How do you diagnose Normal‑Pressure Hydrocephalus?

How to Diagnose Normal Pressure Hydrocephalus (NPH)

Diagnose NPH through the combination of clinical triad (gait disturbance, cognitive impairment, urinary incontinence), neuroimaging showing characteristic features, normal CSF pressure on lumbar puncture, and positive response to CSF tap test. 1

Clinical Presentation

NPH presents with a characteristic triad, though all three symptoms need not be present simultaneously 2:

• Gait disturbance: Appears first and is most severely affected. Look for a magnetic, shuffling gait with broad base and difficulty initiating steps 2
• Cognitive impairment: Develops later, showing a frontal-subcortical pattern (executive dysfunction, psychomotor slowing) 3
• Urinary incontinence: Typically urge incontinence, often the last symptom to appear 1

Critical pitfall: The full triad is present in only a subset of patients—do not require all three symptoms to pursue diagnosis 2. Gait disturbance alone with supportive imaging warrants further evaluation.

Initial Neuroimaging (First-Line)

Order MRI head without IV contrast OR CT head without IV contrast as equivalent first-line options 4. MRI is preferred when available due to higher sensitivity for key findings 1.

Key Imaging Features to Identify:

Required findings 4:

• Ventriculomegaly (Evans index >0.3: maximal width of frontal horns/maximal inner skull width)
• No macroscopic CSF flow obstruction (communicating hydrocephalus)

At least ONE of the following 4:

• Enlargement of temporal horns
• Callosal angle <90° (narrowed posterior callosal angle)
• Evidence of altered brain water content (periventricular white matter changes)
• Aqueductal or fourth ventricle flow void on MRI

Additional supportive features 1:

• Effaced sulci along high convexities with widened Sylvian fissures
• Disproportionately enlarged subarachnoid space hydrocephalus (DESH) pattern 5

MRI advantage over CT: Can detect cerebral aqueduct flow void and small obstructing lesions that CT may miss 1

CSF Pressure Measurement

Perform lumbar puncture to document normal opening pressure (typically 5-18 cm H₂O) 1. This distinguishes NPH from obstructive hydrocephalus with elevated pressure.

CSF Tap Test (Critical Diagnostic Step)

The CSF tap test is the key diagnostic tool to identify shunt-responsive patients 2, 3:

1. Remove 30-50 mL of CSF via lumbar puncture (large-volume tap)
2. Assess gait objectively before and after using timed walking tests (e.g., 10-meter walk time, number of steps, Timed Up and Go test)
3. Document both objective AND subjective improvements 2
4. Positive response: Improvement in gait speed, step count, or patient-reported symptoms within hours to 2-3 days post-tap

Alternative: Extended lumbar drainage (ELD) with temporary spinal catheter for 2-3 days may identify additional responders when single tap test is negative 3.

Ancillary Tests (Second-Line)

DTPA Cisternography

• Shows persistent radiotracer in lateral ventricles and absent activity over convexities on delayed imaging 1
• Important limitation: Evidence is insufficient to proceed with shunting based on cisternography alone—cannot reliably predict shunt response 1
• May support diagnosis but should not be sole decision-making tool

Brain Perfusion SPECT with Acetazolamide Challenge

• Impaired cerebral blood flow reactivity may predict better shunt response 1
• Consider as second-level test for patient stratification 4

Tests NOT Recommended for Initial Diagnosis:

• MR spectroscopy: Cannot distinguish NPH from other dementias or predict shunt outcomes 1
• FDG-PET/CT: Insufficient evidence for initial imaging 1
• Any imaging with IV contrast: Not necessary 1

Differential Diagnosis Considerations

NPH commonly coexists with other neurodegenerative diseases in 20-57% of cases 1. Consider:

• Alzheimer's disease: Look for hippocampal atrophy (typically absent in pure NPH) 1
• Parkinsonian syndromes: Especially progressive supranuclear palsy and dementia with Lewy bodies 2
• Vascular dementia: Check for white matter lesions and microhemorrhages 1
• Cervical stenosis: Common comorbidity that may also require surgical intervention 2

Use dopamine transporter imaging or amyloid PET to exclude misdiagnosis when clinical picture is unclear 6.

Diagnostic Algorithm Summary

1. Clinical assessment: Identify gait disturbance ± cognitive impairment ± urinary incontinence in patient >65 years
2. MRI or CT head without contrast: Confirm ventriculomegaly with characteristic features (Evans index >0.3, callosal angle <90°, temporal horn enlargement)
3. Lumbar puncture: Document normal CSF opening pressure
4. CSF tap test: Remove 30-50 mL CSF and objectively measure gait improvement
5. If tap test positive: Patient is likely shunt-responsive (80-90% success rate) 3
6. If tap test equivocal: Consider extended lumbar drainage or acetazolamide challenge SPECT
7. Exclude comorbidities: Rule out other neurodegenerative diseases and cervical stenosis

The combination of clinical triad, characteristic imaging, normal CSF pressure, and positive tap test establishes the diagnosis and predicts shunt responsiveness 1, 2, 3.