What is the recommended radiologic workup for Normal Pressure Hydrocephalus (NPH)?

Radiologic Workup for Normal Pressure Hydrocephalus

MRI of the brain without IV contrast is the first-line and preferred imaging modality for evaluating suspected NPH, as it provides superior detection of characteristic features that are critical for diagnosis and cannot be visualized on CT. 1, 2

Primary Imaging: MRI Without Contrast

MRI should be obtained first because it identifies the complete constellation of NPH imaging features with higher sensitivity than any other modality 1:

• Ventriculomegaly with Evans index >0.3 (ratio of maximum frontal horn width to maximum internal skull diameter) 1
• Disproportionately enlarged subarachnoid-space hydrocephalus (DESH) pattern: tight high-convexity sulci with effaced medial subarachnoid spaces, enlarged Sylvian fissures, and ventriculomegaly 1, 3
• Narrowed posterior callosal angle <90° 1
• Periventricular white matter changes (detected with much higher sensitivity than CT) 1, 4
• Cerebral aqueduct flow void on T2-weighted sequences, which correlates with shunt responsiveness and cannot be seen on CT 1, 4
• Rounded frontal horns with marked enlargement of temporal horns and third ventricle 1

The DESH pattern is particularly important as it helps differentiate NPH from simple cerebral atrophy, where you would see enlarged convexity sulci rather than tight ones 3, 5.

Alternative Imaging: CT Without Contrast

Use CT head without IV contrast only when MRI is contraindicated or unavailable 1, 2:

• CT can identify ventriculomegaly, narrowed callosal angle, effaced sulci, and widened Sylvian fissures 1
• However, CT has critical limitations: it cannot detect cerebral aqueduct flow void, has lower sensitivity for periventricular white matter changes, and may miss small obstructing lesions 1, 6

Common pitfall: Relying solely on CT findings without attempting MRI when possible can lead to misdiagnosis, as CT misses key features that predict shunt responsiveness 2.

Advanced MRI Techniques (Adjunctive, Not First-Line)

Cine phase-contrast MRI can be used in diagnostically challenging cases to measure CSF flow velocity through the cerebral aqueduct 1, 7:

• Increased ventricular flow rate has high positive predictive value for shunt responsiveness 1
• Measuring peak CSF flow velocity before and after lumbar CSF drainage (50 mL) can help select patients likely to benefit from shunt surgery 7
• In one study, 14 of 15 patients recommended for shunt based on flow velocity changes after drainage improved post-operatively 7

Volumetric MRI measurements of ventricular volume combined with total cortical thickness may improve diagnostic accuracy when distinguishing NPH from other neurodegenerative conditions like Alzheimer's disease, where ventricular volumes can overlap 8.

Secondary Imaging Modalities (Not Recommended as Primary Tests)

DTPA cisternography should NOT be used as a primary diagnostic tool or sole basis for treatment decisions 6:

• While it can show persistent radiotracer in lateral ventricles and absence over cerebral convexities on delayed imaging, evidence is insufficient to proceed with shunting based on cisternography alone 1, 6
• Critical caveat: Never deny shunt surgery to an otherwise appropriate NPH candidate based solely on negative cisternography results 6

FDG-PET/CT and MR spectroscopy are not recommended for routine NPH workup 1:

• FDG-PET may help distinguish NPH from other dementias but does not reliably demonstrate classic NPH imaging features 1
• MR spectroscopy cannot reliably distinguish NPH from other dementias and does not correlate with CSF diversion outcomes 1

Algorithmic Approach to Radiologic Workup

1. Order MRI brain without contrast as first-line imaging 1, 2
2. Evaluate for complete NPH imaging constellation: Evans index >0.3, DESH pattern, callosal angle <90°, periventricular changes, aqueduct flow void 1
3. If MRI contraindicated: obtain CT without contrast, recognizing its limitations 1, 2
4. If diagnosis remains uncertain after standard MRI: consider cine phase-contrast MRI to assess CSF flow dynamics 1, 7
5. Do not order cisternography as a primary test or use it alone to make treatment decisions 6
6. If imaging and clinical features align: proceed to CSF diversion testing (lumbar puncture with gait assessment or external lumbar drainage) 1, 6