MRI markers of idiopathic normal pressure hydrocephalus in a population study with 791 participants: Exploring reference values and associations.

Purpose: Epidemiological studies on idiopathic normal pressure hydrocephalus (iNPH) imaging markers and their normal values are scarce. This population-based study aimed to analyze several morphologic and volumetric iNPH-related imaging markers in a large sample, determining their distribution, diagnostic accuracy, suggested cut-offs, and associations with iNPH symptoms.

Methods: This cross-sectional study included 791 70 year olds, 40 with radiologically probable iNPH (iNPH_Radiol) and 751 without iNPH features (reference). MRI measures included Evans index (EI), z-EI, brain per ventricle ratio at anterior (BVR_AC) and posterior commissures (BVR_PC), sulcal compression, Sylvian fissure enlargement, callosal angle, diameter of temporal horns, 3^rd and 4^th ventricles, midbrain, and pons. Volumes of ventricles, corpus callosum, and brainstem were computed using automated segmentation. ROC analysis determined imaging markers' cut-offs. Symptoms were evaluated clinically and through self-report.

Results: In the reference group, median values (95% CI) for imaging markers were as follows: EI: 0.27 (0.26-0.27), z-EI: 0.28 (0.26-0.31), BVR_AC: 1.69 (1.48-1.90), and BVR_PC: 2.66 (2.24-3.27). Most imaging markers differed significantly between iNPH_Radiol and the reference. Lateral ventricle volumes correlated better with z-EI and BVR than EI (Rs > 0.81 vs 0.68). Optimal cut-off values for z-EI, and BVR_AC and BVR_PC for distinguishing iNPH_Radiol were 0.32, 1.36, and 1.83, respectively. Clinical symptoms correlated moderately with imaging markers (Rs < 0.49 for iNPH_Radiol, p < .01).

Conclusions: We report population-based reference values and propose cut-offs for iNPH-related imaging markers and volumetric measurements. Z-EI and BVR are likely superior markers for assessing ventricular enlargement in iNPH. Imaging markers of iNPH correlate moderately with iNPH symptoms.