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. Hydrocephalus occurs when the ventricles are >15 mm wide. Occasionally, hydrocephalus can present with normal head size but with marked ventricular dilatation.
Ventriculomegaly (VM) is an enlargement of the cerebral ventricles. In a normal fetal brain the ventricles are <10 mm wide (mean diameter of normal atrium is 7.6 mm). In mild VM, ventricles are between 10 and 15 mm wide; in severe VM, ventricles are >15 mm wide. VM may or may not be associated with macrocephaly. Increased ventricular dimension can be due to increased intraventricular pressure (as in hydrocephalus) or the result of passive ventricular enlargement caused by loss of periventricular white matter (such as diffuse leukomalacia). Early diagnosis of fetal VM and hydrocephalus remains a diagnostic dilemma.
Cerebrospinal fluid 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 substances 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 to 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 every 4 to 6 hours. The mean CSF opening pressure in neonates and preterm infants is typically lower (100 mm H2O and 95 mm H2O, 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 from aqueductal stenosis, ventriculitis, or a clot following an extensive intraventricular hemorrhage resulting in noncommunicating hydrocephalus.
Communicating (absorptive) hydrocephalus. Results when ...