Normal pressure hydrocephalus (NPH) is diagnosed through a combination of clinical symptoms, brain imaging, and a spinal fluid drainage test that checks whether removing cerebrospinal fluid improves symptoms. There is no single definitive test. Instead, doctors piece together findings from a neurological exam, an MRI, and one or more fluid-removal procedures to determine whether NPH is the likely cause of a patient’s problems.
The process can feel drawn out, partly because NPH mimics other conditions common in older adults, including Parkinson’s disease and Alzheimer’s. Getting the diagnosis right matters because NPH is one of the few causes of dementia-like symptoms that can actually be reversed with treatment.
The Three Hallmark Symptoms
NPH produces a recognizable cluster of three symptoms: difficulty walking, cognitive decline, and loss of bladder control. All three appear together in roughly 50 to 75 percent of cases. Walking and thinking problems show up in 80 to 95 percent of patients, while urinary symptoms are present in about 50 to 75 percent.
Gait trouble is almost always the first symptom to appear and the most prominent. The walk is wide-based, with feet turned outward and steps that barely clear the floor. People sometimes describe it as feeling like their feet are stuck to the ground, which is why clinicians call it a “magnetic gait.” This pattern differs from Parkinson’s disease, where steps are small but the stance is narrower and the feet point forward. That distinction can be an early clue during a physical exam.
Cognitive changes come next, typically showing up as slowed thinking, forgetfulness, and apathy rather than the language and recognition problems more common in Alzheimer’s. Urinary urgency or incontinence tends to develop last. A patient over 60 who shows at least two of these three symptoms, with no other obvious neurological explanation, meets the threshold for what guidelines call “possible” NPH and warrants further workup.
What the MRI Reveals
Brain imaging is the cornerstone of the diagnostic workup. An MRI (or CT scan if MRI is not an option) looks for ventricular enlargement, meaning the fluid-filled chambers inside the brain are bigger than they should be relative to overall brain size. The standard measurement for this is the Evans index: the ratio of the widest width of the frontal horns of the ventricles to the maximum inner width of the skull at the same level. A value greater than 0.3 confirms ventricular enlargement and is required for an NPH diagnosis.
But enlarged ventricles alone are not enough. The brain naturally loses volume with age, and ventricles can expand simply to fill that space. What sets NPH apart on imaging is a pattern called DESH, or disproportionately enlarged subarachnoid space hydrocephalus. In DESH, the fluid spaces at the base of the brain (around the Sylvian fissures) are widened and pushed upward, while the spaces over the top of the brain are squeezed tight. This mismatch suggests that cerebrospinal fluid is being forced into the wrong areas rather than flowing and absorbing normally.
Radiologists also measure the callosal angle, the angle formed by the two halves of the brain on a coronal MRI slice taken through the corpus callosum. In NPH, pressure from below pushes the brain hemispheres together at the top, narrowing this angle. When the callosal angle falls below 90 degrees, it shows good sensitivity and specificity for NPH and helps distinguish it from Alzheimer’s disease, where the angle stays wider.
The CSF Tap Test
If symptoms and imaging both point toward NPH, the next step is a high-volume lumbar puncture, commonly called a tap test. A needle is placed in the lower spine and 40 to 50 milliliters of cerebrospinal fluid is removed. A trained clinician, often a neurologist or physical therapist, measures the patient’s walking speed, step length, and balance before the procedure and again within the hours or days that follow.
When gait noticeably improves after fluid removal, typically defined as a 20 percent or greater improvement in walking measures, the test is considered positive. A positive result suggests the patient is likely to benefit from a permanent shunt, a surgically placed tube that continuously drains excess fluid from the brain.
The tap test is widely used because it is simple and can be done at most neurological centers. However, it has meaningful limitations. A recent meta-analysis found the test has a pooled sensitivity of about 68 percent and a specificity of only 53 percent. In practical terms, this means a negative tap test does not rule out NPH. Nearly half of patients who showed no improvement on the tap test still benefited from shunt surgery. For this reason, a single tap test alone is not considered sufficient to make or reject a surgical decision in patients whose clinical picture and imaging otherwise support the diagnosis.
Extended Drainage and Infusion Testing
When the tap test is inconclusive or the clinical picture is complicated, specialized centers can perform more involved tests. The most common is extended lumbar drainage, where a small catheter is left in the spine and cerebrospinal fluid is drained continuously over several days while the patient is hospitalized. Because it removes a much larger volume of fluid over a longer period, extended drainage gives a clearer picture of how the brain responds and is considered more sensitive than a single tap test.
Another option is a CSF infusion test, in which fluid is slowly infused into the spinal canal through one needle while a second needle simultaneously measures the pressure response. This gauges how well the brain absorbs cerebrospinal fluid and can reveal abnormal resistance to fluid flow even when resting pressure appears normal.
Both tests carry more risk than a standard lumbar puncture and require specialized nursing and equipment. They are typically available only at tertiary referral centers that manage complex hydrocephalus cases with a multidisciplinary team of neurologists, neurosurgeons, radiologists, and neuropsychologists.
How “Probable” NPH Is Classified
Current guidelines divide the diagnosis into two tiers. “Possible” NPH applies to patients over 60 who have at least two of the three hallmark symptoms, enlarged ventricles with an Evans index above 0.3, and no better-fitting diagnosis. “Probable” NPH requires everything in the possible category plus a lumbar cerebrospinal fluid opening pressure of 200 mm of water or less (confirming the pressure is in the normal range, which is the “normal pressure” in NPH), normal fluid composition, and either imaging consistent with the DESH pattern along with characteristic gait impairment, or measurable clinical improvement after a tap test or extended drainage trial.
The distinction matters for treatment planning. Patients classified as probable NPH have the strongest evidence supporting shunt surgery. Those in the possible category may need additional testing or monitoring before a surgical decision is made.
Why NPH Is Often Missed
NPH is frequently misdiagnosed as Parkinson’s disease, Alzheimer’s, or simply “normal aging.” The overlap in symptoms is substantial: all three conditions can cause slow movement, memory problems, and falls. The wide-based, outward-rotated gait of NPH is a key differentiator from Parkinson’s narrower shuffling pattern, but this distinction requires a clinician who is specifically looking for it.
The cognitive decline in NPH also looks different under neuropsychological testing. It tends to affect processing speed and attention more than memory recall, whereas Alzheimer’s more typically impairs the ability to form new memories. When dementia is the most prominent symptom, NPH can easily be attributed to Alzheimer’s, and the treatable component gets overlooked.
Neurologists play a central role in identifying patients who should be evaluated for NPH and coordinating the diagnostic workup. Complex or ambiguous cases benefit from referral to a center with dedicated hydrocephalus expertise, where the full range of diagnostic tools is available and outcomes from shunt surgery tend to be better.