Classic Bulk Flow Theory

According to the classic bulk flow theory, hydrocephalus occurs as a result of imbalance between the production and the absorption of CSF. An obstruction inside the ventricular system proximal to fourth ventricle foramina of Luschka and Magendie causes obstructive hydrocephalus, whereas an obstruction outside the ventricular system causes communicating hydrocephalus.

Increased CSF production is rare and may occur with choroid plexus papilloma. Decreased CSF uptake occurs as a result of obstruction to the bulk flow anywhere along the CSF pathway. The common sites of obstruction along the CSF pathway are the foramen of Monro, the aqueduct of Sylvius, the fourth ventricle outlet foramina, the basal cisterns, and the arachnoid villi.

Hydrodynamic Model for Csf Circulation

The hydrodynamic model is based on the concept that the absorption of CSF occurs through the capillaries in the CNS rather than through the arachnoid granulations and villi.³ The skull is a nonelastic housing for brain tissue; blood, CSF, and brain tissue are almost incompressible. As stated by the Monro-Kelly doctrine, the total volume of arteries, veins, CSF, and brain confined within the skull cavity and dura mater is constant, and any increase in volume in one or more compartments causes a decrease in volume in the others. Skull and dura mater are more elastic. The elasticity of these structures plays a pivotal role in the hydrodynamic theory of hydrocephalus. During cardiac systole, the expansion of the intracranial arteries increases the ICP, causing CSF displacement into the spinal canal and an increase in the venous outflow. During cardiac diastole, inflow of CSF from the spinal canal occurs, which causes elevation of pressure in the subarachnoid space. Thus increased pressure is present in the CSF spaces during the entire cardiac cycle, which in turn compresses the venous outlets, causing an increase in outlet resistance and venous “counter” pressure. This pressure is necessary to keep the intracerebral veins sufficiently distended to accommodate the normal cerebral flow.

Computed Tomography and Magnetic Resonance Imaging

Computed tomography (CT) and MRI are used as primary modalities to assess ventricular size. Ultrasound of the head is used as the initial study in infants with macrocephaly. Several parameters can help differentiate between hydrocephalus and ex vacuo dilatation of ventricles from cerebral atrophy in infants (Box 32-1).

The most reliable sign of hydrocephalus is enlargement of the anterior and posterior recesses of the third ventricle (Fig. 32-1); this phenomenon does not occur in ex vacuo ventricular enlargement. The disproportionate enlargement of the recesses occurs because the thin hypothalamus and cisterns surrounding these recesses provide relatively little resistance to expansion. In contrast, the body of the third ventricle is restricted by the rigid thalami, which provide more resistance to expansion. The anterior recesses (i.e., the chiasmal and infundibular recesses) expand earlier than the posterior recesses (i.e., the pineal and suprapineal recesses), which is best appreciated on midsagittal MRI.⁵ On axial CT, dilation of the anterior recesses of the third ventricle is detected when the third ventricle is larger at the level of the optic chiasm than at the middle of the ventricle.

The enlargement and inferior displacement of the anterior recesses may cause flattening of the pituitary gland with erosion of the dorsum sella, giving the classic plain film appearance of increased ICP in older children and adults. The recesses may compress the infundibulum, resulting in hypothalamic-pituitary dysfunction. Enlargement of the suprapineal and pineal recesses may displace the pineal gland inferiorly and occasionally elevates the vein of Galen. A large diverticulum of this recess may compress the tectum inferiorly and shorten it in the rostrocaudal direction, mimicking a neoplasm.

Commensurate dilation of the temporal horns with the lateral ventricles also is a strong indicator of hydrocephalus. The dilation of the temporal horns is best viewed on coronal T2-weighted images. The choroidal fissure is enlarged, and the hippocampus is compressed and displaced inferomedially (Fig. 32-2). Studies have suggested that temporal horns dilate less than the bodies of the lateral ventricles in generalized atrophy.⁶ This finding may be related to the small size of the temporal lobes and to their relatively small volume of white matter.

In ex vacuo dilatation of ventricles associated with cerebral atrophy, the superior and inferior walls of temporal horns remain parallel and are smaller than the lateral ventricle body. The hippocampus is normally placed, and the choroidal fissures are not enlarged. The sylvian fissure is enlarged in patients with temporal lobe atrophy, and in these patients, temporal horn enlargement cannot be used to distinguish hydrocephalus from ex vacuo ventriculomegaly.

The mamillopontine distance is measured on MRI from the anterior root of the mamillary body to the top of the pons parallel to the anterior mesencephalon. The normal average distance is 3.8 mm.⁵ The floor of the third ventricle as seen on sagittal MRI is usually concave downward. With enlargement of the third ventricle, it becomes straightened or convex downward, resulting in reduction of the mamillopontine distance (e-Fig. 32-3).

The ventricular angle (e-Fig. 32-4) measures the divergence of the frontal horns.⁶ Concentric enlargement of the frontal horns in a patient with hydrocephalus causes diminution of this angle, as seen on axial or coronal images. This concentric dilation produces an enlargement of the frontal horn radius with a rounded configuration of the frontal horns, or a “Mickey Mouse ears” appearance.

Enlargement of the ventricles disproportionate to enlargement of the cortical sulci favors a diagnosis of hydrocephalus. However, this parameter is not reliable in children, especially in the first years of life, because patients with communicating hydrocephalus and atrophy have enlargement of both of the fluid spaces. In addition, the sizes of the ventricles and the subarachnoid spaces may vary tremendously, as seen in infants with benign macrocephaly. Therefore it is important to evaluate ventricular size in conjunction with the patient’s neurologic evaluation and serial head circumference measurements. A large or rapidly enlarging head would favor a diagnosis of hydrocephalus, whereas a small or diminishing head circumference would suggest atrophy.

The presence of periventricular interstitial edema is indicative of hydrocephalus (Fig. 32-5). With elevation of pressure within the ventricles, the normal centripetal flow toward the ventricles is reversed. The CSF is forced out through the ependyma into the surrounding extracellular spaces to be absorbed by alternative routes. This increase in periventricular fluid constitutes interstitial edema. It is best recognized on MRI with fluid-attenuated inversion recovery and proton density sequences. It is more difficult to appreciate on T2-weighted images because of the bright signal from the ventricles. Periventricular interstitial edema is difficult to appreciate in neonates and young infants because it is masked by a bright signal from immature myelin, with its high water content. On CT, periventricular interstitial edema is seen as hypoattenuation in the periventricular region, with indistinct ventricular margins.

A CSF “flow void” in the third ventricle, aqueduct of Sylvius, and fourth ventricle may be accentuated in persons with hydrocephalus as a result of hyperdynamic flow, although the specificity of this finding is unclear (e-Fig. 32-6).

Hydrocephalus causes mass effect and distortion of adjacent brain structures. Stretching, upward displacement, and smooth, uniform thinning of the corpus callosum occurs as a result of lateral ventricular enlargement. Corpus callosum thinning also occurs with atrophy, but typically it is not elevated superiorly and may not be uniformly thin, as seen with hydrocephalus.

Marked hydrocephalus may lead to the formation of atrial diverticula, which is herniation of the ventricular wall through the choroidal fissure of the ventricular trigone into the supracerebellar and quadrigeminal cisterns. Diverticula may cause compression and distortion of the tectum and may mimic arachnoid cysts in the region of the quadrigeminal cistern.⁷

MRI in persons with hydrocephalus frequently involves utilization of three-dimensional balanced steady-state free precession (SSFP). This sequence is particularly useful in demonstrating arachnoid membranes, intraventricular cysts, and aqueductal stenosis.

Qualitative and quantitative CSF analysis can be performed using phase-contrast cine MRI. The technique is useful in demonstrating pulsatile flow at the craniocervical junction (Fig. 32-7), aqueduct of Sylvius, and across a surgically created third ventriculostomy.¹⁸

Ultrasound

Ultrasound of the head is a useful initial examination for evaluation of macrocephaly in infants if the anterior fontanelle is open. Transcranial Doppler techniques may be helpful in identifying infants with raised ICP and may help determine the need for shunt placement. With elevated ICP, arterial flow tends to be reduced during diastole, resulting in elevated pulsatility of arterial flow. Many researchers have demonstrated elevated resistive indices in infants with raised ICP and a subsequent decrease on ventricular tapping. These results, however, have not been uniformly reproducible.8–10

Plain Radiographs

The changes related to elevated ICP on a plain radiograph of the skull depend on the age of the child. In children up to 8 or 10 years of age, sutural diastasis may occur within a few days of elevated pressures (e-Fig. 32-8). After 12 to 13 years of age, sutural diastasis is uncommon early, and the first sign of long-standing hydrocephalus may be erosion of the sellar cortex caused by enlargement of the third ventricular anterior recesses. The anterior part of the base of the dorsum is the earliest to be eroded, but the erosion may spread to involve the sella floor.

Nuclear Medicine Cisternogram

Occasionally, radionuclide cisternography is helpful in diagnosing communicating hydrocephalus if other imaging studies and a neurologic examination are equivocal. Indium-111–labeled diethylenetriaminepentaacetic acid is injected intrathecally via a lumbar puncture. The opening CSF pressure is determined at the time of injection. Planar and single-photon emission CT imaging is performed at 4 and 24 hours, and if needed, at 48 hours. The protocol involves collection of urine for 4 hours after injection of the isotope to calculate the percentage of isotope excreted in the urine. The interpretation of the cisternogram is based on three pieces of information—the CSF opening pressure, the percentage of isotope excreted in the urine, and the images obtained:

The first two pieces of information are most useful. The results from imaging should be interpreted cautiously.

Newborns and Infants

The common etiologies for ventriculomegaly are listed in Box 32-2.

Infants with ventriculomegaly typically present with macrocephaly. It is critical and often difficult to differentiate between ventriculomegaly caused by communicating hydrocephalus, which requires shunting, and ventriculomegaly related to benign extraaxial fluid of infancy, which does not require intervention. These infants therefore should be assessed clinically for other signs of elevated ICP, such as dilated scalp veins, bulging fontanelles, and sutural diastasis. They may have ocular signs and spasticity in the lower extremities as a result of disproportionate stretching of the corticospinal tracts arising from the motor cortex leg area by the enlarged ventricles. Infants with macrocephaly should be evaluated with imaging to assess for ventricular dilation. Ultrasound is the most common initial modality used to obtain images of infants with macrocrania.

In persons with communicating hydrocephalus, the ventricles typically are disproportionately larger than the subarachnoid spaces. At times, the two conditions cannot be differentiated on imaging. The imaging findings should always be interpreted in the context of the clinical history and serial head circumference measurements. Infants with communicating hydrocephalus usually have other signs of raised ICP and an abnormal neurologic examination. When the imaging findings and clinical examination are equivocal, a CSF cisternogram may help to differentiate between the two conditions. Rarely, the issue needs to be resolved by the gold standard of invasive ICP monitoring.

Older Children

Common causes of ventriculomegaly in older children are listed in Box 32-3. Because of the inability of their cranium to expand as quickly as in infants and young children, older children with hydrocephalus have a more acute presentation. They may have the classic triad of headache, vomiting, and lethargy. Children who have chronic hydrocephalus as a result of slowly expanding lesions typically present with persistent morning headaches and intermittent vomiting. Papilledema often is encountered. Focal neurologic deficits from the primary lesion and pyramidal tract signs, which are more marked in the lower extremities, may be present. Hypothalamic-pituitary dysfunction also may develop as a result of compression of these structures by enlarging anterior recesses of the third ventricle.

Methods of Shunting

Different methods have been used for CSF shunting, with the goal to decrease ICP by providing an alternative pathway for CSF absorption. The shunt catheters have a proximal intracranial segment connected in series to a valve and distal tubing. The proximal catheter may be inserted through the occipital, frontal, or posterior temporal approach. The placement of the ventriculostomy catheter is optimal if the tip and the side holes of the catheter are not in direct contact with the choroid plexus. This technique minimizes the chances of tissue ingrowth and occlusion of the proximal catheter. The distal catheter may be placed in various locations, including the peritoneal cavity, pleural cavity, central veins or right atrium, and gallbladder. The peritoneal cavity is the overwhelmingly favored location. The only relative contraindications to its use are active intraperitoneal infection, significant risk of future peritoneal infections, severe adhesions, and previous failure of peritoneal shunts.

Shunt Malfunction

Hemorrhage from insertion of the proximal catheter occurs in approximately 1% of patients (Box 32-4). It is even more common when an old catheter is removed. Neuronal injury may result in focal deficits if the catheter traverses the internal capsule. Seizures can occur and are more common with catheters placed through a frontal approach. With lumbar catheters, there is a 5% reported risk of radiculopathy and a 1% risk of myelopathy.

It is an axiom in pediatric medicine that when a child with a ventricular shunt has a medical problem, the shunt is the cause of the problem until proven otherwise. The causes of shunt malfunction can be divided broadly into two categories: (1) mechanical shunt failure involving the shunt apparatus and (2) shunt infection. The most common time for the shunt to fail is in the first 6 months after insertion. Multiple studies indicate that patients younger than 6 months are at a higher risk of shunt malfunction. The overall 1-year failure rate is 40% for patients with a corrected median age of 55 days and 30% in older age groups.

Evaluation of Shunt Malfunction

The diagnosis of shunt malfunction is based primarily on the presenting clinical signs and symptoms. Plain radiographs of the shunt and CT of the head are the primary radiologic investigations for evaluating shunt malfunction.

Plain radiographs of the entire shunt are obtained with frontal and lateral views of the skull and with frontal views of the chest and abdomen. These radiographs help assess shunt discontinuity or migration. Calcification along the tubing is common in old shunts, which are prone to fracture (see e-Fig. 32-10). Abdominal complications such as mass effect from pseudocyst formation, bowel perforation, and adhesions resulting in bowel obstruction also are evaluated on plain radiographs. If preperitoneal placement of distal tubing is suspected, a lateral radiograph should be obtained.¹³

Imaging of the head, typically with CT, is performed to assess changes in the shape and size of the ventricles, as well as any other collections or loculated compartments. One problem is that CT usually is obtained only when the patient is sick, and thus there is no valid baseline comparison when the patient is well. The position and course of the catheter can be assessed on CT. Dilation of ventricles compared with previous images is the clearest sign of shunt malfunction. However, some patients with shunt malfunction may demonstrate little if any enlargement, and some patients may have small ventricles. Therefore the ventricular size alone, especially in the absence of previous studies, may be misleading. Enlarged ventricles are seen in approximately 70% of patients with mechanical obstruction and in 30% of those with a shunt infection.

The advantages of CT are that it is widely available and relatively inexpensive, and it allows quick examination and a reproducible assessment of ventricular size and catheter position. However, even a single CT study exposes children to potentially harmful levels of radiation, and children with hydrocephalus are usually exposed to multiple CT procedures, further increasing their cancer risk.

MRI is an alternative to CT for shunt evaluation with limited steady-state gradient-recalled sequence and balanced SSFP sequences (Fig. 32-13). Fast MRI sequences such as single-shot or half-Fourier T2-weighted sequences and T1-weighted spoiled gradient sequences reduce the scan time dramatically, decreasing or eliminating the need for sedation. These sequences provide reliable visualization of the catheter and superior anatomic detail.¹⁴

The subset of symptomatic patients with little or no change in ventricular size, or with small ventricles, may benefit from shunt tapping and, rarely, from a shuntogram. Tapping of the shunt is also helpful for CSF sampling if a shunt infection is suspected. Free flow of CSF may indicate adequate patency of the proximal catheter. Manometric measurements also can be performed. A shuntogram can be performed by injecting contrast material into the valve chamber at the time of shunt tapping and taking radiographs at 1 and 15 minutes. Peritoneal spilling of contrast material proves patency of the distal tubing. The results of the shuntogram may be inconclusive in patients with partially obstructed shunts. Shunt injection studies are more helpful in diagnosing distal malfunction. A 25% to 40% rate of false-positive results is found in patients with proximal malfunction (i.e., failure of proximal CSF to pass through the distal tubing despite patent tubing). Similarly, radioisotope studies occasionally are used to assess shunt patency and obtain physiologic information about CSF flow through the shunt.

In general, small ventricles in a child with a shunt are good, and large ventricles are bad. Approximately 1% to 5% of patients with very small, or slit, ventricles become symptomatic with acute or chronic headaches, nausea, vomiting, and lethargy. These patients have been lumped together under the term “slit ventricle syndrome” (e-Fig. 32-14). This terminology is confusing because it has been used for multiple clinical entities.¹⁵ At one end of the spectrum is a child with small ventricles who is very sick as a result of intracranial hypertension. At the other end of the spectrum is an asymptomatic child with a harmless and inconsequential CT finding.

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