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Cerebrovascular

Brain Arteriovenous Malformation (AVM)

An abnormal tangle of arteries and veins in the brain — most are treatable, with the right approach tailored to the individual.

In brief

An AVM is a short-circuit between arteries and veins that bypasses the brain’s normal capillary network
Many are silent, but they can cause bleeding, seizures or neurological changes
Diagnosed on MRI and confirmed with catheter angiography
Treatment may combine surgery, embolisation and stereotactic radiosurgery — or careful observation
Management decisions require genuine cerebrovascular expertise

What is a brain arteriovenous malformation?

A brain arteriovenous malformation (AVM) is an abnormal tangle of blood vessels within or on the surface of the brain that creates a direct short-circuit connection between arteries and veins, bypassing the normal capillary network. Under normal circumstances, arteries carry high-pressure blood to the brain’s capillary bed, where oxygen and nutrients are exchanged before blood returns via the veins at lower pressure. In an AVM, this exchange does not occur — arterial blood flows at high pressure directly into the venous system, placing the vessel walls under chronic abnormal stress and significantly increasing the risk of rupture and bleeding within the brain. The central portion of the AVM — the core of abnormal vessels — is referred to as the nidus. Feeding arteries carry blood into the nidus, and draining veins carry blood away from it. The surrounding brain tissue may be deprived of its normal blood supply, which can contribute to symptoms even without rupture. Brain AVMs are uncommon. The estimated prevalence in the general population is approximately 10–18 per 100,000 persons, with an annual detection rate of approximately 1.1–1.4 per 100,000 persons per year.¹ They affect males and females approximately equally, though some series report a slight male preponderance. AVMs do not show a strong age-related peak but most commonly come to clinical attention between the second and fourth decades of life. In patients younger than 50 years, up to approximately one-third of intracerebral haemorrhages are attributable to AVM rupture.²

What causes a brain AVM?

The precise cause of brain AVMs is not fully understood. They are generally considered to arise during early foetal vascular development, though the exact triggering mechanism remains unclear. They are not typically inherited in a simple pattern, and the vast majority of AVMs occur sporadically with no identifiable family history. Recent research has identified several genetic pathways involved in AVM formation, most notably disruptions in the transforming growth factor-beta (TGF-β) and Notch signalling pathways, along with somatic activating mutations, most commonly in the KRAS gene and less commonly in BRAF, MAP2K1, or PIK3CA.³ These discoveries have opened avenues for potential future medical therapies targeting the underlying molecular biology of AVM formation and growth. AVMs are strongly associated with hereditary haemorrhagic telangiectasia (HHT), also known as Osler-Weber-Rendu syndrome, a hereditary condition caused by mutations in the ENG or ACVRL1 genes. In patients with HHT, brain AVMs occur in approximately 10% (higher in HHT type 1) of cases and warrant dedicated screening.³

What are the symptoms?

Many AVMs remain entirely silent for years and are discovered incidentally during brain imaging performed for unrelated reasons. When symptoms do occur, they may include:

Haemorrhage

Rupture and bleeding within or around the brain is the most serious presentation of an AVM. Depending on the location and volume of bleeding, this may manifest as a sudden severe headache, vomiting, confusion, weakness, speech difficulty, loss of consciousness, or seizure. Rupture carries approximately a 10–15% risk of death and a 20–30% risk of persistent neurological disability per haemorrhagic event.²In patients who have already suffered one haemorrhage, the risk of re-bleeding is substantially elevated.

Seizures

Seizures are a common presenting feature, occurring in 10–30% of patients at diagnosis.² They may be focal (affecting one limb or one side of the body) or generalised, and typically reflect irritation of the surrounding brain cortex by the AVM or by prior small haemorrhages.

Headaches

Chronic headache is frequently reported in AVM patients, often of a migraine-like character. In most cases, headache is not a reliable indicator of impending rupture, though a sudden severe headache that is different from usual warrants urgent assessment.

Focal neurological deficits

Weakness, numbness, visual disturbance, speech difficulty, or cognitive changes may occur due to chronic steal of blood supply from surrounding brain tissue, or from accumulated effects of prior small haemorrhages.

Incidental discovery

A growing proportion of AVMs are identified on brain MRI or CT scans performed for unrelated reasons such as headache evaluation or head injury, with no prior symptoms attributable to the AVM.

What is the natural history and prognosis?

Understanding the natural risk of an AVM is central to all treatment decisions. The lifetime risk of rupture is determined primarily by the annual haemorrhage rate and the patient’s age at diagnosis.

For unruptured AVMs, the annual haemorrhage risk is approximately 2.2–2.7% per year.^4,5 This accumulates substantially over time: the cumulative 5-year haemorrhage risk is approximately 11%, and the 10-year risk reaches approximately 15%.⁵ For previously ruptured AVMs, the annual re-bleeding risk is higher, approximately 4.5% per year overall, and considerably higher again in the first year following rupture.⁴

Several features of an AVM are known to increase the risk of rupture⁴:

Prior haemorrhage (HR ~3.2)
Deep brain location, e.g. basal ganglia, thalamus, brainstem (HR ~2.4)
Exclusively deep venous drainage (HR ~2.4)
Associated flow-related aneurysms on feeding arteries (HR ~1.8)

If left entirely untreated, the long-term AVM-related mortality rate in conservatively managed cohorts is approximately 8%.⁵ Median overall survival following AVM diagnosis with conservative management alone has been estimated at approximately 19 years in unruptured patients.⁵

Pregnancy and AVMs: women of childbearing age with an untreated AVM should discuss family planning with their neurosurgeon. Whether pregnancy itself significantly increases the risk of AVM rupture remains debated; nonetheless, individualised counselling and planned obstetric management are recommended for women with a known AVM.

How is a brain AVM diagnosed?

AVM diagnosis relies on brain imaging, which is also essential for treatment planning. Tests typically include:

Computed Tomography (CT)

CT of the brain is typically the first investigation in a patient presenting with an acute headache or neurological event. It rapidly identifies brain haemorrhage. CT angiography (CTA) can identify the AVM and its main feeding arteries and draining veins, though with less detail than conventional angiography.

Magnetic Resonance Imaging (MRI)

MRI is the preferred investigation for characterising an AVM in non-emergency settings. T2-weighted sequences demonstrate the flow voids of the AVM’s vessels, while susceptibility-weighted imaging (SWI) can detect small or prior haemorrhages. MR angiography (MRA) provides additional vascular detail. Together, these sequences define AVM size, location, relationship to eloquent brain regions, and the presence of prior subclinical bleeding.³

Digital Subtraction Angiography (DSA)

DSA — conventional catheter-based cerebral angiography — remains the gold standard for definitive AVM characterisation and surgical planning. It provides the highest resolution images of the feeding arteries, the nidus, and the draining veins, and is essential for identifying associated aneurysms and determining the haemodynamic characteristics of the AVM. It typically involves a small puncture in the groin, passage of a fine catheter to the brain vessels, and injection of contrast under X-ray guidance.

How are brain AVMs classified?

The Spetzler-Martin grading system is the most widely used classification, assigning a score from I to V based on three features: AVM size (small, medium, or large), whether the AVM drains into deep veins, and whether it is located in an eloquent brain region (i.e. a functionally critical area such as the motor cortex, language areas, thalamus, or brainstem).⁶

Higher Spetzler-Martin grades (IV and V) indicate greater complexity and surgical risk. The grade is fundamental to treatment planning — low-grade AVMs (I–II) are generally considered for surgery, mid-grade (III) require individualised decision-making, and high-grade (IV–V) are typically managed conservatively unless haemorrhage occurs.⁷ A simplified three-tier classification (Spetzler-Ponce) groups grades I–II as Class A, grade III as Class B, and grades IV–V as Class C, correlating with treatment recommendation categories.⁷

More recently, the Lawton-Young supplementary grading system has been introduced to refine the prediction of surgical risk beyond the Spetzler-Martin grade alone. Rather than replacing it, this system adds three further patient-specific factors — the patient’s age, whether the AVM has previously bled, and whether the nidus is compact or diffuse — to produce a supplementary score that is combined with the Spetzler-Martin grade. Incorporating these features has been shown to predict neurological outcomes after surgery more accurately than the Spetzler-Martin grade alone, helping to identify the patients most likely to benefit from surgery.⁸

What treatments are available?

Management of a brain AVM requires individualised, multidisciplinary decision-making. The goal of treatment is complete elimination of the AVM — obliteration of the nidus — which eliminates the long-term haemorrhage risk. There is currently no effective medical therapy that can eliminate an AVM or reliably reduce haemorrhage risk, though anticonvulsants are used to manage seizures when required. Importantly, partial or incomplete treatment does not reliably reduce the long-term risk of haemorrhage — and in some circumstances may increase it — so the protective benefit is realised only once the AVM has been completely obliterated. The three main treatment options — microsurgical resection, stereotactic radiosurgery, and endovascular embolisation — can be used alone or in combination. The choice between them is determined by the Spetzler-Martin grade, AVM size and location, whether the AVM has previously bled, the patient’s age, and the availability of specialist expertise.

The ARUBA Trial and Unruptured AVMs

Decision-making for unruptured AVMs has been significantly influenced by the ARUBA trial (A Randomised trial of Unruptured Brain Arteriovenous Malformations), which reported in 2014 that medical management alone was superior to interventional therapy for unruptured AVMs in the short term.⁹ However, the trial has been subject to significant criticism regarding its design, follow-up duration (only 33 months), and heterogeneity of the interventional arm. Long-term follow-up data and subsequent observational studies have challenged the applicability of ARUBA findings to all patient types, and notably, a nationwide study found a nearly three-fold increase in AVM rupture and mortality in the post-ARUBA era, raising concerns that conservative management of some unruptured AVMs may carry greater long-term risk than intervention.¹⁰ Most expert consensus supports continued consideration of intervention for appropriately selected unruptured AVMs, particularly in younger patients where lifetime cumulative risk is substantial.³

Microsurgical Resection

Surgical removal of the AVM nidus remains the most definitive treatment, offering immediate cure and elimination of haemorrhage risk following confirmed complete resection. Microsurgery is the preferred approach for low-grade (Spetzler-Martin I–II) AVMs, where complete resection rates exceed 95% and the risk of new permanent neurological deficit is low.¹¹In experienced hands, outcomes for appropriately selected patients are excellent, particularly for ruptured AVMs where the existing haematoma can be simultaneously evacuated. The operation involves a craniotomy (opening of the skull), microsurgical dissection of the AVM under high magnification, progressive interruption of the feeding arteries first, followed by the draining veins — the arterial supply must be interrupted before the main draining veins are divided, and en-bloc removal of the nidus. Postoperative digital subtraction angiography (DSA) is performed to confirm complete obliteration. Surgical risks include stroke, bleeding, infection, and neurological deficit. Risk is closely related to AVM grade; for low-grade lesions in non-eloquent locations, permanent morbidity rates are typically below 5%.¹¹Surgery is generally not recommended as first-line treatment for high-grade (IV–V) AVMs, where the risk of significant neurological harm outweighs the benefit in most unruptured cases.

Stereotactic Radiosurgery

Stereotactic radiosurgery — most commonly delivered by Gamma Knife — is a non-invasive technique that delivers precisely focused beams of high-dose radiation to the AVM nidus, causing progressive vessel occlusion over time. It does not require a surgical incision and is performed as an outpatient procedure under local anaesthetic with a stereotactic head frame. Radiosurgery is particularly well suited to small (generally a maximum nidus diameter of less than 3 cm, or a total nidus volume of less than 10 cm³) AVMs in deep or eloquent locations where surgical access would carry unacceptable risk. Obliteration rates of approximately 70–80% are achieved at three to five years for appropriately sized lesions.³For larger AVMs, obliteration rates are substantially lower, and staged radiosurgery (treating the AVM in two or more sessions over months) may be employed. Importantly, the patient remains at haemorrhage risk during the latency period between treatment and obliteration, which lasts a minimum of two years, and more typically three to five years. Radiosurgery may cause radiation-related swelling in surrounding brain tissue (adverse radiation effects), typically appearing twelve to twenty-four months post-treatment, which may be asymptomatic or may cause transient or, in a minority of cases, permanent neurological symptoms.

Endovascular Embolisation

Embolisation involves passing a fine catheter from the groin through the arterial system to the AVM, where a liquid embolic agent or coils are injected to occlude feeding vessels or the nidus itself. Embolisation alone very rarely achieves complete AVM cure and is therefore most commonly used as an adjunct — to reduce AVM size and blood flow prior to surgery or radiosurgery, or to treat associated flow-related aneurysms that carry an independent risk of rupture. In carefully selected small AVMs with a limited number of feeding arteries, curative embolisation may be attempted, though this remains uncommon.^3,11

Combined Multimodality Treatment

For complex or high-grade AVMs, a staged combination of embolisation, surgery, and/or radiosurgery may offer the best risk-benefit profile. Treatment planning in these cases requires a specialised multidisciplinary neurovascular team.

Who is a candidate for treatment?

Treatment candidacy depends on a careful balance of the natural haemorrhage risk against the procedural risk of intervention. Key considerations include:

Spetzler-Martin grade
Prior haemorrhage (increases natural risk, generally strengthens case for treatment)
Patient age (younger patients carry greater cumulative lifetime risk)
AVM location (eloquent vs. non-eloquent brain)
General health and patient preference

Broadly, intervention is generally recommended for low-grade (I–II) ruptured or unruptured AVMs in surgically accessible locations, and for any ruptured AVM where complete cure is technically feasible.¹¹ For high-grade (IV–V) unruptured AVMs, conservative management with annual imaging surveillance is generally favoured unless haemorrhage occurs or progressive neurological deficit is present. Grade III AVMs represent the most complex decision-making group, requiring individualised assessment.

Children with brain AVMs require particular consideration. They carry a greater cumulative lifetime risk of rupture, and AVM recurrence after apparent surgical cure has been reported at higher rates in children than adults, thought to reflect ongoing cerebrovascular remodelling of the developing brain.¹²Close long-term follow-up with repeat angiography following apparently successful treatment is therefore recommended in the paediatric population, as recurrence can occur even when the initial post-operative angiogram confirmed complete removal.¹²

References

Helpful links

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Brain Arteriovenous Malformation (AVM) — patient information sheet PDF · prepared by Mr Jeremy Russell

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Content on this page is for education and information only and is not a substitute for advice from your doctor. Medically reviewed by Mr Jeremy Russell.