Hydrocephalus is a condition where cerebrospinal fluid builds up inside a baby’s brain, causing the fluid-filled spaces (ventricles) to swell and put pressure on surrounding brain tissue. About 1 in 770 babies develop hydrocephalus, making it one of the more common congenital brain conditions. It can be detected before birth or become apparent in the weeks and months after delivery, and it almost always requires treatment.
How Fluid Builds Up in the Brain
The brain constantly produces cerebrospinal fluid, a clear liquid that cushions the brain, delivers nutrients, and carries away waste. In a healthy baby, this fluid flows through a series of connected chambers (ventricles) inside the brain, then out into the space surrounding the brain and spinal cord, where it gets reabsorbed into the bloodstream. Hydrocephalus happens when something disrupts this cycle.
There are two main ways the disruption occurs. In obstructive hydrocephalus, a physical blockage inside the ventricles prevents fluid from flowing through. The blockage is often at a narrow channel connecting the third and fourth ventricles, a passage so small that even minor abnormalities can seal it off. In communicating hydrocephalus, fluid flows through the ventricles normally but can’t be reabsorbed once it exits. This is typically caused by inflammation or scarring in the membranes surrounding the brain, or by elevated pressure in the veins that would normally drain the fluid away.
What Causes It
Hydrocephalus in babies can be congenital (present at birth) or acquired shortly after. The most common congenital causes include structural brain malformations that form during fetal development, narrowing of the internal brain passages, and spina bifida, which frequently involves hydrocephalus because the brain’s anatomy is altered from early in development.
Genetics play a role in some cases. One well-studied genetic cause involves mutations in the L1CAM gene, which produces a protein essential for brain cells to connect and communicate. When this gene is mutated, the narrow channel between brain ventricles can become blocked, trapping fluid. This form is X-linked, meaning it primarily affects boys: mothers can carry the mutation without symptoms, while sons who inherit it develop hydrocephalus along with other neurological problems.
Acquired hydrocephalus in newborns most often results from bleeding inside the brain (intraventricular hemorrhage), which is particularly common in premature infants. The blood and resulting inflammation scar the fluid-absorption pathways, leading to buildup. Infections like meningitis can cause similar scarring.
Signs Parents Notice
The most visible sign in infants is a rapidly growing head. Because a baby’s skull bones haven’t yet fused together, rising fluid pressure pushes the bones apart rather than compressing the brain immediately. This means the head can enlarge noticeably over days or weeks. The soft spot on top of the head (fontanelle) may bulge outward and feel tense rather than slightly sunken or flat.
Other signs include a downward gaze where the baby’s eyes seem fixed toward the floor, sometimes called “sunsetting” because only the upper part of the iris is visible. Babies may become unusually irritable, feed poorly, or seem excessively sleepy. Vomiting without an obvious cause, particularly when paired with other symptoms, can also signal rising pressure inside the skull.
How It’s Diagnosed
Hydrocephalus is sometimes detected before birth during routine ultrasound. Doctors measure the width of the brain’s ventricles at a specific point. Normal ventricles measure under 10 mm across. Mild enlargement falls between 10 and 12 mm, moderate between 12 and 14 mm, and severe is 15 mm or greater. Detection can occur as early as the second trimester, though some cases don’t become apparent until the third trimester or after birth.
After birth, if a baby’s head is growing faster than expected on growth charts, doctors typically order an ultrasound through the fontanelle or an MRI to see the ventricles directly. These images reveal whether the ventricles are enlarged, where any blockage might be, and whether there are underlying brain malformations contributing to the problem.
Treatment Options
Hydrocephalus doesn’t resolve on its own. The goal of treatment is to redirect excess fluid away from the brain, and there are two main approaches.
The most established treatment is a shunt, a thin tube placed surgically inside one of the brain’s ventricles. The tube runs under the skin, typically down to the abdomen, where the excess fluid drains and gets absorbed by the body. Shunts are effective but come with ongoing maintenance concerns. In a ten-year review of infants who received shunts, mechanical malfunction occurred in about 23.5% of cases, and infection within the first year affected roughly 10%. These complications require additional surgery to repair or replace the shunt. Children with shunts need monitoring throughout their lives because the devices can fail at any age.
A newer alternative is a procedure that creates a small opening in the floor of one of the brain’s ventricles, allowing fluid to bypass the blockage and reach the absorption surfaces naturally. When combined with a technique that reduces the amount of fluid the brain produces, this approach can eliminate the need for a shunt entirely. Success rates are higher in babies older than one month and in those whose hydrocephalus is caused by a clear structural blockage rather than by bleeding from prematurity. For premature infants with hemorrhage-related hydrocephalus, particularly those under one month of corrected age, this procedure is less likely to work.
Warning Signs of Shunt Problems
If your baby has a shunt, recognizing malfunction early is critical. A national survey of 228 caregivers found that the three most common symptoms during confirmed shunt malfunctions were vomiting (23.1%), irritability (20.8%), and excessive sleepiness (17.2%). Other reported signs included poor feeding, a full or bulging fontanelle, decreased consciousness, changes in eye movements, and head swelling.
One challenge caregivers face is that these same symptoms are also the most common during false alarms: vomiting, irritability, and sleepiness topped both lists. Some caregivers described subtler cues like their child “not acting like themselves,” changes in skin color, a visibly distended vein on the scalp, or symptoms that came and went over weeks. Because shunt failure can become dangerous quickly, most pediatric neurosurgeons recommend erring on the side of getting checked.
Long-Term Outlook
The long-term picture for children treated for hydrocephalus varies widely depending on the underlying cause, whether complications like infections occurred, and how many surgeries were needed. In a long-term follow-up study of 137 patients who received shunts during childhood, 69% of survivors completed secondary school, but only about 52% were working in the open job market as adults. Roughly 48% had verbal intelligence within the normal range, while 29% had a severe intellectual disability.
Shunt infections, epilepsy, and the total number of shunt operations were all significantly associated with lower cognitive outcomes. Even among patients with average or above-average verbal intelligence, detailed neuropsychological testing revealed specific deficits in areas like processing speed, memory, or executive function. These subtler difficulties can affect school performance and daily life even when overall intelligence appears normal.
Early treatment, minimizing infections, and ongoing developmental support all improve the trajectory. Many children with hydrocephalus attend regular schools, participate in activities, and live independently as adults, particularly when the condition is caught early and managed without major complications. Regular neuropsychological assessments can help identify specific learning needs so that support is tailored rather than generic.