AANS/CNS Joint Section on Tumors Neuro-Oncology Trainee Award: Revisiting the Evans Index: Radiographic Predictors of Hydrocephalus Risk After Stereotactic Radiosurgery for Vestibular Schwannoma

Introduction: Hydrocephalus following Gamma Knife® stereotactic radiosurgery (SRS) for vestibular schwannoma is an uncommon but manageable complication. Most series report development of communicating hydrocephalus in 1% of patients, thought to be related to an influx of inflammatory or proteinaceous substances into the spinal fluid. Larger tumor size and older patient age have been shown to be associated with hydrocephalus development. However, hydrocephalus risk following stereotactic radiosurgery in patients with baseline enlargement of the ventricular system has not been effectively delineated to date.

Methods: A retrospective cohort study was conducted with patients undergoing Gamma Knife® stereotactic radiosurgery (SRS) for unilateral vestibular schwannoma at a single institution from 2011 – 2021. Patients were included if there was imaging available at both baseline and at least six months of follow-up. Evans index (maximum width of the frontal horns of the lateral ventricles / maximum internal diameter of the skull) was measured on axial T1-weighted MRI images. Univariate analyses were calculated with Mann-Whitney U test and Pearson’s chi-squared test with p < 0.1 considered statistically significant. Binary logistic regression was then performed for further multivariate analyses with p < 0.05 considered statistically significant. All statistical analyses were performed with IBM® SPSS® Version 29.

Results: 378 patients with unilateral vestibular schwannoma met the inclusion criteria for the study. Nine patients (2.4%) underwent shunt placement and six of these patients (1.6%) presented with hydrocephalus after one year. An additional five patients (1.3%) had mild symptoms consistent with communicating hydrocephalus and were observed (time range: 3.2 – 32.7 months). The odds of developing hydrocephalus were 10.5 times higher in association with an Evans Index > 0.3 at treatment (p=0.002). Increasing age (p=0.047), fourth ventricle effacement (p=0.002) and larger tumor volume of 3-4 cm3 (p=0.021) were significantly associated with hydrocephalus incidence in the follow-up period.

Conclusion : Symptomatic communicating hydrocephalus is a potentially latent complication after stereotactic radiosurgery for vestibular schwannoma. Certain radiographic and demographic features can quickly stratify patient risk and guide surveillance strategies for CSF flow diversion.