What MRI findings in normal-pressure hydrocephalus indicate that a ventriculoperitoneal shunt is appropriate?

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MRI Findings Suggesting Shunt Appropriateness in Normal Pressure Hydrocephalus

For patients with suspected NPH, MRI findings that support proceeding with ventriculoperitoneal shunt placement include: Evans Index >0.3, at least one additional feature (temporal horn enlargement, callosal angle <90°, altered brain water content, or aqueductal flow void), and critically, the presence of high-convexity tight sulci with enlarged Sylvian fissures (DESH pattern). 1

Core Diagnostic Imaging Features

The 2025 ACR Appropriateness Criteria establish clear structural requirements for probable NPH that guide surgical decision-making 2:

Required Baseline Features:

  • Ventricular enlargement not attributable to cerebral atrophy or congenital causes
  • Evans Index >0.3 (maximal width of frontal horns/maximal width of inner skull)
  • No macroscopic CSF flow obstruction

At Least ONE of These Additional Features Must Be Present:

  • Enlargement of temporal horns (marked)
  • Callosal angle <90° (narrowed posterior callosal angle)
  • Evidence of altered brain water content (periventricular T2/FLAIR hyperintensity suggesting transependymal CSF flow)
  • Aqueductal or fourth ventricle flow void on standard MRI sequences

Features That Predict Shunt Responsiveness

Beyond diagnostic criteria, specific MRI findings correlate with better surgical outcomes 1:

Strong Predictors of Shunt Response:

  • Increased CSF flow void through the cerebral aqueduct - correlates with good response to shunt surgery 1
  • Hyperdynamic aqueductal CSF on cine MRI - helps identify shunt-responsive patients 1, 3
  • High-convexity tightness (effaced sulci along high convexities) - independently associated with shunt surgery selection 4
  • Focal impingement of corpus callosum - associated with unstable ICP and better outcomes 5

The DESH Pattern (Disproportionately Enlarged Subarachnoid Space Hydrocephalus):

This pattern is particularly important and includes 2:

  • Tight high-convexity sulci (effaced)
  • Widened Sylvian fissures
  • Ventriculomegaly out of proportion to cortical atrophy

A DESH score >2 was associated with shunt surgery selection in recent studies 4.

Quantitative Thresholds

Recent research provides specific cutoffs that improve surgical selection 4:

  • Evans Index >0.36 (not just >0.3) - independently associated with shunt surgery selection
  • Bicaudate Index >0.23 - associated with shunt selection in univariate analysis
  • Mean aqueductal CSF velocity >26 mm/sec - predicts significant shunt responsiveness with 83.3% specificity 6
  • Peak aqueductal velocity >70 mm/sec - predicts gait improvement with 83.3% specificity 6

Critical Exclusionary Features

Absence of or only mild cortical atrophy is essential 1. Extensive cortical atrophy suggests a neurodegenerative process rather than NPH, though 20-57% of NPH patients have comorbid neurodegenerative disease 2.

Supportive But Not Required Features

Additional findings that strengthen the case for shunting 1:

  • Prior MRI showing smaller ventricular size before symptom onset
  • Cine MRI demonstrating increased ventricular flow rate
  • Rounded frontal horns with marked third ventricle enlargement

Common Pitfalls to Avoid

  1. Don't rely on Evans Index alone - continuous MRI measurements (ventricular width indices) alone are insufficient for identifying shunt candidates 4. Integration of qualitative features (especially high-convexity tightness) with quantitative measures is essential.

  2. Callosal angle has limited discriminatory value - despite being part of diagnostic criteria, it showed no discriminatory value for shunt surgery selection in recent studies 4.

  3. MR spectroscopy is not useful - it cannot differentiate NPH from other dementias and doesn't help predict shunt response 1, 2.

  4. Beware of cerebral atrophy - patients with significant cerebral atrophy have higher rates of shunt overdrainage complications (subdural hygromas/hematomas) 7. This doesn't preclude shunting but requires careful valve selection and monitoring.

Algorithmic Approach to MRI Interpretation

Step 1: Confirm ventriculomegaly with Evans Index >0.3 (ideally >0.36)

Step 2: Verify absence of obstructive lesion along ventricular system

Step 3: Assess for DESH pattern:

  • High-convexity tight sulci present?
  • Sylvian fissures enlarged?
  • Minimal cortical atrophy?

Step 4: Count additional supportive features:

  • Temporal horn enlargement
  • Callosal angle <90°
  • Periventricular T2/FLAIR hyperintensity
  • Aqueductal flow void

Step 5: If available, assess CSF dynamics:

  • Hyperdynamic aqueductal flow on cine MRI strongly supports shunting
  • Mean velocity >26 mm/sec or peak velocity >70 mm/sec predicts response

If Steps 1-4 are positive with high-convexity tightness, proceed with clinical correlation and consider shunt placement. The combination of Evans Index >0.36 and high-convexity tightness provides the strongest imaging support for surgical intervention 4.

References

1
Guideline

acr appropriateness criteria® dementia.

Journal of the American College of Radiology, 2020

2
Guideline

acr appropriateness criteria® dementia: 2024 update.

Journal of the American College of Radiology, 2025

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