New concepts in adult brain arteriovenous malformations

Development of De Novo Arteriovenous Malformation Following Ischemic Stroke: Case Report and Review of Current Literature

BACKGROUND

Arteriovenous malformations (AVMs) are hypothesized to be static, congenital lesions developing as early as 4 weeks of fetal life. New literature has shown that AVMs may represent dynamic and reactive vascular lesions arising from cerebral infarction, inflammation, or trauma. A literature search reveals 17 previously reported cases of new AVM formation after previous negative imaging studies. This reactive development or "second hit" theory suggests that at a molecular level, growth factors may play a vital role in aberrant angiogenesis and maturation of an arteriovenous fistula into an AVM.

CASE DESCRIPTION

A 52-year-old female presented with a ruptured left frontal AVM demonstrated by computed tomography angiography and digital subtraction angiography. The patient had suffered an acute ischemic stroke in the similar cerebral vascular territory 8 years prior due to left internal carotid artery occlusion. Detailed neuroimaging at that time failed to reveal any vascular malformation, suggesting that the AVM might have developed in response to initial vascular insult.

CONCLUSIONS

We believe that there might exist a subset of AVMs that display dynamic characteristics and could potentially appear, grow, or resolve spontaneously without intervention, especially in the presence of local growth factors and molecular signaling cascades. When combined with a previous cerebral insult such as stroke, trauma, or inflammation, de novo AVM formation may represent a "second hit" with abnormal angiogenesis and vessel formation.

Symptomatic de novo arteriovenous malformation in an adult: Case report and review of the literature

BACKGROUND

Cerebral arteriovenous malformations (AVMs) have been long thought to be a congenital anomaly of vasculogenesis in which arteries and veins form direct connections forming a vascular nidus without an intervening capillary bed or neural tissue. Scattered case reports have described that AVMs may form de novo suggesting they can become an acquired lesion.

CASE DESCRIPTION

The current case report describes a patient who presented with new-onset seizures with an initial negative magnetic resonance imaging (MRI) of the brain and subsequently developed an AVM on a MRI 9 years later.

CONCLUSION

This case joins a small, but growing body of literature that challenges the notion that all AVMs are congenital.

Relevance of bleeding pattern on clinical appearance and outcome in patients with hemorrhagic brain arteriovenous malformations

Although several descriptions of the angioarchitectural features of brain arteriovenous malformations (AVMs) associated with higher hemorrhagic risk have been reported, the prognostic value of the different bleeding patterns still needs to be elucidated. This study evaluated the influence on clinical appearance and outcome of the parenchymal and non-parenchymal (subarachnoid hemorrhage-SAH-and intraventricular hemorrhage-IVH) bleedings associated with ruptured AVMs. Clinical records and neuroradiological examinations of 30 patients with hemorrhagic AVMs were reviewed in order to identify their angioarchitectural features and the associated bleeding pattern. These data along with demographic characteristics and treatment modality were dichotomized and their relationship with clinical status at admission and follow-up was tested. IVH as well as parenchymal hematomas larger than 20 cm(3) appeared associated with a severe clinical status at admission, whereas SAH involving basal cisterns was significantly associated with unfavorable outcome. Age, sex and angioarchitectural features did not show significant association with the severity of the prognosis. However, none of these bleeding patterns appeared as an independent risk factor of poor outcome at multivariate analysis. In conclusion, our data emphasized the possibility that non-parenchymal bleeding may worsen the outcome of patients with hemorrhagic AVMs.

De novo formation of large arteriovenous shunting and a vascular nidus mimicking an arteriovenous malformation within an anaplastic oligodendroglioma: treatment with embolization and resection

The authors report the de novo occurrence and treatment of an arteriovenous lesion within an anaplastic oligodendroglioma in a patient with previously unremarkable brain imaging. Intracranial arteriovenous malformations (AVMs) are believed to be congenitally acquired lesions, and their association with brain neoplasms is extremely rare. Diagnostic imaging revealed a mass lesion with large arteriovenous shunts and a vascular nidus mimicking a true AVM. Histological and immunohistochemical testing showed an anaplastic oligodendroglioma mixed with an AVM. The clinical, radiological, and operative data are reviewed, as are the histopathological findings. To the authors' knowledge this is the first case of de novo occurrence of an arteriovenous lesion with large shunts and a vascular nidus within an anaplastic oligodendroglioma.

De novo cerebral arteriovenous malformation: case report and literature review

We describe a rare case of a de novo cerebral arteriovenous malformation (AVM) in a 9-year-old girl. MR imaging at 6 years of age demonstrated band heterotopia. Follow-up MR imaging 3 years later demonstrated a new 3.5-cm AVM in the left parietol-occipital region, confirmed by conventional angiography. This report, along with limited previous reports, suggests that AVMs can be acquired lesions and that AVM development is a dynamic process extending into the postnatal period.

Neurologie des malformations artérioveineuses cérébrales

INTRODUCTION

Brain arteriovenous malformations (AVMs) constitute a neurovascular disorder that comes to clinical attention mainly in young adults in their mid thirties. Associated symptoms often require neurological treatment for symptomatic seizures (focal or generalized), headaches (episodic or chronic), progressive neurological deficits, or spontaneous AVM rupture leading to intracerebral, intraventricular, and/or subarachnoid hemorrhage.

STATE OF ART

Little data exist in the medical literature regarding the natural history risk of the disease and no controlled studies are available on the risk of invasive AVM treatment (endovascular, neurosurgery, radiotherapy).

PERSPECTIVES

This review focuses on all aspects of neurological brain AVM management and discusses possible predictors of the natural history risk as well as the benefit and risk of invasive treatment.

CONCLUSIONS

AVM patient management is ideally based on a trans-disciplinary approach via a neurovascular team of neurologists, neuroradiologists, neurosurgeons, and radiotherapists. A newly diagnosed AVM does not necessarily represent an a priori indication for interventional treatment. The decision in favor or against therapy mainly depends on clinical criteria (ruptured versus unruptured AVM, neurological exam, patient age and co-morbidity, etc.) and the angioarchitecture of the malformation. The ARUBA study is going to be the first randomized clinical trial comparing the risk of invasive treatment versus non-invasive management.

Clinical Outcome After First and Recurrent Hemorrhage in Patients With Untreated Brain Arteriovenous Malformation

Background and Purpose— The morbidity from spontaneous hemorrhage of untreated brain arteriovenous malformations (AVM) is not well described.

Methods— The 241 consecutive AVM patients (mean age 37±16 years, 52% women) from the prospective Columbia AVM Databank initially presenting with hemorrhage were evaluated using the Rankin Scale (RS) and the National Institute of Health Stroke Scale (NIHSS). From the 241 AVM patients, 29 (12%) had subsequent intracranial hemorrhage during follow-up. For further comparisons, 84 non-AVM patients with intracerebral hemorrhage from the Northern Manhattan Study (NOMAS) served as a control group.

Results— In 241 AVM patients presenting with hemorrhage the median RS was 2 and the median NIHSS was 1 (49% RS 0 to 1, 61% NIHSS <2). The median time between hemorrhage and clinical evaluation was 11 days (mean 219 days). Recurrent AVM hemorrhage during follow-up resulted in no significant increase in morbidity (median RS 2, P =0.004; median NIHSS 3, P =0.322; time between hemorrhage and study evaluation: median 55 days, mean 657 days). Among AVM-hemorrhage subtypes, parenchymatous AVM hemorrhage was associated with higher stroke morbidity (odds ratio, 2.9; 95% CI, 1.5 to 5.8 for NIHSS ≥2) than nonparenchymatous hemorrhages. Parenchymatous AVM hemorrhage had a significantly better outcome (median NIHSS 1) than non-AVM related hemorrhage (median NIHSS 12; P <0.0001).

Conclusions— Hemorrhage, either at initial presentation or during follow-up of untreated AVM patients appears to carry a lower morbidity than intracranial hemorrhage from other causes. These findings support a careful weighing of risks from interventional treatment and natural history.

Fatal hemorrhagic complication following endovascular treatment of a cerebral arteriovenous malformation

Evaluation of the natural history of brain Arteriovenous Malformations (AVMs) including its morbidity and mortality is a crucial point in the management of patients having a cerebral AVM. The risks associated with the AVM natural history, especially regarding the occurrence of an hemorrhage, have to be compared to the risks due to the therapeutic approach. In the literature, the risk of annual bleeding of an AVM is estimated from 2 to 4%. Morbidity from AVM rupture is estimated from 13% to 50% with a risk of mortality reported from 3 to 30%. Endovascular treatment is an efficient tool in the therapy of these lesions. However, AVM embolization remains a difficult procedure. Complications of the endovascular treatment must be evaluated in relation to the potential risk associated to the AVM natural history. After AVM endovascular treatment, morbidity with permanent neurological deficit is reported in 0.4% to 12.5% of patients and mortality in 0.4% to 7.5%. In more recent reports, after brain AVM embolization, a permanent neurological deficit is estimated to occur in 9% of patients and death in 2%. Hemorrhage appears the most frequent and serious complication in the endovascular treatment of a brain AVM. We report a case of fatal hemorrhagic complication following endovascular treatment of a cerebral AVM in a 20 year old patient. This case contributes to remind that embolization, even in specialized centers with experience in the management of this pathology, can be followed by a poor and even fatal outcome. In most cases, the treatment is performed in order to protect the patient of a potential risk. Consequently, the complication of the embolization must always be carefully considered and discussed between the medical team, the patient and its family for planning the AVM endovascular treatment.

Brain arteriovenous malformations in adults

Arteriovenous malformations of the brain (AMB) can cause stroke when they rupture. Epidemiological and imaging research has found that about 50% of patients with AMB present with haemorrhage, and the other 50% either present with non-focal symptoms, such as headache, seizure, or focal neurological deficit, or have no symptoms and the lesion is found during unrelated investigations. Treatment for arteriovenous malformations aims to prevent and resolve haemorrhage and is a growing interdisciplinary challenge. Although treatment uses enormous resources, there have been few studies on the risk-benefit ratios for treatment of unruptured AMB and the best approaches.

De novo presentation of an arteriovenous malformation. Case report and review of the literature

The authors report the case of a patient with a de novo arteriovenous malformation (AVM), indicating that the origin of these lesions may not always be congenital. A 3-year-old girl who was struck by a car suffered a mild head injury and experienced posttraumatic epilepsy. The initial magnetic resonance (MR) image obtained in this child revealed only a small contusion in the left frontal lobe. Intractable epilepsy subsequently developed. A second MR image obtained almost 4 years after the injury demonstrated an AVM in the right posterior temporal lobe that was verified using angiography. The lesion was classified as a Spetzler-Martin Grade III AVM. The patient underwent embolization of the feeding vessels followed by gamma knife surgery. Fourteen months after treatment she was asymptomatic. Follow-up MR images demonstrate no evidence of an AVM and no changes in the white matter. This case presents a de novo AVM that developed within approximately 4 years. The findings indicate that AVMs may not always be congenital and reinforce the concept that the natural history of AVMs is dynamic. Lesions may appear de novo, grow, and thrombose spontaneously.

[Cerebral arteriovenous malformations: evaluation of the hemorrhagic risk and its morbidity]

Evaluation of the hemorrhagic risk associated with intracranial arteriovenous malformation is a keypoint question for physicians involved in the management of these patients. Indeed, this hemorrhagic risk including its morbid-mortality consequences has to be balanced with the therapeutic risk. All the published studies have estimated the annual bleeding risk between 2 to 4%; a first hemorrhagic event is associated with an increased risk of a new bleeding. Mortality associated with AVM bleeding is estimated between 10 to 15%, with a 50% risk of morbidity. Factors associated with an increased risk of bleeding include existence of aneurysms (intranidal or flow related pedicular, proximal or distal) and some venous drainage patterns such as deep venous drainage or venous stenosis.

[Intracranial arteriovenous malformations: review of epidemiologic and genetic data]

Estimation of prevalence and incidence rates in a disease is crucial to estimate the risk of the natural evolution of the disease. We have reviewed published data on intracranial AVM and have noted that reported estimated risks were quite variable mainly due to the lack of population-based prospective data. Nonetheless, estimation of these parameters can be made from selected populations. We will note also that there is no published data confirming a genetic origin for the most frequent group of sporadic AVM.

Clinical characteristics and surgical results of patients with cerebral arteriovenous malformations

BACKGROUND

Cerebral arteriovenous malformation (AVM) is a common vascular disease in neurosurgery, and the indication for alternative treatments remains controversial. In a review of a series of 2086 patients with AVMs, the clinical characteristics and surgical results were assessed.

METHODS

Collected data of 2086 consecutive patients with AVMs from January 1956 to October 2001 were analyzed. All patients were divided into 2 groups: traditional surgery group (from 1956 to 1991) and microsurgery group (from 1992 to 2001). The variables assessed for clinical characteristics in our study included age (at presentation), sex, Spetzler-Martin grade, and first presentations. Surgical complications were assessed between different surgery groups by chi(2) test.

RESULTS

Cerebral AVMs are more commonly diagnosed at age of 20 to 40 years, which comprises almost one half of the whole population. The size of the AVMs ranged from 1 to 9 cm. There were 77 cases of giant AVMs in this series that were treated by a combination of surgical resection and intraoperative embolization. Hemorrhage (43.4%), headache (24.9%), and seizure (17.3%) were the first 3 common presentations. Regarding Spetzler-Martin grading system, the percentage of grade 3 to 5 patients increased, whereas that of grade 1 patients decreased in the microsurgery group (P = .00). However, compared with the traditional surgery group, the incidence of main surgical complications (death, hemiparalysis, cranial nerve dysfunction, and gastrointestinal hemorrhage) decreased significantly in the microsurgery group (P = .00). Although the incidence of main surgical complications had no statistical difference between early (from 1992 to 1996) and late microsurgery subgroup (from 1997 to 2001) (P = .796), the incidence really decreased with increase of higher-grading patients (grade 3-5) in the late microsurgery group (P = .00).

CONCLUSIONS

Cerebral AVM is one of the important reasons for spontaneous intracranial hemorrhage in patients younger than 40. Spetzler-Martin grading system is helpful to predict the surgical risk. Microsurgical technique has made surgical treatment safer and become the best choice for patients with cerebral AVM.

Multimodal ultrasound assessment of cerebral hemodynamics in a patient with a diffuse cerebral angiomatosis

Recent newly developed ultrasound (US) techniques extend our ability to study the cerebral hemodynamics in patients with arteriovenous malformations (AVM) beyond the conventional cerebral blood flow velocity (CBFV) analysis. We present US data of global cerebral blood flow (CBF) and global cerebral circulation time (CCT) in a patient with a unique bihemispherial diffuse cerebral angiomatosis and compare them with 10 age-matched controls. In addition, the estimation of an US-derived global cerebral blood volume (CBV) is proposed. Duplex sonographic CBF analysis revealed 2620 mL/min in the patient and 754 +/- 93 mL/min in controls. Doppler sonographic CCT was 2.9 s and 6.3 +/- 1.5 s and CBV 126 mL and 79 +/- 19 mL, respectively. US allows a simple, minimal invasive bedside analysis of several global hemodynamic parameters that might provide valuable additional information in patients with diffusely altered cerebral hemodynamics.

Adult cranial dura II: venous sinuses and their extrameningeal contributions

This is the second of two articles designed to provide detailed user-friendly schematics of the adult dural vascular anatomy. It supplements the first article on the intrinsic meningeal arteries and veins by describing the dural venous sinuses along with their three remaining connections: to the brain, to the diploë, and to the extracranial soft tissues. The discussion of this anatomy is supplemented by illustrative pathologic insights. This information will assist in interpreting neuroimaging studies, communication with clinicians, and teaching of this complex subject.

Arteriovenous Malformations in the Brain

Arteriovenous malformations (AVM) are a leading cause of intracerebral hemorrhage, especially among the young. Because they pose a lifelong risk of serious bleeding, definitive treatment to obliterate the AVM should be pursued in the majority of patients. Microsurgical resection of a small AVM located in the superficial or non- eloquent brain achieves high cure rates with low morbidity, and is the recommended choice for such lesions. Radiosurgery with gamma knife, linear accelerator, or heavy ion beam irradiation is an alternative therapy for AVM treatments less than 3 centimeters in diameter located in brain regions where surgery is likely to produce major neurologic deficits, or for patients unable or unwilling to undergo craniotomy and resection. Cure rates are lower than with microsurgery, and obliteration of the lesion may take 2 to 3 years, during which time the patient remains at risk for hemorrhage. Because rates of recurrent hemorrhage are higher than rates of initial bleeding, radiosurgery may be a good option for patients who have not yet had an intracranial hemorrhage. Endovascular embolization as sole therapy is curative only in a small percentage of cases, but is recommended as part of a multimodal approach to reduce the size of a large AVM, and decrease bleeding risk of lesions with multiple or inaccessible feeding vessels or associated aneurysms prior to surgery or radiotherapy. Currently, treatment decisions must rely solely on Class III evidence from case series and expert opinion. Randomized clinical trials are needed to provide objective guidelines for the future management of patients with an AVM.

Adult cranial dura I: intrinsic vessels

This is the first of two articles designed to provide user-friendly schematics of the adult dural vascular anatomy. It describes the intrinsic meningeal arteries and veins of the skull base/cranial vault and the dural partitions (the tentorium, falx cerebelli, and falx cerebri). The discussion of this anatomy is supplemented by illustrative pathologic insights. The second article focuses on the dural sinuses and their remaining tributaries from the brain, diploë, and emissary veins from the extracranial soft tissues. This information will assist in interpreting neuroimaging studies, communications with clinicians, and teaching of this difficult subject.

De novo cerebral arteriovenous malformation: case report

OBJECTIVE AND IMPORTANCE

Arteriovenous malformations (AVMs) are generally thought to have a congenital cause. This is the first report of an angiographically proven de novo cerebral AVM in an adult patient without previous vascular abnormality.

CLINICAL PRESENTATION

A 26-year-old African-American woman developed multiple cranial nerve deficits and ataxia over the course of a few days after a streptococcal throat infection. T2-weighted magnetic resonance imaging scans revealed a hyperintense signal in the midbrain with extension into the diencephalon. A cerebral angiogram performed at that time to exclude vasculitis revealed normal cerebral vasculature. The patient was treated with corticosteroids, and symptoms resolved. Subsequently, at the age of 32, this patient presented with a severe headache and emesis, but with no focal neurological deficit.

INTERVENTION

The patient's cranial computed tomographic and magnetic resonance imaging scans revealed a right posterior temporal intraparenchymal hemorrhage, and cerebral angiography revealed a new 3- by 2-cm AVM. The patient underwent microsurgical resection of the AVM and associated hematoma. Postoperative angiography revealed no evidence of residual AVM.

CONCLUSION

This study details the case of a woman who developed a de novo cerebral AVM during a 6-year period. This report challenges the conventional belief that all AVMs have a congenital cause.

Incidence of adult brain arteriovenous malformation hemorrhage in a prospective population-based stroke survey

BACKGROUND

Brain arteriovenous malformations (AVMs) represent a potential source of intracranial hemorrhage, especially in young adults, but prospective population-based incidence data on AVM hemorrhage are lacking. We investigated the incidence of first-ever AVM hemorrhage in adults based on population data from the Northern Manhattan Stroke Study (NOMASS).

METHODS

NOMASS is a prospective, population-based, stroke incidence survey collecting all hospitalized and nonhospitalized cases with first-ever (incident) stroke over the age of 20 in a ZIP code-defined area. All patients undergo CT and/or MR brain imaging and clinical data are systematically collected from the medical records. For this study, data on all cases with incident intracranial hemorrhage, i.e. any intracerebral, intraventricular and/or subarachnoid hemorrhage, occurring between July 1, 1993 and June 30, 1997 were used. Patients with intracranial hemorrhage due to trauma, tumor or intracranial vascular malformations other than a previously unknown AVM were excluded from the study.

RESULTS

Of the 207 patients diagnosed with a first-ever intracranial hemorrhage, 3 cases (1.4%) with an underlying brain AVM were identified. The crude incidence rate for first-ever AVM hemorrhage in our adult population was 0.55 per 100,000 person-years (95% confidence interval 0.11-1.61).

CONCLUSIONS

Our results support prior findings from retrospective surveys. Population-based studies providing a prospective design for AVM detection and diagnosis are needed to confirm the data.

Dopplersonographic measurement of global cerebral circulation time using echo contrast-enhanced ultrasound in normal individuals and patients with arteriovenous malformations

Echo-contrast "bolus tracking" by ultrasound (US) is an exciting new tool to study cerebral haemodynamics. In the present study, a global cerebral circulation time (CCT) was measured by extracranial Doppler as the time difference of contrast bolus arrival between the internal carotid artery and internal jugular vein. A total of 64 healthy volunteers and 9 patients with an angiographically diagnosed arteriovenous malformation (AVM) were studied. CCT in volunteers and patients was calculated as the time interval between the points of 10% rise (CCT(1)) and 90% rise (CCT(3)) of the total intensity increase and between the turning points (CCT(2)) of the resulting time-intensity curves. In the volunteer group, CCT(1) was 5.4 +/- 1.8 s, CCT(2) was 7 +/- 1.3 s and CCT(3) 7.5 +/- 1.8 s. CCT results in the AVM group were 2.8 +/- 2.5 s, 3.0 +/- 1.3 s and 4.5 +/- 2.1 s, respectively, and differed significantly from the controls. For the first time, we could confirm a significant shortening of CCT in patients with cerebral AVM by US. The presented test might become a new, additional tool for AVM evaluation and follow-up of treatment in these patients.

Incident hemorrhage risk of brain arteriovenous malformations located in the arterial borderzones

BACKGROUND AND PURPOSE

We sought to assess the relative risk of hemorrhagic presentation of brain arteriovenous malformations (AVMs) located in the arterial borderzone territories.

METHODS

The 464 consecutive, prospectively enrolled patients from the New York AVM Databank were analyzed. AVM borderzone location was coded positive when the malformation was supplied by branches of at least 2 of the major circle of Willis arteries (anterior, middle, and/or posterior cerebral arteries). AVMs fed by branches of only 1 major pial or any other single artery served as a comparison group. Clinical presentation (diagnostic event) was categorized as (1) intracranial hemorrhage, proven by brain imaging, or (2) seizure, focal neurological deficit, headache, or other event with no signs of AVM hemorrhage on brain imaging.

RESULTS

In 48% (n=222) of the patients, AVMs were located in the arterial borderzone territories; in 52% (n=242) a non-borderzone location was found. Hemorrhage was the presenting symptom in 44% (n=205); 28% (n=132) presented with seizures, 11% (n=52) with headaches, 7% (n=34) with a neurological deficit, and 9% (n=41) with other or no AVM-related symptoms. The frequency of incident AVM hemorrhage was significantly lower in borderzone AVMs (27%, n=61) than in non-borderzone malformations (60%, n=144; P:<0.001). This difference remained significant in a multivariate model controlling for age, sex, AVM size, deep venous drainage, and presence of aneurysms (odds ratio, 0.4; 95% CI, 0.25 to 0.66).

CONCLUSIONS

Our findings suggest that borderzone location is an independent determinant for a lower risk of AVM hemorrhage at initial presentation.

Demographic, morphological, and clinical characteristics of 1289 patients with brain arteriovenous malformation

BACKGROUND AND PURPOSE

The purpose of this study was to assess demographic, clinical, and morphological characteristics of patients with brain arteriovenous malformations (AVMs).

METHODS

Prospectively collected data of 1289 consecutive AVM patients from 3 independent databases (1 multicenter [Berlin/Paris/Middle and Far East, n=662] and 2 single centers [New York, n=337, and Toronto, n=290]) were analyzed. The variables assessed were age at diagnosis, sex, AVM size, AVM drainage pattern, AVM location in functionally important brain areas ("eloquence"), and type of presentation (hemorrhage, seizure, chronic headache, or focal neurologic deficit). Comparisons were made by ANOVA, contingency tables, and log-linear models.

RESULTS

Overall, mean age at diagnosis was 31.2 years (95% CI 30.2 to 32.2 years), and 45% of the patients were female (95% CI 42% to 47%). AVM maximum diameter was <3 cm in 38% (95% CI 35% to 41%). Deep venous drainage was present in 55% (95% CI 52% to 59%). An eloquent AVM location was described in 71% (95% CI 69% to 74%). AVM hemorrhage occurred in 53% (95% CI 51% to 56%). Generalized or focal seizures were described in 30% (95% CI 27% to 33%) and 10% (95% CI 8% to 12%), respectively. Chronic headache was recorded in 14% (95% CI 12% to 16%). Persistent neurological deficits were found in 7% (95% CI 6% to 9%), and progressive neurological deficits in 5% (95% CI 4% to 6%). Significant differences between centers were found for age (P<0.001), sex (P=0.04), eloquence (P=0.04), size (P<0.001), hemorrhage (P=0.006), persistent neurological deficit (P<0.001), and reversible neurological deficit (P=0.013). The intercenter difference found for hemorrhage frequency did not remain after adjustment for AVM size.

CONCLUSIONS

Baseline characteristics differed considerably between centers. The differences found in patient age and AVM size may be explained by center-specific referral patterns and the influence of access to treatment resources, whereas those found for other characteristics may be attributable to center-specific definitions. Analysis of natural history data from tertiary referral center databases may be improved by consistent definitions applicable to the entire population of AVM patients.