Hydrocephalus develops when cerebrospinal fluid (CSF) builds up inside the brain’s cavities, called ventricles, faster than the body can drain it. This can happen because something blocks the fluid’s normal path, because the body loses the ability to reabsorb it, or, rarely, because the brain produces too much of it. The result is the same: pressure builds inside the skull, and the ventricles swell. How you get hydrocephalus depends largely on your age and whether the condition was present at birth or triggered later by injury, infection, or disease.
How Cerebrospinal Fluid Normally Flows
Your brain constantly produces a clear fluid that cushions it, delivers nutrients, and carries away waste. This fluid is made inside the ventricles by tissue called the choroid plexus. From there, it flows through a series of narrow channels from the lateral ventricles into the third ventricle, then through a tight passage called the cerebral aqueduct into the fourth ventricle. It then exits through small openings into the space surrounding the brain and spinal cord, where it gets reabsorbed into the bloodstream.
The whole system runs on a pressure gradient: higher pressure where the fluid is produced, lower pressure where it drains. If anything disrupts production, flow, or absorption at any point along this route, fluid accumulates and the ventricles begin to expand.
Congenital Hydrocephalus: Present at Birth
Some people are born with hydrocephalus. Globally, it occurs in roughly 68 to 316 per 100,000 births, depending on the region, with higher rates in low- and middle-income countries. The most common congenital cause is aqueductal stenosis, a narrowing of the tiny passage between the third and fourth ventricles. This is frequently linked to variations in a gene called L1CAM.
Other causes present at birth include brain or spinal cord malformations, bleeding inside the ventricles (which can happen before or during delivery), prenatal infections, and exposure to certain harmful substances during pregnancy. Neural tube defects, including spina bifida, are also associated with hydrocephalus because they can disrupt the normal structure of the brain’s fluid pathways.
Premature Birth and Brain Bleeding
Premature infants face a specific risk. The fragile blood vessels in their developing brains are prone to rupture, causing bleeding into the ventricles. These bleeds are graded from I (mild) to IV (severe). In one prospective study of very preterm infants, none with grade I or II bleeds developed hydrocephalus. Among those with grade III or IV bleeds, 22.4% did. The blood and resulting inflammation can physically block the narrow fluid channels or damage the tissue responsible for reabsorbing fluid, leading to a dangerous buildup.
Acquired Causes in Children and Adults
Hydrocephalus that develops after birth is called acquired hydrocephalus. Several things can trigger it:
- Meningitis. Bacterial or viral infections of the membranes surrounding the brain cause inflammation that can scar and clog the absorption sites where CSF drains back into the bloodstream.
- Traumatic brain injury. Severe skull fractures or brain trauma can cause bleeding and swelling that block fluid pathways. This is sometimes called post-traumatic hydrocephalus.
- Brain hemorrhage. A stroke or ruptured blood vessel can flood the ventricles or surrounding spaces with blood, which physically obstructs CSF flow and triggers inflammation that impairs reabsorption.
- Brain tumors. A tumor growing near one of the narrow passages between ventricles can act like a dam, preventing fluid from moving through.
In all of these cases, the underlying problem is the same: something creates an imbalance between how much CSF is produced and how much is absorbed or drained.
Obstructive vs. Communicating Types
Doctors classify hydrocephalus into two categories based on where the problem occurs, and the distinction matters because it affects treatment.
Non-communicating (obstructive) hydrocephalus happens when the flow of CSF is physically blocked inside the ventricle system itself. The most common site is the cerebral aqueduct, the narrowest point in the pathway. A tumor, a stricture, or blood from a hemorrhage can all cause a blockage here. Because fluid can’t get past the obstruction, the ventricles upstream of the block expand rapidly.
Communicating hydrocephalus means fluid can still move freely between the ventricles, but absorption is impaired after the fluid exits into the space around the brain. This often happens when blood, infection debris, or inflammatory substances damage the arachnoid villi, the tiny structures that act as one-way valves to drain CSF into the bloodstream. Meningitis, severe head trauma, and subarachnoid hemorrhage are common triggers for this type.
Normal Pressure Hydrocephalus in Older Adults
There is a distinct form of hydrocephalus that typically appears in people over 60. In normal pressure hydrocephalus (NPH), the ventricles enlarge even though the fluid pressure, when measured at a single point in time, reads as normal or only mildly elevated. The exact mechanism is still being studied, but a leading theory points to changes in how blood vessels in the brain handle pressure pulses from each heartbeat. As arteries stiffen with age and lose their elasticity, the extra pulse pressure gets transmitted to smaller vessels and ultimately disrupts the normal back-and-forth movement of CSF through the aqueduct. In people with NPH, the net flow of fluid actually reverses direction, pushing CSF back toward the larger ventricles and forcing it to seep into surrounding brain tissue.
NPH produces a recognizable pattern of three symptoms: difficulty walking (usually the first to appear), urinary incontinence, and cognitive decline that can resemble dementia. This combination is sometimes mistaken for Alzheimer’s disease or Parkinson’s disease, which matters because NPH is one of the few causes of dementia-like symptoms that can potentially be treated.
How Hydrocephalus Is Detected
In infants, the most visible sign is a rapidly growing head, since the skull bones haven’t yet fused and can expand under pressure. In older children and adults, the skull can’t expand, so pressure builds internally, causing headaches, nausea, vision problems, difficulty with balance, and changes in thinking or personality.
Brain imaging confirms the diagnosis. On an MRI or CT scan, doctors measure the ratio of ventricle width to skull width, known as the Evans Index. A value of 0.3 or higher has traditionally indicated abnormal ventricular enlargement. In older adults, though, ventricles naturally enlarge somewhat with age, so newer research suggests age-adjusted thresholds. For example, a healthy person in their mid-70s might have an Evans Index of 0.36 or 0.37 without it being pathological. For a diagnosis of NPH specifically, enlarged ventricles must be paired with at least gait problems and either urinary or cognitive symptoms.
Risk Factors That Increase Vulnerability
Certain groups face higher risk. Premature infants, especially those born before 32 weeks, are vulnerable because of the fragility of their brain’s blood vessels. People who have had bacterial meningitis carry an elevated risk even after the infection clears, because the scarring it leaves behind can permanently impair CSF absorption. Anyone who suffers a significant traumatic brain injury or stroke involving bleeding in or around the brain is at risk in the weeks and months that follow. And older adults with vascular disease and stiffening arteries are the population most likely to develop NPH.
There is no single lifestyle factor that causes hydrocephalus. It is not contagious, and in most cases it cannot be prevented. The congenital forms are driven by genetics and prenatal development, while the acquired forms are consequences of other medical events. Understanding how each type develops helps explain why treatment approaches differ and why early recognition of symptoms matters at every age.