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