01Core principlesThe concepts and mechanisms needed to understand the subject.
Normal-pressure hydrocephalus describes chronic ventriculomegaly with a characteristic clinical syndrome despite pressure being within a conventional range when sampled. The name does not mean intracranial pressure is continuously normal, and one lumbar opening pressure neither proves nor excludes the disorder. Idiopathic NPH has no obvious preceding causative event and is generally a disease of older adults. Secondary NPH follows an acquired process such as subarachnoid haemorrhage or meningitis. Congenital or developmental ventriculomegaly presenting later is a separate category.
Gait disturbance is the most prominent feature. Patients may take short shuffling steps, broaden their base, appear magnetically stuck to the floor, struggle to initiate or turn, and lose postural stability. The upper limbs are often less affected than walking. Cognitive change tends to be frontal-subcortical: slowed processing, impaired attention, apathy and executive dysfunction can dominate rather than an isolated amnestic syndrome. Bladder urgency may progress to incontinence. These features overlap with Parkinsonian disorders, vascular cognitive impairment, Alzheimer disease, cervical myelopathy, peripheral neuropathy and urological disease, so the triad is a prompt for structured evaluation rather than a diagnostic shortcut.
DESH is the key imaging pattern in the 2021 Japanese guideline. It combines ventriculomegaly, typically Evans index at least 0.3, with tight sulci over the high convexity and midline plus disproportionately enlarged Sylvian fissures. This distribution differs from simple diffuse atrophy, where sulci are broadly enlarged in proportion to ventricular size. A narrowed callosal angle can support the pattern, but measurements are technique-sensitive. Absence of DESH has limited negative predictive value and should not be used alone to close the diagnosis.
Response to temporary CSF removal provides functional evidence. Assessment must be objective and repeated: timed walking, turning, video and a cognitive baseline are more useful than asking whether the patient feels better. The guideline reports tap-test sensitivity around 58% and specificity around 75% across reviews; improvement supports iNPH, but a negative test can be false negative, particularly after a long symptom duration. Gait may improve before cognition or continence, so evaluation on the following day and more than once in the first week can reveal a delayed response.
Shunting can improve appropriately selected patients, but it exposes them to haemorrhage, infection, obstruction and overdrainage including subdural collections. “Definite iNPH” is therefore a retrospective label for objective improvement after shunt surgery, not a criterion that can be known beforehand. Referral decisions integrate disability, imaging, objective drainage response, comorbidity, frailty, goals and the probability that other diseases account for persistent symptoms.
Key points
- Think gait first: iNPH typically causes a small-step, shuffling, broad-based, unstable gait with difficulty initiating and turning; gait is often the earliest and most treatment-responsive domain.
- The triad is gait disturbance, cognitive impairment and urinary urgency or incontinence, but all three need not be equally advanced before referral.
- Ventriculomegaly is necessary context, not proof. DESH combines an Evans index of at least 0.3 with tight high-convexity or midline subarachnoid spaces and enlarged Sylvian fissures.
- Possible iNPH requires more than one triad symptom, no better complete explanation and no obvious preceding cause of ventricular dilatation.
- Probable iNPH in the Japanese third-edition criteria adds CSF pressure of 200 mmH2O or less with normal contents plus either DESH with characteristic gait or improvement after CSF drainage.
- A positive tap test supports likely shunt responsiveness; a negative test does not exclude iNPH or later benefit and must be interpreted with timing, objective testing, imaging and comorbidity.
02Mechanisms and patternsImportant relationships and how to distinguish them.
Observe initiation, stride length, base, foot clearance, turning and postural recovery. Small-step, magnetic, broad-based and unstable walking with multi-step turns is characteristic; quantify it rather than relying on a verbal description.
Ask about slowing, attention, planning, apathy and loss of independence. Prominent early episodic-memory encoding loss, hallucinations, marked language dysfunction or rapid progression should strengthen alternative or coexisting diagnoses.
Urgency and frequency can precede incontinence. Check infection, retention, prostate or pelvic disease, mobility barriers, diuretics and functional access rather than attributing every bladder symptom to NPH.
Seek enlarged ventricles with tight high-convexity or midline spaces and widened Sylvian fissures. Diffuse proportional sulcal enlargement favours atrophy; neither an Evans index nor a callosal angle is diagnostic in isolation.
An obvious preceding SAH, meningitis, head injury, congenital condition or aqueductal stenosis excludes the idiopathic label and redirects investigation and prognosis.
Sudden coma, focal deficit, seizure or vomiting is not explained by routine iNPH progression. Investigate immediately for acute hydrocephalus, bleeding, stroke, infection or shunt complication.
03Interpreting evidenceInformation, measurements and their limitations.
Consider the information, its meaning and its limitations before deciding what follows.
- 01
Objective gait assessment - Why
- Document the dominant impairment and create a reproducible baseline before and after CSF drainage.
- Interpretation and limitations
- Use a timed up-and-go or short straight-walk test with recorded steps, speed, turn quality and video where appropriate. A meaningful change must exceed day-to-day variability and be interpreted with pain, fatigue and practice effects.
- 02
MRI brain - Why
- Confirm ventriculomegaly, assess DESH and callosal angle, and identify vascular, degenerative, obstructive or structural alternatives.
- Interpretation and limitations
- DESH increases diagnostic confidence when paired with the characteristic gait. Its absence does not exclude iNPH; severe diffuse atrophy, strategic infarcts or an obstructing lesion may better explain the syndrome.
- 03
Cognitive, functional and continence baseline - Why
- Characterise all symptom domains and separate potentially reversible disability from comorbidity.
- Interpretation and limitations
- Use the same instrument before and after drainage. Gait often changes first; cognitive and bladder change may be delayed and should not be judged from a single same-day conversation.
- 04
Lumbar tap test - Why
- Test for objective improvement after temporary CSF removal in a selected patient without an obstructive pressure gradient.
- Interpretation and limitations
- Improvement strongly supports the pathway, but sensitivity is limited. Assess at about 24 hours and repeatedly within the first week as locally arranged; a negative result does not by itself rule out shunt response.
- 05
Specialist drainage or CSF-dynamics testing - Why
- Refine shunt selection when suspicion remains despite an equivocal tap test.
- Interpretation and limitations
- Repeat tap, external lumbar drainage and infusion testing vary by specialist service. No test supplies certainty; risks and results must be integrated with imaging, objective phenotype and comorbidity.
04Applied reasoningWorked examples connecting principles to decisions.
01Worked case: negative tap testGait-led syndrome with DESHAn older adult has progressive magnetic gait, executive slowing and urgency, MRI shows DESH, but no improvement is seen immediately after a tap test.+
- 1Confirm that more than one triad symptom is present, no preceding cause is obvious and no competing disorder completely explains the disability.
- 2Review MRI for true DESH—ventriculomegaly with tight high-convexity or midline spaces and enlarged Sylvian fissures—rather than using ventricular size alone.
- 3Check that gait and cognition were measured objectively before drainage and reassess on the following day and, through the specialist pathway, more than once within the first week.
- 4Do not exclude iNPH because the immediate tap response is negative; discuss possible false negativity and further specialist testing or shunt selection in context.
- 5Balance potential functional gain against comorbidity and shunt risks, agree meaningful patient-centred goals, and retain alternative diagnoses if response remains incomplete.
02Diagnostic pathwayProgressive gait, cognition and bladder changeAn older adult develops a chronic gait-led syndrome with cognitive and urinary symptoms.+
- 1Observe and quantify gait, examine for Parkinsonism, neuropathy, myelopathy and focal deficits, and establish cognitive, functional and continence baselines.
- 2Obtain brain imaging to assess ventriculomegaly, DESH and competing pathology, then identify any preceding event that would make the syndrome secondary rather than idiopathic.
- 3Refer a plausible, functionally important syndrome for specialist CSF-drainage assessment and shunt discussion rather than diagnosing from the triad or Evans index alone.
- 4After any intervention, repeat the same objective measures and monitor for overdrainage, haemorrhage, infection or mechanical failure.
03Mimic pathwayVentriculomegaly without a convincing syndromeImaging shows large ventricles but gait or cognitive findings are atypical or absent.+
- 1Compare prior imaging and assess whether sulcal enlargement is proportionate, whether DESH is present and whether an obstructive lesion exists.
- 2Investigate the dominant clinical problem—degenerative, vascular, spinal, peripheral nerve, musculoskeletal or urological—rather than labelling imaging alone as NPH.
- 3Monitor clinically if uncertainty remains and re-refer if a characteristic progressive gait-led syndrome develops.
05Checking understandingVerify the reasoning, revisit uncertainties and apply feedback.
- Use reproducible gait measures, including speed, step count, turning and timed up-and-go, at baseline and after drainage or shunting.
- Track executive function, attention, continence and independence separately because improvement can occur at different times and to different degrees.
- After shunting, watch for headache related to posture, new focal deficit, drowsiness, fever, wound change or symptom recurrence as possible overdrainage, subdural collection, infection or obstruction.
- Review valve settings and imaging only through the specialist shunt pathway; device type and local protocol determine adjustment and follow-up.
- Continue management of coexisting Parkinsonism, vascular risk, neuropathy, arthritis, cognitive disease and bladder pathology even when NPH treatment helps.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
Gait carries signal
The combination of short steps, broad base, initiation difficulty and unstable turning is more informative than the word “unsteady,” and it provides the clearest response measure.
DESH is disproportion
Tight high-convexity spaces beside enlarged Sylvian fissures distinguish altered CSF distribution from uniformly enlarged sulci of atrophy.
Negative is not no
Limited tap-test sensitivity means no immediate improvement cannot close a convincing iNPH diagnosis. Timing and objective measurement are part of the test.
Normal pressure is a name
A sampled pressure within range is compatible with NPH but does not explain the full dynamics or establish diagnosis independently.
Definite is retrospective
The Japanese criteria reserve definite iNPH for objective improvement after shunt; preoperative categories express probability, not certainty.
07Common pitfallsFrequent interpretation and management errors.
- 01
Diagnosing iNPH from the triad alone without imaging, objective gait assessment and evaluation of mimics.
- 02
Calling ventriculomegaly diagnostic when diffuse atrophy or long-standing stable enlargement may explain the scan.
- 03
Excluding iNPH after one negative same-day tap-test assessment.
- 04
Calling post-SAH, post-meningitic, post-traumatic or congenital disease idiopathic.
- 05
Promising cognitive recovery without discussing comorbidity, incomplete response and shunt complications.
- 06
Sending a patient with abrupt drowsiness, focal deficit, seizure or vomiting down a routine NPH clinic pathway.