- Department of Neurosurgery, Division of Cerebrovascular Neurosurgery, The Johns Hopkins University, School of Medicine, Baltimore, Maryland, USA
Joao Paulo Almeida
Department of Neurosurgery, Division of Cerebrovascular Neurosurgery, The Johns Hopkins University, School of Medicine, Baltimore, Maryland, USA
DOI:10.4103/2152-7806.152140Copyright: © 2015 Almeida JP. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
How to cite this article: Almeida JP, Medina R, Tamargo RJ. Management of posterior fossa arteriovenous malformations. Surg Neurol Int 25-Feb-2015;6:31
How to cite this URL: Almeida JP, Medina R, Tamargo RJ. Management of posterior fossa arteriovenous malformations. Surg Neurol Int 25-Feb-2015;6:31. Available from: http://sni.wpengine.com/surgicalint_articles/management-of-posterior-fossa-arteriovenous-malformations/
Background:Posterior fossa arteriovenous malformations (AVMs) are rare vascular lesions, representing 7–15% of all intracranial AVMs. Although less frequent than supratentorial AVMs, they present higher rupture, morbidity, and mortality rates. Microsurgery, radiosurgery, and endovascular neurosurgery are treatment options for obliteration of those lesions. In this paper, we present a critical review of the literature about the management of posterior fossa AVM.
Methods:A MEDLINE-based search of articles published between January 1960 and January 2014 was performed. The search terms: “Posterior fossa arteriovenous malformation,” “microsurgery,” “radiosurgery,” and “endovascular” were used to identify the articles.
Results:Current data supports the role of microsurgery as the gold standard treatment for cerebellar AVMs. Brainstem AVMs are usually managed with radiotherapy and endovascular therapy; microsurgery is considered in cases of pial brainstem AVMs.
Conclusions:Succsseful treatment of posterior fossa AVMs depend on an integrated work of neurosurgeons, radiosurgeons, and endovascular neurosurgery. Although the development of radiosurgery and endovascular techniques is remarkable, microsurgery remains as the gold standard treatment for most of those lesions.
Keywords: Arteriovenous malformation, endovascular, microsurgery, management, posterior fossa, radiosurgery
Posterior fossa arteriovenous malformations (AVMs) are for a rare group of vascular lesions located in the brainstem and cerebellum. The first report of these lesions date to1908 when Clingestein published a case report presenting the clinical manifestations of this entity.[
Most cerebellar AVMs are favorable for surgical resection. Brainstem AVMs are often treated with radiosurgery and/or endovascular therapy, given the high risks of major postoperative deficits after resection of AVMs in this region.[
We present a review of the management of posterior fossa AVMs. The roles of surgery, radiosurgery, and endovascular treatment as well as treatment outcomes are discussed based on the analysis of the current literature.
Studies included in this review were selected from a MEDLINE-based search of articles published between January 1960 and January 2014. The search terms: “Posterior fossa arteriovenous malformation,” “microsurgery,” “radiosurgery,” and “endovascular” were used to identify the articles. Analysis was restricted to articles that included patients diagnosed with posterior fossa AVM despite its location or age. Articles about the natural history, diagnostic work up, management, surgical, endovascular, and radiosurgical therapies were evaluated. For centers with multiple reports on the management of posterior fossa AVM, the most recent and/or most comprehensive study was used for analysis.
Classification of posterior fossa AVMs
The infratentorial space is one of the most complex regions of the nervous system. A thorough knowledge of the anatomy of this area is necessary for adequate resection of posterior fossa AVMs. Only then the surgeon is able to adequately evaluate the spatial location of the malformation (i.e. its relationships within the cerebellum and/or brainstem to the posterior fossa cranial nerves, arteries, and veins) based on the preoperative catheter digital subtraction angiography (DSA), magnetic resonance imaging (MRI) scans, and computed tomography (CT)[
Based on the anatomy of the posterior fossa, infratentorial AVMs can be classified in eight groups [
According to Lawton et al.,[
Posterior fossa AVMs, represent 7–15% of all intracranial AVMs.[
In recent years, important contributions have been made to our understanding of the prognosis and behavior of posterior fossa AVMs.[
Patients with posterior fossa AVMs commonly present with infratentorial hemorrhages (60-86%).[
Adequate preoperative evaluation and patient selection are important aspects of the clinical work up. CT, MRI scan, and DSA are necessary for evaluation and treatment planning. CT scan evaluation is helpful to diagnose brain hemorrhage and to assess potential associated complications, such as hydrocephalus or mass effect. Contrast-enhanced CT scan can detect up to 95% of all intracranial AVMs,[
MRI studies define with higher accuracy the anatomical location of the AVM. Additionally, it is useful for evaluation of intraventricular and brain parenchyma extensions. MRI studies can evaluate the size of the nidus and its precise location as well as demonstrates recent and old hemorrhages and the presence of intralesional and perilesional gliosis.[
The gold standard study for diagnosis and evaluation of posterior fossa AVMs is DSA. The study is usually performed in a 2-step manner: First a selective angiographic evaluation of the AVM is performed. Then, a superselective evaluation of the arterial nidus with microcatheters[
Surgical intervention is performed according to the patient overall and emergency status. Although most cases can be treated conservatively during the acute hemorrhagic period, in some cases the initial treatment may require insertion of intraventricular catheters, craniectomy, and duraplasty. In cases in which partial evacuation of a hematoma is pursued, it is important to avoid resecting the AVM at this time considering the difficult visualization of normal structures, the presence of brain edema and blood products, and the loss of autoregulation of the brain vasculature after the hemorrhage. Surgical resection of the AVM should be attempted in the acute period only as a last resort in cases of uncontrolled and persistent bleeding.[
Treatment selection must be performed according to the characteristics of each case and the final objective must be the complete resection or obliteration of the AVM. Microsurgical resection remains the gold standard for treatment of most of these lesions. It is associated with excellent outcomes when performed by dedicated vascular microneurosurgeons.[
In contrast, patients with lesions at the deep cerebellar nuclei and brainstem parenchyma and those in poor medical or neurological condition are not good surgical candidates. In these cases, stereotactic radiosurgery (SRS) and/or embolization are preferable. Embolization is used in preparation for either SRS or surgery, and is typically not curative when used alone. Simply continuing to follow patients with no interventions is reasonable in cases of large AVMs (Spetzler–Martin grade 4–5), with involvement of the brainstem and cerebellum, where the chances of complete occlusion are very low and surgical resection is associated with high risks of major postoperative deficits.[
The basic strategy for microsurgical resection of AVMs can be described in five steps: (i) Creation of a wide craniotomy centered over the nidus, (ii) adequately exposing the feeding vessels and the draining veins,[
In our experience, intraoperative monitoring is essential in these cases. Routinely, brainstem auditory-evoked potential and somatosensory-evoked potentials are used for surgical approaches of posterior fossa AVMs. For lesions involving the floor of the fourth ventricle, facial nerve monitoring is mandatory. Intraoperative angiography is encouraged by the authors to confirm the complete resection of the AVM before finishing the procedure. If a residual lesion is observed, the surgery is continued in order to achieve the complete obliteration of the lesion.[
The lateral decubitus (“park bench”) position is recommended for the surgical treatment of posterior fossa AVMs. This position is superior to the full prone position because it decreases the intrathoracic pressure and lowers intracranial pressure.[
In cases of vermian, tonsillar, and fourth ventricular AVMs, a midline incision from the occiput to about C3 is performed. A lateral “sigmoidal” incision centered over the cerebellar hemisphere is used for cerebellar hemispheric AVMs or placed slightly more laterally for petrosal surface and anterolateral brainstem lesions. A wide midline or retrosigmoid suboccipital craniectomy is usually performed for posterior fossa AVMs. The far lateral transcondylar extension can be used in cases of laterally located lesions, such as cerebellopontine angle (CPA) AVMs and anterolateral brainstem surface AVMs. The dura overlying the draining veins might be close adherent to these structures and should be carefully dissected to avoid venous bleeding. In cases where the dura is adherent to either a draining vein or to the nidus itself, it is best to cut around this portion of dura and to leave it attached to the AVM.
Adequate exposure of the AVM, careful inspection of the lesion and detailed analysis of its anatomy with correlation to the angiogram must be performed. The surface draining veins are usually the key to orientation regarding the location of the nidus.
The approach to the malformation itself should be initiated by identification and dissection of the major feeding vessels until their point of entry into the nidus. After identification, the arterial feeders can be coagulated or ligated with the use of small hemostatic clips. Adequate occlusion is followed by reduction of the pulsation and turgidity of the nidus. However, erroneous occlusion of a draining vein often results in increased pulsation, turgidity, and, even, hemorrhages. The “en passage” vessels must be carefully identified and preserved in order to avoid vascular injuries and subsequent ischemic complications.
The superficial portion of the AVM must be dissected away from the parenchyma after division of the superficial feeding vessels, freeing the lesion at its interface with the surrounding cerebellum/brainstem. Then a circumferential dissection around the nidus down to its apex must be performed. Special attention should be given to small vascular loops (Hashimoto's U-shaped channels) that are encountered in the periphery of the AVM. These loops emerge from the nidus and reenter the lesion, consisting of shunting vessels or draining venules that should be preserved until the arterial supply has been completely obliterated. Cottonoids can be used to mark the progression of the dissection, separating the interface between the parenchyma, and the nidus. The dissection and bleeding control in the periventricular region can present as a significant problem in this stage of the surgery. Application of multiple small temporary microaneurysm clips followed by coagulation, lower bipolar coagulation, broader bipolar tips, coating the tips with wax, and efficient entry into the ventricles to eliminate the small ventricular feeders are techniques that might help in this part of the procedure.
The final part of the AVM resection consists of coagulation of the draining veins and resection of the lesion. These vessels are ligated as close as possible to a normal vein or sinus to avoid creating a blind sac. Then the AVM is extirpated and the cavity of the lesion is inspected. Valsalva maneuvers are performed to peak pressures of 30–40 mm Hg and a 5-min period of sustained systolic blood pressure to 140 mmHg to test the resection cavity. Copious lavage of the subarachnoid space and ventricular system is done, followed by water-tight dural closure and subsequent closure of the operative wound in the usual fashion.
All patients are admitted for 2–3 days to the neurosurgical intensive care unit for monitoring of the blood pressure and possible signs of acute intracranial hypertension secondary to hemorrhages or thrombosis of the venous system. Strict blood pressure control for at least the first 3 days after surgery is recommended for the management of posterior fossa AVMs. This hemodynamic regimen prevents excessive perfusion of the chronically ischemic brain tissue surrounding the resection bed and avoid reperfusion injury.[
Intra- and postoperative hemorrhages, venous and arterial ischemia, cranial nerves, and long tract deficits are potential complications associated to the surgical resection of posterior fossa AVMs.[
Surgical resection remains the gold standard for treatment for posterior fossa AVMs. In one of the largest, early series of posterior fossa AVM surgery, Drake et al.[
The goal of SRS is to achieve complete obliteration of the AVM nidus while avoiding adverse radiation effects. It is the main treatment modality for brainstem and deep cerebellar AVMs, given that surgery is usually associated with significant postoperative deficits.[
Endovascular embolization of posterior fossa AVMs may be useful as an adjuvant treatment before surgical resection or radiosurgery. Cases of large hemispheric cerebellar AVMs and brainstem-cerebellar AVMs might specially benefit from a multidisciplinary treatment that includes adjuvant endovascular therapy.[
Posterior fossa AVMs are among the most challenging lesions in neurosurgery. Management of these AVMs requires knowledge of their natural history, thorough clinical and radiological evaluation, and collaboration between neurosurgeons, endovascular surgeons, and radiosurgeons.
Surgical resection is the optimal treatment option for most cerebellar AVMs, whereas most cases of brainstem AVMs will require radiosurgical treatment. Endovascular therapy has an important role as an adjuvant treatment in cases of large cerebellar AVMs and represents an important treatment option for occlusion of superficial brainstem lesions, particularly since the development of modern embolic agents.
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