Neuroradiology

Arteriovenous malformation

usually a congenital anomaly involving an abnormal communication between arteries and veins of the brain, spinal cord or face, head and neck region They are a subcategory of the wider group of vascular malformations (see vascular malformation intracranial).

Gross pathology

There is a complex network of abnormal vascular channels that consists of arterial feeders, a nidus and enlarged draining veins. In the brain pial collaterals may be demonstrated.

Usually, the arteriovenous malformation (AVM) has no intervening brain tissue. There may be secondary changes around the AVM with loss of cells, atrophy, gliosis and haemosiderin due to subclinical focal bleeding. Intranidal or flow-related aneurysms on the feeding arteries may be found as well as focal dilatations of the draining veins (venous varices); they usually represent preferential sites of rupture and cause of bleeding. They may be located anywhere in the nervous tissue; 15% are infratentorial, the remaining being supratentorial. Different types of classification have been proposed according to size and location (Spetzler and Yasargil); in general a brain AVM extends from the pial surface of the brain to the deep white matter, sometimes down to the ventricular wall. AVM are usually solitary lesions although 2% may be multiple.

Clinical presentation

Often asymptomatic for many years or throughout life. When symptomatic they may present with seizures, headaches, progressive neurological signs of weakness or sensory loss. When they rupture the onset may be acute, with severe headache, severe neurological deficits and coma.

Imaging

The gold standard is cerebral angiography.

CT may show haemorrhage, if ruptured, or calcifications and faint areas of hyperdensity when large draining veins are present, or denser areas where calcifications are seen. Sometimes, particularly in large AVMs, a mass effect is also seen. Contrast injection shows the characteristic pattern of marked enhancement of nidus with serpiginous structures and dilated cortical or deep draining veins. MRI is superior to CT in showing signs of previous haemorrhage, in different stages of evolution with T2 shortening due to haemosiderin, T1 shortening due to methaemoglobin, as well as the ischaemic and atrophic changes of the adjacent brain. The most characteristic picture, in the standard spin-echo sequence, is that of an area with a nidus of flow void due to the flow within the vessels. The signal varies according to the velocity of flow. MR angiography has not yet reached a resolution sufficient to clearly depict feeding pedicles, nidus and draining veins with the accuracy needed to plan surgical or endovascular treatment.

The gold standard for diagnosis remains selective intra-arterial catheter angiography (Fig.1). With the development of selective and superselective angiography, including direct catheterization of feeding pedicles within the brain, it is now possible to characterize with extreme precision all the components of an AVM: feeding arterial pedicles, dysplastic arteries and flow-related aneurysms, nidus, intranidal direct fistulas, intranidal aneurysms, draining veins, varices and stenosis of the draining veins, neoangiogenesis and pial recruitment. All information is needed to plan endovascular treatment with different types of embolic agents, surgical treatment or radiotherapy.

Differential diagnostic problems are very rare. Some hypervascular tumours or enhancement in ischaemic areas of the brain as shown by CT and MR may mimic an AVM but arterial feeders and enlarged draining veins are usually absent.

GS

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Fig.1

a. Axial CT showing intraventricular haemorrhage. b. MRI, sagittal T1-weighted image. Flow void serpiginous structures in the corpus callosum represent callosal arteriovenous malformation. c. MR, proton density axial view, shows midline callosal arteriovenous malformation. d, e. Lateral and AP right internal carotid angiogram; the callosal malformation is mainly fed by the right pericallosal artery; venous drainage is through the internal cerebral vein, vein of Galen, straight sinus and mesial frontal cortical veins and superior sagittal sinus. f. Embolization, selective injection of cyanoacrylate in the nidus. g. Three-dimensional CT cast of glue, following multiple pedicles embolizations. h. Axial CT showing midline hyperdensity of the glue within the malformation. i. Midline, sagittal T1-weighted image showing disappearance of the flow void of the AVM. Only a small posterior residual portion in the splenium is seen. l, m. Postembolization angiogram. The portion of the AVM fed by the internal carotid artery is completely obliterated. n. Residual posterior portion fed by posterior callosal and posterior choroidal arteries.
Arteriovenous malformation, Fig.1 (a)
Arteriovenous malformation, Fig.1 (b)
Arteriovenous malformation, Fig.1 (c)
Arteriovenous malformation, Fig.1 (d)
Arteriovenous malformation, Fig.1 (e)
Arteriovenous malformation, Fig.1 (f)
Arteriovenous malformation, Fig.1 (g)
Arteriovenous malformation, Fig.1 (h)
Arteriovenous malformation, Fig.1 (i)
Arteriovenous malformation, Fig.1 (l)
Arteriovenous malformation, Fig.1 (m)
Arteriovenous malformation, Fig.1 (n)