Arteriovenous malformations (AVMs) are complex and rare cerebral vascular dysplasia. The main purpose of treatment is to avoid the neurological impairment caused by hemorrhagic stroke. The Spetzler-Martin (SM) grading system is widely used to estimate the risk of postoperative complication based on maximum AVM nidus diameter, pattern of venous drainage, and eloquence of location. Generally, grade I and II are amenable to surgical resection alone. Grade III is typically treated via a multimodal approach, including microsurgical resection, embolization, and radiosurgery (SRS). Grade IV and V are generally observed unless ruptured. However, some previous studies indicated that despite the high rate of poor outcomes for high-level unruptured AVMs, the mortality for high-level unruptured AVMs are likely lower than untreated patients. With the development of new embolic materials and new intervention strategies, patients with high-level AVMs may have more opportunities to underwent more aggressive interventions. The OHAVM study aims to clarify the clinical outcomes for patients with SM grade IV and V AVMs after different management strategies.