Wet. Wacky. Wobbly. Three words describe one of the only treatable dementias in medicine. Learn the mechanism, the exam trick, and how to tell it from Parkinson, Alzheimer, and cerebellar disease.
WetWackyWobbly
The Wet-Wacky-Wobbly Triad
Three problems, one disease, one treatable cause. Tap each card to see the full clinical picture for each part of the triad.
Wet
Urinary Incontinence
Urgency and urge incontinence: the patient cannot hold urine when the urge hits. Frontal lobe and corona radiata fibers that normally suppress the bladder detrusor are impaired by the stretched periventricular white matter. The bladder contracts before the patient can respond. This is a cortical uninhibited bladder, not a bladder problem.
Wacky
Cognitive Impairment
A frontal-subcortical pattern: slowed processing, poor executive function (planning, sequencing, shifting attention), and psychomotor slowing. Unlike Alzheimer disease, early NPH is not an amnestic dementia. Memory is often relatively spared early. The frontal fibers are compressed, so the patient looks slow and disorganized, not confused and forgetting faces.
Wobbly
Gait Apraxia
The magnetic gait: wide-based, shuffling, short steps, difficulty initiating movement, feet appear glued to the floor. The key exam finding: the patient walks poorly but can make near-normal stepping movements when seated, because the motor cortex and cerebellum are intact. The problem is in the descending frontal fibers, not in the legs themselves.
From the Attending
The triad does not always appear together, and the gait is usually first. A patient who comes in for memory problems but shuffles into the room and mentions new leaky bladder should immediately get brain imaging. The question is not whether they have dementia: it is whether they have the one dementia with a surgical fix. Gait first, incontinence second, cognitive last is the usual order of onset. When you see all three, the ventricular system on imaging is your next move.
Gait apraxia that normalizes when seated is the one exam finding that separates NPH from everything else on the differential. Know it cold.
The Mechanism: Why the Ventricles Injure White Matter
NPH is not about the ventricles themselves. It is about what the enlarged ventricles do to the tissue around them.
Step by Step
1
CSF accumulates in the ventricular system. In NPH, CSF absorption at the arachnoid granulations is impaired. Pressure does not build sky-high (hence "normal pressure" on LP opening pressure), but over time the ventricles slowly expand.
2
Enlarged ventricles stretch the corona radiata. The corona radiata is the fan of white matter fibers that descend from the motor cortex through the deep hemispheres. The fibers serving the legs and bladder run closest to the medial wall of the lateral ventricles, exactly where the CSF pressure is applied.
3
Stretched fibers lose their ability to transmit signals reliably. The cortex is intact. The legs are intact. The cerebellum is intact. But the cables running between them are physically stretched and compressed by the expanding fluid balloon surrounding them.
4
The result: frontal disconnection syndrome. The frontal cortex cannot drive the legs efficiently for walking (gait apraxia), cannot suppress the bladder detrusor (urgency incontinence), and cannot maintain processing speed and executive function (frontal cognitive impairment).
5
VP shunt relieves the pressure. Diverting CSF away from the ventricles reduces the balloon effect. The stretched corona radiata fibers can recover. In good candidates, gait improves first and most dramatically, followed by urinary control, then cognition. This recovery sequence mirrors the injury sequence.
From the Attending
The exam classic: seated stepping test. Sit the patient down and ask them to demonstrate walking with their legs. The legs move nearly normally. Now stand them up: the gait is magnetic and shuffling. That contrast proves the motor cortex and cerebellum are working. The wiring between them is what is compressed. That is a deep white matter problem, not a cerebellar problem. If the cerebellum were damaged, the legs would be just as dyscoordinated seated as standing. The seated normalization is your anatomical fingerprint.
Ventriculomegaly out of proportion to atrophy on imaging + the seated stepping test = NPH until proven otherwise. Every time.
Diagnosis and Workup
Two tests do most of the work: brain imaging and the tap test.
Imaging
CT
First step. Look for enlarged lateral and third ventricles that are disproportionate to any cortical atrophy. The key word is "disproportionate": if the ventricles are big just because the brain has shrunk (as in AD or aging), that is not NPH. NPH ventricles are too large for the amount of atrophy present. CT also rules out obstructive causes (tumor, aqueductal stenosis).
MRI
Better detail. Shows periventricular signal changes (transependymal flow of CSF), aqueductal flow void on T2 (high-velocity CSF jet), and the Evans index (ratio of lateral ventricle width to brain diameter at the same level, abnormal above 0.3). MRI also characterizes white matter injury and distinguishes NPH from communicating versus obstructive hydrocephalus.
LP
High-volume tap test: the key diagnostic and predictive test. Remove 30 to 40 mL of CSF. Reassess gait within 30 to 60 minutes. Improvement in gait predicts a good VP shunt response with sensitivity around 70%. Opening pressure will be in the normal range (5 to 18 cmH2O), which is the paradox of NPH. Normal pressure does not mean the CSF is not causing harm, only that intermittent pressure waves (B-waves) rather than sustained elevation drive the injury.
From the Attending
The tap test result is the most important single predictor of surgical outcome. A patient who improves meaningfully after 40 mL is drained is a shunt candidate. A patient who does not improve is either not NPH or has irreversible white matter injury from years of untreated compression. Operate early: the longer NPH is untreated, the more permanent the damage.
Tap test gait improvement = shunt it. Know the logic, know the test, know the anatomical reason gait is first to respond.
Dementia + Gait: Who Is It?
Four diagnoses. One tree. Tap through the discriminators to land on the right answer. The leaf tells you the diagnosis and the one finding that seals it.
Step 1: What was the first or most prominent presenting symptom? (Read all three options before picking.)
Pick the one that best fits the vignette you are working through.
ALZHEIMER DISEASE. The hippocampus fails first. Early amnestic dementia is the signature: forgetting recent events, names, getting lost. Gait is typically preserved until late in the disease. No ventriculomegaly out of proportion to atrophy on imaging. No tap-test response. No shunt benefit.
VASCULAR DEMENTIA. Stepwise decline tied to ischemic events, not a smooth progression. Each step corresponds to a new infarct. The discriminator: focal neurological signs at each step + vascular risk factor history (HTN, DM, smoking, AF) + infarcts or white matter lesions on imaging. Not a gradual downhill slope; it drops in stairs.
Gait first, or gait alongside mild frontal slowing, without prominent early amnesia. Continue to Step 2.
Step 2: What are the dominant motor findings?
PARKINSON DISEASE. Resting tremor that stops with movement is Parkinson until proven otherwise. The triad: resting tremor + bradykinesia + cogwheel rigidity. The substantia nigra loses dopaminergic neurons. Responds to levodopa. The gait is shuffling, but it features a resting pill-rolling tremor and rigidity, not the magnetic stuck-to-floor pattern with seated normalization of NPH.
Seated near-normal leg movement is the anatomical fingerprint of NPH. The motor cortex and cerebellum are intact. The corona radiata, the white matter cable between them and the legs, is what is compressed by the enlarged ventricles. The cortex can drive the legs when upright balance demands are removed. Continue to Step 3.
Step 3: What do imaging and the tap test show?
NORMAL PRESSURE HYDROCEPHALUS. Mechanism: impaired CSF absorption at the arachnoid granulations expands the ventricles, which stretches the periventricular corona radiata and compresses the descending leg and bladder fibers. Treatment: VP shunt. The tap test (30-40 mL LP, then re-assess gait within 1 hour) predicts shunt response with about 70% sensitivity. Gait improves after the tap = shunt it.
Ex vacuo hydrocephalus: the brain has shrunk from another cause (Alzheimer, chronic alcohol, aging), and the ventricular space fills the void. Ventricles are large because the brain is small, not because of CSF pressure. The periventricular white matter is not compressed. No shunting benefit. The tap test is negative because removing CSF does not change the fundamental brain-shrinkage problem.
NPH Triad: Flip the Cards
Tap each card to see the mechanism behind the symptom.
WetTap to flip
Urinary urgency incontinenceFrontal and corona radiata fibers normally suppress the bladder detrusor. When periventricular white matter is compressed, that suppression fails. The detrusor contracts before the patient can respond. This is a cortical uninhibited bladder, not a bladder or sphincter problem. Wet first = same fibers that drive the legs also inhibit the bladder, and both are failing.
WackyTap to flip
Frontal-subcortical cognitive impairmentSlowed processing, poor executive function, psychomotor slowing. Not an amnestic dementia. Memory is relatively spared early because the hippocampus is not the direct target. The compressed frontal fibers make the patient look slow and disorganized, not confused and forgetful. This frontal pattern is what separates NPH cognition from Alzheimer cognition on boards.
WobblyTap to flip
Magnetic gait + seated normalizationWide-based, shuffling, short steps, feet stuck to floor. The seated test: ask the patient to walk with their legs while seated. Movements are near-normal. Proves the motor cortex and cerebellum are intact. Only the white matter cable between them is compressed. Gait normalizes when seated = corona radiata injury, not cortex, not cerebellum.
Clinical Walkthrough
Board-style vignettes. Tap an answer to see the full explanation.
Medically reviewed by Kaitlyn Cocuzzo, MD and Fatima Ali, DO · Last reviewed June 2026
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