Hydrocephalus is a condition marked by dilatation of the cerebral ventricles due to the excessive accumulation of cerebrospinal fluid. The disorder is characterized by an imbalance in CSF production and reabsorption. This article reviews both the congenital and acquired forms of hydrocephalus and discusses their diagnosis and treatment.
The skull and the spinal canal are rigid containers that hold the brain, spinal cord parenchyma, intravascular blood, and the CSF in the ventricles and the subarachnoid spaces. The total volume of the brain, CSF and vascular compartments in the cranium must remain constant. When blood flow to the brain increases because of metabolic demand, CSF volume is reduced by compression of the ventricles and the subarachnoid spaces, which leads to increased reabsorption of CSF.
CSF is primarily produced in the choroid plexus of the ventricular system. It circulates from the lateral ventricles to the third ventricle of the brain through the foramen of Monro and then to the fourth ventricle by way of the aqueduct of Sylvius. CSF then passes through the roof of the fourth ventricle into the subarachnoid spaces, where it circulates to the primary site of reabsorption, the arachnoid granulations of the sagittal and transverse sinuses (Figure 1). The emissary veins of the dura and the lymphatic drainage system of the skull are other sites of CSF reabsorption. These sites may be important secondary CSF pathways in patients with intracranial hypertension.
The production of CSF is constant, at a rate of about 0.3 mL per minute. For a constant volume to be maintained, and equivalent amount of CSF must be reabsorbed by the arachnoid granulations and other sites (Figure 2). If reabsorption does not occur, the ventricles enlarge at the expense of the brain parenchyma. If enlargement continues, the lining of the ventricle and then the underlying white matter
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are disrupted. Expansion of the skull and thinning and atrophy of the brain are resultant compensatory mechanisms. 
Etiology and Classification
Hydrocephalus may be congenital or acquired (Table 1). Congenital hydrocephalus occurs in approximately four of every 1,000 live births and is usually overt at birth. It may be genetic, as in X-linked hydrocephalus, which is an autosomal dominant disease affecting new-born males. Congenital hydrocephalus also may be caused by a chronic intra-uterine infection, such as toxoplasmosis, cytomegalovirus infection or rubella. These condition usually cause narrowing of the aqueduct of Sylvius; because of its reduced size, the Sylvian aqueduct is unable to accomodate the required volume of CSF.
The Dandy-Walker syndrome is a congenital disorder that accounts for less than 5 percent of all cases of hydrocephalus. Children with this syndrome most often present with progressive macrocrania and hydrocephalus.  Imaging studies reveal enlargement of the fourth ventricle and an absence of cerebellar vermis, although the cerebellar hemispheres are present. Hydrocephalus occurs because there is no opening in the roof of the fourth ventricle. Consequently, CSF builds up in the ventricle, causing membranous distention of the ventricular roof (Figure 3).
Acquired hydrocephalus develops after birth as a result of infection, neoplasm, vascular pathology or trauma that produces subarachnoid bleeding. Most forms of hydrocephalus are the result of obstructed CSF flow somewhere in the ventricular system. In one condition, choroid plexus tumor, the cause of hydrocephalus may be overproduction of CSF.
Hydrocephalus also may be classified according to CSF flow dynamics (Figure 4). The disorder is termed "communicating" when CSF flows out of the fourth ventricle to the subarachnoid space but cannot be reabsorbed by the arachnoid granulations. This form of hydrocephalus may be seen with subarachnoid hemorrhage or following bacterial meningitis. The reaction from the blood or the infection creates an inflammatory response, with subsequent scarring that impairs the passage of CSF in the basal cisterns, over the surface of the hemispheres or in the arachnoid granulations. In "noncommunicating," or "obstructive," hydrocephalus, CSF flow in the ventricular system is impaired. This type of hydrocephalus may occur with fourth ventricular tumors or aqueductal stenosis.
Congenital hydrocephalus is often detected prenatally on ultrasound examinations. During the ultrasound examination, it is important to search for associated anomalies, such as spina bifida. Craniopelvic disproportion may mandate a cesarean delivery of a hydrocephalic infant. The hydrocephalus may be severe, and transillumation of the infant's skull may reveal an absence of visible parenchyma (Figure 5).
In difficult vaginal deliveries, stress on the infant's cranium may cause subarachnoid hemorrhage. Subsequently, the breakdown products of blood obstruct the subarachnoid fluid channels. The hydrocephalus that may follow presents as increasing head circumference on well-baby examinations. As intracranial pressure rises, the cranial sutures separate, the anterior and posterior fontanelles become full, and the scalp veins engorge. Finally, the infant becomes increasingly lethargic or irritable, and vomiting may ensue.
Intracranial hemorrhage can occur in premature infants, and subependymal rupture of the hemorrhage into the ventricle may impair CSF flow.  Initially, the ventricles may enlarge progressively, with no visible increase in head size or fontanelle fullness. However, the increased pressure causes episodes of bradycardia and/or apnea, which may be noted in the nursery or the neonatal intensive care unit. Cranial ultrasonography will show hydrocephalus. 
Intracranial tumors in children are most common between the ages of five and 10 years, with 75 percent of these tumors occurring in the posterior fossa.  As an intracranial tumor enlarges, it can impair CSF flow through the fourth ventricle, creating obstructive hydrocephalus. Intermittent headaches (frequently in the early morning), vomiting, irritability, and subsequent fatigue and lethargy are common presenting symptoms. Examination of the eyes may reveal papilledema. If the obstruction of CSF flow is very slow and gradual, macrocrania develops and separation of the cranial sutures will be visible on skull films.
Communicating hydrocephalus may follow bacterial meningitis. Early recognition is important because of the potential for severe intracranial hypertension due to the combination of brain swelling (as a response to inflammation) and acute hydrocephalus.
In any age group, hydrocephalus should be differentiated from other diseases. When an infant presents with progressive head enlargement, the physician also needs to consider subdural hematoma from trauma, familil macrocephaly, the initial stages of a storage disease, a congenital brain tumor and benign subdural hematoma of infancy. [5,6]
Children who present with headache, vomiting and lethargy may be suffering from acute viral or bacterial encephalitis or meningitis. However, fever usually accompanies these infections. Similar symptoms may be seen in children with pseudotumor cerebri, a condition characterized by intracranial hypertension of unknown etiology. In pseudotumor cerebri, imaging studies demonstrate a very small ventricular system and subarachnoid space, but no other obvious pathology.  Acute cerebellar ataxia of childhood and intoxications such as lead poisoning should also be included in the differential diagnosis.
The selection of a diagnostic modality is based on the patient's age, the type of hydrocephalus, the presence or absence of abnormalities in the formation of the brain, and the possibility that an intracranial neoplasm is present. In hydrocephalus, the most commonly used cranial imaging techniques are ultrasonography, computed tomographic (CT) scanning and magnetic resonance imaging (MRI).
For an ultrasound examination to be successful, a cranial window is necessary. Therefore, ultrasonography is applicable for infants with patent fontanelles. This imaging modality is reliable and noninvasive, and it can be performed in the awake, unrestrained infant. Ultrasonography is particularly useful in hospital nurseries, since ultrasound equipment is portable and the examination can be performed at cribside. Ventricular size can be readily measured on ultrasonography. 
A closed cranium obviates the use of ultrasonography in older infants and children. Futhermore, the surface of the brain and some of the posterior fossa structures cannot be readily demonstrated with this imaging modality.
CT scanning demonstrates the complete cranial and intracranial anatomy in detail. The subarachnoid spaces, as well as the structures of the posterior fossa, are more readily seen with CT scanning than with ultrasonography. Current CT scanning equipment uses low doses of ionizing radiation.
An important disadvantage of cranial CT scanning is that the patient must remain motionless during the study. Therefore, CT scanning may require general anesthesia or supervised sedation if the child is uncooperative.
MAGNETIC RESONANCE IMAGING
Compared with CT scanning, MRI studies provide better detail of white and gray matter, as well as the ventricular system. Furthermore, areas situated in or surrounded by bone are more readily visualized with MRI than with CT scanning. MRI studies provide superior detail of the brainstem, posterior fossa and foramen magnum regions (Figure 6). With both CT and MRI studies, contrast material may help delineate structural abnormalities, particularly posterior fossa tumors.
Like CT scanning, MRI studies require immobilization of the uncooperative patient. Therefore, general anesthesia or supervised sedation may be required.
Whenever possible, treatment of hydrocephalus should be directed at the underlying cause. For example. when hydrocephalus is due to a posterior fossa tumor that obstructs CSF pathways, the best treatment may be surgical removal of the tumor, rather than placement of a shunt.
In other situations, hydrocephalus may be transient and may occur, for example, following subarachnoid hemorrhage in a premature infant. In this case, the initial treatment is decompression by means of intermittent lumbar or ventricular punctures. This technique not only helps clear the CSF of blood products and minimizes the particles obstructing the CSF pathways, but it also transiently relieves increased intracranial pressure. [3,4]
When hydrocephalus causes rapid clinical deterioration and immediate treatment is needed to reduce intracranial hypertension, the neurosurgeon may choose to perform external ventricular drainage. After twist-drill entry into the skull has been obtained, a ventricular catheter is inserted and connected to a closed external drainage system.  External ventricular drainage is also used when the patient has a shunt infection or ventriculitis. 
When treatment of the underlying cause of hydrocephalus is not possible, a permanent CSF shunt is indicated (Figure 7). The basic function of the CSF shunt is to bypass the site of obstruction and drain the CSF to another location, where it is reabsorbed. Currently used silastic shunts cause minimal tissue reaction.
The ventriculoperitoneal shunt is most commonly used to treat obstructive hydrocephalus. The advantage of this shunt is that abundant additional peritoneal tubing can be placed to allow for growth of the patient. 
The lumboperitoneal shunt is indicated in communicating hydrocephalus. This shunt may be placed in the lumbar subarachnoid space by percutaneous puncture, thus avoiding laminectomy and cranial surgery.  Abundant tubing may also be placed in the peritoneal cavity to allow for the patient's growth. The lumboperitoneal shunt has a lower incidence of infection than cranial shunts. 
Ventriculojugular and ventriculoatrial shunts are used when peritoneal surgery is contraindicated or when the peritoneum no longer absorbs the CSF. Both of these conditions are usually associated with local infection and/or major abdominal surgery performed for reasons other than the underlying hydrocephalus.
Ventriculojugular and ventriculoatrial shunts are placed in a lateral ventricle and into the jugular vein or the right atrium (through the internal jugular vein). One disadvantage of these shunts is that additional tubing cannot be placed to allow for the patient's growth. Consequently, lengthening procedures are required during the patient's development.
The child with a shunt system must be followed regularly by a family physician, with intermittent assessment by a neurosurgeon. Both physicians should observe the child for any of several potential complications associated with a shunt.
Signs of acute shunt obstruction include rapid onset of headache, anorexia, vomiting and varying degrees of lethargy. The child's parents usually realize that something different or serious is happening and contact their family physician. The physician should promptly assess the situation. If shunt obstruction is suspected, the neurosurgeon should be contacted immediately so that neurologic damage from acutely elevated intracranial pressure can be avoided.
Shunt obstruction can be caused by growth of the choroid plexus into the lumen of the ventricular catheter or by debris in any portion of the shunt system. The majority of shunt malfunctions occur in the first few years of life, and the incidence of these malfunctions diminishes in late childhood.
Shunt obstruction may also occur slowly, without overt signs of intracranial hypertension. This problem can be detected by periodic measurements of the
circumference of the patient's head. These measurements should be performed in all hydrocephalic children well into adolescence. 
Infection and colonization of shunt systems occur in up to 15 percent of patients with hydrocephalus. [8,12] About 75 percent of shunt infections occur within the first six months after shunt placement or revision. Most of these infections are caused by Staphylococcus epidermidis. [8,12] Patients with infection of the shunt system may manifest irritability, intermittent fever or signs of peritonitis, without obvious cause. Infection is confirmed by aspiration of the CSF from the shunt reservoir, followed by Gram stain or culture of the organism.
Intravenous antibiotic therapy should be initiated for shunt system infection. However, definitive treatment requires removal of the shunt system and placement of a temporary external closed system for CSF drainage. [8,12]
Compartmentalized hydrocephalus is a complication caused by progressive gliosis of the aqueduct of Sylvius and sclerosis of the roof of the fourth ventricle (Figure 8). As a result, the CSF produced by the choroid plexus of the fourth ventricle has no outflow and the ventricle progressively enlarges.
The clinical presentation of compartmentalized hydrocephalus is usually insidious. Symptoms may include dysarthia, diplopia, ataxia, hearing loss, vomiting or difficulty swallowing,  The CT or MRI study reveals a very large, distended fourth ventricle, which is out of proportion to the size of the lateral and third ventricles.
Compartmentalized hydrocephalus is treated by placing a shunt catheter in the fourth ventricle and connecting the catheter to the existing lateral ventricular shunt. Sometimes it is possible to remove the obstruction of the aqueduct of Sylvius, which may be just a thin membranous occlusion.
The outcome of hydrocephalus varies with the etiology, the severity of the disorder, and the promptness of diagnosis and treatment. As a general rule, the more simple forms of uncomplicated hydrocephalus are associated with a better outcome. In many patients, early diagnosis and treatment result in normal motor and intellectual development.
The outcome for children with hydrocephalus and spina bifida has steadily improved.  One study of an unselected group of children who had spina bifida with hydrocephalus  found that 75 percent were functioning independently and were at a comparable level with their peers who did not have spina bifida. In part, this improved outcome is the result of better school and social support, as well as on-going physical and occupational therapy.
In premature infants with intracranial hemorrhage and subsequent hydrocephalus, the outcome is variable.  This poorer outcome is probably related to the presence of other diseases, such as broncho-pulmonary dysplasia and the extent of intraparenchymal hemorrhage superimposed on the hydrocephalus.  In a follow-up study of children with significant hemorrhage,  50 percent had normal developmental milestones, with no significant motor handicaps.
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HECTORE E. JAMES, M.D. is a senior staff member at Children's Hospital, San Diego, and a clinical professor of neurosurgery and pediatrics at the University of California, San Diego, School of Medicine. Dr. James received his medical degree from the University of Buenos Aires and completed a residency in neurosurgery at the University of Pennsylvania, Philadelphia, and at Children's Hospital of Philadelphia.
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