1.3.2.8 Radiotherapy
As mentioned above, radiotherapy is usually carried out when AVMs are in an inoperable location. Following the appropriate planning, the treatment is carried out using a linear accelerator or gamma knife with a stereotactic technique. One disadvantage of radiotherapy is the long latency period until the onset of effect—1–3 years are usually required before complete thrombosis takes place. In long-term follow-up, the rate of complete occlusions is much lower compared with surgery. The size of the AVM or residual AVM is a limitation for radiotherapy: good results are achieved with AVMs < 3 cm in size. Undirected and noncompact embolization before radiotherapy can actually have a negative effect on the treatment results. Embolization before radiotherapy is only useful if it compactly obliterates a defined part of the nidus.
1.3.2.9 Clinical picture in DAVFs
Dural arteriovenous fistulas (DAVFs) are acquired arteriovenous shunts on the wall of the dural sinus. In contrast to intracerebral pial AVMs, dural fistulas are mainly supplied by dural branches of the afferent cerebral vessels (“dural AVMs”). Pathophysiologically, it is assumed that there is a prior thrombosis in the affected sinus that has recanalized “incorrectly”—i.e., when the body attempts to recanalize the occluded vessel, arteries sprout into the sinus wall and the fistula arises when the sinus reopens.
The age at manifestation is 40–60 years. The symptoms depend on the location and type of venous outflow. A basic distinction should be made between three groups: fistulas at the large sinuses (transverse sinus and superior sagittal sinus); tentorial fistulas and ethmoidal fistulas; and thirdly, cavernous sinus fistulas. The Cognard classification (Table 1.3-4) grades dural fistulas in relation to the type of venous outflow, from which the risk of bleeding is inferred depending on whether or not there is reflux of arterial blood into the cerebral veins. Such reflux is usually associated with obstruction of regular venous outflow, with stenosis or occlusion of the sinuses.
Tentorial and ethmoidal fistulas represent a rare and special form where the arterial supply and forms of venous drainage need to be managed on a case by case basis, since this type of DAVF is associated with a higher risk of bleeding.
The Barrow classification (Table 1.3-5) grades cavernous sinus fistulas in relation to their arterial afferents. Type A is a special form in which the pathogenesis mentioned above in connection with sinus thrombosis does not apply. Barrow type A fistulas arise as a result of rupture of an aneurysm in the cavernous course of the internal carotid artery or injury to the vessel in that area. They are also referred to as direct fistulas, in contrast to indirect fistulas, which are caused by the thrombosis described above.
Table 1.3–4 Cognard classification of dural arteriovenous fistulas relative to venous drainage (orthograde: no symptoms or only slight symptoms; reflux: raised intracranial pressure, bleeding risk 10% in type IIb, 40% in type III, 65% in type IV).
| Type I | Orthograde drainage into the sinus |
| Type II | Retrograde drainage (reflux) |
| Type IIa | Reflux into the sinus |
| Type IIb | Reflux into cortical veins |
| Type IIc | Reflux into the sinus and cortical veins |
| Type III | Direct drainage into cortical veins |
| Type IV | Drainage into cortical veins with venous ectasia |
| Type V | Reflux into spinal veins |
Table 1.3–5 Barrow classification of cavernous sinus fistulas.
1.3.2.10 Clinical findings in DAVFs
In dural fistulas, the symptoms depend on the fistula’s position and the extent to which venous drainage is compromised. Tinnitus and a pulse-synchronous bruit occur with lateral dural fistulas, and visual disturbances or double vision with cavernous sinus fistulas. Headache, hydrocephalus, neurological deficits, and even dementia have been attributed to chronic venous hypertension due to disturbance of intracranial venous drainage. However, the most severe clinical manifestation is spontaneous intracranial bleeding. As mentioned above, in dural fistulas the risk of spontaneous intracranial bleeding depends on the extent and type of venous reflux intracranially, as well as on the location of the fistula.
1.3.2.11 Diagnosis of DAVFs
Dural fistulas are rarely asymptomatic, incidental findings; as with cavernous sinus fistulas, it is more often the case the clinical findings tend to be incorrectly assessed. The typical situation is a patient with a pulse-synchronous bruit behind the ear that stops when the occipital artery is compressed, or which is position-dependent. The clinical examination already points the way here. DSA is the method of choice for diagnosis and classification. With cavernous sinus fistulas, the emphasis is on ocular symptoms, the leading ones being exophthalmos, chemosis, ciliary injection and pareses of the extraocular muscles.
Fig. 1.3–12a-c Dural fistula in the transverse sinus (MRA and DSA). (a) The T1-weighted MRI after intravenous contrast administration shows extensive vascularization in the left occipital lobe, with an urgent suspicion of cortical drainage in the presence of a dural arteriovenous fistula. (b) The DSA shows a view over the left common carotid artery, with confirmation of the suspected fistula on the left transverse sinus and extensive cortical drainage (arrows). (c) Appearance after occlusion of the fistula site in the transverse sinus using platinum coils.
If vision is impaired as a result of venous drainage disturbances, there is an urgent indication for treatment. CT or MRI of the orbit can often reveal dilation of the superior ophthalmic vein. Here as well, however, DSA is the method of choice for diagnosis and classification. If the patients develop conspicuous symptoms as a result of intracranial bleeding or symptoms resembling infarction, a differential diagnosis of dural fistula needs to be considered on the basis of CT and/or MRI. Indicative signs here may include visible pathological vessels, signs of congestion in unusual locations such as the edge of the tentorium, or multiple collaterals, which can be well imaged using time-of-flight magnetic resonance angiography (TOF-MRA) (Fig. 1.3-12).
1.3.2.12 Indication for invasive treatment of DAVFs
Invasive therapy is indicated:
