Head CT Findings in Normal Pressure Hydrocephalus
On head CT without contrast, normal pressure hydrocephalus demonstrates ventriculomegaly with a narrowed posterior callosal angle (<90°), effaced sulci along the high convexities with widened sylvian fissures, and periventricular white matter hypoattenuation 1.
Key Imaging Features
The classic CT findings form a distinctive pattern that helps differentiate NPH from other causes of dementia:
Ventricular Changes
- Ventriculomegaly out of proportion to cortical atrophy - This is the hallmark finding 2
- Evans index >0.3 (maximal width of frontal horns divided by maximal width of inner skull) 2
- Rounded frontal horns with marked enlargement 2
- Enlarged temporal horns - This finding is particularly important and correlates with better shunt response 3
- Enlarged third ventricle - Also associated with better treatment outcomes 3
Sulcal Pattern (Disproportionately Enlarged Subarachnoid Space Hydrocephalus - DESH)
- Effaced/tight sulci over the high convexities - Sulci appear compressed or absent at the vertex 1
- Widened sylvian fissures - In contrast to the tight high convexity sulci 1
- This discordant pattern (tight high sulci with wide sylvian fissures) is highly characteristic of NPH
White Matter Changes
- Periventricular white matter hypoattenuation (hypodensity) - Represents transependymal CSF flow 1
- Note: MRI has higher sensitivity for detecting this finding 1
Callosal Angle
- Narrowed posterior callosal angle - Typically <90° 1
- Recent automated measurements suggest a threshold of 95° may optimize diagnostic accuracy (85.9% sensitivity and specificity) 4
Important Clinical Caveats
CT cannot detect cerebral aqueduct flow void, which is better visualized on MRI and may help predict shunt responsiveness 1. Additionally, CT may miss small obstructing lesions along the ventricular system that could indicate noncommunicating rather than communicating hydrocephalus 1.
Ventricular size alone does not correlate with CSF absorption dysfunction - Multiple studies demonstrate no relationship between ventricular dimensions and resistance to CSF outflow 5, 6. This is why the pattern of findings matters more than absolute ventricular size.
Periventricular hypodensity, when present, suggests impaired CSF absorption (low conductance to outflow) 6, but its absence doesn't exclude NPH.
Distinguishing NPH from Cerebral Atrophy
The critical distinction is the ventricle-to-sulcal discordance:
- NPH: Large ventricles with small/absent high convexity sulci
- Atrophy: Both ventricles and sulci are proportionally enlarged
However, 43% of patients with cerebral atrophy may show no cortical atrophy on CT 7, making this distinction challenging in some cases. The most consistent CT finding favoring NPH over atrophy is temporal horn dilatation 7.
Recent automated volumetric analysis using the ventricle-to-subarachnoid volume ratio (VSR) demonstrates 94.1% sensitivity and 92.5% specificity for NPH detection, with the advantage of not being age-dependent 8.
Limitations and Next Steps
CT is appropriate for initial screening 2, but when CT findings are equivocal or when planning shunt surgery, MRI without contrast is preferred as it provides superior visualization of:
- Periventricular white matter changes
- Aqueductal flow void (correlates with shunt response)
- Small obstructing lesions
- Temporal horn enlargement
CT findings alone cannot determine shunt candidacy - Clinical correlation with the classic triad (gait disturbance, urinary incontinence, cognitive impairment) and potentially CSF dynamics testing (lumbar tap test, cisternography) are necessary 1, 2.