Idiopathic Normal Pressure Hydrocephalus: Causes and Care

Idiopathic normal pressure hydrocephalus (iNPH) is a progressive brain condition in which the fluid-filled spaces inside the brain, called ventricles, gradually enlarge and press on surrounding tissue, even though the fluid pressure itself measures in the normal range on a standard spinal tap. The word “idiopathic” means no clear trigger, such as a head injury or brain bleed, can be found. It strikes older adults almost exclusively and produces a recognizable triad of trouble walking, cognitive decline, and urinary problems. The condition is widely considered one of the few treatable causes of dementia-like symptoms, yet it remains dramatically underdiagnosed.

The Classic Symptom Triad

Gait trouble is the hallmark. People with iNPH develop a broad-based, shuffling walk with difficulty turning, and it tends to be the first symptom to appear. Roughly 85 to 95 percent of patients show gait impairment at the time of diagnosis.¹ Friends and family often describe it as a magnetic gait, as though the person’s feet are stuck to the floor. Falls become frequent, and many patients begin using a walker or wheelchair before anyone suspects a treatable condition.

Urinary dysfunction appears in about 75 to 90 percent of patients and usually involves urgency and incontinence rather than difficulty starting a stream.¹ The problem originates from the brain’s impaired control over the bladder muscle, not from a prostate or pelvic-floor issue, which is one reason it gets overlooked or attributed to aging.

Cognitive impairment is reported in 60 to 80 percent of patients. Unlike Alzheimer’s disease, which tends to erode memory for recent events early on, iNPH preferentially dulls attention, processing speed, and the ability to plan and shift between tasks.¹ This frontal-subcortical pattern of slowing is an important clue, though in practice the distinction from other dementias is far from easy.

How Common Is iNPH and Why Does It Go Undiagnosed?

Population studies consistently find that iNPH is far more prevalent than clinical referral rates suggest. A Swedish study of people aged 80 and older found probable iNPH in nearly 6 percent of participants, yet only 2 of those 26 identified individuals had ever been treated.² A Norwegian population study pegged prevalence at about 3.7 percent in adults over 65, with the rate climbing to nearly 9 percent in those over 80.³ Similar underdiagnosis has been reported in Chinese urban populations.⁴

The gap between how many people have iNPH and how many get treated is enormous. One reason is that the symptoms overlap heavily with more familiar conditions. Shuffling gait looks like Parkinson’s disease. Cognitive slowing looks like Alzheimer’s or vascular dementia. Incontinence gets chalked up to age. Primary care providers may never order the brain scan that would reveal the enlarged ventricles. By the time a specialist sees the patient, years of treatable decline may have passed.

What Actually Causes It

The honest answer is that nobody fully knows, which is precisely what “idiopathic” means. But research over the past two decades has converged on a few overlapping mechanisms that explain how cerebrospinal fluid (CSF) dynamics go wrong in these patients.

Normally, CSF is produced inside the ventricles, flows outward through the brain, and gets absorbed at several sites, including structures on the brain’s surface and a recently described waste-clearance network called the glymphatic system. In iNPH, this absorption appears to break down. Brain imaging studies in iNPH patients have shown reduced flow of contrast agent into and out of the spaces surrounding blood vessels, suggesting that glymphatic clearance is impaired.⁵ A separate study using diffusion-based MRI found significantly lower glymphatic activity in iNPH patients compared to healthy controls.⁶

One theory tying these observations together proposes that many iNPH patients were born with slightly reduced capacity to absorb CSF through the traditional surface pathway. For most of their lives, the glymphatic route picks up the slack. Late in life, when small-vessel disease damages the deep white matter, that backup pathway becomes blocked. The damaged white matter loses its normal lipid coating, and the exposed protein attracts CSF in a way that increases resistance to flow, causing fluid to accumulate.⁷ This model neatly explains why iNPH is a disease of aging: the original vulnerability is present for decades, but the trigger only arrives when vascular damage accumulates.

The Vascular Connection

Hypertension appears strikingly overrepresented in iNPH. One case-control study found that 83 percent of iNPH patients had high blood pressure, compared to 36 percent of age-matched controls. In a multivariate analysis adjusting for other risk factors, hypertension was the only vascular factor that remained significantly linked to iNPH. The same study found a positive association between the severity of clinical symptoms and the presence of hypertension, particularly for gait impairment.⁸ This fits the white-matter-damage theory: chronic high blood pressure accelerates small-vessel disease, which in turn compromises the glymphatic backup pathway.

That said, intracranial pressure dynamics in most iNPH patients appear to be driven more by local brain physiology than by blood-pressure swings. One study found that in about three-quarters of patients with elevated intracranial pressure wave amplitudes, those amplitudes were not correlated with central vascular pulsation, suggesting the problem is predominantly within the brain’s own fluid compartments rather than just a reflection of systemic cardiovascular disease.⁹

Genetic Susceptibility

iNPH is not a single-gene disorder, but genetics clearly play a role. Genome-wide studies, exome sequencing, and copy-number analyses have now identified more than a dozen risk loci. These genes cluster around pathways that matter for the blood-brain barrier, CSF-producing cells, and the hair-like cilia that help move fluid through brain cavities.¹⁰ A large Finnish cohort study pinpointed four loci that reached genome-wide significance in iNPH specifically, including protective variants near the genes SLCO1A2 and MLLT10.¹¹

One of the most concrete genetic findings so far involves a partial deletion within the SFMBT1 gene. About a quarter of patients with shunt-responsive definite iNPH carried this deletion, compared to roughly 4 percent of healthy elderly controls, yielding an almost eightfold increase in odds.¹² None of these findings are ready for clinical screening, but they reinforce the idea that certain people are biologically predisposed to develop iNPH as they age.

How iNPH Is Diagnosed

Diagnosis rests on a combination of clinical symptoms, brain imaging, and a CSF drainage trial. No single test confirms iNPH beyond doubt, which is one reason the condition sparks so much debate among neurologists and neurosurgeons.

On MRI, clinicians look for ventricles that are enlarged out of proportion to any general brain shrinkage. The Evans Index, a simple ratio of ventricular width to skull width, is the most commonly used measure. One study found that combining an Evans Index above 0.30 with a narrow callosal angle (under 90 degrees) distinguished iNPH from Alzheimer’s disease with about 96 percent accuracy.¹³ Still, imaging alone cannot make the diagnosis. It narrows the field and raises suspicion.

Clinical overlap with other neurodegenerative disorders is the biggest diagnostic hurdle. Alzheimer’s, vascular dementia, and parkinsonian syndromes, particularly progressive supranuclear palsy and dementia with Lewy bodies, can all mimic iNPH.¹⁴ The strongest clinical clue favoring iNPH is when gait disturbance appears first and dominates the picture, rather than cognitive or behavioral changes leading the way.

The Tap Test and Extended Lumbar Drainage

The most direct way to gauge whether someone will improve with surgery is to remove a sample of CSF and watch what happens to their walking. This is called the tap test. It can be done in an office with a standard lumbar puncture needle. Interestingly, one study found no relationship between the volume of CSF removed and subsequent gait improvement, suggesting that even a relatively modest removal can reveal a positive response.¹⁵

For cases where the single tap is ambiguous, extended lumbar drainage over several days provides more information. In one series, a positive response to extended drainage correctly predicted improvement after shunt surgery in about 88 percent of patients.¹⁶ However, extended drainage requires a short hospital stay and carries its own risks, so it is typically reserved for patients in whom the clinical picture is uncertain.

CSF Biomarkers

Researchers have been hunting for spinal-fluid markers that can separate iNPH from its mimics. A promising pattern has emerged: in iNPH, a protein marker of white-matter damage called neurofilament light (NFL) tends to be elevated, while amyloid and tau proteins are lower than in both healthy controls and patients with other dementias.^{17 18} This combination may reflect the fact that in iNPH, the primary damage hits white matter tracts rather than the cortical neurons that accumulate amyloid and tau in Alzheimer’s. Biomarker panels are not yet routine in clinical practice, but they represent a promising direction for reducing diagnostic uncertainty.

Shunt Surgery

The primary treatment for iNPH is a surgically implanted shunt, most commonly a ventriculoperitoneal (VP) shunt, which diverts excess CSF from the brain’s ventricles to the abdominal cavity where it gets reabsorbed. The operation itself is relatively straightforward by neurosurgical standards and typically takes under an hour. The bigger questions are which patients will benefit and how to manage the shunt over time.

A key decision involves the type of valve placed in the shunt. Programmable valves, whose resistance setting can be adjusted from outside the body using a magnetic device, have become the standard at many centers. A study comparing programmable to fixed-pressure valves found that patients receiving a programmable valve had lower overall revision rates (about 13 percent versus 24 percent) and were far less likely to need a complete valve swap during follow-up.¹⁹ Costs did not differ significantly between the two valve types in that analysis, which makes the programmable option appealing from both a clinical and economic standpoint.

One retrospective study, however, reported somewhat different complication profiles, with the programmable-valve group experiencing numerically higher infection and revision rates, though those differences did not reach statistical significance, and the sample was smaller.²⁰ The balance of evidence still favors programmable valves for most patients because they allow clinicians to fine-tune drainage without additional surgery.

Complications of Shunting

The most worrisome complication is overdrainage, which can cause the brain to pull away from the skull and produce subdural fluid collections or hematomas. In one series, about 5 percent of patients developed subdural hematomas serious enough to require surgery. Patients whose opening pressure on the preoperative lumbar puncture was above 160 mm of water were at substantially higher risk of radiological overdrainage (38 percent versus 21 percent).²¹ This is one reason that knowing the baseline opening pressure matters and why programmable valves, which can be turned up to slow drainage, are so useful.

A 17-year single-center review tracking different valve generations found subdural hematomas requiring surgery in about 4 percent of patients overall, with valve revisions performed in roughly 16 percent of all shunted patients across the full follow-up period.²² These numbers are not trivial, especially in an elderly population, but they need to be weighed against the progressive decline that untreated iNPH causes.

Endoscopic Third Ventriculostomy as an Alternative

Some neurosurgeons have explored endoscopic third ventriculostomy (ETV), a procedure that creates a small opening in the floor of the third ventricle to reroute CSF flow without implanting permanent hardware. A systematic review and meta-analysis that pooled available evidence, though from only a handful of studies, found no significant difference in improvement rates between ETV and VP shunting (about 68 percent versus 73 percent). However, the complication rate was markedly lower with ETV, at roughly 8 percent compared to about 51 percent for VP shunts.²³ The evidence base is thin, and ETV has not become the standard of care for iNPH, but the lower complication profile makes it worth discussing with a neurosurgeon, especially for patients who are poor candidates for implanted hardware.

Rehabilitation After Surgery

Shunt surgery can produce dramatic gait improvement within days, but residual deficits are common, and targeted rehabilitation appears to help. A randomized controlled trial found that a structured dual-task exercise and gait training program produced significantly better gait-and-balance recovery than standard exercise or no intervention, with lower fall rates at follow-up.²⁴ Another trial testing a high-intensity functional exercise program after shunt surgery did not find a statistically significant boost to overall outcomes on the iNPH symptom scale, but it did show long-term balance improvements and higher goal achievement among participants who completed the program.²⁵

The practical takeaway is that surgery is not the finish line. People recovering from iNPH benefit from working with physical therapists on gait retraining, balance exercises, and fall prevention. The brain needs help rebuilding motor patterns that may have degraded over years of illness.

Is Shunt Surgery Worth the Cost?

For a condition that mainly affects people in their 70s and 80s, healthcare systems reasonably ask whether the expense of surgery plus lifelong shunt management is justified. Two independent economic analyses came to the same conclusion: yes. A European cost-utility model found that shunt surgery added roughly 2.2 life-years and 1.7 quality-adjusted life-years per patient, at an incremental cost of about €7,500 per quality-adjusted life-year gained, well below standard thresholds for cost-effective care.²⁶ A Japanese analysis based on two multicenter trials estimated that the total cost of caring for an iNPH patient showed a positive return on investment within about 18 months of VP shunt placement, driven largely by reduced need for institutional care.²⁷

Beyond the economics, there is meaningful relief for families. Caregiver burden scores drop substantially after successful shunting. Data from the SINPHONI trial showed that improvement in cognitive function was the single biggest driver of reduced caregiver burden, presumably because cognitive decline is the symptom that creates the most day-to-day dependency.²⁸

Brain Stiffness and What MR Elastography Reveals

One of the more intriguing recent findings involves the mechanical properties of the brain itself. MR elastography, a specialized MRI technique that measures tissue stiffness, has consistently shown that brains affected by iNPH are stiffer than normal. One study found significantly increased stiffness in the cerebrum overall and in the occipital, parietal, and temporal lobes.²⁹ A follow-up study confirmed increased parenchymal stiffness across similar brain regions but found decreased stiffness in the tissue immediately surrounding the ventricles, likely reflecting the mechanical stretching and damage in that zone.³⁰

These stiffness changes may help explain a longstanding puzzle: why iNPH patients have enlarged ventricles despite normal fluid pressure. If the brain tissue itself becomes less compliant, even normal-range pressures could be enough to push the ventricles outward over time. MR elastography is not yet a standard diagnostic tool for iNPH, but it could eventually help clinicians identify patients earlier or predict who will respond to shunting.

Telemetric Monitoring and Emerging Technology

Managing a shunt over years requires knowing whether it is working correctly, and traditionally that means waiting for symptoms to change, then ordering imaging or invasive testing. Telemetric intracranial pressure sensors, implanted alongside or built into the shunt system, offer continuous monitoring without repeated hospital visits. A systematic review found growing adoption of these devices and evidence that they improve diagnostic accuracy and allow earlier adjustment of shunt settings, potentially reducing emergency admissions and invasive testing.³¹ The technology still faces hurdles including high upfront cost and sensor calibration drift over time, but it represents a meaningful step toward proactive rather than reactive shunt management.

For patients and families, the practical implication is that iNPH care is moving beyond the binary of “shunt or no shunt.” Advances in valve design, remote monitoring, and rehabilitation science are gradually transforming the condition from one that is diagnosed too late and managed crudely into one where early detection and individualized adjustment are within reach. The bottleneck remains awareness: if primary care providers and families learn to recognize the gait-predominant symptom pattern and push for imaging, far more people stand to benefit from treatments that already exist.