Morphological Heterogeneity of Large (Koos Grade 4) Vestibular Schwannomas: Surgical Consequences and Predictors of Facial Nerve Outcome

Background/Objectives: Microsurgery is the principal treatment for large vestibular schwannomas (VS) causing symptomatic brainstem compression. Koos grade 4 tumors remain anatomically heterogeneous. We evaluated surgical and functional outcomes and explored whether a study-specific subdivision, adapted from the Hannover T4a/T4b distinction, provides prognostic information beyond established clinical and surgical variables. Methods: This retrospective single-center study included 139 adults treated for unilateral Koos grade 4 VS between January 2013 and December 2024. Tumors were categorized as Koos 4a (brainstem compression without fourth ventricular compression) or Koos 4b (brainstem displacement with fourth ventricular compression). The primary endpoint was facial nerve function at the last documented clinical follow-up. Resection extent was determined by preoperative and early postoperative contrast-enhanced MRI. Results: Seventy-five patients (54.0%) had Koos 4a and 64 (46.0%) Koos 4b tumors. Koos 4b tumors were larger (median 38 vs. 30 mm; p < 0.001). Gross-total resection was achieved in 73.5%, and complications occurred in 13.7%. Favorable facial nerve function (House–Brackmann grades I–II) decreased from 98.5% preoperatively to 36.0% immediately postoperatively and was documented in 58.7% at the last available follow-up. In exploratory complete-case multivariable analysis (n = 104), longer operative duration was associated with lower odds of favorable facial nerve function (adjusted OR per 30 min, 0.84; 95% CI, 0.74–0.95; p = 0.005). This association should not be interpreted causally because operative duration likely reflects tumor complexity. The study-specific subtype was not associated with outcome after adjustment, but collinearity with tumor diameter and limited model precision preclude a definitive conclusion. Conclusions: This study-specific subdivision describes differences in anatomical extent and preoperative clinical burden. Its independent prognostic value was not demonstrated in this single-center cohort and requires external validation. Operative duration appears to be a marker of surgical complexity rather than an actionable causal risk factor.