Infantile hydrocephalus can be associated with congenital anomalies, such as aqueduct stenosis, spina bifida, and Arnold-Chiari malformation; it can also be associated with less common conditions, such as Dandy-Walker syndrome and encephaloceles. Of these, the most frequent is aqueduct stenosis. It can be associated with acquired conditions such as perinatal intraventricular hemorrhage and meningitis. It can also occur after closed head injury.

History of the Procedure: In the 1960s, before shunting was established, children with hydrocephalus had a poor prognosis. Most patients were not offered treatment, and only 20% of children who did not receive operation for hydrocephalus reached adulthood. Furthermore, the children who did survive had a 50% chance of remaining permanently brain damaged. After the introduction of shunting, outcomes improved. Currently, if appropriate care of the shunt has occurred, most children with hydrocephalus reach adulthood. In a 20-year follow-up survey of children who received shunting in the 1970s, more than half of them graduated from normal schools.

The outcome of patients with spina bifida has also improved. In a review of a cohort of patients treated in the 1970s for spina bifida aperta, 52% of the patients were alive at 20 years. Most of the deaths occurred in the first year of life, mostly because of renal and respiratory problems associated with the spina bifida. Only a few of the deaths were related to hydrocephalus. In a similar but more recent review of children treated in the 1980s, only 27% died, most of them in the first year of life, from causes not related to hydrocephalus but to the spina bifida. In a recent survey of adults with spina bifida, 6% of patients died because of shunt-related problems or after craniovertebral decompression for Chiari malformation.

Problem: Hydrocephalus can be caused by increased production of CSF or impaired circulation and absorption. Hydrocephalus caused by impaired circulation is called obstructive hydrocephalus because an anatomic block to CSF circulation is present. Hydrocephalus caused by increased production or impaired absorption is called communicating hydrocephalus because no anatomic block to CSF circulation exists. According to some authorities, all cases of hydrocephalus are obstructive, ie, the cases with communicating hydrocephalus have a functional obstruction at the final stage of absorption at the arachnoid granulations.

Frequency: The incidence of infantile hydrocephalus is estimated at 3-5 cases per 1000 live births. The peak ages of presentation for this group are the first few weeks of life, age 4-8 years, and early adulthood. The latter 2 peaks represent delayed presentations of infantile hydrocephalus. An estimated 750,000 people have hydrocephalus, and 160,000 ventriculoperitoneal shunts are implanted each year worldwide.

The incidence of myelomeningocele ranges from 0.2-2 per 1000 live births, with regional and racial variations. The overall incidence of myelomeningocele has declined significantly in the last 2 decades because of improved maternal nutrition during pregnancy, including the addition of folic acid, a wider availability of prenatal diagnosis, and therapeutic termination of pregnancy. In a significant proportion of patients with open spina bifida, hydrocephalus is absent at birth but develops in the first few weeks or months of life. Hydrocephalus occurs in 15-25% of children with open myelomeningocele at birth; however, in most surgical series, the proportion of patients with myelomeningocele who require shunting reaches 80-90%. No obvious correlation between the level of the lesion and the presence of hydrocephalus has been shown.

Etiology: The simplistic distinction of obstructive and communicating hydrocephalus is historically related to the different conditions that are causing or are associated with impairment of CSF circulation. Examples of conditions with obstructive hydrocephalus are congenital aqueduct stenosis, tumors of the ventricular system (eg, colloid cyst of the third ventricle, astrocytoma of the third ventricle), and tumors of the posterior cranial fossa (eg, cerebellar astrocytoma, medulloblastoma). An example of communicating hydrocephalus caused by CSF overproduction is the presence of choroid plexus papilloma in one of the ventricles. Examples of conditions with communicating hydrocephalus caused by impaired CSF absorption are spina bifida, intraventricular hemorrhage, meningitis, and head injury. The distinction between obstructive and communicating hydrocephalus sometimes cannot be made because an element exists of both mechanisms operating.

Pathophysiology: Production of CSF is an active process at a rate of 0.35 mL/min. Absorption of CSF at the arachnoid granulations also is an active process and requires at least 6.8 mm of water pressure to overcome the venous blood pressure inside the sagittal sinus. During a 24-hour period, a total of 500 mL in adults or 250 mL in children of CSF are produced and absorbed. At any given time, a total of 140 mL for adults or 70 mL for children of CSF exists inside the head. Normal CSF pressure inside the ventricles is 110 mm of water pressure. When an impairment of CSF circulation exists, the resulting CSF accumulation leads to ventricular enlargement and rise in intraventricular (and hence intracranial) pressure. In infants with open fontanelles, some of this rise in pressure is counteracted with enlargement of the head. When the maximum capacity for head enlargement has been used, rapid deterioration follows because of raised intracranial pressure.

In children with aqueduct stenosis, the aqueduct of Sylvius is narrower than usual or is completely occluded, and an obstruction to CSF flow is present. A debate exists whether the aqueduct stenosis is the primary deformity, which leads to ventriculomegaly, or whether the ventriculomegaly is the primary deformity (because of altered abnormal ventricular wall compliance), which leads to secondary aqueduct stenosis caused by continuing compression of the midbrain.

In children with myelomeningocele, several factors are implicated in the pathogenesis of hydrocephalus, including the Chiari type II malformation, a degree of aqueduct stenosis, anomalous venous drainage in the posterior fossa caused by compression of the sigmoid sinuses, the open myelomeningocele, and the presence of other CNS malformations

In the context of the Chiari type II malformation, extensive deformity of the posterior fossa and its structures exists. The brain stem has abnormal disposition with respect to the midbrain and the tentorial hiatus, the posterior fossa has smaller capacity than usual, the fourth ventricle is displaced caudally, and significant prolapse of the cerebellar tonsils is present through the foramen magnum. All these anatomic factors contribute to the impairment of CSF circulation.

The development of hydrocephalus is temporally related to the closure of the myelomeningocele. In a small proportion of patients with open myelomeningocele, dramatic deterioration occurs after closure of the defect. The impaction of the hindbrain hernia plays a significant role in this acute deterioration.

The development of hindbrain hernia during gestation is believed to be caused by the progressive caudal migration of the hindbrain in association with the low-pressure conditions that the open myelomeningocele creates in the spine. Continued loss of CSF soon after birth results in further deterioration of hindbrain hernia and the associated hydrocephalus. This can lead to acute neurologic deterioration caused by a combination of raised intracranial pressure related to the ventriculomegaly and acute bulbar dysfunction caused by compression of the brain stem in the region of the foramen magnum. The neurologic state usually improves after ventricular shunting. In most patients, ventriculomegaly develops gradually within the first few weeks or months of life.

The incidence of hydrocephalus was decreased from 91% to 59% in the small number of children who underwent intrauterine repair of the myelomeningocele compared to historic control subjects who had traditional treatment. The incidence of hindbrain hernia present at birth in this group of children was considerably smaller than that in historic control subjects. Therefore, the lower incidence of hydrocephalus was postulated to be caused by the absence of the obstructing effect of the hindbrain hernia at the level of the foramen magnum to the flow of CSF. However, a variety of factors certainly are implicated in this mechanism.

In children with postmeningitic or posthemorrhagic hydrocephalus, the arachnoid granulations are occluded by protein (meningitis) or blood degradation products (intraventricular hemorrhage), rendering CSF absorption relatively ineffective.

Clinical: Infants with hydrocephalus develop enlarging head with bulging fontanelle, enlarged scalp veins, macrocrania, suture diastasis, and positive Macewen (ie, cracked-pot) sign. If left untreated, these infants develop sunset eyes, recurrent vomiting, and later, respiratory arrest.

The particular consideration of children with myelomeningocele is the presence of hindbrain hernia in the context of the Arnold-Chiari malformation, which can cause early clinical symptoms of bulbar palsy and can remain unnoticed by inexperienced observers. Poor feeding, recurrent vomiting, poor sucking, generally subdued behavior with poor crying, high-pitched cry or stridor caused by vocal cord paralysis, episodes of apnea, and recurrent aspiration (often manifesting with recurrent pneumonia) can all be manifestations of brain stem dysfunction caused by hindbrain hernia and aggravated by ventricular dilatation. Lastly, persistent CSF leak from the repaired spinal wound almost invariably indicates active hydrocephalus, even if the ventricular size is only modestly enlarged and the anterior fontanelle is not bulging.

Children with a clinical picture of active hydrocephalus and significant ventriculomegaly (often with evidence of periventricular lucency indicating raised CSF pressure in the ventricular system) require treatment early in life. In contrast, children with mild or moderate ventriculomegaly and head circumference within the reference range may not require initial treatment. In these children, an observation policy can be adopted for the first few months of life, and monitoring of the head circumference and repeat ultrasound, CT, or magnetic resonance (MR) scanning are helpful to decide whether shunting is finally required.

A significant consideration for timing of treatment is the child's age, prematurity status, and weight. As a general rule, shunting is avoided, if possible, in children younger than 6 months because they have an increased risk of infection. For the same reason, in premature babies weighing less than 1 kg, deferring shunting, if possible, until they have gained weight is best. These considerations often arise in children with posthemorrhagic hydrocephalus, who often are premature and small-for-dates.

In children with open myelomeningocele, simultaneous shunting and closure of myelomeningocele should be performed, if feasible. It appears to protect patients from CSF leak from the spinal wound, which can lead to shunt infection, and it improves the chances for better development by reducing intracranial hypertension early. One of the signs of oncoming hydrocephalus after closure of myelomeningocele is persistent CSF leak. In children with open myelomeningocele, in which closure of the defect has been delayed, CSF infection may have already taken place. In such circumstances, CSF microbiological testing should be performed, and if CSF infection is present, external ventricular drainage should be employed for 7-10 days in conjunction with antibiotic treatment, until CSF infection is controlled and a shunt can be inserted.

In children with intraventricular hemorrhage, although active hydrocephalus may have been present early in life, contemplating shunting is often difficult because the CSF is heavily blood stained and/or the protein content is too high (>1 g/dL). In such cases, the traditional attitude is to insert an external ventricular drain for a period until the CSF clears and a shunt can be inserted. Efforts to accelerate the clearance of blood from the CSF have been made by injecting streptokinase in the ventricles, with moderate success. This therapeutic maneuver still has not gained universal acceptance.

An issue that merits attention is the need to decide whether shunting is indicated in older children or young adults who have myelomeningocele and untreated ventriculomegaly. Patients are sometimes observed with the typically shaped ventricles of hydrocephalus that are caused by spina bifida but do not appear to have tension, with no periventricular lucency, and with no symptoms (eg, headache, drowsiness, diplopia, bulbar features) suggesting active hydrocephalus. If these patients never receive shunting, serial monitoring with intelligence and psychometric testing should be undertaken.

If a patient has no clinical symptoms and their psychometric test results indicate stability, the neurosurgeon should be discouraged from contemplating shunting solely on the basis of radiologic appearances, keeping in mind that these patients have a high risk of bilateral subdural hematomas. In a similar manner, in patients who already have received shunting, caution should be exercised for considering any intervention on the shunt. Because shunts may be disconnected or appear to not have been working for years, regarding the situation as compensated hydrocephalus and choosing to remove the shunt, especially if it is causing local discomfort in the neck, is tempting.

However, shunts that have been implanted for years acquire a tube of strong fibrous tissue surrounding them along their entire length. Even though the tube may appear fractured on radiographs, CSF is bridging the gap guided by the encircling fibrous tube. Such shunts actually are functioning, and any attempt to remove them without instituting any alternative means of CSF drainage (eg, third ventriculostomy) may be lethal. In contrast, if subtle symptoms or clear measured intellectual decline is present, treatment should be offered. In such cases, patients who have not received shunting should be shunted, and those who have shunts should have their shunts revised.

• For the last 2 decades, MR scanning has played an increasing role in the management of hydrocephalus in children; however, unfortunately, it still is not as widely available as CT scanning.

• A major consideration, especially in very young infants, is the need for sedation, or even general anesthesia, to obtain good images because acquisition takes several minutes or more and any movement severely deteriorates picture quality. This is particularly problematic in children with associated congenital conditions that cause poor respiratory drive.

• MR scanning shows structures of the brain with superior anatomic detail and assists the surgeon in choosing the best treatment. MR scanning is especially necessary in endoscopic treatment, in which the surgeon must verify the presence of aqueduct stenosis or any intracranial cysts that may be fenestrated, and in determining the relationship of important anatomic structures (eg, the floor of the third ventricle to the basilar artery) in children with aqueduct stenosis.

• The use of phase-contrast cardiac gated sequences can provide information on CSF flow through the aqueduct, and ventriculostomy can provide information on patency of the stoma.

• Children born with open myelomeningocele have a very high incidence of hindbrain hernia, close to 100% . In up to 15-20%, marked hydrocephalus is present, and a further 20% have moderate ventriculomegaly at birth. A large proportion of patients (up to 90%) develop clinical hydrocephalus at some point. The lateral ventricles have a characteristic appearance in almost all patients with spina bifida, ie, the occipital horns are more dilated than the frontal horns, and the long axis of the lateral ventricles tend to be parallel. A contributing factor may be the partial or complete absence of the falx and absence of the septum pellucidum in a very large proportion of these patients. MR scanning produces a good image of the hindbrain hernia, the small posterior fossa, and the midbrain deformity with kinking of the aqueduct.

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