John M. Barkley
L. Anne Hayman
Pedro J. Diaz-Marchan
Hydrocephalus is dilatation of the ventricles that contain cerebrospinal fluid (CSF). It involves an increase in the volume of CSF within the subarachnoid space, ventricles and cisterns (1).
Hydrocephalus ex vacuo
Ventriculomegaly is a generic radiological term meaning enlargement of the ventricles, while hydrocephalus implies a pathologic state that may lead to elevated intracranial pressure. Cerebral atrophy or old age may cause ventriculomegaly secondarily. The CSFspaces dilate to compensate for the loss in brain parenchyma over time. Patients over the age of 65 usually have some degree of brain volume loss, or atrophy.
The enlargement of the ventricles in this case is termed hydrocephalus ex vacuo. The ventricles, sulci and cisterns appear larger radiologically because of underlying brain parenchymal loss. Hydrocephalus ex vacuo occurs physiologically and does not result in elevated intracranial pressure.
There are two types of pathological hydrocephalus: communicating hydrocephalus and noncommunicating hydrocephalus. In these conditions, there is an increase in CSF volume without brain atrophy.
Communicating hydrocephalus is a global dilatation of all of the ventricles, while the connections between the ventricles remain patent. Typically, there is disruption of CSF reabsorption at the level of the arachnoid granulations of the dural venous sinuses. Conditions that cause communicating hydrocephalus include subarachnoid hemorrhage, meningitis and meningeal carcinomatosis (2).
Communicating hydrocephalus can be differentiated from hydrocephalus ex vacuo by signs of elevated intracranial pressure and radiologically by dilated ventricles in the setting of cisternal and sulcal effacement.
Normal pressure hydrocephalus (NPH)
Increased production of CSF may also cause communicating hydrocephalus, as in normal pressure hydrocephalus (NPH) and tumors of the choroid plexus. Prior to sampling CSF, clinicians often obtain an imaging study, usually a noncontrast CT of the brain to evaluate for hydrocephalus or mass effect. If too much CSF is removed in the acute setting, intracranial hypotension or herniation syndromes may develop (3–5).
Noncommunicating hydrocephalus (Obstruction hydrocephalus)
Noncommunicating hydrocephalus is a condition in which there is obstruction of the ventricular system somewhere between the lateral ventricles and the fourth ventricle. Congenital causes such as cerebral aqueductal stenosis may result in dilated lateral and third ventricles. Intraventricular hemorrhage, meningitis and infection may also result in obstruction of the ventricles. Obstruction may also occur at the level of the foramina of Monro leading from the paired lateral ventricles to the midline third ventricle. Thus, non-communicating hydrocephalus would be expected to cause asymmetric dilatation of the ventricles, while communicating hydrocephalus causes diffuse dilatation of the ventricular system on MRI and CT.
Original: Brain Injury Medicine. Principles and Practice
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