Pathology:

An AVM is a cluster of congenital arteriovenous communications without intervening capillaries; the arteries and veins are tortuous and dilated.

In most, the AVM is visible in the cortex. It fans out subcortically.

They are more commonly supratentorial, particularly in the parietal lobe; middle cerebral, posterior cerebral, and anterior cerebral territories are involved in declining frequencies.10% of them are infratentorial.

They derive blood supply from one or a combination of vessels.

Those supplied by the epicerebrals (perforators from the pial vessels) are confined to the cortex and are drained by cortical veins.

Those supplied by the transcerebrals (major the parenchymal vessels) are wedge shaped with its apex reaching the ventricles and drained by superficial and deep veins.

The centrally located AVMs mostly receive feeders from the anterior as well as the posterior circulation.

They grow apace with the growth of the brain.

In the presence of large draining veins, and the arterial feeders are submerged within the brain parenchyma and the AVMs present as SOLs with mass effect.

Some may be so compact and resemble a cavernoma.

Most have a gliotic core with a nidus and a gliotic wall forming a ‘pseudocapsule’.

Calcification is not uncommon.

Natural history:

Growth of the AVMs occurs in about 20% because of repeated hemorrhages, gradual dilatation of the vessels and recruitment of new supply.

In the elderly, especially small AVMs with a single feeder may diminish in size and on occasions, disappear.

In an unruptured AVM, the incidence of first bleed and the annual rebleed is about 4%.

The annual mortality rate due to an AVM is 1%, with the mortality at the first bleed being 10%. The morbidity with each bleed occurs in 20-30% per episode of bleed, with long term morbidity being 2.7% per year. It has been reported that, in a patient presenting with seizures, there is a 25% chance of the first bleed within 15 years, whereas in patients presenting with a bleed, the possibility of a second bleed was 25% in the next four years, and that of a third bleed is 25%within one year of the second episode.

Studies suggest that only 34% of patients with AVM remained symptom free; 26% become symptomatic and partially disabled; 11% are severely disabled.

Untreated posterior fossa AVMs carries a poorer prognosis.

The risk of bleeding is greater in children.

Clinical features:

Hemorrhage: It is the commonest presentation with an incidence of about 70%. Unlike an aneurysm, AVMs bleed, more frequently during sleep and it is unrelated to stress, trauma, or hypertension.

It is widely believed that they tend to rupture during pregnancy; but there is no convincing evidence.

Children bleed more often and the risk declines after the age of 40 years.

Small AVMs, because of the higher pressure in the feeding artery, are more at risk.

The posterior fossa and the periventricular AVMs are more likely to bleed.

There is a higher risk of second bleed in the first year following a bleed.

High arterial pressure, suggest a higher risk. Smaller the feeding arterial segment, and the smaller AVMs will have high-pressure feeders.

Single venous or deep venous drainage, or venous obstruction suggests a high risk as a result of high flow arterial feeders. The risk is less in cases of peripheral or mixed venous drainage and in the presence of an angiomatous change, because of resultant dilated cortical and leptomeningeal vessels with low flow.

Associated aneurysms are due to mechanical or venous outflow obstruction and suggest a high risk.

Seizures: It is the second commonest (about 30%) and associated with subclinical bleed in about 7%. The average age of onset is 25 years. They are more common with large, superficial, high flow AVMs.

Arterial steal and resultant ischaemia, gliosis around the lesion, and the mass effect (due to venous ectasia and retrograde dural sinus hypertension resulting in hydrocephalus or raised ICT) are the possible causes for the seizure.

Focal neurological deficit: About 10% of AVMs present with focal deficit alone and about 25% with seizure or hemorrhage in addition. The deficit may be due to arterial steal, or mass effect or hydrocephalus.

Headache: The exact nature of mechanism of headache in unruptured AVM is not known. It is often seen in AVMs with dural or pial component.

Other features: In large AVMs, the scalp veins may enlarge, and a thrill associated with a bruit over the neck may be detected. Retinal angiomas may be present. Posterior fossa angiomas may cause trigeminal neuralgia. High output cardiac failure, especially in children, may be the presenting symptom occasionally. Investigations: CT scan –may suggest a nidus as a low density within the hematoma (nidus sparing sign). A serpiginous enhancing lesion with an early draining vein; associated hypoperfused areas may be evident as low-density areas. MRI scan –T1 and T2 images may show areas of flow void; associated hemorrhage including subclincal  hemorrhage and areas of cortical atrophy and hypo perfusion are better seen. MRAngiography and 3D CT may outline the AVM; better suited for follow up studies. Digital angiography is still the imaging mode of choice. A detailed study of the arterial feeders, the nidus and venous drainage is mandatory. SPECT and functional PET scanning are useful for assessment of cerebral perfusion. Grading of AVMs:

Bleed due to pericallosal AVM-CT            Midline AVM-CT                       Giant AVM-MRI         Vermian AVM-MRI                 3D CT--temporal AVM Midline AVM- MRangiography

No ideal grading system exists. Spetzler and Martin grading system (1986) is widely followed, but ignores arterial feeders. There are 5 grades, arrived at adding the scores. Grade 1 has the best prognosis and grade 5 has the worst.

Spetzler and Martin grading system:

Management:

The aim is to compete obliteration of the AVM on follow up angiography with no morbidity. 

Surgical excision, endovascular procedures and stereotactic radiosurgery are the accepted modalities. Conventional radiotherapy, electrothrombosis, and cryosurgery have not been accepted in modern practice.

The strategy for a given patient must decided on the patient’s age and associated conditions, the AVM’s size, site and the venous drainage etc, and the available facilities and experience.

Surgical excision: Click for intraoperative video clippings

This remains the gold standard; other modalities are considered only if a safe surgical excision without any long-lasting morbidity is not feasible. Ideally, cortical AVMs in non eloquent sites are best treated with surgery.

Surgery is usually delayed for a few weeks (as the rebleed risk is much less unlike in aneurysms) unless the hematoma requires emergency evacuation.

Large, high flow AVMs with multiple deep feeders may need to be embolized before surgery. Some prefer to do it in stages without embolization. Intraoperative embolization is not popular anymore.

A generous craniotomy is advised. Associated dural component, if any, should be excised. Any injury to an adherent vein while opening the dura must be avoided.Prominent landmarks at surgery are the large arterialized veins, which need to be protected until the arterial  feeders are coagulated.Bleeding vein may be controlled with gelfoam and cottonoids and dissection should be continued. Intermittent hypotension helps on occasions. As the major feeders are coagulated, the malformation shrinks. Clipping a feeder shrinks the draining vein whereas clipping the arterialized vein produces venous engorgement.  Temporary clipping helps in differentiation of the feeders from the arterialized veins, which, perhaps, is the most important part of the surgery. Presence of hematoma helps in delineation of the malformation and the adjacent gliotic ‘pseudocapsule’ offers a plane for dissection. Such gliotic areas are encountered, more often, in deeper areas. Dissection is kept close to the malformation. As the superficial feeders are secured, the deeper ones appear to collateralize and coagulation may be difficult. Use of gel foam and judicious use of hypotension help. In case of persistent oozing from the bed, a residual nidus must be looked for. Recent advances in anesthesia, laser photocoagulation, evoked potential monitoring, facilities for intra operative DSA and cortical mapping have contributed in total excision of these lesions. Stereotactic localizing helps in deep seated AVMs.

Parietal AVM at surgery    Post excision at surgery     Pericallosal AVM-angio(lat)    Post Excision -angio (lat)      Pericallosal AVM-angio AP    Post excision- angio AP

In the early postoperative period, brain swelling may be troublesome. Perfusion pressure breakthrough syndrome is, most often, blamed. The rapid restoration of perfusion to a chronically ischemic area with dilated vessels, which is not able to respond to the increased flow, is assumed to be the cause. They are more likely in patients with CT evidence of hypoperfusion and / or atrophy. Many studies suggest hemorrhage from a residual nidus is the cause for the brain swelling.

Stereotactic Radiosurgery:

Conventional radiotherapy has no role.

Radiosurgery using Gamma knife or Linear accelerator is found to be effective either in a single sitting or repeated sittings.

It is indicated in (1) Small (<2cm) AVMs in eloquent areas. (2) Poor surgical candidates and in patients who are not willing for surgical excision. (3) Post surgical inaccessible residual AVMs. Deep seated AVMs are ideally treated with radiosurgery.

A success rate of 80% at two years of follow-up is claimed.

Limiting factor, in addition to the small size of the AVM, is the risk of bleeding which persists (3-4%) during the latency period of 2 to 3 years till the AVM gets obliterated, and may even be enhanced due to change in hemodynamics. Permanent neurological deficit due to delayed radiation necrosis occurs in 1%.

Embolization:

Embolization of the nidus or the feeders as definitive treatment, or as a part of the multimodality approach, prior to microsurgery or radiosurgery, is getting popular. However, at the present time, despite the recent technical advances, rate of complete obliteration of the nidus with embolization alone is low, about 20% in a recent study. The procedure carries a 20% risk of hemorrhagic and ischemic complications.

Multimodality Treatment:

Although it is difficult to make generalizations about specific uses of multimodality treatment, such treatment does appear to play a helpful role in larger lesions. It is done as either a planned maneuver, typically with embolization followed by surgical resection or radiosurgery, or as an unplanned maneuver where one treatment modality fails and a second treatment modality is necessary to obliterate the AVM. This can occur in situations such as residual AVM after subtotal surgical resection or resection of an AVM after incomplete radiosurgical treatment. The aim is total obliteration of the AVM.