Hydrocephalus happens when cerebrospinal fluid (CSF) builds up inside the brain’s ventricles, the hollow chambers where this fluid normally flows. The buildup creates pressure that pushes outward against brain tissue. The causes range from structural problems present at birth to infections, head injuries, tumors, and age-related changes in how the brain handles fluid. Some cases have no identifiable cause at all.
To understand what goes wrong, it helps to know what’s supposed to happen. Your brain continuously produces CSF, which cushions and nourishes the brain, then circulates through the ventricles and the spaces surrounding the brain and spinal cord before being reabsorbed. Most of this absorption happens through tiny blood vessels in the brain and spinal cord tissue, with the lymphatic system also playing a role. Hydrocephalus develops when something disrupts this cycle, either by blocking the flow, reducing absorption, or, rarely, causing overproduction.
Two Main Types of Fluid Buildup
Doctors classify hydrocephalus based on where the problem occurs. In non-communicating (obstructive) hydrocephalus, something physically blocks the narrow passages that connect the brain’s ventricles. The fluid can’t get through, so it backs up. The most common blockage point is the aqueduct of Sylvius, a slender channel connecting the third and fourth ventricles. Tumors, cysts, and congenital narrowing of this passage are typical culprits.
In communicating hydrocephalus, the pathways between ventricles are open, but the brain can’t reabsorb fluid efficiently. This often follows infections or bleeding that damage the absorption surfaces. The distinction matters because it influences which surgical approach works best, but many cases involve some combination of both problems.
Congenital Causes in Infants
Infantile hydrocephalus affects between 1 and 32 out of every 10,000 births, a wide range that reflects differences in how it’s measured across populations. Several structural problems present at birth can trigger it.
Aqueductal stenosis is one of the most common. The narrow channel connecting the brain’s middle ventricles is abnormally tight, sometimes from a web of tissue, a congenital membrane, or an unusual forking of the passage. Fluid simply can’t move through fast enough, and pressure builds in the ventricles above the blockage.
Spina bifida with Chiari II malformation creates a more complex chain of events. When the spinal cord doesn’t close properly during fetal development (myelomeningocele), cerebrospinal fluid leaks through the open defect. This chronic leakage reduces pressure inside the skull, which causes the posterior fossa, the compartment at the base of the skull, to develop smaller than normal. The cerebellum and brainstem get crowded downward through the opening where the skull meets the spine. That displacement blocks CSF pathways at the base of the brain, leading to fluid accumulation in the ventricles above.
Dandy-Walker malformation involves abnormal development of the cerebellum and the fluid-filled space behind it. A large cyst forms in the back of the brain, obstructing CSF drainage and causing the ventricles to swell.
Maternal infections during pregnancy also raise the risk. Cytomegalovirus and toxoplasmosis, a parasitic infection commonly acquired from undercooked meat or cat litter, can damage developing brain tissue and disrupt the structures that handle CSF flow. Maternal diabetes and hypertension have been linked to congenital brain malformations as well.
Acquired Causes in Children and Adults
Hydrocephalus that develops after birth usually traces back to an event that damages the brain’s fluid-handling system.
Meningitis is a well-known trigger. Bacterial infection of the membranes surrounding the brain causes inflammation that can scar the surfaces responsible for CSF absorption. About 5% of people with community-acquired meningitis develop hydrocephalus, and when it occurs, it signals a more serious course of illness. The scarring from infection reduces the tissue’s ability to reabsorb fluid, and the buildup can happen weeks or months after the initial infection resolves.
Head injury is another major cause, particularly severe traumatic brain injury. Post-traumatic hydrocephalus is surprisingly common on imaging. Studies using CT scans find enlarged ventricles in 30 to 86% of severe head injury patients, though not all of these cases cause symptoms. The mechanism involves blood and inflammatory debris clogging the absorption pathways, or swelling that disrupts CSF circulation through the spaces at the base of the brain. When head trauma also causes meningitis, a known complication of skull fractures, the risk of hydrocephalus compounds.
Brain tumors can physically block the narrow channels between ventricles, especially tumors in the brainstem or near the center of the brain. Even a small mass in the right location can obstruct flow completely. Bleeding within the brain, whether from a stroke, a ruptured aneurysm, or bleeding into the ventricles themselves, can also clog the absorption surfaces with blood products.
Normal Pressure Hydrocephalus in Older Adults
Normal pressure hydrocephalus (NPH) is a distinct form that typically affects people over 60. The ventricles enlarge, but standard pressure measurements come back in the normal range, which is part of why it’s frequently missed or mistaken for dementia or Parkinson’s disease. The classic triad of symptoms is difficulty walking, urinary incontinence, and cognitive decline.
The causes of NPH are still debated, but several theories have gained traction. One leading explanation involves a “hydraulic press” effect: an initial, temporary spike in CSF pressure stretches the ventricles. As the ventricles expand, their larger surface area allows more fluid to be absorbed, which brings pressure readings back to normal. But the damage from the stretching persists, and the enlarged ventricles continue pressing on surrounding brain tissue.
More recent research points to blood vessel changes in the brain. NPH is strongly associated with intracranial arteriosclerosis, the same kind of artery stiffening that causes cardiovascular disease elsewhere in the body. Healthy arteries in the brain act as natural shock absorbers, cushioning the pulse of blood with each heartbeat. When those arteries stiffen, the pulsation energy that would normally be absorbed gets redirected into the brain’s fluid spaces and ventricles. The ventricles may enlarge as an adaptive response, expanding to create more room for fluid to buffer these abnormal pulses. This vascular theory helps explain why NPH shares so many risk factors with heart disease and stroke, including high blood pressure, diabetes, and aging itself.
In many older adults, no clear triggering event is identified, and the condition is labeled idiopathic. It likely develops gradually over years as age-related changes in blood vessels and brain tissue slowly shift how CSF pulses and drains.
How Symptoms Differ by Age
In infants, the skull bones haven’t yet fused together, so rising pressure causes the head to grow abnormally large. A bulging soft spot on top of the head, eyes that seem to look permanently downward (“sunsetting”), irritability, poor feeding, and vomiting are common signs. Because the skull can expand, infants may tolerate fluid buildup longer before showing obvious distress, but the pressure still damages developing brain tissue.
In older children and adults, the skull is rigid, so pressure rises faster and symptoms tend to be more acute. Headaches (often worse in the morning), nausea, blurred or double vision, balance problems, and difficulty concentrating are typical. In acute cases from a sudden blockage, symptoms can escalate within hours to drowsiness and loss of consciousness.
In older adults with NPH, the presentation is more subtle. Walking becomes shuffling and unsteady, resembling a magnetic gait where the feet seem stuck to the floor. Memory and thinking slow down in ways that look like early dementia. Bladder urgency progresses to full incontinence. These symptoms develop over months to years and are often attributed to “just getting old” before the real cause is identified.
How It’s Treated
Most hydrocephalus requires surgical intervention because there’s no medication that reliably reduces CSF buildup long-term. The two main procedures are shunt placement and a minimally invasive alternative called endoscopic third ventriculostomy (ETV).
A shunt is a thin tube placed into the ventricle that drains excess fluid to another body cavity, usually the abdomen, where it’s harmlessly reabsorbed. Shunts are effective but not permanent solutions. Within the first year after placement, 11 to 25% fail. Early failures are typically caused by infection, while later ones usually result from the tube becoming blocked. Many people with shunts will need one or more revisions over their lifetime, which is why close follow-up matters.
ETV works differently. A surgeon creates a small opening in the floor of the third ventricle, giving CSF an alternate route to bypass a blockage. It avoids the need for implanted hardware, which means no device to malfunction or get infected. However, primary ETV failure rates range from about 17 to 61% depending on the patient population and cause of hydrocephalus. It works best for obstructive cases where there’s a clear blockage, and it’s less reliable for communicating hydrocephalus where the absorption system itself is impaired.
For NPH, shunt surgery can dramatically improve symptoms, particularly gait problems, which tend to respond best. Cognitive symptoms may also improve, though the degree of recovery depends on how long the condition went undiagnosed. This is one of the few forms of “dementia” that is potentially reversible with treatment, making accurate diagnosis especially important.