Hydrocephalus is a condition in which cerebrospinal fluid (CSF) builds up inside the brain’s ventricles, the hollow chambers deep within the brain. This fluid buildup increases pressure on the brain tissue, and without treatment, it can cause serious neurological damage. The condition can develop at any age, from before birth through late adulthood, and it takes different forms depending on what’s causing the fluid to accumulate.
How Fluid Builds Up in the Brain
Your brain continuously produces cerebrospinal fluid, a clear liquid that cushions the brain, delivers nutrients, and carries away waste. This fluid is made primarily inside the ventricles by a structure called the choroid plexus. Under normal conditions, CSF flows through the ventricles, exits into the spaces surrounding the brain and spinal cord, and gets reabsorbed into the bloodstream. The brain produces and reabsorbs roughly equal amounts, keeping the total volume stable.
Hydrocephalus develops when something disrupts that balance. Either a physical blockage prevents the fluid from draining out of the ventricles, the body can’t reabsorb it fast enough, or (rarely) the brain overproduces it. The ventricles swell with trapped fluid, compressing surrounding brain tissue. In infants whose skull bones haven’t yet fused, the head itself can enlarge. In older children and adults, the rigid skull means pressure rises quickly with nowhere to go.
Types of Hydrocephalus
The condition is broadly divided into two categories based on where the problem occurs. In obstructive (non-communicating) hydrocephalus, a physical blockage somewhere in the narrow channels connecting the ventricles prevents fluid from flowing out. Common blockage points include the aqueduct of Sylvius, a tiny canal connecting the third and fourth ventricles, and the openings at the base of the fourth ventricle. Tumors, cysts, and congenital narrowing of these passages can all cause obstruction.
In communicating hydrocephalus, there’s no physical blockage. Fluid flows freely through the ventricles but isn’t reabsorbed properly once it reaches the outer surface of the brain. This form often develops after an infection like meningitis, a brain hemorrhage, or head trauma, all of which can scar or inflame the tissue responsible for reabsorption.
Normal Pressure Hydrocephalus
Normal pressure hydrocephalus (NPH) is a distinct form that primarily affects people over 60, with an average age of onset around 70. Unlike other types, fluid pressure readings may appear normal or only intermittently elevated, even though the ventricles are clearly enlarged. NPH produces a recognizable pattern of three symptoms known as the Hakim triad: progressive difficulty walking, cognitive decline, and urinary urgency or incontinence. The full triad appears in 50 to 75 percent of patients, though walking and thinking problems show up in 80 to 95 percent of cases.
NPH matters because it’s one of the few reversible causes of dementia. The walking difficulty typically comes first, often described as a shuffling, magnetic gait where the feet seem stuck to the floor. When recognized and treated, many patients improve significantly. When missed, it’s frequently mistaken for Alzheimer’s disease or simply attributed to aging.
What Causes It
In babies, hydrocephalus is most often congenital, meaning the brain’s fluid pathways didn’t develop normally before birth. Aqueductal stenosis, a narrowing of the channel between the third and fourth ventricles, is one of the most common congenital causes. Some cases are linked to genetic mutations, while others result from infections the mother contracted during pregnancy or from developmental problems like spina bifida.
In children and adults, hydrocephalus more commonly develops after another event. Brain hemorrhage (particularly common in premature infants), meningitis, traumatic brain injury, and brain tumors are the leading acquired causes. After a traumatic brain injury, for instance, radiological signs of ventricular enlargement appear in 30 to 86 percent of patients, though not all of these cases become symptomatic. Meningitis causes hydrocephalus in about 5 percent of community-acquired cases, typically by scarring the surfaces where fluid would normally be reabsorbed.
Symptoms by Age
Hydrocephalus looks very different depending on when it develops. In infants, the most visible sign is a rapidly growing head, since the skull bones can still spread apart under pressure. A bulging, tense soft spot (fontanelle) on top of the head is another hallmark. One of the earliest clinical signs is the “setting sun” eye phenomenon, where the baby’s eyes are driven downward so that white sclera is visible above the iris. This downward gaze problem actually appears before the head starts to enlarge noticeably. Other infant symptoms include irritability, a high-pitched cry, poor feeding, vomiting, and visible veins stretched across the scalp.
In older children and adults, the skull can’t expand, so symptoms reflect rising intracranial pressure: persistent headaches, nausea and vomiting, blurred or double vision, difficulty with balance and walking, personality changes, and declining mental sharpness. Urinary incontinence, seizures, and excessive sleepiness can develop as the condition progresses.
How It’s Diagnosed
Brain imaging is the primary tool. CT scans or MRI reveal enlarged ventricles, and doctors use specific measurements to confirm the finding isn’t just normal variation. The most widely used is the Evans Index: the ratio of the widest point of the front horns of the ventricles to the widest inner diameter of the skull. A normal ratio falls between 0.20 and 0.25. Values between 0.25 and 0.30 suggest early or borderline enlargement, and anything above 0.30 indicates definite ventricular enlargement. That 0.30 threshold is the standard diagnostic cutoff used in clinical guidelines for NPH.
Newer measurements supplement the Evans Index. The anteroposterior diameter of the lateral ventricle index (ALVI) measures the ventricles from front to back and, at a threshold above 0.5, may be more effective at detecting enlargement than the Evans Index alone. For suspected NPH, additional tests like a lumbar puncture to remove a volume of fluid can help predict whether a patient will respond to treatment. If symptoms temporarily improve after fluid is drained, surgical treatment is more likely to help.
Treatment: Shunts
The most common treatment is a ventriculoperitoneal (VP) shunt, a system of tubing that drains excess fluid from the brain’s ventricles into the abdominal cavity, where the body reabsorbs it naturally. The system has three components: a thin tube placed inside a ventricle, a valve that controls how much fluid drains, and a longer tube tunneled under the skin down to the abdomen.
The valve is the critical piece. Fixed-pressure valves drain at a set rate, while programmable (adjustable) valves can be tuned after surgery using an external magnetic tool, without another operation. This flexibility is particularly useful in young children, whose fluid dynamics change as they grow. Gravitational and flow-regulated valves are designed to prevent overdrainage, a common problem that happens when standing upright causes too much fluid to siphon out. However, these designs can sometimes underdrain when the patient is lying down, leaving them with persistent headaches and nausea.
No single valve type has proven clearly superior overall. Adjustable valves tend to reduce the need for early repeat surgeries, especially in younger patients. Gravitational valves lower the risk of overdrainage complications like collapsed ventricles and fluid collections under the skull, but may increase the chance of underdrainage.
Treatment: Endoscopic Third Ventriculostomy
For patients with obstructive hydrocephalus, an alternative to shunting exists. In an endoscopic third ventriculostomy (ETV), a surgeon creates a small hole in the floor of the third ventricle, allowing trapped fluid to bypass the blockage and flow directly into the spaces around the brain where it can be reabsorbed. The advantage is that no permanent hardware stays in the body, which eliminates the risk of shunt malfunction and infection over a lifetime.
In one large pediatric series, 74 percent of children remained shunt-free after ETV over the follow-up period. When patients were carefully selected using clinical and radiological criteria, the success rate climbed to 92 percent. About 30 to 35 percent of patients who undergo ETV will eventually need a shunt placed anyway, usually because the opening closes or because the underlying problem turns out to involve absorption rather than obstruction. ETV is generally not effective for communicating hydrocephalus, since the issue in those cases isn’t a blockage.
Signs of Shunt Problems
Shunts are life-sustaining devices, but they can malfunction, and recognizing the signs quickly is important. Blockage is the most common problem. When a shunt stops draining properly, the original symptoms of hydrocephalus return: headaches, nausea, vomiting, vision changes, irritability, and sleepiness. In infants, look for a bulging fontanelle, downward eye deviation, swollen scalp veins, poor feeding, and a high-pitched cry.
Infection produces its own pattern, often including a low-grade fever, soreness in the neck or shoulder muscles, and redness or tenderness along the path of the tubing under the skin. Overdrainage can be tricky to distinguish from underdrainage because the symptoms overlap: both cause headaches that can be severe. Overdrainage headaches often worsen when standing and improve when lying down, while underdrainage headaches tend to be more constant. Any return of hydrocephalus symptoms in someone with a shunt warrants immediate medical evaluation.
Long-Term Outlook
Hydrocephalus is a chronic condition. Even with successful treatment, many patients need lifelong monitoring and periodic shunt revisions. The long-term cognitive and developmental picture varies widely and depends more on the underlying cause and any associated brain conditions than on the hydrocephalus treatment itself. In one study of children with VP shunts, about 37 percent showed age-appropriate development, 17 percent had learning disabilities, and 47 percent had significant developmental delays or cognitive impairment. Roughly 43 percent attended regular schools, while 53 percent required special education.
Interestingly, the type of valve used and the number of shunt revision surgeries did not significantly affect cognitive outcomes or school placement in that same study. The cause of the hydrocephalus, whether from a brain bleed, a tumor, or a congenital malformation, also didn’t predict cognitive trajectory in a statistically meaningful way. What this suggests is that the brain injury driving the hydrocephalus matters more than the mechanical details of how the fluid is managed. For adults with NPH, early diagnosis and treatment offer the best chance of meaningful improvement, particularly in walking ability, which tends to respond better than cognitive symptoms.