AVMs usually become clinically manifest in middle age. The bleeding risk is estimated at approximately 2–4% per year. The leading diagnostic syndrome is intracranial bleeding (ICB; 50%), followed by headache and seizures at almost equal frequency. Neurological deficits without ICB as the first symptom are rare with these malformations. In children, ICB is the initial symptom more often. In contrast to aneurysmal bleeding, the course of bleeding from an AVM is clinically less severe and the risk of early secondary bleeding is much lower. Seizures occur particularly with large AVMs in supratentorial and cortical locations. Headache often occurs when the external vessels are involved. The clinical findings in patients with AVM are much more heterogeneous than in those with aneurysms and they depend much more on the size and position of the AVMs.
1.3.2.3 Diagnosis of AVMs
Starting from a size of at least 1 cm in diameter, there is no difficulty in diagnosing an AVM. Modern tomographic imaging procedures are all able to detect the lesions. A node of convoluted vessels with blood flowing through them is usually found, along with one or several large veins and hypertrophic afferent vessels. On CT, AVMs may show dilated vessels, calcification and hemorrhage. After contrast administration, there is strong enhancement of the vascular structures. MRI shows pathological vascular structures even before contrast administration (Fig. 1.3-10), particularly the angioma nidus and dilated efferent veins. For treatment planning, however, DSA with complete intracranial angiography is necessary, even after previous CTA or MRA. DSA not only identifies the afferent and efferent vessels in the vascular malformation, but also delineates the normal vascular supply to the brain, allowing assessment of the extent of the arteriovenous shunt and detection of aneurysms and stenoses in the vessels involved. In addition, DSA makes it possible to classify the malformation, and important treatment decisions usually depend on this. When there is spontaneous intracerebral bleeding, the following criteria should suggest a vascular malformation as the bleeding source: an “atypical” bleeding location (i.e., not in the white matter, basal ganglions, or cerebellum), age under 50, and no risk factors present (such as hypertension, coagulation disturbances, or amyloid angiopathy). Smaller AVMs showing bleeding or thrombosis can sometimes not be demonstrated during the initial diagnosis when there is intracerebral bleeding. In such cases, repeating CTA, MRA, or DSA after blood resorption can be recommended.
1.3.2.4 Indication for invasive therapy for AVMs
There is no conservative treatment approach for AVMs. Only supportive treatment, with anticonvulsant medication and symptomatic treatment for possible headache, is possible. A direct indication for invasive treatment is present when bleeding has taken place. The indication for invasive treatment in unruptured AVMs is a matter of debate.
1.3.2.5 General treatment considerations
There are three invasive treatment approaches for intracranial AVMs, which in principle can be used either alone or in combination in the framework of a multimodal concept. The decision for or against the various approaches is heavily dependent on local availability and expertise. In principle, endovascular occlusion, surgical removal, and stereotactic radiotherapy are available. The aim in all these procedures is to definitively eliminate the arteriovenous shunt in the AVM. With all of them, the difficulty of achieving this goal increases relative to the size and location of the AVM. It is certainly true to say “anyone can do the small ones.” The neuroradiologist can occlude the vessels using embolization; the surgeon can expose the AVM, identify all of the arterial afferents and can remove the AVM completely; and the radiotherapist can induce sclerosis of the AVM vessels. Most medical centers will use a multimodal approach in which the first step consists of endovascular reduction in the size of the AVM, followed by removal of operable AVMs and radiotherapy for AVMs in inoperable locations. Endovascular treatment can only lead to obliteration of an AVM to a limited extent. Depending on the location, radiotherapy or surgery are thus usually required. The advantage of radiotherapy is the lack of direct invasiveness, but the disadvantage is that the full effect of the treatment only follows after a latency period of 2-3 years and success rates of only 80-90% are possible, whereas incomplete surgical removal of AVMs tends to be the exception.
Fig. 1.3–10a-c MRI and DSA in the patient shown in Fig. 1.3-5 before treatment of either the aneurysm or the AVM. (a) T2-weighted and (b) T1-weighted images after intravenous contrast administration, showing the AVM (white arrow) and the aneurysm (black arrow). (c) DSA showing the aneurysm (white arrow) and the AVM in AP and lateral projections.
1.3.2.6 Endovascular treatment
In principle, endovascular treatment for AVMs is carried out in the same way as for aneurysms. However, the catheter materials are slimmer and different embolic agents are used. Corpuscular and liquid embolic agents are used.
Particulate embolic agents:
Liquid embolic agents:
The materials differ with regard to the level of occlusion (capillary or precapillary), the duration of the occlusion, and in their physical properties. For example, acrylates as a liquid embolic agent create a capillary occlusion that is permanent and seals the vessels using a polymerization process. Particulate embolic agents create a precapillary occlusion that is not permanent. Onyx™ behaves like acrylate at the occlusion level, but it is not a glue and has other different properties. With all of the embolic agents, the initial aim is to advance the catheter as close as possible to the AVM. When embolizing with Onyx™, it is best to position the catheter tip intranidally so that the nidus can be filled from the center. The aim of embolization is to achieve compact occlusion of all the angioma structures (Fig. 1.3-11).
Fig. 1.3–11a-d Endovascular treatment of an AVM. (a) Superselective imaging of the AVM via the posterior cerebral artery. (b) Positioning of the microcatheter in the posterior cerebral artery. (c, d) Step-by-step embolization of the AVM.
1.3.2.7 Surgical treatment
The indications for surgical removal of an AVM are prior bleeding, difficult-to-treat seizures, and prophylaxis against cerebral hemorrhage (Fig. 1.3-5). In small and easily accessible AVMs (Spetzler grades 1-3), this is the method of choice for complete obliteration, and it can often be done without prior interventional treatment. However, surgery or radiotherapy can also be carried out at lower risk after previous—and if necessary multiple—sessions of interventional treatment. In large AVMs (Spetzler grades 4–5), the risk of postoperative neurological deficits needs to be weighed against the natural course (influenced by variables such as the bleeding risk and the patient’s age and condition). Treatment is therefore contra-indicated in older patients with multimorbid conditions with no history of bleeding but with extensive AVMs occupying large parts of the cerebral hemisphere. The view has become generally accepted in recent years that Spetzler–Martin grade 5 AVMs should only be treated in exceptional cases.
The aim of the operation is to excise the AVM completely. Partial removal is not useful and even increases the risk of bleeding. During the operation,