Learning Objectives:After participating in this CME activity, the neurosurgeon should be better able to:
- Define the anatomic features of posterior fossa arteriovenous malformations (AVMs).
- Assess the adapted management of ruptured posterior fossa AVMs.
- Compare the various therapeutic options for posterior fossa AVMs, including their respective benefits and risks.
Posterior fossa arteriovenous malformations (AVMs) represent only 7% to 15% of brain AVMs and are defined as AVMs localized into the brainstem or the cerebellum. Considering their angioarchitecture and their difficult management, mesencephalic AVMs could be included in this subtype of brain AVMs.
Although no major or randomized study identifies the location of the AVM in the posterior fossa as a risk factor for bleeding, we noted a most important tendency to bleed for posterior fossa AVMs compared with supratentorial AVMs. This could be due in part to the absence of seizure presentation in this population. The natural bleeding risk of this subtype of brain AVMs is estimated as more as 11% per year by extrapolation of large retrospective studies.
The posterior fossa is a small area that contains many highly functional neurologic structures. This explains the high frequency of neurologic deficit after a bleeding event of an AVM or related to complications of any treatment. One-third of patients (24%–39%) present a permanent neurologic deficit (modified Rankin Score >2) after the first hemorrhage.
Posterior fossa AVMs are challenging lesions for their particular angioarchitecture, their high tendency to bleed, and for the concentration of eloquent neurologic systems in the posterior fossa. All of these factors give particular importance to multidisciplinary treatment and treatment planning.
Infratentorial AVMs usually occur in populations similar to those that have brain AVMs. Mean patient age at presentation is 35 years (28–46) with no clear sex preponderance. The AVM nidus may be located in the cerebellar hemisphere (49%–68%), in the vermis (15%–30%), or in the brainstem (12%–21%). A large nidus localized both in the brainstem and in the cerebellum is found in 10% to 15% of cases.
The most common presentation of infratentorial AVMs is hemorrhage, and it occurs in 74% to 91% of cases. Accumulating data have demonstrated an association between infratentorial AVM location and hemorrhagic presentation, but this tendency was not confirmed by the unique randomized prospective study (ARUBA) that investigated this aspect. The first hemorrhage often leads to the development of a neurologic deficit, and the hemorrhagic mortality rate is estimated at 15% to 25%. More than 25% of patients require intubation and intensive care support for more than 7 days, and about one-third of patients have a permanent neurologic deficit (cerebellar syndrome, diplopia, or trigeminal neuralgia). This hemorrhagic event is equally divided into subarachnoid hemorrhage (26%–34%) (generally due to an associated aneurysm rupture), intraparenchymal hematoma (31%–37%), or both (27%–34%). The natural history of the AVM and the risk of rebleeding are difficult to estimate, but, according to Stapf et al., it could be up to 35% within 5 years.
Progressive neurologic deficit is the second most common presentation of a posterior fossa AVM. Symptoms and clinical signs may be nonspecific, such as headache (4%–8%) and confusion (2%–3%). Or, symptoms and sign may be more specific, including localizing neurologic signs such as trigeminal neuralgia (6%–10%) or cerebellar deficit (3%–5%). These various clinical signs may be caused by hydrocephalus (3%–7%), venous mass effect (2%–4%), secondary ischemia (1%–3%), or neurovascular conflict (2%–4%).
The incidence of asymptomatic posterior fossa AVMs has increased during the past 2 decades, and these represent 10% to 15% of patients in recent studies.
Vascular Anatomy of the Posterior Fossa
Detailed knowledge of the arterial and venous anatomy is indispensable to understand the angioarchitecture and the anatomy of an AVM. The normal arterial and venous anatomy of the posterior fossa is illustrated in the angiograms shown in Figures 1 and 2. Figure 3, which is an artist's illustration, highlights the importance of anastomotic veins to understand the venous drainage of the posterior fossa. In Figure 3, the superior (C) and inferior (B) veins provide anastomosis to the superior and inferior petrosal sinuses, respectively, and the lateromesencephalic vein (A) is an anastomotic vein between the petrosal and galenic venous systems. An AVM does not create a new vascular system, but it uses the existing vascular system, which is dilated and elongated by the increased flow.
In the absence of anatomic variants, arterial vascularization of the posterior fossa depends exclusively on the vertebrobasilar system. The brainstem is vascularized by short and long perforating vessels that originate from the vertebral arteries (medulla oblongata) or from the basilar artery (pons and mesencephalon). The cerebellum receives its vascular support from the 3 cerebellar arteries [superior (SCA), anteroinferior (AICA), and posteroinferior (PICA)], which correspond, respectively, to 1 of the 3 faces of the cerebellum (tentorial, petrosal, and occipital). The posterior part of the brainstem is vascularized by perforators that originate from the SCA and the PICA, and which could have a recurrent course.
Venous drainage is organized in 3 different systems. The galenic system, with the great cerebral vein of Galen, permits the venous drainage of the superior part of the cerebellum and the mesencephalon. The petrosal system, with the superior and inferior petrosal veins, drains the petrosal part of the cerebellum and anterior two-thirds of the pons and medulla oblongata. The tentorial group drains the occipital part of the cerebellum and the region of the fourth ventricle.
Posterior fossa AVMs often involve the cerebellum (80%–95%), with a predilection for cerebellum hemispheres (55%–68%). The vermis is involved in one-third of cases (28%–37%) and cerebellar peduncles in approximately 20%. AVMs involving the brainstem are less common and represent only 10% to 15% of infratentorial AVMs.
The majority of posterior fossa AVMs present a nidus smaller than 3 cm (65%–78% of cases), and in other cases, nidus size is between 3 and 6 cm. A nidus larger than 6 cm is uncommon because of the small size of neurologic structures in the posterior fossa.
Cerebellar AVMs are most commonly located in the superior part of the cerebellar hemispheres, and the SCA is the most common involved artery (58%–73%). The PICA is also involved in approximately 50% of cases. Cerebellar AVMs often have a double compartment, with feeders from both SCAs and PICAs. The AICA is less commonly involved in AVMs, but we have found its participation in about 20% to 30%, especially in large cerebellar AVMs or in brainstem AVMs. The presence of inflow aneurysm is more common for a posterior fossa AVM than for a supratentorial AVM, with an incidence of 20% to 28%. On the other side, the incidence of intranidal aneurysms is similar to that of supratentorial AVMs, which is 8% to 14% if 3D digital subtraction angiography (DSA) is performed systematically.
Venous drainage of infratentorial AVMs presents some challenges. The first is the high frequency of a unique venous drainage, which occurs in more than 75% of cases. Only largest AVMs have multiple venous drainage. The presence of venous ectasia is also more frequent for other brain AVMs, with an incidence of more than 30%. Deep venous drainage, which includes all veins located anterior to the brainstem and the internal cerebral veins, is present in approximately one-third of cases.
The neurologic structures in the posterior fossa that are considered eloquent area are cerebellar peduncles and nuclei and the brainstem. Consequently, patients with infratentorial AVMs have commonly a Spetzler-Martin grade of 3 or less, with a typical nidus smaller than 3 cm located in eloquent area, and with deep venous drainage. Grade I, II, and III AVMs have an incidence of 30% each, and grade IV AVMs represent the other 10% of infratentorial AVMs.
Management of patients with posterior fossa AVMs can be addressed in 2 phases: 1) acute management after a hemorrhagic event; and 2) management of the AVM, which depends on its anatomy and its angioarchitecture. Figures 4 and 5 summarize management of ruptured and unruptured AVMs, respectively. Anatomic details and the natural history of this subtype of brain AVMs guide patient management.
Radiologic assessment of a posterior fossa AVM is similar to that of other brain AVMs. In the setting of acute presentation with suspicion of hemorrhage, a brain CT scan is indicated to evaluate not only the type of hemorrhage (intraparenchymal, intraventricular, or subarachnoid) but also its location, volume, and the presence of an associated hydrocephalus. The undertaking of angio-CT permits diagnosis of the AVM and its location, which is useful if emergent surgical evacuation of the hematoma is required. The organization of brain DSA takes too much time before an emergent surgery and is done when the patient is neurologically stable.
Brain MRI and DSA comprise complete assessment of an AVM. MRI allows exact localization of the nidus and its anatomic relationship with the brainstem, cerebellar peduncles, nuclei, and cranial nerves. The presence/absence of associated stroke is also seen on MRI. DSA remains the gold standard to study the AVM, the presence of associated aneurysms, nidal aneurysms, venous ectasia, reflux, and thrombosis. 3D DSA images centered on the nidus have a great importance to the study of the details of the nidus and the presence of risk factors for (re)bleeding. These include the presence of a flow-related or intranidal aneurysm, venous stenosis or ectasia, and dural sinus thrombosis. Selective microcatheterism of cerebellar arteries also is of great importance and helps to better understand the angioarchitecture of the nidus, especially if endovascular treatment is planned.
Acute Phase of Bleeding
Management in the acute phase is crucial to the outcome of the patient. Standard reanimation therapies must be performed urgently to protect airways and the brain. After the native brain CT scan, a rapid decision to proceed with ventriculostomy, posterior fossa decompressive craniectomy, or both must be taken to avoid a possible intracranial hypertension. In general, in the setting of obstructive hydrocephalus secondary to posterior fossa hypertension, we advise decompressive craniectomy performed at the same time as ventriculostomy to avoid upward engagement of the culmen.
Radiologic assessment of the AVM, as mentioned previously, can be performed after intracranial hypertension is controlled.
Ruptured Associated Ane m (Inflow Aneurysm)
In approximately 30% of cases, presentation of a posterior fossa AVM is subarachnoid hemorrhage due to rupture of an associated aneurysm. Usually, this aneurysm is localized at the origin of one of the cerebellar arteries, which fed the AVM distally. In 20% of cases, the aneurysm has a fusiform appearance. The recommendation is to treat the ruptured aneurysm first to avoid a second hemorrhage. For saccular and proximal aneurysms, simple coiling is the first choice. For a more distal aneurysm or a fusiform aneurysm, parent artery occlusion with glue or coils is indicated, and this generally allows occlusion of a part of the nidus at the same time. Distal cerebellar stroke is the most common risk of performing an endovascular arterial occlusion and should be considered beforehand.
In the rare patients in whom the anatomy is not favorable to endovascular treatment, alternative treatments are surgical clipping or trapping with extracranial-intracranial bypass. In this case, we advise to plan in a second treatment of the AVM, as illustrated in Figure 4. The 3 therapeutic options (surgical excision, endovascular occlusion, or radiosurgery) remain always possible. We reserve radiosurgical treatment for situations when surgery or embolization is technically impossible. The first line of therapy is surgical excision for cerebellar superficial nidus. If there is a deep cerebellar nidus or a large AVM, endovascular therapy is the first treatment considered. For a brainstem AVM, endovascular treatment and radiosurgery should be considered.
AVM Nidus Hemorrhage
The hemorrhage comes from the nidus itself in 50% of cases, especially in patients with nidal aneurysmal dilatation or venous stenosis/thrombosis. In this situation, we advise to plan complete occlusion/resection of the AVM. This is a multidisciplinary decision, and a combination of various treatment modalities could be necessary in the patient with a large nidus. Figure 4 illustrates the various treatment choices, principally depending on the location of the AVM. Other factors such as an inflow aneurysm, an intracerebellar hematoma cavity, and the dimension of the nidus also are important in deciding on the treatment strategy. The experience of each physician and center also are important factors in the choice of the first-line treatment.
In case of complete occlusion of an AVM, an unruptured associated aneurysm (inflow aneurysm) does not need to be treated if its dimensions are reasonable (<5 mm). The AVM normally will decrease in size in a few months secondary to the absence of a high-flow arteriovenous (AV) shunt.
Progressive Neurologic Signs
The presence of or worsening of a neurologic deficit could argue for treatment of the AVM. Before initiating treatment, the most important step is to understand the anatomic reasons for the neurologic sign and its pathophysiology. The deficit can be caused by various events: obstructive hydrocephalus, neurovascular conflict, and venous congestion. Treatment depends directly on the pathophysiology of the symptoms, and the options are ample. If the decision is to treat the AVM, Figure 5 illustrates the various treatment lines depending on the location the AVM nidus. An alternative therapy to direct treatment of the AVM is indirect symptomatic treatment. For example, obstructive hydrocephalus due to a complex unruptured AVM could be treated with a third ventriculocisternostomy or a ventriculoperitoneal shunt, without treatment of the AVM. Another example is functional treatment (balloon compression or thermocoagulation) of trigeminal nerve neuralgia due to a neurovascular conflict by venous drainage from a dilated AVM. Although these therapies are less effective than direct treatment of the underlying cause and do not allow treatment of the AVM, they can be of great help in complex cases.
A posterior fossa AVM is encountered occasionally in only 10% of cases. Although some authors think that the infratentorial location is an independent risk factor for bleeding, we advise a wait-and-see approach in the absence of angiographic risk factors for bleeding (ie, nidal aneurysm, venous ectasia/stenosis/thrombosis, and inflow aneurysm).
In the patient with symptoms or angiographic risk factors, Figure 5 gives an idea of which treatment option to choose as the first-line treatment, depending on the location of the AVM.
Poor Neurologic Outcome after Initial Therapy
After the first therapeutic phase of a hemorrhagic event that corresponds to intensive care therapy, the possible treatment of intracranial hypertension, possible occlusion of the bleeding (inflow aneurysm), and neuroreanimation, the treatment strategy of the patient with a posterior fossa AVM is discussed in a multidisciplinary staff meeting. Neurologic status a few weeks/months after the initial hemorrhage is a determinant factor to guide the treatment strategy. In the patient with a poor neurologic outcome (modified Rankin Score of 4 or 5), it may be reasonable to decide on therapeutic abstention of the AVM and accept the natural history of the pathologic entity.
A 46-year-old man presented with a spontaneous loss of consciousness after left side ataxia. He presented a Glasgow Coma Scale score of 7 (M3; Y2; V2) at the initial neurologic examination and was immediately intubated. Figure 6 shows the brain CT scans obtained in the emergency department.
As for other brain AVMs, the outcome of patients with posterior fossa AVMs depends on many factors. The most important factor is the patient's neurologic status at the time of diagnosis. Generally, 10% of patients will die in the 3 months after the initial hemorrhage, and about 30% of patients will have a permanent neurologic deficit with a long-term modified Rankin Score higher than 2. When the modality of treatment is well chosen with regard to all AVM factors, treatment does not influence the neurologic outcome of the patient, but each treatment option has a 5% to 10% of risk of neurologic complication. Globally, the outcome of patients with posterior fossa AVMs is worse than that of patients with supratentorial AVMs.
Table 1 summarizes the radiologic follow-up advised after each treatment modality. Radiologic follow-up depends on the treatment chosen. After surgical resection of an AVM, follow-up DSA is advised 6 months later, and the patient is then followed by brain MRI. After embolic occlusion of an AVM, 5-year follow-up DSA is necessary to exclude a new AV shunt. The patient is also followed with brain MRI. The first DSA after radiosurgical treatment is generally performed 3 years after therapy. The patient is then followed with brain MRI if the AV shunt is occluded.
Posterior fossa AVM is a particular subtype of brain AVM. The high eloquence of the posterior fossa with the brainstem and all particularities of posterior fossa AVM makes its management more challenging and reserved to experimented neurovascular teams.
The authors thank Dr Philippe Sylvestre for the artist's illustration.
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Keywords:Copyright © 2019 Wolters Kluwer Health, Inc. All rights reserved
Arteriovenous malformations; Cerebral angiography; Posterior fossa; Vascular anatomy