Normal pressure hydrocephalus is primarily a disorder of the elderly. This condition is characterized by episodic elevation of CSF pressure, hence subarachnoid space volume is not affected. Characteristic symptoms include urinary incontinence, cognitive dysfunction, and gait apraxia (motor planning deficit leading to magnetic or shuffling gait). These characteristic symptoms are caused by the distortion of corona radiata fibers by the enlarged ventricles. CSF drainage via lumbar puncture or shunt placement can reverse these symptoms, hence normal pressure hydrocephalus is considered a reversible cause of cognitive dysfunction.
Communicating hydrocephalus describes hydrocephalus that's caused by decreased CSF absorption at subarachnoid granulations. Etiologies include scarring from infections (e.g., meningitis), subarachnoid hemorrhage, or neoplasms. Rarely communicating hydrocephalus can also be caused by decreased venous outflow or overproduction of CSF. Communicating hydrocephalus manifests with increased intracranial pressure and headache, papilledema (optic disc edema), and brain herniation.
Non-communicating hydrocephalus describes hydrocephalus that's caused by some type of obstruction in the ventricular system. This obstruction can have multiple etiologies. Examples include mass effect from tumors or cysts, as well as congenital or acquired stenosis of the aqueduct of Sylvius.
Ex vacuo ventriculomegaly is a medical condition characterized by an increase in the volume of CSF and enlargement of cerebral ventricles on imaging, but does not represent true hydrocephalus. The two therefore must be distinguished. This condition is usually caused by encephalic volume loss and neuronal atrophy seen in conditions like Alzheimer's disease, Huntington's disease, frontotemporal dementia, and advanced HIV/AIDS. It is important to note that these patients do not develop the typical triad of apraxia, dementia, and urinary incontinence and intracranial pressure remains normal.
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