MRI-based cerebrospinal fluid volumetric indices for predicting tap test response in idiopathic normal pressure hydrocephalus.

Idiopathic normal pressure hydrocephalus (iNPH) is a potentially reversible disorder; however, predicting symptom-specific improvement after cerebrospinal fluid (CSF) tap testing remains challenging. This exploratory study investigated whether MRI-derived CSF compartment volumes are differentially associated with gait and cognitive responses after tap testing. In this retrospective observational study, 50 patients with clinically diagnosed iNPH underwent 3.0-T MRI with 3D T1WI prior to tap testing. Third ventricle and bilateral choroid plexus volumes were quantified using automated FreeSurfer segmentation. Gait was assessed using the Timed Up and Go (TUG) test (completion time and step count), and cognition was assessed using the Mini-Mental State Examination (MMSE). Multivariable logistic regression adjusted for age, sex, and total intracranial volume was performed with false discovery rate correction. Receiver operating characteristic (ROC) analysis with bootstrap internal validation was conducted to evaluate discriminatory performance. Larger third ventricle volume was significantly associated with a lower probability of TUG step response (odds ratio per 100 mm³ increase = 0.857, 95% CI 0.760-0.965; FDR-adjusted p = 0.022), whereas no association was observed with TUG time response. Third ventricle volume demonstrated good discriminatory performance in this cohort for identifying TUG step responders (AUC = 0.876, 95% CI 0.772-0.958). In contrast, choroid plexus volume was not significantly associated with MMSE response after correction and showed limited discriminatory ability (AUC = 0.651, 95% CI 0.472-0.805). These findings suggest that third ventricle volumetry may serve as an exploratory imaging-based reference for gait pattern improvement after tap testing; however, the proposed cutoff values remain exploratory.