Posterior Fossa Arteriovenous Malformations

Posterior fossa arteriovenous malformations (AVMs) comprise about 12% of all brain AVMs. About 70% present with hemorrhage into the brain parenchyma, the subarachnoid space, and/or the ventricular system. Current treatment modalities include surgical resection, radiosurgery, and endovascular embolization. Asymptomatic and unruptured AVM can be managed electively with assessment of the risk of the natural history compared to the risk:benefit of treatment. Ruptured posterior fossa AVMs may be associated with acute hydrocephalus and require ventricular drainage and if there is a large cerebellar hematoma, posterior fossa craniotomy, evacuation of the hematoma, and possibly resection of the AVM, depending on the complexity of the AVM. The principles of AVM surgery include adequate exposure, isolation, and control of surface feeding arteries with preservation of venous drainage and arteries en passage followed by circumferential progressively deeper dissection around the AVM, coagulating arterial feeders, and finally transection of venous drainage and completion of removal.

Keywords: cerebral arteriovenous malformation, intracranial hemorrhage

27.1 Patient Selection

About 70% of posterior fossa arteriovenous malformations (AVMs) present with hemorrhage. Headache and incidental detection make up most of the remaining 30% of presentations, and seizures directly caused by posterior fossa lesions are rare. The hemorrhage is most often parenchymal but can also be subarachnoid or intraventricular. The presence of nonparenchymal blood should raise suspicion that the hemorrhage may not be due to the AVM itself but rather to an associated vascular lesion such as an aneurysm on a feeding artery. This occurs in 10% of posterior fossa AVMs.

In patients presenting with hemorrhage the first study obtained is usually a computed tomographic (CT) scan. Cerebellar parenchymal hemorrhage, intraventricular hemorrhage, or subarachnoid blood may be seen. Much less commonly blood is seen within the brainstem itself. A CT angiogram can be obtained at the same time as the initial CT scan once hemorrhage is diagnosed, assuming there are no contraindications to administration of radiographic contrast. Serpiginous vasculature in proximity to the hemorrhage suggests the etiology of the hemorrhage is an AVM. Magnetic resonance imaging (MRI) is useful to better define the anatomy of the hemorrhage and any underlying cause. A catheter angiogram including external carotid injections should be done acutely in case urgent surgery is necessary. Radiographic studies should be scrutinized to determine the location and size of the lesion, the vascular supply, and drainage as well as the presence of any associated vascular abnormalities. Thorough knowledge of the venous drainage pattern is helpful in the planning of surgical and adjunctive therapy.

27.2 Indications and Contraindications for Surgery

The most important factors that determine whether to treat an AVM are the grade of the AVM and whether or not it has ever hemorrhaged ( ▶ Table 27.1, ▶ Fig. 27.1, ▶ Fig. 27.2, ▶ Fig. 27.3). ¹^, ² The goal of treatment is to completely obliterate the AVM so that there is no arteriovenous shunting. Treatment is more indicated if there is a history of hemorrhage and low-grade lesions, whereas as the grade increases or if there is no history of hemorrhage then the risk of the natural history which is the risk of hemorrhage begins to be outweighed by the risk of treatment.

**Table 27.1** The Spetzler–Martin and Spetzler–Ponce grading systems for brain arteriovenous malformations. ¹^, ² Spetzler–Martin grade is calculated by summing the points (grade is the sum of the allocated points)
Spetzler–Martin grading system points allocation
AVM-related features	Points
Size
Small (< 3 cm)	1
Medium (3–6 cm)	2
Large (> 6 cm)	3
Situation
Non-eloquent area	0
Eloquent area^a	1
Venous Drainage
Superficial only	0
Deep draining vein(s)	1
^aEloquent areas: sensorimotor, visual, and language cortices; diencephalon; internal capsule, brainstem, cerebellar peduncles, and deep cerebellar nuclei

Spetzler–Ponce classes
Class	Spetzler–Martin grade	General management
A	1, 2	Usually single modality treatment (surgical resection, stereotactic radiosurgery, complete embolization)
B	3	Multimodality treatment
C	4, 5	No treatment (exceptions include recurrent hemorrhage, neurological deficits steal-related symptoms and associated aneurysms)

Fig. 27.1 An aneurysm on the distal part of the posterior inferior cerebellar artery that is a feeding artery to a cerebellar hemispheric arteriovenous malformation.

Fig. 27.2 (a) A typical arterial feeding to a cerebellar hemispheric arteriovenous malformation (AVM) with supply from the superior cerebellar artery (SCA) and the posterior inferior cerebellar artery (PICA). (b) Arterial feeding to an AVM in the cerebellar vermis with supply from the SCA and PICA.

Fig. 27.3 This 76-year-old man presented with a sudden decreased level of consciousness. Computed tomography (CT) scan of the head (a) showed an intraventricular hemorrhage, associated hydrocephalus and a thalamic/midbrain lesion suggestive of an arteriovenous malformation (AVM). A CT angiogram was done and a ventricular drain inserted, resulting in neurological improvement (b). Anteroposterior and lateral vertebral cerebral angiography (c, d) revealed a high grade (Spetzler–Martin grade 5 or 6), essentially inoperable diencephalic AVM with deep venous drainage. Associated intranidal and distal flow-related microaneurysms were identified, however, they were not amenable for endovascular repair. After removal of the ventricular drain, hydrocephalus and cognitive dysfunction persisted and a programmable ventriculoperitoneal shunt was performed. Follow-up assessments showed very good functional recovery to living at home with minimal assistance with daily living activities due to poor short-term memory.

If the AVM has ruptured, surgical treatment of the lesion is best delayed until some of the hemorrhage has resolved and the acute cerebral edema has lessened. Waiting 6 to 8 weeks can dramatically decrease the difficulty of surgery and minimizes the chances of injury to the surrounding cerebellum. The rate of recurrent hemorrhage from the AVM may be slightly increased during this period compared with the risk of bleeding from an unruptured AVM or one that has not bled within 6 months, but in general AVMs do not have the same morbid natural history as an untreated aneurysm. Thus the benefits of delayed surgery are thought to outweigh the risks of rebleeding. Emergency surgery includes insertion of an external ventricular drain (EVD) in patients with neurological compromise due to acute hydrocephalus. Cerebellar hematomas greater than 3 cm in maximum diameter or more than about 10 mL volume are usually associated with acute hydrocephalus, brainstem compression, and neurological deterioration and need to be evacuated emergently. ³ The AVM can be resected at the same time if it has been adequately defined by preoperative angiographic imaging and is judged to be a simple surgical lesion ( ▶ Fig. 27.4, Video 27.1). If an associated vascular lesion such as a feeding artery aneurysm is the cause of the rupture it can be treated at the time of craniotomy if it is near enough to the AVM to reach through the same exposure, otherwise it can generally be treated prior to treatment of the AVM ( ▶ Fig. 27.1).

Fig. 27.4 This 58-year-old man presented with a sudden onset of headache, nausea, and vomiting without loss of consciousness. He was on warfarin because of a prior deep vein thrombosis that developed after liver transplantation for hepatitis C, all occurring 6 months ago. Computed tomographic scan of the head showed a right cerebellar hemispheric hemorrhage extending into the vermis (a). Cerebral angiography (b) revealed a Spetzler–Martin grade 1 AVM within the right inferior cerebellar hemisphere, fed by right posterior inferior cerebellar artery branches and drained by superficial bilateral cerebellar hemispheric veins and a vermian vein. Because of the hematoma location in the posterior fossa and the arteriovenous malformations (AVM) size, accessibility, and straightforward anatomy, a suboccipital craniotomy with simultaneous AVM excision and hematoma evacuation was performed 48 hours after the hemorrhage when anticoagulation was reversed (Video 27.1). The patient’s symptoms improved, and follow-up neuroimaging assessments (c, d) revealed complete removal of the AVM. He recovered fully and remained off warfarin since it had been 6 months from venous thromboembolism.

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