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Chapter 90

1. I. Definition. Hydrocephalus is dilation of the cerebral ventricular system secondary to an accumulation of cerebral spinal fluid (CSF) and a disturbance in CSF circulation. It is usually associated with increased intracranial pressure (ICP) and an enlarging head. The measurable occipital-frontal head circumference (OFC) may exceed the growth percentiles for gestational age or chronological age. Typically, an OFC of >2 standard deviations of normal is consistent with macrocephaly due to hydrocephalus. Occasionally hydrocephalus may be present with normal head size but with marked ventricular dilation.

CSF is produced largely from the choroid plexus of each ventricle throughout the cerebral ventricular system. Approximately 80% is choroid plexus in origin, and the remainder is contributed from the substance of the brain and spinal cord. CSF flows posterior from the lateral ventricles through the third ventricle into the aqueduct of Sylvius and through to the fourth ventricle. It ultimately reaches the subarachnoid space by way of the foramina of Magendie and Luschka. CSF enters the venous circulation by way of the absorptive arachnoid villi that line the superior sagittal sinus. Any disturbance of the flow or absorption of CSF leads to accumulation and hydrocephalus. Two mechanisms exist to explain pathologic CSF accumulation:

1. A. Noncommunicating hydrocephalus is the result of obstruction anywhere along the ventricular CSF pathway from the third ventricle to the subarachnoid cisterna (ie, cisterna magna). Also known as obstructive hydrocephalus, noncommunicating hydrocephalus is an obstruction to the progressive flow of CSF. It is any blockage of CSF that keeps it from reaching the subarachnoid space and the normal resorptive function of the arachnoid villi (eg, blockage may be aqueductal stenosis, ventriculitis, or a clot following an extensive intraventricular hemorrhage).

1. B. Communicating hydrocephalus results when CSF is able to pass through all the foramina, including the foramina at the base of the brain (cisterna magna), but is not absorbed into the venous drainage of the cerebral circulation because of obliteration of the arachnoid villi. Various disease processes can result in obliteration of the arachnoid villi causing CSF accumulation throughout the cerebral ventricular system (ie, following bacterial meningitis or an extensive subarachnoid hemorrhage).

2. II. Incidence. The incidence of neonatal hydrocephalus alone is unknown. When included in the diagnosis of spina bifida it occurs in 2–5 births in 1000.

3. III. Pathophysiology

1. A. Congenital hydrocephalus (CH) refers to progressive ventricular enlargement, starting prior to birth and readily apparent on the first day of life. All typical forms of CH are noncommunicating or obstructive in presentation. CH results from developmental malformations of the brain that disturb CSF pathways. Most malformations occur between 6 and 17 weeks of gestation. Fetal hydrocephalus is usually severe and accompanied by other serious anomalies of the brain, namely holoprosencephaly or encephalocele. Fifty percent of CH cases presenting as fetal hydrocephalus are associated with myelomeningocele, Arnold Chiari malformation, aqueduct stenosis, or the Dandy-Walker malformation.

1. B. Postinfectious hydrocephalus may be either communicating or noncommunicating. Bacterial inflammation of the meninges and subsequent arachnoiditis sets the stage for loss of the CSF ...

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