Perinidal hypervascular network on immediate postoperative angiogram after removal of large arteriovenous malformations located distant from the arterial circle of Willis

Unexplained intraoperative or postoperative hemorrhage occurs in certain cases in the surgery of large arteriovenous malformations (AVMs), which are defined as those with maximal diameter of the nidus larger than 5 cm. Immediate postoperative angiograms were evaluated with reference to whether such hemorrhage could be predicted. Immediate postoperative angiograms were taken in 12 patients with large AVMs on the day after the operation during which the AVM was removed. In five of six AVMs located in the distal cerebral hemisphere, a hypervascular network was seen in the vicinity of the removed nidus on the immediate postoperative angiogram. The unexplained hemorrhage occurred in four of the five cases with the hypervascular network. In the remaining six patients with AVMs, the hypervascular network was not observed and they did not develop hemorrhage; the AVMs were located either close to the arterial circle of Willis or in the cerebellum. We believe that the hypervascular network seen on the immediate postoperative angiogram after the removal of a large distal AVM is an important sign predicting the likelihood of unexplained intraoperative and postoperative hemorrhage development. We call this perinidal hypervascular network "modja-modja vessels" because it resembles shaggy hair. The mechanism of its appearance and the pathogenesis of the unexplained intraoperative and postoperative hemorrhage are discussed.