal variation in CSF production, with an increase at night to as high as twice diurnal values at 2:00 A.M.⁵ CSF flow and circulation are maintained by the arterial systolic pulsations of the brain and by continuous CSF secretion, as well as by changes in central venous pressure through respiration. These mechanisms combine as a “CSF pump.”

The arachnoid villi of the dural sinuses are traditionally regarded as the main site of CSF absorption.⁶ However, perineural lymphatic pathways, including those at the cribriform plate, and transependymal pathways, with absorption into capillaries or venules in the brain parenchyma, may also contribute.^7,8 These alternative routes of CSF absorption may become increasingly important in hydrocephalus.

The CSF circulation is shown in Figure 63-1. Flow is from the lateral ventricles through the foramen of Monro into the third ventricle, via the aqueduct of Sylvius into the fourth ventricle and then via its outlets (foramina of Luschka and Magendie) into the subarachnoid space and basal cisterns. CSF then circulates throughout the spinal subarachnoid space and basal cisterns up through the tentorial hiatus. It flows over the cerebral hemispheres and is absorbed at the sites identified previously.

Figure 63-1 Flow pattern of cerebrospinal fluid within the ventricles of the brain and the surrounding subarachnoid space.

(From Burt AM: Textbook of Neuroanatomy. Philadelphia: WB Saunders, 1993, Fig. 9-4.)

HYDROCEPHALUS

Classification and Etiology

Hydrocephalus has a variety of etiologies, outlined in the following sections. In most cases, it is caused by obstruction of CSF circulation or reduced absorption of CSF. In rare cases, there is overproduction of CSF (see Kaye, 2005).

Hydrocephalus can be classified into obstructive or communicating types. Obstructive hydrocephalus results from obstruction to CSF flow within the ventricles. In communicating hydrocephalus, as originally defined, there is communication between the ventricles and the lumbar CSF, but CSF flow over the surface of the brain is obstructed, or there is a failure of absorption of CSF.

Obstruction of Cerebrospinal Fluid Circulation

Sites and common causes of obstruction within the ventricular system (obstructive hydrocephalus) are as follows:

• Lateral ventricle obstruction by tumors, including basal ganglia gliomas, and thalamic gliomas.

• Third ventricle obstruction, caused by colloid cysts or other tumors of the third ventricle.

• Obstruction of the aqueduct of Sylvius, either by congenital stenosis or secondary to lesions such as pineal tumors (Fig. 63-2).

• Fourth ventricular obstruction caused by posterior fossa tumors, such as medulloblastomas, ependymomas, and acoustic neuromas.

Figure 63-2 Hydrocephalus secondary to a pineal tumor.

Rights were not granted to include this figure in electronic media. Please refer to the printed book.

(Copyright Andrew H. Kaye.)

CSF flow may also be obstructed at the basal cisterns, often as a result of congenital developmental abnormalities, such as those associated with spina bifida.

Reduction of Cerebrospinal Fluid Absorption

Reduced CSF absorption through the arachnoid granulations may be caused by

• Infection or meningitis. This can lead to adhesive arachnoiditis and permanent fibrosis of the CSF absorptive surfaces.⁹ This is a common cause of hydrocephalus in children. Newborns are particularly at risk because of severe inflammatory responses to leptomeningeal infection by agents, including Haemophilus influenzae, Streptococcus pneumoniae, and group B streptococci.¹⁰

• Cerebral hemorrhage. Both subarachnoid hemorrhage and intraventricular hemorrhage may result in adhesive arachnoiditis. Intraventricular hemorrhage is often associated with prematurity and is a frequent cause of hydrocephalus in infants.²

• Carcinomatous meningitis (see Pattisapu, 2001).¹¹

• Various other, uncommon causes of reduced CSF absorption, including maternal malnutrition and an increase in CSF viscosity caused by high CSF protein content.¹¹

Overproduction of Cerebrospinal Fluid

Choroid plexus papillomas are rare ventricular tumors that can produce excessive volumes of CSF. In some cases, the hydrocephalus persists despite tumor resection, which is probably related to chronic pressure changes or proteinaceous debris within the spinal fluid. This tumor type is the only known cause of overproduction of CSF^10,12 (see also Kaye, 2005.)

Radiological Investigations

The most important investigation is either a computed tomographic (CT) scan or magnetic resonance imaging (MRI) of the brain (Figs. 63-3 and 63-4), which demonstrate enlargement of the ventricles. The type and cause of the hydrocephalus may also be determined on imaging.¹² For example, an enhanced CT scan or MRI may demonstrate a tumor obstructing the ventricles. In communicating hydrocephalus, all the ventricles are dilated, whereas in obstructive hydrocephalus caused by a lesion at or above the level of the aqueduct of Sylvius, the fourth ventricle is of normal size.

Figure 63-3 Brain computed tomographic scan, demonstrating hydrocephalus of the lateral and third ventricles, caused by aqueduct stenosis.

Rights were not granted to include this figure in electronic media. Please refer to the printed book.

(Copyright Andrew H. Kaye.)

Figure 63-4 Brain magnetic resonance imaging (MRI), demonstrating obstructive hydrocephalus in a patient. A, Axial T1-weighted MRI showing dilated lateral ventricles. B, Coronal T1-weighted MRI with arrow pointing to dilated third ventricle.

Rights were not granted to include this figure in electronic media. Please refer to the printed book.

(Copyright Andrew H. Kaye.)

MRI is particularly helpful when performed in the sagittal plane, because it may demonstrate aqueduct stenosis and lesions around the third ventricle. Flow of CSF through the aqueduct can be determined through the use of dedicated flow sequences on MRI.^16,17

Ultrasonography through the open anterior fontanelle is useful in assessing ventricular size in infants and may obviate the need for repeated CT scans.

Records of the head circumference and its comparison with body weight and length percentile charts are an integral part of the postnatal follow-up of any child.

In adult neurology, the question often arises as to whether dilated ventricles are a result of atrophy of the brain parenchyma (sometimes termed hydrocephalus ex vacuo) or are a result of acquired hydrocephalus (typically NPH, discussed later). A number of features are suggestive of a diagnosis of hydrocephalus rather than atrophy, including enlargement of the temporal horns (however, hippocampal and parahippocampal atrophy in Alzheimer’s disease can look similar), a convex third ventricle with dilated anterior recesses, an acute rather than obtuse angle between the frontal horns on axial scans, enlargement of the ventricles out of proportion to sulcal enlargement (or frank sulcal effacement), and periventricular smooth high signal on fluid-attenuated inversion recovery imaging (representing transependymal edema).¹⁸