Distinguishing iNPH from Alzheimer Disease: Key Imaging Differences
Distinguishing iNPH from Alzheimer Disease:
Key Imaging Differences
A structured neuroimaging reference for clinicians — grounded in NEJM 2025, meta-analytic data, and computational evidence
Ventriculomegaly is nonspecific — it occurs in normal aging, Alzheimer disease, and other conditions. The central diagnostic challenge is recognizing the pattern of CSF distribution and structural change that distinguishes treatable iNPH from irreversible neurodegeneration.
According to a 2025 invited review in the New England Journal of Medicine (Johnson MD, Williams MA), ventricular enlargement alone cannot differentiate iNPH from cerebral atrophy. The critical distinction lies in the disproportionate and regionally specific redistribution of CSF seen in iNPH, as opposed to the globally symmetric parenchymal loss of Alzheimer disease. No single imaging biomarker is sufficient; clinical judgment must integrate multiple features.
Quantitative Screening Tools
Two standardized measurements serve as the initial quantitative screen. Both are measured on axial or coronal MRI sequences and should be interpreted alongside the clinical triad and overall imaging gestalt.
Evans Ratio vs. Callosal Angle
Reference: Johnson MD, Williams MA. N Engl J Med. 2025;393(22):2243–2253.
Sensitive · Less Specific
Frontal horn width ÷ inner skull diameter. Detects ventriculomegaly broadly but cannot distinguish iNPH from other etiologies. Sens 96%, Spec 83% (AUC 0.87).
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