Normal pressure hydrocephalus (NPH) results from a buildup of cerebrospinal fluid (CSF) in the brain’s ventricles, the hollow chambers deep inside the brain. What makes it puzzling is that the fluid pressure, when measured, reads as normal or only slightly elevated. In most cases, the underlying cause is never identified, though several mechanisms involving aging blood vessels, impaired fluid drainage, and prior brain injuries are well supported.
Two Types With Different Origins
NPH falls into two categories. The more common form, called idiopathic NPH, has no clear triggering event. It develops gradually, typically in people over 60, and researchers believe it stems from slow, age-related changes in how the brain circulates and absorbs fluid. A population-based MRI study of 70-year-olds in Sweden found the prevalence of possible idiopathic NPH at about 1.5%, with men affected roughly twice as often as women. Reported prevalence rates among people 65 and older range from 0.5% to nearly 9%, depending on the diagnostic criteria used.
Secondary NPH, on the other hand, follows a known medical event that disrupts fluid flow. Common triggers include bleeding in or around the brain (from a ruptured aneurysm or other hemorrhage), traumatic brain injuries, brain infections like meningitis or encephalitis, stroke, and brain tumors. Even injuries caused by previous brain surgeries can set it off. In each of these cases, the event leaves behind scarring or inflammation that interferes with the brain’s ability to reabsorb cerebrospinal fluid.
How Fluid Builds Up Despite Normal Pressure
Your brain constantly produces cerebrospinal fluid, which cushions the brain, delivers nutrients, and carries away waste. Normally, the fluid circulates through the ventricles, flows over the brain’s surface, and is reabsorbed into the bloodstream through small structures called arachnoid granulations. In NPH, the reabsorption process slows down. Fluid accumulates in the ventricles, gradually stretching them larger than they should be.
The paradox that gives NPH its name is that this enlargement happens without the dramatic pressure spike you’d expect. In the 1960s, the physicians who first described the condition found CSF pressures of just 150 to 180 mm of water in their patients, well within the normal range. One explanation, proposed by neurosurgeon Salomón Hakim, is what he called the “hydraulic press effect.” Even at normal pressure, enlarged ventricles have a much greater surface area. That means the total force pushing outward against brain tissue is significant, even though the pressure per unit of area looks unremarkable on a spinal tap.
For the ventricles to expand inside a rigid adult skull, something else has to give. The brain compensates by losing volume in other compartments: a slight reduction in blood volume within the brain’s vessels, shrinkage of the fluid-filled spaces on the brain’s outer surface, or changes in the brain tissue itself, including loss of myelin (the insulation around nerve fibers) and cellular shrinkage. This is why NPH produces real neurological symptoms even though a pressure reading may look reassuringly normal.
The Role of Aging Blood Vessels
One of the most compelling theories for idiopathic NPH centers on arterial stiffness, the same process behind high blood pressure and cardiovascular disease in older adults. Healthy, flexible arteries in the brain act as shock absorbers. With each heartbeat, a pulse of blood enters the skull, and compliant arteries expand slightly to buffer that pulse. The brain tissue, fluid, and venous drainage system all work together to dampen the impact.
When arteries stiffen from aging or atherosclerosis, they lose that cushioning ability. The force of each heartbeat travels further downstream into smaller vessels, capillaries, and veins. This disrupts the delicate balance of fluid movement in and around the brain. Research shows that in NPH, this loss of compliance produces exaggerated back-and-forth CSF motion through the narrow passages connecting the ventricles, particularly during each heartbeat cycle. The result is a gradual distortion of normal fluid dynamics that, over months or years, leads to enlarged ventricles.
Reduced venous drainage compounds the problem. Studies have found that people with idiopathic NPH have significantly narrowed cerebral veins and sinuses compared to healthy people of the same age. When venous outflow is restricted, pressure rises inside the brain’s blood vessels, which in turn makes it harder for cerebrospinal fluid to be reabsorbed through its normal pathways. The system essentially backs up.
Waste Clearance Breaks Down
The brain has its own waste removal network, sometimes called the glymphatic system, that uses cerebrospinal fluid flowing along the outside of blood vessels to flush metabolic byproducts out of brain tissue. This system depends on healthy arterial pulsations to drive fluid through the spaces between cells. When arteries stiffen, the pump weakens.
In NPH, this waste clearance system appears to malfunction. The buildup of waste products in brain tissue may increase resistance to fluid outflow, which could partly explain why cerebrospinal fluid reverses direction, flowing back into the ventricles instead of draining out of the brain. This creates a vicious cycle: poor clearance leads to more fluid accumulation, which leads to more tissue compression, which further impairs clearance.
How Enlarged Ventricles Damage the Brain
As the ventricles expand, they compress the surrounding brain tissue, particularly the deep white matter that connects different brain regions. This compression squeezes blood vessels, reducing blood flow to these areas. The tissue becomes starved of oxygen, a condition called ischemia. This is especially damaging to the nerve fibers that control walking and bladder function, which run close to the ventricles.
NPH produces a characteristic triad of symptoms: difficulty walking (often a slow, wide-based, shuffling gait), urinary incontinence, and cognitive decline resembling dementia. Gait problems are usually the earliest and most prominent symptom. The combination of mechanical compression, reduced blood flow, and inflammatory changes in the tissue surrounding the ventricles accounts for this specific pattern. Because the damage starts with compression rather than cell death, it is often at least partially reversible with treatment, which is what makes NPH distinct from degenerative conditions like Alzheimer’s disease.
Genetics and Family History
NPH can run in families. At least 32 families with two or more affected members have been documented. In studies from Canada and Finland, 10% to 16% of NPH patients had a relative who showed at least two of the three hallmark symptoms. Genomic research has identified risk variants in or near 15 genes and protective variants near two others. Many of the implicated genes are tied to the blood-brain barrier, the blood-CSF barrier, and the function of ependymal cells, the ciliated cells that line the ventricles and help move cerebrospinal fluid.
Dysfunction of these cilia may impair the coordinated flow of fluid through the ventricular system. Other cilia on these cells act as sensors that detect mechanical forces, and when those sensors malfunction, the signaling pathways that maintain healthy ventricle size can go awry. While no single gene has been identified as a definitive cause, the evidence points to a genetic component that at minimum increases a person’s susceptibility.
Why It Often Goes Unrecognized
Because the three core symptoms of NPH, walking difficulty, incontinence, and memory problems, overlap heavily with normal aging, Parkinson’s disease, and Alzheimer’s disease, NPH is frequently missed or attributed to something else. The diagnosis typically requires brain imaging showing enlarged ventricles (measured by the Evans index, a ratio of ventricle width to skull width, where a value above 0.3 indicates enlargement) combined with the right clinical picture. Not all three symptoms need to be present. Many people initially develop only gait changes, with the other symptoms appearing later. The fact that NPH is treatable, usually with a surgically placed shunt to drain excess fluid, makes recognizing its causes and risk factors especially important.