Arteriovenous Malformations Research Articles

Hypofractionated Gamma Knife Radiosurgery in Big Tumors

Objective: This research aimed to determine the most effective dosages and achieve improved outcomes without surgical intervention, discomfort, or blood loss. Its objective was also to regulate the formation of both benign and malignant tumours and reduce tumour size. Patients and Methods: A study involving 50 patients at Dr. Saad Al-Witry Hospital for Neurosciences and Al-Altaj Hospital examined brain tumours using MRI models, CBCT, and a gamma knife device. Patients with large tumours were included, while those receiving single fractions of treatment and pregnant women were excluded. Patients were prepared using Leksell frames, thermoplastic masks, and personalised stereotactic masks. Gamma Knife radiosurgery sessions were administered based on tumour size, location, and patient tolerance. Results: This study analysed the use of hypofractionation gamma knife treatment for six brain tumours: Meningioma, Glioma, Schwannoma, Arteriovenous Malformations (AVM), Pituitary Adenoma, and Metastasis. The mean dose distribution was higher in metastasis, followed by AVM, meningioma, glioma, schwannoma, and pituitary adenoma. The maximum dose was found in metastasis, followed by meningioma, glioma, pituitary adenoma, AVM, and schwannoma. The coverage of the target reached excellent value for metastasis, followed by AVM and pituitary adenoma. The beam on time for the treatment was highest in meningioma. Following six months of treatment, the study found a significant difference in tumour volume before and after six months. The highest reduction was observed in meningioma cases, followed by glioma and vestibular schwannoma. A study involving ten patients showed a significant reduction in tumours after a year of gamma knife treatment, with meningioma cases showing the highest reduction. Conclusion: The study found that metastasis was the most common cancer, followed by meningioma, glioma, schwannoma, AVM, and schwannoma. After six months, meningioma showed the highest reduction, and meningioma showed a better response to hypofractionation, followed by AVM.

Management of Giant Type II Arteriovenous Malformation of the Neck With Heart Failure in a Pediatric Patient With Coil and NLE Embolization: A Case Report.

Although arteriovenous malformations (AVMs) are typically benign lesions, massive ones can lead to severe complications, including heart failure. Notably, there is no standardized treatment for AVMs. Moreover, management of AVMs can be challenging when these lesions involve vital organ systems, such as the head and neck. In this report, we describe embolization of a massive cervical AVM in a pediatric patient with heart failure. A 10-year-old boy presented with severe heart failure secondary to a massive AVM in the neck (right side). Despite 3 unsuccessful arterial embolization procedures, venous embolization of the dominant outflow vein using a coil and n-butyl-2-cyanoacrylate, lipiodol, and ethanol proved effective, and symptoms, cerebral natriuretic peptide level (568-29 pg/mL), and echocardiography were significantly improved. Although residual AVM persisted in the right upper extremity for 6 months after surgery, his cardiac function remained preserved, and his activities of daily living have improved. Venous embolization is a valuable treatment modality for type II AVMs with a dominant outflow vein. However, considering the patient burden associated with large AVMs, multiple therapeutic interventions at appropriate intervals and long-term follow-up are important. We present a pediatric patient with a type II massive arteriovenous malformation (AVM) in the neck and severe heart failure. Despite undergoing 3 arterial embolization procedures, his condition remained unchanged. However, venous embolization of the dominant outflow vein resulted in a significant improvement in symptoms. This case emphasizes the importance of considering multiple and appropriately timed therapeutic interventions, particularly for massive AVMs, to optimize patient outcomes while minimizing treatment burden.

Headache and Visual Aura Outcomes After Stereotactic Radiosurgery for Patients With Occipital Lobe Arteriovenous Malformations: A Single-Center Retrospective Cohort Study.

Patients with brain arteriovenous malformations (BAVMs) sometimes experience migraine-like chronic headaches. Reportedly, this symptom is common in occipital lobe BAVMs; however, the exact prevalence has not been clarified, and the details of visual auras have been scarcely reported. Furthermore, no comprehensive studies exist on the improvement of migraine associated with visual auras in occipital lobe BAVMs after stereotactic radiosurgery (SRS). Therefore, this study investigated the prevalence of headache with visual aura in patients with occipital lobe BAVMs and evaluated the impact of SRS on preexisting symptoms. As a single-center retrospective cohort study, a total of 104 patients with occipital lobe BAVMs treated with SRS from 1990 to 2024 were analyzed. Pre-SRS and post-SRS symptoms of headache and visual aura were documented. The outcomes measured were headache severity, frequency, aura characteristics, and BAVM obliteration rates. Of the 104 patients, 41 (39%) reported chronic headache pre-SRS, and 37 (36%) experienced migraine with visual aura. Compared with pre-SRS, 90% (28/31) of the patients experienced a >50% reduction in headache severity and frequency at a median follow-up of 11 months, and 89% (25/28) experienced a >50% reduction in aura frequency. Complete headache remission was achieved in 35% (13/31) of patients. Complete obliteration of BAVMs was confirmed in 75% (76/102) of the patients at a median of 33 months. A smaller nidus volume was associated with both complete headache remission and BAVM obliteration, whereas age and sex were not significant predictors of headache outcome. Patients with occipital lobe BAVMs frequently experience migraine-like headaches with visual aura, which may be more prevalent than in the general population. SRS for occipital lobe BAVMs could not only achieve favorable BAVM obliteration with minimal adverse effects but also significantly improve migraine-like symptoms, including visual aura, particularly in patients with smaller nidus volumes.

Treatment of iliac arteriovenous malformation associated with trisomy 21: a case report

Both arteriovenous malformation (AVM) and trisomy 21 are rare diseases. Studies have shown that individuals with trisomy 21 may have potential vascular malformations, with the main site of onset being the portal system. This case involved an adult patient with trisomy 21 who presented with pelvic iliac arteriovenous malformations. The patient underwent interventional treatment, and the prognosis was favorable. It is important to enhance the screening for vascular malformations in patients with trisomy 21 syndrome. At the same time, interventional treatment for AVMs is effective and increasingly important for managing AVMs.

Hereditary haemorrhagic telangiectasias with recurrent ischemic stroke hinted by manganese deposition in the basal ganglia: a case report and literature review

BackgroundHereditary hemorrhagic telangiectasia (HHT) is a rare autosomal dominant inherited vascular disorder that can involve multiple organs, thus can be associated with so many clinical departments that proper screening and diagnosis of HHT are needed for providing better management of both patients and their family members.Case presentationWe present a 58-year-old female patient with recurrent paradoxical brain embolism due to HHT. She received aspirin therapy and underwent pulmonary arteriovenous malformation embolization, recovering well and discharged 3 days postoperatively. Though ischemic stroke caused by HHT-induced vascular disorders has been reported, our patient presented with both recurrent paradoxical brain embolisms and radiologic findings of bilateral globus pallidus manganese deposition at the same time, a combination rarely reported. We also review the literature on the clinical features and management of HHT for prompt diagnosis of this genetic disease behind paradoxical embolism.ConclusionsWhen patients with ischemic stroke, especially recurrent ischemic stroke, have combined arteriovenous malformations (AVMs) in single or multiple organs, or clues for AVMs like manganese deposition in globus pallidus, genetic diseases such as HHT may be the reason for ischemic stroke and shouldn’t be missed in the evaluation of embolic sources.

Reporting of angiographic studies in patients diagnosed with a cerebral arteriovenous malformation: a systematic review

A cerebral arteriovenous malformation (cAVM) is an abnormal tangle of cerebral blood vessels. The consensus document by the Joint Writing Group (JWG) highlighted which cAVM features should be recorded. Subsequent publications have reported cAVM angioarchitecture, but it is unknown if all followed the JWG recommendations. The aim of this systematic review was to describe use of the JWG guidelines. A database search, using the PRISMA checklist, was performed. We describe the proportion of publications that used JWG reporting standards, which standards were used, whether the definitions used differed from the JWG, or if any additional angiographic features were reported. Out of 4306 articles identified, 105 were selected, and a further 114 from other sources. Thirty-three studies (33/219; 15%) specifically referred to using JWG standards. Since the JWG publication, few studies have used their standards to report cAVMs. This implies that the angioarchitecture of cAVMs are not routinely fully described.

8409 Thyroid Nodules A to V: A Discussion on Thyroid Nodules and a Case Report of a Rare Thyroid Arteriovenous Malformation

Abstract Disclosure: M. Townsend: None. M. Brennan: None. C. Kazmierczak: None. P. Czako: None. Introduction: Thyroid nodules are an increasingly common finding on imaging, and the differential is broad. Arteriovenous malformations (AVMs) are a very rare cause of thyroid lesions, with only 6 reported cases to date. Given their scarcity, there is a lack of information regarding these lesions and how they are diagnosed and managed. Our case aims to address these gaps in information. Case: A 58-yo female with a history of hypertension was found by her PCP to have an enlarged left thyroid lobe with a new left sided bruit and palpable thrill. She had undergone prior thyroid ultrasonography for the enlarged left thyroid lobe which noted dilated and tortuous vascular structures at the posterior aspect of the left thyroid consistent with varices. These were being monitored clinically. Thyroid function tests were all normal. Due to concern for atherosclerotic disease, carotid ultrasound was obtained and showed patent carotid arteries with a large left sided thyroid mass. CTA of the neck revealed the mass to be a large (6.0 x 3.0 x 2.4 cm) AVM centered at the left thyroid gland. She underwent diagnostic angiography that confirmed the size of the mass and showed predominant arterial supply from the left superior thyroidal artery as well as bilateral inferior thyroidal arteries. Since the patient was asymptomatic with no reported neck pain, dysphagia or dyspnea, non-operative management with angioembolization was pursued given the risk of significant blood loss with surgical intervention. Following angioembolization, the results and adequacy of treatment are to be followed with repeat imaging given the high risk of reoccurrence. Discussion: AVMs of the thyroid are a very rare but clinically significant cause of thyroid lesions. Left untreated, they can grow in size and lead to compression of the esophagus or trachea, serious bleeding and high output heart failure. Their presentation is often non-specific and on routine thyroid imaging they can be difficult to differentiate from malignant tumors or varices, as in the case of our patient. Management prioritizes embolization, either alone or in conjunction with surgery, depending on the clinical scenario. Regardless of the treatment modality, reoccurrence rates remain high. Conclusion: Our case aims to add to the available information about thyroid AVMs and discuss the clinical presentation, challenges in diagnosis, imaging findings and treatment modalities available to manage these lesions. Presentation: 6/2/2024

7819 Hypoglycemia with Hereditary Hemorrhagic Telangiextsia in a Boy After Liver Transplantation

Abstract Disclosure: H. Miyagi: None. M. Igarashi: None. E. Tanimoto: None. R. Horikawa: None. Introduction: Hereditary hemorrhagic telangiectasia (HHT) causes arteriovenous malformation (AVM) in liver, lung, and brain. It is not clear whether it also causes hypoglycemia. We show a case of HHT combined with hypoglycemia after liver transplantation, improved by nutrition management and growth hormone treatment. Case: 11 months old boy born from a mother with HHT was diagnosed as having with AVM prenatally. He was once operated for the liver AVM but developed a new collateral circulation which caused severe heart failure and lead to liver transplantation. He showed severe growth retardation and malnutrition. His body weight was 4.6 kg and height were 60 cm (-5.8 SD) when he was 1 year old, and blood test showed low albumin for 3 g/dL (3.4 - 4.7 g/dL), and total protein for 4 g/dL (5.7 - 7.5 g/dL). Hypoglycemia was found while switching his nutrition from parenteral to enteral nutrition after operation. His insulin was under 0.3 µU/mL when his blood glucose was 49 mg/dL IGF1 was 17 ng/mL (ref.11 - 149 ng/mL), suggested growth hormone (GH) deficiency. GH stimulation test showed blunted response. GH treatment was started. Other pituitary hormones were within normal range. Nutritional support was added using high-calory milk. After GH treatment, IGF1 level increased to 87 ng/mL. GH and nutritional management improved not only his weight and height gain but also his glycemic control. Discussion: Hypoglycemia may be caused by both GHD and malnutrition. GHD and low IGF1 might be caused by malnutrition. Retesting GH secretion after improvement of nutrition is planned to clarify his pituitary function. Presentation: 6/2/2024

Clinical case of an atypical variant of Labbe's vein structure in a patient with pharmacoresistant temporal epilepsy

Among the commonly recognised structural lesions that cause drug-resistant temporal lobe epilepsy, such as mesial temporal sclerosis, focal cortical dysplasia, benign intracerebral tumors, arteriovenous malformations and cavernous angiomas, researchers have recently noted other pathologies, such as, encephalocele. There are a number of publications on the connection between epilepsy and cerebral venous angiomas. However, there are no published cases of the influence of the abnormal structure and location of normal cerebral vessels on the development of epilepsy. This article presents a case of an atypically large and unusually located vein of Labbe that may have been involved in epileptogenesis and also complicated an anteromedial temporal lobectomy. Forced coagulation and excision of the Labbe vein during surgery did not lead to venous infarction and neurological deficit.

Safety and Efficacy of Multisession Gamma Knife Radiosurgery for (Large Benign Intracranial Lesions > 3cm)

Background: Stereotactic radiosurgery is delivery of an ablative dose of ionizing radiation to a focused target with 3D localization to elicit a specific radiobiological response and sparing the surrounding normal brain tissue. Objective: to evaluate safety and efficacy of Gamma knife radiosurgery by volume staging of large benign intracranial lesions. Patients and methods: prospective cohort study is conducted at Saad Alwitry neuroscience hospital, gamma knife department between June 2017 to February 2021, 60 patients (25 case meningioma ,15 case vestibular schwannoma ,10 case arteriovenous malformation and 10 patients with pituitary macro-adenoma) having large lesions of >3cm were treated by multisession radiosurgery (2-3 sessions), time between sessions (24hr-6m), follow up time ranged from(6ms-3yr). Results: rate of growth control for meningioma, vestibular schawannoma, AVM and pituitary macro-adenoma was 76 %,53.3% ,30% and 60% while reduction in lesion size is 20%, 33.3%, 70% and 30% respectively and clinical improvement was 86.4% for meningioma ,52.6% for VS ,73.2% for AVM and 88% for PA and the complications reported were 15.2%. Conclusion: volume staged multisession GKSR is effective, safe and well tolerated with low morbidity modality for these large lesions.

Atypical venous drainage system and distinct vascular characteristics in pediatric intracerebral hemorrhage caused by multiple micro arteriovenous malformations.

Hemorrhagic brain micro-arteriovenous malformations (micro-AVMs) are considered to constitute a relatively significant portion of pediatric AVMs, though they are often associated with a low bleeding rate, as seen in hereditary hemorrhagic telangiectasia, which frequently involves multiple micro-AVMs. We present a rare case of a 10-year-old girl with multiple hemorrhagic micro-AVMs. Intraoperative findings during the emergency operation for hematoma evacuation and post-operative superselective angiography highlighted the unique angioarchitecture of three micro-AVMs (two lesions in the superficial areas and one lesion in the deep-seated area) and the atypical bleeding source due to the complex congestive venous drainage system. One micro-AVM was successfully occluded by a transarterial emboliozation, and remaining two micro-AVMs underwent gamma knife irradiation as a salvage therapy. Superselective angiography is crucial for detecting micro-AVMs, offering detailed insights into small, localized abnormal vascular drainage systems, and guiding therapeutic strategy. Additionally, micro-AVM-associated unique vascular hypersensitivity, such as vasospasm, requires careful consideration, as invasive procedures may significantly alter the visibility of the entire micro-AVM network.

Genotypes and phenotypes of capillary malformation-arteriovenous malformation: characterization and correlation analysis.

Capillary malformation-arteriovenous malformation (CM-AVM) is a rare genetic disorder characterized by multiple small capillary malformations (CMs) and arteriovenous malformations (AVMs), which has been linked with pathogenic variants in RASA1 and EPHB4. However, more data are needed to explore the phenotypic characteristics and the association between genotypes and clinical phenotypes. Our aim was to investigate the phenotypic and genetic characteristics of CM-AVM in East Asians, identify potential unique phenotypes, and conduct genotype-phenotype association analyses. This is a single-center study prospectively collecting CM-AVM patients' clinical data, with genetic data from blood or tissue samples. A total of 59 patients were enrolled. Thirty-two individuals had a leading CM greater than Schobinger stage II. The trigeminal nerve branches and greater auricular, transverse cervical, and lesser occipital nerves' somatosensory innervation zones divided head and neck CMs into six zones: V1, V2, V3, GA, TC, and LO zones. GA, TC, and LO zones had a positive correlation with one another but a negative correlation with V2 zone involvement. The RASA1 and EPHB4 pathogenic variants were detected in 41 out of 59, which showed two types of variant allele frequency (VAF) distributions. VAF above 30% made RASA1 pathogenic variants more susceptible to multifocal CMs than those below 30%. Leading CMs in the head and neck exhibit two segmentation patterns, anterior and lateral, which may differ in ear involvement and progression. Germline RASA1 pathogenic variants increased multifocal CM risk more than the somatic variants.

Comparing stand-alone endovascular embolization versus stereotactic radiosurgery in the treatment of arteriovenous malformations with Spetzler-Martin grades I–III: a propensity score matched study

BackgroundArteriovenous malformations (AVMs) are uncommon cerebral lesions that can cause significant neurological complications. Surgical resection is the gold standard for treatment, but endovascular embolization and stereotactic radiosurgery (SRS) are viable...

Commentary on Mosaic Copy Number Variation in a Patient with Cerebral and Pulmonary Arteriovenous Malformations and Recurrent Epistaxis.

Commentary on Mosaic Copy Number Variation in a Patient with Cerebral and Pulmonary Arteriovenous Malformations and Recurrent Epistaxis.

Adjunctive venous sinus stenting in transvenous embolization of vein of Galen malformations.

Vein of Galen malformations are congenital arteriovenous malformations primarily treated by endovascular embolization via transarterial or transvenous approaches. transvenous embolization can be utilized to close the malformation but may be difficult in patients with venous stenosis or blockages, which drive venous hypertension and lead to significant neurologic consequences. Here, we illustrate the atypical placement of an intracranial venous sinus stent to improve outflow after transvenous embolization in pediatric patients with the vein of Galen malformation. A retrospective review of clinical databases at two high-volume endovascular centers from January 2018 to March 2023 identified all vein of Galen malformation patients who received a venous sinus stent during transvenous embolization. Clinical data, imaging, angioarchitecture, operative details, postoperative management, and follow-up were reviewed. Three patients presented for transvenous embolization after multiple staged transarterial embolizations of their vein of Galen malformation. Transvenous access was complicated by lateral sinus stenosis, which was temporarily relieved by balloon angioplasty. After transvenous embolization by pressure cooker technique, the dural sinuses were stented using the existing venous guide catheter. Venous angiography demonstrated improved flow across the stenosed areas and post-embolization angiography demonstrated normalized venous drainage with widely patent stents. One patient experienced postoperative oculomotor nerve palsy unrelated to the stent placement. All patients demonstrated a complete cure of their vein of Galen malformations with patent venous sinus stents on follow-up. In patients with the vein of Galen malformation and venous hypertension receiving transvenous embolization, venous sinus stenting may be a safe and effective option to reduce aberrant cortical venous drainage and improve normal outflow. Further studies are warranted to investigate its benefit in high-flow vascular malformations.

Mosaic Copy Number Variation in a Patient with Cerebral and Pulmonary Arteriovenous Malformations and Recurrent Epistaxis.

Mosaic Copy Number Variation in a Patient with Cerebral and Pulmonary Arteriovenous Malformations and Recurrent Epistaxis.

Coil Embolization of a Ruptured Anterior Spinal Artery Aneurysm Associated with Spinal Cord Arteriovenous Malformation.

The present report describes a patient with spinal cord arteriovenous malformation (AVM) and an associated anterior spinal artery aneurysm presenting with subarachnoid hemorrhage. Diagnostic spinal angiography revealed an intramedullary AVM, located at the T10-T11 level, and a prenidal saccular aneurysm at the junction of the radiculomedullary artery and the anterior spinal axis, fed by the right T8 segmental artery. The patient underwent successful selective coil embolization of the aneurysm. Follow-up angiography 3 months postoperatively showed no recurrence of the aneurysm.

Comparison of a Novel Liquid Embolic System with Commonly Used Embolic Agents in the Endovascular Treatment of Intracranial Dural Arteriovenous Fistulas: A Single-Center Experience

Background/Objectives: Endovascular embolization is an effective treatment option for cerebral arteriovenous malformation (AVM) and dural arteriovenous fistulas (DAVFs). The objective of this study was to assess the safety and efficacy of MenoxTM in patients with cranial dural arteriovenous fistulas. Methods: From January 2021 to January 2023, 19 patients with intracranial DAVFs underwent embolization procedures. All patients were treated by embolization with MenoxTM or/and in combination with other embolization products such as Onyx (Covidien, Irvine, California), PHIL (MicroVention, Tustin, California), and Squid (Balt Extrusion, Montmorency, France). Treatment approaches were selected depending on the anatomical location of the fistula. Patients were monitored and followed-up for 12 months. Results: The patients’ mean age was 56.26 ± 16.49 years. Of these 19 patients, 58% (n = 11) were treated with the MenoxTM liquid embolizing agent (LEA) alone or in combination with different LEAs, while n = 7 were treated with other LEAs and 1 patient was treated solely with coils. Complete occlusion of DAVFs with MenoxTM and other agents was evident in 68.4% (n = 13/19) of patients. Complete occlusion (100%) was observed in the sinus rectus, transverse sinus, and diploic veins of the orbital roof, while complete occlusion was observed in 50% of falcotentorial patients and 60% of superior sagittal sinus patients. The lowest rate of complete fistula obliteration was observed in the dural carotid cavernous fistula (CCF) group (25%). An intra-procedural adverse event occurred in one patient. No other post-procedural adverse events were noted. Furthermore, in patients treated with MenoxTM, total occlusion was achieved in 72.7% (n = 8) of patients, whereas the non-MenoxTM group had 62.5% (n = 5) of patients with 100% occlusion and 37.5% (n = 3) of patients with subtotal occlusion. Conclusions: Outcomes using MenoxTM alone and in combination with other agents were effective, and it is safe for the treatment of dural arteriovenous fistulas.

Arteriovenous malformation-associated aneurysms in the pediatric population: the University of Pittsburgh Medical Center experience.

Arteriovenous malformations (AVMs) are the most common cause of spontaneous intracranial hemorrhage (ICH) in children, often leading to devastating complications. The current literature from the adult AVM population suggests that both younger age and associated aneurysms carry an increased risk of hemorrhagic presentation. However, detailed analysis of pediatric AVM-associated aneurysms and their link to ICH is relatively unknown, with the literature largely consisting of case reports. This study aimed to determine whether AVM-associated aneurysms predispose pediatric patients to ICH. A retrospective cohort study of 238 pediatric patients with AVMs who presented to the Children's Hospital of Pittsburgh from 1988 to 2023 was performed. Hospital records, patient charts, and radiographic imaging studies were reviewed for patient demographic characteristics, presentation status, and AVM architecture. Of the 238 total patients, 44 (18.5%) children with AVM had associated aneurysms. There were 19 (38.8%) feeding artery aneurysms, 8 (16.3%) intranidal aneurysms, 21 (42.9%) postnidal aneurysms, and 1 (2.0%) unrelated aneurysm of 49 aneurysms. Five patients had venous varices. One hundred forty (58.8%) children presented with hemorrhage. Twenty-one of 44 (47.7%) patients with an AVM-associated aneurysm presented with hemorrhage, whereas 119 of 194 (61.3%) with a solitary AVM presented with hemorrhage (p = 0.10). On multivariate analysis, postnidal aneurysm (OR 0.36, p = 0.037) and an increased number of draining veins (OR 0.66, p = 0.049) were significantly associated with a decreased likelihood of hemorrhagic presentation. Deep venous drainage was associated with an increase in hemorrhagic presentation (OR 2.25, p = 0.0045) on multivariate analysis. Approximately one-fifth of children with AVMs in this study had accompanying aneurysms, and in this patient population, those with postnidal aneurysms and increased number of draining veins had a decreased incidence of hemorrhage on presentation. Feeding artery and intranidal aneurysms were not associated with an elevated risk of hemorrhagic presentation.

Conservative management of brain arteriovenous malformations: results of the prospective observation registry of a pragmatic trial.

Many patients recruited in the Treatment of Brain Arteriovenous Malformations Study (TOBAS) are managed conservatively. The aim of this study was to monitor what happened to those patients. TOBAS comprises two randomized controlled trials and multiple prospective registries. All patients with brain arteriovenous malformations (AVMs) can participate. This report concerns patients selected for conservative management. The primary trial outcome measure is related death or dependency (modified Rankin Scale [mRS] score > 2) at 10 years. Secondary outcomes include intracranial hemorrhages, nonhemorrhagic neurological events, and serious adverse events (SAEs). For this report, outcome results are presented using patient-years, Kaplan-Meier survival curves, and Cox log-rank tests. There was no blinding. From June 2014 to May 2021, 1010 patients were recruited, of whom 498 (49%) were proposed 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 (52%) were low grade (Spetzler-Martin grade I or II). During a mean follow-up period of 3.2 years (total 1368 patient-years), the primary outcome occurred in 23 of 434 (5%) patients, corresponding to an incidence of 1.7 (95% CI 1.1-2.5) per 100 patient-years. For unruptured AVMs the incidence was 1.1 (95% CI 0.7-1.9) per 100 patient-years, and for low-grade unruptured AVMs it was 0.6 (95% CI 0.2-1.7) per 100 patient-years. Poor outcomes were more frequent in patients with a history of rupture (HR 5.6 [95% CI 2.4-13.0], p < 0.001), infratentorial AVMs (HR 2.9 [95% CI 1.1-7.3], p = 0.027), and age ≥ 55 years (HR 3.2 [95% CI 1.4-7.6], p = 0.007). Major intracranial hemorrhage occurred in 35 of 434 (8%) patients (incidence of 2.6 [95% CI 1.9-3.6] per 100 patient-years; 2.0 [95% CI 1.3-2.9] per 100 patient-years for unruptured AVMs and 1.3 [95% CI 0.6-2.6] per 100 patient-years for low-grade unruptured AVMs). Major AVM hemorrhages were more frequent in ruptured (HR 4.4 [95% CI 2.1-8.9], p < 0.001), large (HR 2.6 [95% CI 1.1-6.6], p = 0.039), and high-grade (HR 2.5 [95% CI 1.2-5.3], p = 0.013) AVMs and those with deep venous drainage (HR 2.1 [95% CI 1.1-4.2], p = 0.032). SAEs occurred in 48 of 434 (11%) patients (incidence of 3.6 [95% CI 2.7-4.8] per 100 patient-years). For unruptured AVMs the incidence was 2.8 (95% CI 2.0-4.0) per 100 patient-years, and for low-grade unruptured AVMs it was 1.8 (95% CI 1.0-3.2) per 100 patient-years. Nearly half of TOBAS participants were observed. Rates of untoward neurological events were within expected boundaries.