Hydrocephalus refers to abnormal dilation of the ventricular system in association with narrowing of the subarachnoid spaces. Elevation of the intraventricular pressure may or may not be present. Hydrocephalus is not synonymous with ventriculomegaly. Most forms of hydrocephalus are due to an imbalance between the formation and absorption of cerebrospinal fluid, that is, inadequate passage of cerebrospinal fluid (CSF) from its point of production within the ventricular system to its point of absorption into the systemic circulation. The excess intraventricular CSF that results from this imbalance in turn causes an increase in intracranial pressure. The degree and pattern of enlargement of the CSF pathways and the amount of damage to central nervous system (CNS) structures depend on both the severity and the pathogenesis of the hydrocephalus.^1–3

An important site of CSF absorption is into the sagittal sinus via the arachnoid villi, that is, the pacchionian granulations. Absorption of CSF at the pacchionian granulations is passive. The valve system of the arachnoid villi has an opening pressure of approximately 5 mm of mercury. Above this opening pressure, the rate of fluid absorption is directly proportional to the intracranial pressure. Equilibrium between CSF production and absorption occurs at approximately 10 mm of mercury (14 cm water). Capillaries throughout the CNS also contribute substantially to CSF absorption. In particular, there is relatively free bidirectional mixing of CSF with fluid in the interstitium of the brain.⁴

Approximately 60% of CSF production is via the choroid plexus, with the remainder by other mechanisms such as parenchymal capillaries and the ependyma. The rate of CSF production in the brain is relatively constant in most individuals; overproduction of CSF is not the cause of hydrocephalus in the great majority of cases (choroid plexus papilloma is an important exception). Acute hydrocephalus is generally due to pathology that blocks the flow of CSF or results in impaired resorption of fluid. Typically, this is related to blockage of flow in the ventricular system, in the basal cisternae, or in the subarachnoid space along the cerebral convexities. Diminished absorption can also result from abnormalities of the arachnoid villi. Ventriculomegaly due to mechanical obstruction of CSF drainage pathways causes brain expansion, compression of cortical veins, secondary venous congestion, impaired brain capillary absorption of CSF, and elevated intracranial pressure. This sequence of events may exacerbate hydrocephalus and cause symptomatic reduction in cerebral oxygenation.

Acute obstructive hydrocephalus causes elevation of intraventricular pressure, ventricular enlargement, and brain expansion. Compression of cortical veins by the expanding brain leads to intracranial venous congestion and elevation of the intracranial pressure that counteracts ventricular expansion. As the intracranial pressure increases in the patient with hydrocephalus, there is increased CSF absorption through the arachnoid membrane and the stroma of the choroid plexus. Fluid may also egress through the extracellular spaces of the cortical mantle, that is, transependymal CSF flow that appears as periventricular edema on CT and MR examinations. These compensatory absorptive pathways establish a new equilibrium between the production and absorption of CSF at an elevated pressure.

Transependymal flow of CSF through the white matter in the patient with hydrocephalus leads to neuronal and astrocytic swelling in the gray matter, and atrophic changes in the nerve fibers of the cerebral hemispheres. The cilia that normally cover the ependymal surface of the ventricular system may disappear in the presence of substantial long-standing hydrocephalus. Marked elevation of the intraventricular pressure can cause reduction of cerebral blood flow; this reduction is most pronounced in the distribution of the anterior cerebral arteries. Compromised blood flow can in turn lead to ischemic injury of the basal forebrain and medial cerebral hemispheres.

In the chronic phase of communicating hydrocephalus, compensatory absorption pathways result in a CSF pressure that is normal or only mildly increased. Because the compressed intracranial arteries have diminished compliance, there is abnormal increased pulse pressure in the brain capillaries. This, in turn, increases the intraventricular pulse pressure and thereby serves to maintain and exacerbate ventriculomegaly even if the mean CSF pressure is normal.⁴

Several factors influence the clinical manifestations of hydrocephalus, including the age of the patient at the time of onset and the duration of the increased intracranial pressure. Also important are the rate of increase of pressure and the presence or absence of associated structural abnormalities. Hydrocephalus that develops prior to the age of 2 years generally leads to macrocephaly. Chiari II malformation and aqueductal stenosis account for approximately 80% of cases of hydrocephalus in this age group. Other relatively common causes of hydrocephalus in infants include intrauterine infection, perinatal hemorrhage, and neonatal meningitis. Rare etiologies include midline tumor, choroid plexus papilloma, arachnoid cyst, and CNS arteriovenous malformation.

In infants with hydrocephalus, the head circumference increases at an abnormal rate, that is, the macrocephaly is progressive. The head circumference in infants with idiopathic “benign” macrocephaly usually follows an age appropriate rate of growth. A disproportionately large forehead and a thin skull often accompany long-term infantile hydrocephalus. Other potential clinical findings include widening of the cranial sutures, bulging of the anterior fontanelle, and dilation of the scalp veins. Ocular disturbances that can occur in these patients include paralysis of upward gaze (Parinaud syndrome), nystagmus, proptosis, and a diminished pupillary light reflux. Spasticity of the lower extremities is common.

Hydrocephalus that first develops later in childhood is usually due to aqueductal stenosis or a posterior fossa tumor. The most important determinants of the clinical features in these patients are the severity of hydrocephalus and the nature of any underlying primary lesion. A common presentation is that of early morning headaches that improve in the upright position. Vomiting, particularly in the morning, is another frequent clinical sign of increased intracranial pressure. This presentation sometimes leads to a mistaken diagnosis of GI tract pathology.

Papilledema and strabismus are common findings of hydrocephalus at presentation. In the lower extremities, spasticity and cerebellar signs are the predominant manifestations. Endocrine abnormalities may result from compression of the hypothalamic-pituitary axis by the enlarged anterior recesses of the third ventricle; potential manifestations include short stature, gigantism, menstrual irregularities, hypothyroidism, and diabetes insipidus.

The mainstay imaging technique for the diagnosis and characterization of hydrocephalus is CT. Sonography is valuable for the detection and monitoring of hydrocephalus in young infants. MR provides optimal depiction of the pathological anatomy in patients with hydrocephalus. MR is also an option to CT for followup of patients with known hydrocephalus, thereby avoiding repeated exposure to ionizing radiation.^5,6

Cross-sectional imaging studies of hydrocephalus show dilation of the ventricular system proximal to the obstruction. Differentiation from ex vacuo ventriculomegaly is crucial. Features that favor true hydrocephalus include effacement of cortical sulci, enlargement of the temporal horns in conjunction with prominence of the remainder of the lateral ventricles, inferior displacement of the floor of the third ventricle, and dilation of the anterior and/or posterior recesses of the third ventricle (in those patients with obstruction beyond the level of the third ventricle) (Figure 15-1). Clinically, there is macrocephaly in most patients with hydrocephalus. Evidence of transependymal flow of CSF (periventricular edema), when present, confirms elevation of intraventricular pressure (Figure 15-2). Fluid-attenuated inversion recovery (FLAIR) MR images are most sensitive for this finding. CT images may show periventricular hypoattenuation (Figure 15-3). Periventricular edema sometimes causes hyperechogenicity in the periventricular white matter on sonography.^7–9

Two-dimensional cine phase-contrast MRI provides information about CSF flow dynamics. This can be useful for selected patients to document an obstruction, such as at the cerebral aqueduct or foramen magnum. This technique is also useful for assessing patency of a third ventriculostomy. Image acquisition is performed parallel and/or perpendicular to the expected direction of flow. CSF that is flowing downward appears white and upward flow is in black.^10,11

Skull radiographs of infants with hydrocephalus show nonspecific macrocephaly. There often is widening of the cranial sutures. With chronic hydrocephalus in older children, there occasionally is a “beaten silver” or “hammered silver” appearance of the skull.

There are 2 major types of hydrocephalus: that due to overproduction of CSF and that due to obstruction to the flow of, or the inadequate absorption of, fluid. Overproduction of CSF is rare; the major cause is papilloma or carcinoma of the choroid plexus. The great majority of cases of hydrocephalus are due to obstruction of normal CSF flow or inadequate absorption. There are 2 subcategories of this type: (1) communicating hydrocephalus, in which there is an extraventricular (beyond the foramina of Luschka and Magendie) obstruction to flow or diminished absorption, and (2) noncommunicating (or obstructive) hydrocephalus, which is caused by intraventricular obstruction, most often at the aqueduct of Sylvius. Other common sites of obstruction include the foramina of Monro and the fourth ventricular outlet foramina.¹²

Hydrocephalus Due to Excessive Cerebrospinal Fluid Formation

Choroid plexus papilloma and carcinoma can cause hydrocephalus by way of CSF overproduction and/or mechanical ventricular outflow obstruction. Occasionally, a papilloma on a pedicle leads to manifestations of intermittent obstruction. Imaging evaluation of a choroid plexus tumor shows a prominently enhancing, lobulated intraventricular mass. Parenchymal invasion can occur, particularly with carcinoma. These tumors are most often located in the lateral ventricle near the trigone.

Diffuse villous hyperplasia is a rare developmental lesion, in which there is generalized choroid plexus enlargement without a localized mass. Overproduction of CSF causes communicating hydrocephalus in these patients.¹³

Intraventricular Obstruction (Noncommunicating Hydrocephalus)

Noncommunicating hydrocephalus (i.e., intraventricular obstructive hydrocephalus) can result from obstruction in any portion of the ventricular system, from the foramina of Monro to the foramina of Magendie and Luschka. The cause of the obstruction may be congenital or acquired, and the obstruction can be intrinsic or extrinsic to the ventricular system (Table 15-1). CT, sonography, and MR all serve important roles for the detection and characterization of obstructive hydrocephalus. Because of the potential for even small neoplasms to cause obstructive hydrocephalus, MR is often useful for evaluation of patients who have clinical and CT findings suggestive of idiopathic hydrocephalus.

Tumors

Various neoplasms and congenital cysts can cause ventricular obstruction at the level of the foramina of Monro. Midline parenchymal brain lesions tend to obstruct both foramina, whereas unilateral or asymmetric tumors more often cause unilateral obstruction. Intraventricular tumors or cysts (e.g., intraventricular arachnoid cyst and colloid cyst of the third ventricle) can also obstruct the foramen of Monro. Suprasellar masses can displace the floor of the third ventricle superiorly and cause extrinsic obstruction of the foramina of Monro and third ventricle. In children with tuberous sclerosis, a subependymal nodule or giant cell astrocytoma that originates adjacent to the foramen of Monro can grow medially to obstruct the foramen.

A second potential site of neoplastic ventricular system obstruction is at the aqueduct of Sylvius. Pineal tumors that can cause extrinsic aqueductal obstruction in children include germ cell tumors (germinoma, endodermal sinus tumor, and teratoma), pineocytoma, and pineoblastoma. Gliomas and hamartomas of the tectum are uncommon tumors that, even when quite small, can produce obstructive hydrocephalus by compression of the aqueduct; these lesions are often subtle on neuroimaging studies. A vein of Galen aneurysm is a nonneoplastic lesion that can present with signs of hydrocephalus due to compression of the aqueduct. A large arachnoid cyst of the quadrigeminal plate cistern can compress the tectum and obstruct the aqueduct (Figure 15-4). Obstruction due to a pineal cyst is rare.

At the level of the fourth ventricle, various posterior fossa masses can cause obstructive hydrocephalus (Figure 15-5). Posterior fossa tumors may obstruct the fourth ventricle or aqueduct by way of invasion or extrinsic compression. Hydrocephalus is relatively common with ependymal and cerebellar tumors (e.g., medulloblastoma and ependymoma); this complication is less frequent with intrinsic brainstem tumors.

Foramen of Monro Obstruction

Hydrocephalus due to obstruction of the foramen of Monro can occur as a developmental lesion (e.g., congenital atresia or stenosis of the foramen) or due to a mass of the anterior aspect of the third ventricle or adjacent portion of the lateral ventricle (e.g., hypothalamic glioma, colloid cyst of the third ventricle, craniopharyngioma, suprasellar germinoma, or giant cell astrocytoma). Imaging studies show unilateral or bilateral dilation of the lateral ventricles, without third ventricular enlargement (Figure 15-6). With long-standing obstruction in an infant, such as occurs with congenital stenosis of the foramen of Monro, there often is enlargement of the ipsilateral hemicranium.

Aqueductal Stenosis

Intrinsic aqueductal stenosis accounts for 15% to 20% of cases of pediatric hydrocephalus. The prevalence is approximately 1 in 1000 livebirths. Both developmental and acquired forms of aqueductal stenosis occur; the latter are less common. Acquired stenosis is usually due to fibrillary gliosis related to prior hemorrhage or infection. Aqueductal stenosis occurs in 50% to 75% of individuals with Chiari II malformation. A rare X-linked form of congenital aqueductal stenosis has been described.^14–16

Intrinsic obstruction of the aqueduct is due to a membrane, focal or long-segment stenosis, or forking (replacement of the aqueduct by multiple narrow channels). Focal stenosis most often occurs at the level of the superior colliculi or at the intercollicular sulcus. Forking and stenosis of the aqueduct are frequently accompanied by fusion of the quadrigeminal bodies, fusion of the third nerve nuclei, and molding or beaking of the tectum. In some patients, the shape of the molded tectum is congruent with that of the medial aspects of the dilated adjacent temporal lobes.

Neuroimaging studies suggest the diagnosis of aqueductal stenosis when there is dilation of the lateral and third ventricles in association with a normal or small fourth ventricle. MR provides the most detailed characterization of the morbid anatomy (Figure 15-7). Contrast-enhanced images are helpful to exclude a small neoplasm. Normal CSF pulsation in the region of the aqueduct is lacking on T2-weighted and cine phase-contrast MR sequences. In patients with severe hydrocephalus due to aqueductal stenosis, rupture of the septum pellucidum can occur. Stenosis of the proximal portion of the aqueduct, either at the level of the superior colliculus or at the entrance of the aqueduct immediately inferior to the posterior commissure, usually leads to severe hydrocephalus. Stenosis of the more distal portions of the aqueduct tends to be associated with mild or moderate severity hydrocephalus. In these patients, imaging studies show dilation of the proximal aspect of the aqueduct and posterior displacement of the quadrigeminal plate (Figure 15-8).¹⁷

Outlet Obstruction of the Fourth Ventricle

Intraventricular obstructive hydrocephalus is occasionally due to an obstruction of the fourth ventricular foramina of Magendie and Luschka. The cause of the obstruction can be developmental (e.g., Chiari I malformation), mechanical (e.g., posterior fossa tumor), or postinflammatory. Imaging studies usually show dilatation of the entire ventricular system, and disproportionate enlargement of the fourth ventricle (Figure 15-9). Syringohydromyelia may occur in patients with outlet foramina occlusion when the opening into the central canal is patent.