Normal pressure hydrocephalus

Definitions

• Normal-pressure hydrocephalus (NPH): clinical triad + ventriculomegaly disproportionate to sulcal atrophy.
• DESH: disproportionately enlarged Sylvian fissures with tight/high-convexity sulci.
• Transependymal oedema: periventricular T2/FLAIR hyperintensity from CSF seepage—can occur in NPH but is neither sensitive nor specific (more florid in acute obstructive hydrocephalus).

Cut-offs

• Evans index (EI): ≥ 0.30 = ventriculomegaly (screening threshold).
• Callosal angle (CA): ≤ 90° supports iNPH; ≤ 80° is more specific.
  (Normal adults typically have a wider angle.)

How to measure

Evans index (EI)

1. Use axial CT/MR aligned to AC–PC.
2. Choose slice with maximal frontal horn width (often at/just above foramen of Monro).
3. Measure A = maximal frontal horn span.
4. On same slice, measure B = maximal inner skull diameter.
5. EI = A / B (report to two decimals).

Callosal angle (CA)

1. Create coronal reformat perpendicular to AC–PC.
2. Use slice through the posterior commissure (bodies of lateral ventricles visible).
3. Draw lines tangent to the superomedial ventricular walls.
4. Measure the inner angle where lines meet above the corpus callosum.

Practical use

• Primary discriminator: CA.
• Rule-in sign: DESH (high specificity when clearly present).
• Screening flag: EI ≥ 0.30 → confirm with CA ± DESH + clinical context.

Pitfalls

• Off-axis planes → falsely ↑EI / ↓CA ⇒ always align to AC–PC.
• Ex-vacuo ventriculomegaly from atrophy → ↑EI without NPH.
• Extensive white-matter disease → obscures margins; be consistent in slice choice.
• Mixed patterns (e.g., prior infarcts, shunts) → interpret with caution.