# Normal Pressure Hydrocephalus > Normal pressure hydrocephalus for NEET-SS Neurology: wet-wobbly-wacky triad, Evans index, tap test protocol and shunt response prediction. - Canonical URL: https://prepelephant.com/topics/neet-ss/neurology/normal-pressure-hydrocephalus - Exam / course: NEET-SS · Subject: Neurology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Normal Pressure Hydrocephalus", PrepElephant, https://prepelephant.com/topics/neet-ss/neurology/normal-pressure-hydrocephalus ## Direct answer A wet, wobbly and wacky elderly patient — urinary incontinence, a magnetic gait and slowing cognition — with ventricles enlarged out of proportion to cortical atrophy defines possible normal pressure hydrocephalus, one of the genuinely treatable dementias. The gait disorder usually arrives first and responds best: short shuffling steps, broad base, feet glued to the floor, arms still swinging, impaired dual-task walking. Imaging shows an Evans index above 0.3 with tight high convexity sulci and enlarged Sylvian cisterns — the DESH pattern — and CSF pressure and constituents are normal. The diagnostic pivot is the CSF tap test: remove 30-50 mL by lumbar puncture and measure gait before and after; a demonstrable improvement predicts shunt responsiveness, with prolonged external lumbar drainage used when the tap test is equivocal but suspicion stays high. Ventriculoperitoneal shunting with a programmable valve is the treatment, and the realistic hierarchy of benefit is gait first, cognition next, continence last. ## What you must remember - **Triad order matters:** gait apraxia is typically first, most prominent and most shunt-responsive; cognition slows as subcortical-type deficit (bradyphrenia, executive failure) rather than amnesia; incontinence is late. - **Imaging criteria:** Evans index (maximum frontal horn width divided by inner skull width) above 0.3; DESH — dilated Sylvian cisterns with tight high convexity sulci; absent or mild hippocampal atrophy argues away from Alzheimer disease. - **Tap test protocol:** standardised gait assessment (10-metre walk time, step count, turn), lumbar puncture removing 30-50 mL, repeat assessment within 24-72 hours; a 10-20% gait improvement is the usual positive threshold per guidelines. - **Secondary NPH:** up to a minority follow subarachnoid haemorrhage, meningitis, head injury or posterior fossa surgery — these patients respond better than the idiopathic group. - **Comorbidity caveat:** vascular parkinsonism and Alzheimer pathology frequently coexist with NPH and cap the achievable improvement; white matter hyperintensity burden tempers prognosis. - **Shunt choices and complications:** programmable valve preferred in the elderly; principal risks are subdural haematoma (over-drainage), infection, blockage and low-pressure headache; valve adjustment is non-invasive. - **Alternative interventions:** endoscopic third ventriculostomy has a limited role; serial lumbar punctures are a bridge, not therapy; lumbar-peritoneal shunt is an option in selected patients. ## From suspicion to shunt: the pathway walked A 72-year-old retired teacher has spent 18 months slowing: her family noticed shuffling and freezing at doorways first, then repeated urinary accidents, then forgotten conversations. Examination shows a broad-based magnetic gait with preserved arm swing, no rigidity or tremor, brisk but symmetric reflexes, and frontal-type cognitive slowing with intact delayed recall relative to her processing speed. MRI: Evans index 0.36, tight convexity sulci, dilated Sylvian cisterns, hippocampi preserved. The pathway runs: baseline timed 10-metre walk (28 seconds, 24 steps); lumbar puncture with opening pressure normal at 14 cm of water, 40 mL removed; next morning the same walk takes 19 seconds. Positive tap test, idiopathic NPH, and after counselling about the one-in-ten risk of subdural collection, a programmable ventriculoperitoneal shunt is inserted. At three months she walks unaided and is continent by day; her memory has brightened only at the edges — the expected hierarchy. Had the tap test been negative with strong clinical conviction, external lumbar drainage over 2-3 days would decide; had her MRI shown hippocampal atrophy and a low-pressure-flow void pattern of small vessel disease, the family conversation would have been about realistic expectations instead. ## Where candidates slip Two questions dominate the viva: "Is it NPH or vascular parkinsonism?" — the latter shows lower-body-predominant bradykinesia with a history of strokes and does not have DESH — and "What does the tap test actually predict?" — it predicts gait response to shunting, moderately; a negative tap test does not exclude benefit, which is why extended drainage exists as a second gate. In India, the practical reality is that programmable valves and revision surgery are out-of-pocket costs for most families, so patient selection is not merely academic: tapping the patient most likely to benefit is an economic decision as much as a clinical one, and exam cases increasingly hint at that with "family can afford one intervention" phrasing. ## Frequently asked questions ### What is the Evans index and its diagnostic threshold? The maximum width of the frontal horns divided by the inner skull width at the same level; a value above 0.3 indicates ventriculomegaly disproportionate to atrophy. ### How is a CSF tap test performed and interpreted? Remove 30-50 mL of CSF after baseline gait timing and repeat the assessment within 24-72 hours; meaningful gait improvement predicts shunt responsiveness. ### Which component of the NPH triad responds best to shunting? The gait disorder — magnetic apraxia improves first and most reliably, followed by cognition, with continence least dependable. ### What is DESH on MRI? Disproportionately enlarged subarachnoid space hydrocephalus — enlarged Sylvian cisterns with tight high-convexity sulci supporting the hydrocephalus pattern. ### What is the most feared shunt complication in the elderly NPH patient? Subdural haematoma from over-drainage in a brain with stretched bridging veins — risk reduced by programmable valves set to higher opening pressures.