Cerebellar AVM, tonsillar subtype. The tonsillar AVM is a small, paramedian AVM in the tonsil, easily circumscribed in the cisterna magna by separating the tonsils, opening the cerebellomedullary fissure, and splitting the tonsillobiventral fissure. The AVM is fed unilaterally by PICA (left lateral view), which can be interrupted early in the dissection. It is drained by tonsillar and inferior vermian veins (posterior view). Abbreviations: BA = basilar artery; PICA = posterior inferior cerebellar artery; SS = straight sinus; TS = transverse sinus; Torc = torcula; IVV = inferior vermian vein; ToV = tonsillar vein; RToV = retrotonsillar vein.