Hydrocephalus is the progressive enlargement of the ventricular system secondary to excessive cerebrospinal fluid (CSF) volume. It is caused by an imbalance between CSF production, absorption, and impaired CSF circulation. Hydrocephalus is associated with increased intracranial pressure (ICP) and an enlarging head.
Typically, an occipitofrontal head circumference of >2 standard deviations of normal is consistent with macrocephaly due to hydrocephalus. Occasionally, hydrocephalus can present with normal head size but with marked ventricular dilatation.
CSF is primarily produced in the choroid plexus that lines the ventricles (mostly by lateral ventricles in humans). Approximately 80% is choroid plexus in origin, and the remainder is contributed from substance of the brain and spinal cord. Cerebral fluid acts as a buffer between the brain and the skull. Normally secretion of CSF occurs at a rate of 0.3–0.4 mL/min (500 mL/d). Total volume of CSF ranges from 10 to 30 mL for preterm infants and 40 mL for full-term infants; 99% of CSF is water. Sodium is a major cation. Replacement occurs in every 4–6 hours.
The mean CSF opening pressure in neonates and preterm infants is typically lower (100 mmH2O and 95 mmH2O, respectively). CSF values for cell count, protein, and glucose concentrations vary with gestational age (GA) and postmenstrual age (PMA). CSF protein concentrations decrease with both advancing PMA and postnatal age. The white blood cell count is higher in the CSF of neonates as compared with older children.
CSF drains from lateral ventricles via the foramen of Monro into the third ventricle, via the aqueduct of Sylvius into the fourth ventricle, and then into the subarachnoid space via the foramina of Luschka and Magendie. CSF enters the venous circulation by way of the absorptive arachnoid villi that line the superior sagittal sinus. Disruption in this pathway can cause hydrocephalus. Two mechanisms exist to explain the pathologic accumulation of CSF:
Noncommunicating (or obstructive) hydrocephalus. This may be any blockage along the ventricular CSF pathway that keeps it from reaching the subarachnoid space or disrupts the normal resorptive function of the arachnoid villi. For example, blockage may be aqueductal stenosis, ventriculitis, or a clot following an extensive intraventricular hemorrhage resulting in noncommunicating hydrocephalus.
Communicating (absorptive) hydrocephalus. Results when CSF is able to pass through all the foramina, including the foramina at the base of the skull (cisterna magna), but is not absorbed into the venous drainage of the cerebral circulation because of the obliteration of the arachnoid villi, as in bacterial meningitis or following an extensive subarachnoid hemorrhage.