MMP9 and schwannoma: MMP9-producing leukocytes promote neo-angiogenesis30 via the upregulation of vascular endothelial growth factor (VEGF) secretion and disruption of the blood-brain barrier.31 Furthermore, MMP9 degrades type IV and V collagen which disrupts vascular permeability and facilitates cellular migration during metastasis.32 We hypothesized that MMP9 in VS may enhance vascular permeability, break down arachnoid planes, and facilitate the outgrowth of schwannoma cells onto the brainstem, thereby making the tumor more adherent.