Anteromedial extension and early facial nerve outcome after retrosigmoid vestibular schwannoma resection: a retrospective cohort study

Tumor size is associated with early facial nerve status after vestibular schwannoma surgery, but diameter, volume, and Koos grade provide limited information about the direction in which tumor burden is distributed. We evaluated anteromedial extension (AE), a simple MRI-derived directional measure, and examined its relationship with early facial nerve outcome alongside standard morphometric measures. This retrospective single-center cohort included 129 adults undergoing retrosigmoid resection of unilateral vestibular schwannoma (2019–2023). Preoperative measures were AE, maximal diameter, 3D volume, and Koos grade. Outcomes were anatomical facial nerve transection and postoperative House-Brackmann (HB) status at discharge and 1 year; functional analyses included 120 patients with preserved nerve continuity. The 8-mm threshold was derived using the Youden criterion in the same cohort and was treated as exploratory. Nine patients (7.0%) had facial nerve transection. Among continuity-preserved patients, 34 (28.3%) had HB grade ≥ III and 20 (16.7%) had HB grade ≥ IV at discharge. AE correlated strongly with diameter and volume (ρ = 0.810 and 0.827). In parsimonious two-predictor models, AE remained associated with early postoperative HB status when paired with Koos grade but not when paired with continuous diameter or volume. In the exploratory continuity-preserved Koos IV subgroup (n = 48; 14 HB grade ≥ IV events), AE was associated with HB grade ≥ IV (OR 1.22/mm, 95% CI 1.05–1.42). A same-cohort Youden threshold of 8 mm separated observed HB grade ≥ IV rates of 5/33 (15.2%) versus 9/15 (60.0%); sensitivity was 50.0% and specificity 94.0%. For HB grade ≥ IV, 3D volume had higher discrimination than AE (AUC 0.824 vs 0.768). AE shares substantial information with conventional tumor size but may provide a simple directional description that is not represented by Koos stage alone. In Koos IV tumors, greater AE was associated with severe early postoperative facial nerve dysfunction. The internally derived 8-mm cut point is hypothesis-generating and requires independent validation before clinical use.