Hydrocephalus (from the Greek words hydor "water" and kephale "head") occurs in 15-25% of children with open myelomeningocele (a form of spina bifida) at birth, though in most surgical series, the proportion of patients with myelomeningocele who require shunting reaches 80-90%. An estimated 750,000 people have hydrocephalus, and 160,000 ventriculoperitoneal shunts are implanted each year worldwide. About 56,600 children and adolescents younger than age 18 years have a shunt in place.
Hydrocephalus is defined as excess cerebrospinal fluid (CSF) accumulation in the head caused by a disturbance of formation, flow, or absorption. Although there are a number of causes of infantile hydrocephalus, the condition is most associated with the congenital anomalies spina bifida and aqueductal stenosis.
Hydrocephalus is caused by either increased production of CSF or impaired circulation and absorption. Hydrocephalus caused by impaired circulation is called obstructive hydrocephalus because CSF circulation is anatomically blocked. Hydrocephalus caused by increased production or impaired absorption of CSF is called communicating hydrocephalus because CSF circulation is not anatomically blocked.
According to some authorities, all cases of hydrocephalus are obstructive (ie, patients with communicating hydrocephalus have a functional obstruction at the final stage of absorption at the arachnoid granulations).
Spina bifida is a midline defect in the mesenchymal-derived tissues and is classified as either a closed or open neural tube defect (NTD). Closed NTDs do not involve exposed neural tissue and do not leak CSF. Open NTDs are subclassified into myelomeningocele (most common), myeloschisis, or hemimyelomeningocele (most rare). The goals of surgery are to preserve neural function, to prevent infection, and to prevent long-term complications such as an epidermal or dermal inclusion cyst and cord tethering. 
CSF is typically produced by the choroid plexus of the ventricles and circulates in one direction from the lateral ventricles to the third ventricle and through the aqueduct of Sylvius to the fourth ventricle.
From the fourth ventricle, CSF exits the brain through three separate openings: one in the midline (foramen of Magendie) and one on either side (foramina of Luschka). It enters the subarachnoid space at the foramen magnum, circulates down to the spine, and then circulates up again to the surface of the brain, where it is absorbed at the arachnoid granulations. These are sievelike structures where the CSF enters the venous circulation, leading to the sagittal sinus.
Several factors are implicated in the etiology of hydrocephalus in children with myelomeningocele, including the following:
Arnold-Chiari II malformation
A degree of aqueductal stenosis
Anomalous venous drainage in the posterior fossa caused by compression of the sigmoid sinuses
Presence of other central nervous system (CNS) malformations
Extensive deformity of the posterior fossa and its structures is associated with Arnold-Chiari II malformations. The brainstem has an 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 the cerebellar tonsils through the foramen magnum are significantly prolapsed. These anatomic factors all contribute to the impairment of CSF circulation.
Although the development of hindbrain herniation during gestation in myelomeningocele was once thought to be the result of cord tethering “pulling” on the brain tissue, hindbrain herniation is now believed to be caused by the progressive caudal migration of the hindbrain in association with the low-pressure conditions created in the spine by the open myelomeningocele. Continued loss of CSF soon after birth exacerbates the 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 brainstem in the foramen magnum region. The neurologic state usually improves after ventricular shunting. In most patients, ventriculomegaly gradually develops within the first few weeks or months of life.
Hydrocephalus development can be temporally related to the closure of the myelomeningocele. In a small group 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.
In addition to the presumed effect of the low-pressure leak during gestation on the development of Arnold-Chiari II malformation, the abnormal and exposed spinal tissue is theorized to sustain damage in utero through trauma and exposure to neurotoxic substances in amniotic fluid. This is thought to lead to neurologic deficits that are worse than if the exposure did not occur.
The incidence of infantile hydrocephalus is estimated at 3-5 cases per 1000 live births. The peak ages of presentation in this group include the first few weeks of life, age 4-8 years, and early adulthood. The latter two peaks represent delayed presentations of infantile hydrocephalus.
An estimated 750,000 people have hydrocephalus, and 160,000 ventriculoperitoneal shunts are implanted each year worldwide. About 56,600 children and adolescents younger than age 18 years have a shunt in place.
The incidence of myelomeningocele is in the range of 0.2-2 per 1000 live births. The overall incidence of myelomeningocele has significantly declined in the past two 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%.
In a retrospective chart review, shunt placement was shown to vary according to the level of the lesion, with a greater number of patients with thoracic lesions requiring shunts than those with lumbar or sacral lesions. Lesions at levels of T12 and above have also been associated with increased incidence of brain abnormalities and lower scores on psychometric testing than lesions at L1 or below.
In the 1940s, 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 undergo surgery for hydrocephalus reached adulthood. Furthermore, children who survived had a 50% chance of having permanent brain damage. Outcomes improved after the introduction of valved shunt systems by Nulsen and Spitz in 1952 and after the development of silicone systems by Holter and Pudenz in the 1960s.
The mortality associated with initial shunt insertion is approximately 0.1%; mortality due to shunt failure ranges from 1-4%. Although shunting is necessary in children with myelomeningocele and hydrocephalus, those who require shunting have been shown to have a shorter lifespan than those who do not. This is likely due to the severity of the lesion and complications of shunting.
Most children with hydrocephalus currently reach adulthood if the shunt is appropriately maintained. In a 20-year follow-up survey of children who received shunting in the 1970s, more than half of them graduated from mainstream education.
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 20 years after treatment. Most of the deaths occurred in the first year of life, mostly due to renal and respiratory problems associated with spina bifida. Only a few of the deaths were related to hydrocephalus. Chern et al concluded that surveillance imaging of children with spina bifida aperta and shunted hydrocephalus decreased emergencies during follow-up, but had no clear effect on mortality and morbidity. 
In a review of children treated in the 1980s, only 27% died; most of them died in the first year of life from causes related to spina bifida rather than hydrocephalus.
In a survey of adults with spina bifida, 6% of patients died of shunt-related problems or died after craniovertebral decompression for Arnold-Chiari II malformation.
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