What is the recommended aqueductal stroke volume (ASV) cutoff for predicting a positive response to shunting in patients with suspected Normal Pressure Hydrocephalus (NPH)?

Aqueductal Stroke Volume in Normal Pressure Hydrocephalus

There is no established definitive cutoff value for aqueductal stroke volume (ASV) that reliably predicts shunt response in patients with suspected normal pressure hydrocephalus (NPH), as current evidence shows inconsistent correlation between ASV measurements and clinical outcomes after shunting.

Diagnostic Imaging for NPH

MRI brain without IV contrast is the first-line imaging test for suspected NPH, as recommended by the American College of Radiology 1. Key radiological findings include:

• Ventriculomegaly out of proportion to cortical atrophy
• Rounded frontal horns
• Enlarged temporal horns and third ventricle
• Evans index > 0.3
• Callosal angle < 90°
• Evidence of altered brain water content
• Aqueductal or fourth ventricle flow void

CT head without contrast is an acceptable alternative if MRI is contraindicated, though less sensitive for detecting transependymal CSF flow and subtle ventriculomegaly 1.

ASV Measurements and Predictive Value

The use of ASV measurements via phase-contrast MRI (PC-MRI) has yielded conflicting results:

• Some studies suggest that elevated ASV (≥42 μL) may predict good response to shunting 2
• However, a prospective study found no statistically significant difference in outcomes between patients with different ASV ranges (low: 0-50 μL, middle: 51-100 μL, high: >100 μL) 3
• Another study found that many NPH patients actually demonstrate net retrograde (negative) aqueductal flow, which often reverses toward antegrade direction after successful shunting 4

More Reliable Predictors of Shunt Response

Instead of relying solely on ASV, the following tests have shown better predictive value:

1. CSF drainage tests:

   • Lumbar tap test (removal of 30-50 mL CSF)
   • External lumbar drainage
   • Assessment of clinical improvement after CSF removal 1

2. CSF dynamics assessment:

   • Lumbar infusion test measuring resistance to CSF outflow (Rcsf)
   • Positive predictive value of 92% for Rcsf ≥18 mm Hg/ml/minute 5
   • Highest likelihood ratio of 3.5 at this threshold 5

Recommended Diagnostic Algorithm

1. Begin with MRI brain without contrast to confirm radiological features of NPH
2. If radiological findings support NPH, proceed with CSF dynamics testing:
   • Lumbar infusion test to measure Rcsf
   • Consider shunting if Rcsf ≥18 mm Hg/ml/minute (highest positive predictive value)
3. If Rcsf is lower, perform supplementary tests:
   • Lumbar tap test or external lumbar drainage
   • Evaluate for clinical improvement in gait, cognition, and urinary function
4. PC-MRI for ASV measurement may provide additional information but should not be the sole determinant for shunt candidacy

Clinical Pearls and Pitfalls

• ASV measurements alone should not determine shunt candidacy due to inconsistent correlation with outcomes
• Extensive comorbidity significantly impacts prognosis and should be carefully evaluated 5
• Untreated NPH may show variable patterns of ASV over time, initially increasing then eventually decreasing as the condition progresses 2
• Technical difficulties in obtaining accurate ASV measurements occur in approximately 15% of patients 3
• The presence of retrograde aqueductal flow (negative ASV) does not rule out NPH or potential benefit from shunting 4