What Is Hydrocephalus?

Hydrocephalus is a neurological condition in which cerebrospinal fluid accumulates within the ventricular system of the brain, distending the ventricles and, in most forms, raising intracranial pressure. The name comes from the Greek words for water and head, and the older description of the disorder as water on the brain is still encountered. It is a disorder of fluid mechanics rather than of fluid composition: the choroid plexus secretes roughly 500 milliliters of cerebrospinal fluid per day into a system that holds only about 150 milliliters at any moment, so the entire volume turns over several times daily and any sustained mismatch between production, circulation, and absorption produces accumulation.

Fluid flows from the lateral ventricles through the foramina of Monro into the third ventricle, then through the cerebral aqueduct to the fourth ventricle, and out into the subarachnoid space, where it is absorbed at the arachnoid granulations and along perivascular and lymphatic routes. Hydrocephalus is classified by where that circuit fails. Obstructive or non-communicating forms involve a blockage within the ventricular system, commonly aqueductal stenosis or a posterior fossa tumor. Communicating forms involve impaired absorption downstream, as after subarachnoid hemorrhage or meningitis. Congenital cases are often associated with myelomeningocele and Chiari II malformation, while acquired cases follow hemorrhage of prematurity, infection, trauma, or tumor.

Normal Pressure Hydrocephalus

A distinct form appears in older adults in which the ventricles enlarge without a structural blockage and measured intracranial pressure remains within the ordinary range. Idiopathic normal pressure hydrocephalus presents with a characteristic triad of gait disturbance, cognitive decline, and urinary incontinence, though the complete triad is not required for diagnosis. Because those symptoms overlap with Alzheimer disease, Parkinson disease, and vascular dementia, selecting patients likely to benefit from treatment is the central clinical problem. Supplementary tests include a high-volume lumbar tap with gait assessment before and after, extended external lumbar drainage, and lumbar infusion studies that estimate cerebrospinal fluid outflow resistance.

Diagnosis and Imaging

Computed tomography and magnetic resonance imaging establish ventricular size and identify obstruction. The Evans index, the ratio of the maximum width of the frontal horns to the maximum inner skull diameter on an axial slice, remains the standard morphometric criterion, with values above 0.3 indicating ventriculomegaly. Magnetic resonance imaging adds information that computed tomography cannot supply: phase-contrast sequences quantify pulsatile flow through the aqueduct, and heavily weighted sequences show whether the floor of the third ventricle is patent. In neonates, transfontanelle ultrasound provides repeated bedside measurement without ionizing radiation. Invasive intracranial pressure monitoring, using an intraparenchymal strain gauge or an external ventricular drain coupled to a pressure transducer, resolves cases where imaging is ambiguous, and the recorded waveform carries diagnostic information in its pulse amplitude and slow waves.

Shunts and Endoscopic Treatment

The standard treatment diverts fluid through an implanted shunt, most often a ventriculoperitoneal shunt that carries fluid from a lateral ventricle to the peritoneal cavity for absorption. A shunt comprises a proximal catheter, a valve that sets the pressure or flow characteristic, and a distal catheter. Valve design has moved from fixed differential pressure units to externally programmable valves and antisiphon or gravitational devices that compensate for the hydrostatic column created when the patient stands. Reported shunt outcomes in normal pressure hydrocephalus show symptomatic improvement in most carefully selected patients, but shunts remain among the highest revision-rate implants in medicine, failing through obstruction by choroid plexus or debris, disconnection, infection, and both over- and underdrainage. Endoscopic third ventriculostomy offers a shunt-free alternative by opening a channel in the floor of the third ventricle, and combining it with choroid plexus cauterization in infants has been evaluated prospectively by the Hydrocephalus Clinical Research Network as a way to avoid shunt dependence.

Applications

Work on hydrocephalus draws on a range of technical fields, including:

• Implantable valve and catheter design
• Biomaterials engineering for antibiotic and antithrombogenic catheter coatings
• Medical imaging and automated ventricular volume segmentation
• Pressure sensing and telemetric implant instrumentation
• Computational modeling of cerebrospinal fluid dynamics
• Neuroendoscopy instrumentation and surgical navigation
• Global health engineering, where low-cost shunt and endoscopy programs serve infant hydrocephalus