Programmatic Strategies in the Retrosigmoid Approach for Koos Grade IV Vestibular Schwannomas (KG4VS): Exploring Surgical Anatomy, Technical Nuances, and Case Series

Koos Grade 4 Vestibular Schwannomas (KG4VS) pose significant surgical challenges due to their complex anatomy and proximity to cranial nerves. Despite advances in neurosurgery, preserving facial and cochlear nerve functions remains difficult. This study introduces a novel surgical strategy for KG4VS, focusing on minimizing mechanical traction and maximizing functional outcomes through systematic tumor dissection guided by anatomical landmarks in the cerebellopontine angle (CPA) region. We reviewed 105 patients with KG4VS and 206 patients with Koos grade 1-3 vestibular schwannomas (KG1-3VS) to assess the efficacy of our surgical strategy. Surgical anatomy and techniques were detailed. Statistical analysis was performed to compare outcomes between KG4VS and KG1-3VS groups. The surgical protocol initiates with intratumoral decompression to establish a thin capsule, enabling arachnoid dissection from superficial regions (inferior pole, cerebellopontine, and superior pole) down to the brainstem and internal auditory canal (IAC). Critical steps involve IAC adhesion release and precise brainstem depth assessment. Gross total resection (GTR) rates differed significantly between KG4VS (74.3%) and KG1-3VS (95.1%) (p<0.001). KG4VS demonstrated lower hearing preservation (38.5% vs 83.5%) and facial nerve function rates (87.6% vs 97.5%) compared to KG1-3VS (p<0.001). Tumor diameter >3cm (p<0.001), intraoperative morphology alterations (p<0.001), and elevated neurostimulation thresholds (p=0.043) independently predicted long-term facial dysfunction. Postoperative complications comprised infection (3.2%), CSF leak (2.6%), hemorrhage (2.5%), cranial palsy (1.9%), diplopia (1.0%), and cerebellar edema (0.6%). The protocol optimizes surgical efficiency and function outcomes (facial/cochlear) in KG4VS by reducing intraoperative variability and clarifying complex anatomy.