Normal pressure hydrocephalus is a form of communicating hydrocephalus characterized by ventricular dilation with normal CSF pressure, classically presenting with the Hakim triad: gait disturbance, cognitive decline, and urinary incontinence. It results from impaired CSF absorption at the arachnoid granulations without a significant rise in intracranial pressure. Differentiation from atrophy-related (ex vacuo) ventriculomegaly is crucial.

CT (Non-contrast, preferred for initial evaluation):

Ventricular enlargement:

Evans Index (ratio of maximal frontal horn width to maximal internal skull diameter on same axial slice): >0.3 diagnostic; >0.4 strongly suggestive.

Temporal horn dilatation: Out of proportion to cortical sulcal enlargement.

Sylvian fissures: Prominent and widened.

High-convexity sulci: Narrowed or effaced (“disproportionately enlarged subarachnoid space hydrocephalus,” DESH pattern).

Cortical atrophy: Minimal compared to degree of ventricular enlargement (helps distinguish from ex vacuo dilation).

Periventricular lucency: Hypoattenuation due to transependymal CSF flow.

No obstructing lesion (confirms communicating type).

MRI (Preferred for detailed evaluation):

Ventricular morphology:

Evans Index >0.3 confirms ventriculomegaly.

Callosal Angle (measured on coronal image at posterior commissure):

Normal: 100–120°

NPH: <90° (usually 50–80°).

DESH pattern:

Enlarged Sylvian fissures + tight high-convexity sulci + ventriculomegaly.

Flow void sign: Prominent aqueductal and fourth ventricular flow void on T2 or phase-contrast sequences (reflects hyperdynamic CSF flow).

Periventricular T2/FLAIR hyperintensity: CSF seepage (not necessarily ischemic).

Phase-contrast cine MRI:

Measures CSF stroke volume at the aqueduct.

Stroke volume >42 µL/cycle suggests shunt-responsive NPH.

Nuclear Medicine (if performed):

Delayed radionuclide cisternography may show prolonged ventricular retention and delayed cortical activity (rarely used in modern practice).

Classification / Diagnostic Criteria (Imaging):

Evans Index >0.3

Callosal Angle <90°

DESH pattern present

No major cortical atrophy or obstructive lesion