Background Many patients recruited in the Treatment of Brain AVM Study (TOBAS) are managed conservatively. Our aim was to monitor what happened to those patients.

Methods TOBAS comprises two RCTs and multiple prospective registries. All brain AVM patients can participate. This report concerns patients selected for conservative management. The primary trial outcome measure is related to death or dependency (mRS>2) at 10 years. Secondary outcomes include intracranial hemorrhages and non-hemorrhagic neurological and serious adverse events (SAEs). For this report, outcome results are presented using patient-years, Kaplan-Meyer survival curves, and Cox log-rank tests. There was no blinding.

Results From June 2014 to May 2021, 1010 patients were recruited; 498 (49%) were proposed for the prospective observation registry. After exclusions, 434 (87%) patients remained for analysis. The majority of patients had unruptured AVMs (378/434 (87%), of which 195/378 (52%) had a low (Spetzler-Martin 1–2) grade. During a mean follow-up period of 3.2 years (total 1368 patient-years), the primary outcome occurred in 23/434 (5%) patients, corresponding to an incidence of 1.7 per 100 patient-years(95%CI: 1.1–2.5); 0.6 per 100 patient-years (95%CI: 0.2–1.7) for low-grade unruptured AVMs. Poor outcomes were more frequent in patients with a history of rupture (HR, 5.6 (95%CI: 2.4–13.0; P<.001),infratentorial AVMs (HR, 2.9 (95%CI: 1.1–7.3; P=.027), and aged ³55 years (HR, 3.2 (95%CI: 1.4–7.6);P=.007). Major intracranial hemorrhage occurred in 35/434 (8%) patients (2.6 per 100 patient-years (95%CI: 1.9–3.6); 1.3 per 100 patient-years (95%CI: 0.6–2.6) for low-grade unruptured AVMs). Major AVM hemorrhages were more frequent in ruptured (HR, 4.4 (2.1–8.9); P<.001), large (HR, 2.6 (1.1–6.6);P=.039), high-grade (HR, 2.5 (1.2–5.3); P=.013) AVMs with deep venous drainage (HR, 2.1 (1.1–4.2);P=.032). SAEs occurred in 48/434 (11%) patients (3.6 per 100 patient-years (95%CI: 2.7–4.8)).

Conclusion Nearly half of TOBAS participants were observed. Rates of untoward neurological events were within expected boundaries.

Disclosures T. Darsaut: None. H. Alhazmi: None.