A Coronal Hydrocephalic Index (Eo Index) for Predicting Shunt Responsiveness in Idiopathic Normal Pressure Hydrocephalus

The cerebrospinal fluid tap test is commonly used to select shunt candidates in idiopathic normal pressure hydrocephalus; however, it is invasive and has limited predictive value, as a substantial proportion of patients who tested positive on tap test do not improve after shunt surgery. This limitation highlights the need for noninvasive imaging markers to support preoperative decision-making. We retrospectively analyzed 80 patients with ventriculomegaly evaluated at a single center between 2014 and 2022, including 40 patients with idiopathic normal pressure hydrocephalus who tested positive on tap test and underwent shunt surgery, and 40 control patients with ventriculomegaly without idiopathic normal pressure hydrocephalus. Shunt responsiveness was defined as an improvement of at least 1 point on the idiopathic normal pressure hydrocephalus grading scale at 6-12 months. Evans index, z-Evans index, callosal angle, disproportionately enlarged subarachnoid-space hydrocephalus, and the coronal hydrocephalic index (Eo index = z-Evans index/Evans index) were measured on magnetic resonance imaging. Among patients who underwent shunt surgery, 28/40 (70%) demonstrated clinical improvement. In univariate analyses within the shunted subgroup, Evans index and z-Evans index were not associated with shunt responsiveness, whereas callosal angle, disproportionately enlarged subarachnoid-space hydrocephalus, and the coronal hydrocephalic index were significant discriminators. The discriminative performance of the coronal hydrocephalic index was comparable to callosal angle and was complementary to disproportionately enlarged subarachnoid-space hydrocephalus. In the full ventriculomegaly cohort, the coronal hydrocephalic index also discriminated patients with idiopathic normal pressure hydrocephalus who were shunt-responsive from others who were not. Receiver operating characteristic analyses were used to derive within-sample cut-off values for scenario-based clinical decision support. The coronal hydrocephalic index is a simple, noninvasive magnetic resonance imaging-derived parameter that may support stratification of shunt responsiveness in patients with idiopathic normal pressure hydrocephalus, and requires external validation in independent cohorts before broader clinical application.