Clinical outcome of brainstem arteriovenous malformations after incomplete nidus obliteration

•Brainstem AVMs are challenging lesions due to frequent residual nidus post-treatment.•We found a rupture risk of 4.9% per patient-year over median follow-up of 72 months.•Of the five patients who ruptured after treatment, 80% had ruptured once already.•Of the five patients who ruptured after treatment, 80% had an intranidal aneurysm.•These data suggest that partial obliteration still carries risk of future rupture. Brainstem arteriovenous malformations (AVMs) present a formidable therapeutic challenge, and a variety of surgical and non-surgical treatment strategies can be used to obliterate the AVM nidus, eliminating its risk of hemorrhage. However, complete obliteration of brainstem AVMs is often not possible. We aimed to investigate the natural history of brainstem AVMs with incomplete nidus obliteration after initial treatment. Data from consecutive patients who presented to our institution during the study period with a brainstem AVM and residual nidus after treatment were retrospectively reviewed. We evaluated patients for the incidence of AVM rupture and calculated the risk of rupture after treatment resulted in incomplete nidus obliteration. A total of 14 patients were included, five of whom suffered rupture after incomplete nidus obliteration (36%). Annual risk of rupture was 4.9% (95% CI: 1.60–11.5) per patient over a median follow-up of 72 months. The most common treatment modality of these patients was SRS-alone (n = 6), and two (33%) patients who underwent this treatment later ruptured after 103 and 130 months. Of the five patients who ruptured after treatment, 80% had already ruptured once, and 80% had an intranidal and/or feeding artery aneurysm. Brainstem AVMs with incomplete nidus obliteration are at high risk of future rupture. Patients with brainstem AVMs who have a residual nidus after treatment should be counselled about the risk of AVM rupture and be recommended to undergo close follow-up imaging studies to monitor the nidus.