Hydrocephalus

David M. Frim and Nalin Gupta Introduction

Hydrocephalus, loosely translated as "water in the head," is the most frequent clinical problem encountered by the pediatric neurosurgeon. On average, 40% of the procedures performed by a pediatric neurosurgical practice will be related to a diagnosis of hydrocephalus. An understanding of cerebrospinal fluid (CSF) dynamics, as well as the pathophysiology of CSF malabsorption, greatly facilitates the decision-making used in the treatment of hydrocephalus. In this chapter, we describe a practical approach to the treatment of various forms of hydrocephalus.

Definitions

Hydrocephalus is defined as a mismatch between CSF production and absorption, often leading to an abnormal accumulation of fluid within the ventricular system and an increase in intracranial pressure (ICP). This must be contrasted with ventriculomegaly, or simple enlargement of the ventricles, which can occur in the situation of decreased brain volume from atrophy or brain injury (also known as hydrocephalus ex vacuo), or may be a normal variation in some individuals. A variety of classification systems are used to describe hydrocephalus and have led to confusion regarding the underlying physiology and pathogenesis. Historically, hydrocephalus has been divided into two types: communicating and noncommunicating. Communicating hydrocephalus typically refers to uniform enlargement of all four ventricles occuring when CSF is not absorbed through the arachnoid villi, with normal 'communication' within the ventricular system. An enlargement of some of the four ventricles is usually referred to as noncommunicating hydrocephalus. One example of noncommunicating hydrocephalus is in the setting of aqueductal stenosis, which leads to an enlargement of the two lateral ventricles and the third ventricle in the absence of fourth ventricular enlargement.

Other classifications, such as obstructive and absorptive, are based on the patho-physiology of the underlying hydrocephalus. Obstructive is generally synonymous with noncommunicating hydrocephalus and absorptive with communicating hydro-cephalus. A gradual increase in ventricular size caused by hydrocephalus can stabilize by reaching a new equilibrium. The patient has no signs or symptoms of raised ICP. This situation is referred to as compensated hydrocephalus. Uncompensated hydro-cephalus, by comparison, is associated with increased ICP, neurological symptoms and signs, and usually, progressive dilatation of the ventricles. This is the clinical situation where treatment is indicated.

Pediatric Neurosurgery, edited by David Frim and Nalin Gupta. ©2006 Landes Bioscience.

Finally, hydrocephalus can be present at birth, in which case it is termed congenital. Hydrocephalus occurring later in life is termed acquired. Neither of these terms specify cause and the same etiology can cause either congenital or acquired hydrocephalus.

Physiology of CSF Production and Absorption

CSF is formed by the choroid plexus located in the lateral, third and fourth ventricles. Approximately 50% to 80% of CSF is produced by the choroid plexus through an energy-dependent active transport process involving the enzyme carbonic anhydrase. The remainder is derived from the extracellular space of the brain. In adults, the total production of CSF is 0.3 mL/min, or approximately 400 to 500 mL/day. The production of CSF is mainly dependent upon perfusion of the choroid plexus and, therefore, the patient's blood pressure. CSF production remains constant across the normal range of ICP; ICP has to approach mean arterial pressure before CSF production is affected. At this range of ICP, cerebral tissue perfusion is also markedly affected. Pharmacological agents such as acetazolamide (DiamoxO) that inhibit carbonic anhydrase can temporarily affect CSF production, but are not effective for the long-term treatment of hydrocephalus.

The choroid plexus is found in each of the 4 ventricles and is a specialized epithelial tissue. A blood-brain barrier does exist in the choroid plexus because the epithelial cells have tight junctions between them. The bulk flow of CSF is from the lateral ventricles through the foramina of Monro into the third ventricle. CSF then flows through the aqueduct of Sylvius into the fourth ventricle (Fig. 1). There are 3 outlets from the fourth ventricle: the foramen of Magendie and the paired foramena ofLuschka. The exact pattern of CSF flow through the spinal subarachnoid space is not precisely understood. There is some flow of CSF into the central canal of the spinal cord, and both up and down in the area surrounding the spinal cord. Eventually, CSF flows over

Figure 1. A diagram demonstrating the general direction of CSF flow from within the ventricular system to the basal cisterns, and then finally over the brain convexities to the superior sagittal sinus.

Figure 1. A diagram demonstrating the general direction of CSF flow from within the ventricular system to the basal cisterns, and then finally over the brain convexities to the superior sagittal sinus.

the convexities of the cerebral hemispheres where it is absorbed in a pressure-dependent fashion into the intracranial venous sinus system through the arachnoid villi. This process is passive and not energy-dependent. Normally, there is a 5 to 7 mm Hg difference in pressure between the dural venous sinuses and the subarachnoid space. This is presumably the hydrostatic driving force behind the absorption of CSF. Raised venous pressures can interfere with CSF absorption and can also create hydrocephalus.

By definition, hydrocephalus will occur when the brain's ability to absorb CSF is exceeded by production of CSF. The only situation in which CSF production is increased enough to cause hydrocephalus is caused by choroid plexus papillomas. These tumors contain functional choroid epithelium and can produce very large amounts of CSF. For virtually all other types of hydrocephalus, a pathological interference or obstruction to CSF flow leads to high ICP, which in turn leads to the symptoms associated with this condition.

Clinical Presentation

Etiology

A physical obstruction of the ventricular system can lead to hydrocephalus. In children, this is usually due to a neoplasm, congenital closure of portions of the ventricular system, or an intraventricular hemorrhage. The foramina of Monro are small and located immediately above the suprasellar area. Tumors in this region are common in children (see Chapter 4) and, if large enough, will obstruct the outflow of the lateral ventricles leading to their enlargement. The aqueduct of Sylvius connects the third and fourth ventricles and is normally about 1 mm in diameter. The aqueduct can fail to form or close in utero, leading to massive hydrocephalus that is readily apparent at birth. The clue on imaging studies is enlargement of the lateral and third ventricles with a normal-appearing fourth ventricle (Fig. 2). Tumors of the

Figure 2. MR images demonstrating a massively dilated ventricular system with congenital hydrocephalus secondary to acqueductal stenosis. The sagittal image (left) shows a normal-appearing fourth ventricle. The tremendous enlargement of the lateral ventricles has led to compression of the cerebral mantle with only the frontal lobes being visible (right).

Figure 2. MR images demonstrating a massively dilated ventricular system with congenital hydrocephalus secondary to acqueductal stenosis. The sagittal image (left) shows a normal-appearing fourth ventricle. The tremendous enlargement of the lateral ventricles has led to compression of the cerebral mantle with only the frontal lobes being visible (right).

Table 1. Causes of hydrocephalus*

Causes Percent

Intraventricular hemorrhage 24.1

Myelomeningocele 21.2

Tumor 9.0

Aqueductal stenosis 7.0

Infection 5.2

Head injury 1.5

Other 11.3

Unknown 11.0

Two or more causes 8.7

*Modified from: Drake JM, Kestle JR, Milner R et al. Randomized trial of cerebrospinal fluid shunt valve design in pediatric hydrocephalus. Neurosurgery 1998; 43:294.

pineal region and upper brainstem, and intraventricular hematoma can also obstruct the aqueduct. Posterior fossa tumors commonly cause hydrocephalus by obstructing CSF flow through the fourth ventricles. Finally, inflammatory conditions such as meningitis can affect CSF flow. Granulomatous inflammatory disorders such as tuberculosis or sarcoidosis can cause an obliterative meningitis that prevents CSF egress from the basal cisterns around the brainstem and posterior fossa. Bacterial meningitis usually leads to obliteration of the arachnoid villi, leading to what is commonly called communicating hydrocephalus. The majority of new shunts inserted during infancy are related to either spina bifida or intraventricular hemorrhage associated with prematurity (see Chapter 5 for a description of posthemorrhagic hydrocephalus).

Signs and Symptoms

The clinical presentation of hydrocephalus depends on the age of the child. Neonates with hydrocephalus develop progressive head enlargement, a bulging fontanelle, and splitting of the cranial sutures. Often, typical symptoms of ICP such as bradycardia, lethargy and apnea are absent. This is particularly the case when the hydrocephalus is slowly progressive. Congenital hydrocephalus in the newborn is not very difficult to diagnose, and is often discovered on an antenatal basis by ultrasonography. Later in infancy, hydrocephalus often presents as increasing head circumference beyond normal centiles, with or without a bulging fontanelle and splitting of sutures. In older children, symptoms and signs of hydrocephalus are similar to those seen in adults. They include headache, vomiting, diplopia, ataxia, visual loss, or behavioral changes. With severely raised ICP, central brain hernia-tion is preceded by the so-called Cushing's triad of bradycardia, hypertension and decreased respiratory rate.

Diagnostic Evaluation

In infants and small children, typical symptoms such as irritability and vomiting occur with many other medical problems. Imaging studies are indicated when these symptoms occur in the context of findings suggestive of an intracranial process (e.g.,

Figure 3. Ultrasound images taken from the anterior fontanelle in an infant with moderate ventriculomegaly and early hydrocephalus. The coronal (left) and sagittal (right) views are easily obtained in a single study. The abnormal ventricular size is apparent.

lethargy, seizures and increasing head circumference). The initial diagnostic study is often a plain computed tomography (CT) scan of the head. This study is available at many facilities and often does not require sedation of the patient. It will clearly demonstrate the ventricular size and usually identifies whether a mass lesion is present or not. Ventricular size can be easily determined in infants with a patent fontanelle using ultrasound (Fig. 3). Ultrasound has the advantage that sedation is not required and the procedure can be repeated frequently without any adverse effects.

If the patient's clinical course is rapidly progressive, an intervention must be performed before other diagnostic studies are obtained. Usually this means placement of a ventricular drain through a frontal burr hole. Once the patient is stabilized, or in cases where the clinical picture is stable without need for immediate intervention, there must be an attempt to identify the cause of the hydrocephalus. In certain cases the cause is clear from the clinical setting. For example, in premature infants with evidence of intraventricular hemorrhage, progressive ventricular enlargement and hydrocephalus are clear consequences of the original event. Similarly, hydrocephalus occurring after bacterial meningitis is presumed to be due to obliteration of the subarachnoid spaces and/or arachnoid villi. If hydrocephalus is diagnosed without a clear precipitating cause, a magnetic resonance imaging (MRI) scan of the brain should be done to look for common causes of hydrocephalus such as a mass lesion, congenital brain anomaly or hemorrhage.

Treatment

General

Hydrocephalus resulting from reversible causes such as intraventricular hemorrhage or meningitis can be treated by temporary means, such as an external ventricular drain. Once the underlying cause is treated, the hydrocephalus can also resolve in some cases. Medical treatment using diuretics or acetazolamide is generally unsuccessful. If the hydrocephalus is persistent, then the standard treatment is placement of a CSF diversionary device with a pressure-regulating valve, commonly known as a shunt. The simplest shunt device would be a plain tube that would begin in the

ventricular system and carry CSF to any absorptive surface outside of the brain, such as the peritoneum, the pleura, or the vascular tree. For reasons of safety, reduced complications and ease of access, the peritoneal cavity is the distal site of choice. There are other sites that lead to excretion of CSF, but are not preferred because of more significant complications. These sites include the gallbladder and the ureter.

CSF Shunts

The construction of reliable valves that regulate CSF flow is a significant focus for companies involved in the manufacture of these devices. Shunt valves were initially designed as simple differential pressure valves that open if the ICP is above a set pressure and close if the ICP is below that pressure (Fig. 4). This design has been modified by the addition of other antisiphoning components that address some of the physiological limitations. For example, antisiphoning devices do not allow the pressure within the shunt tubing to become negative relative to atmospheric pressure, and thereby prevent overdrainage when patients are sitting or standing. Other components include on-off switches, inline telemonitoring devices and tapping

Figure 4. A differential pressure shunt valve with a reservoir located proximal to the actual valve mechanism. The arrowhead on the actual valve is radioopaque and indicates the direction of flow. This type of valve sits flat on the calvarium while other designs place the reservoir directly above the burr hole.

Figure 4. A differential pressure shunt valve with a reservoir located proximal to the actual valve mechanism. The arrowhead on the actual valve is radioopaque and indicates the direction of flow. This type of valve sits flat on the calvarium while other designs place the reservoir directly above the burr hole.

reservoirs. There are also designs that maintain a constant flow of CSF ('flow-regulated' valves). Finally, programmable valves can be percutaneously reset to variable opening pressures in order to tailor a setting that minimizes symptoms. Fortunately, most patients tolerate fluctuations in ICP and are asymptomatic with a medium pressure setting (an opening pressure between 8 and 15 mm Hg). Only a minority of patients require ongoing readjustments in valve settings to achieve symptom control.

Technique of CSF Shunt Placement

Standard placement of a shunt in the occipital location begins with preoperative antibiotic administration and positioning the patient in the supine position with a roll under the lower cervical spine biased toward the side of shunt placement. Generally, a midline upper abdominal incision is made for peritoneal access, although subcostal incisions for lower quadrant access are acceptable. Meticulous preparation of the skin, preventing the shunt tubing from touching exposed skin, and double gloving of all surgical personnel are important to reduce the risk of shunt infection.

For ventriculoperitoneal (VP) shunts, a muscle-splitting dissection is used to reach the peritoneum. In most children this can be accomplished with an incision of approximately 1.5 cm in length. Percutaneous placement of a conduit such as a laparoscopic introducer into the peritoneum is an acceptable alternative. A curvilinear incision is then made in the occipital region 3 cm from the midline and 5 to 7 cm above the inion. Placement lateral to the lambdoid suture on the flat part of the parietal bone is a useful landmark for a smaller child or infant. At this point a hollow metal tunneling trocar is used to create a passage from the cranial incision to the abdominal incision. The shunt valve and peritoneal catheter are placed into an optimal location, and then the ventricular catheter is introduced into the lateral ventricle. In infants, this can be done under ultrasound guidance. External landmarks can also be used (see Fig. 2, Chapter 14), although for children with abnormal anatomy or small ventricles, neuronavigation based upon a preoperative imaging study is sometimes necessary. Once the shunt system is connected to the ventricular catheter and spontaneous flow of CSF is observed from the distal tubing, the peritoneal catheter is placed into the peritoneal cavity. A frontal approach can also be used to place a ventricular catheter.

Ventriculoatrial (VA) shunts can be placed in children of all ages. The technique for VP shunt placement is modified to allow access to the internal jugular vein either with a percutaneous introducer or open exposure of the facial vein or jugular vein. The distal catheter is advanced under fluoroscopic guidance to the junction of the right atrium and the superior vena cava. Special distal cathters are required to minimize the possibility of thrombus formation. For ventriculopleural (VPl) shunt placement, an incision is made over the second or third rib above the nipple and the pleural space is reached by dissecting through the intercostal musculature. After the pleural catheter is placed, several positive pressure ventilations help to reinflate the lung and reduce the likelihood of a pneumothorax; a small pneumothorax is commonly seen after placement of a VPl shunt. Intrapleural placement of 20 to 25 cm of additional tubing allows free movement of the catheter within the pleural space and also allows for growth in small children.

Lumboperitoneal Shunting

An alternative to placement of the proximal catheter into the ventricles is placement into the lumbar subarachnoid space. There are devices available for lumboperitoneal (LP) shunting that regulate pressure in a posture-dependent fashion. The subarachnoid catheter itself can be placed through a larger incision or percutaneously. The distal catheter is tunneled from the lumbar incision to an upper abdominal incision for peritoneal access.

Endoscopic Third Ventriculocisternostomy

Using endoscopes adapted for neurosurgical use, surgical fenestration of portions of the ventricular system for the purpose of bypassing areas of CSF obstruction is now possible. The most common procedure, endoscopic third ventriculostomy or third ventriculocisternostomy, is the creation of an opening in the floor of the third ventricle allowing CSF to pass directly into the prepontine cistern. This is the procedure of choice for lesions obstructing the aqueduct of Sylvius or with a posterior fossa mass. The surgical technique involves using a coronal burr hole to pass a small (3 to 6 mm diameter) endoscope into the lateral ventricle and then through the foramen of Monro into third ventricle. The retro-chiasmatic space and mamillary bodies are identified and an ostomy is created in the anterior floor of the third ventricle posterior to the retro-chiasmatic space.

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