13 Step-by-Step Microsurgical Resection of Arteriovenous Malformations
Cerebral arteriovenous malformations (AVMs) represent a highly morbid and formidable neurosurgical challenge. Cerebral AVMs are managed with a unimodal or multimodal strategy, including microsurgical resection, stereotactic radiosurgery, and/or endovascular embolization. Microsurgical resection of these lesions is the most definitive strategy for occlusion but requires immense surgical prowess and endurance. The general principles for AVM microsurgical resection begin with an intimate understanding of the three-dimensional angioarchitecture involved. This serves as a basis for preoperative surgical planning and determination of appropriate endovascular preoperative interventions. The next fundamental step involves a surgical approach with a wide operative field providing optimal working angles for the nidal disconnection, and unanticipated maneuvers are potentially necessary if complications are encountered. Feeding arteries must then be sequentially dissected, occluded, and disconnected from the nidus. Identifying and disconnecting the arterial feeding vessels involves perinidal circumdissection with immense importance reliant on preserving the dominant draining veins. Following circumdissection and occlusion of the feeding arteries, the dominant draining veins are disconnected to complete the nidal resection. These lesions provide an immense surgical challenge hindering intraoperative efficiency. Therefore, the surgeon must have knowledge of time-saving and decision-making maneuvers to provide an efficient strategy for the resection. This chapter highlights the details and nuances of these fundamental steps for successful AVM microsurgical management, which are applicable to all cerebral AVMs.
Keywords: microsurgical, dissection, angioarchitecture, nidus, circumdissection, efficiency, fundamental, craniotomy
- Understand the unique three-dimensional anatomic distribution of the arteriovenous malformation prior to attempting excision.
- Plan a generous exposure utilizing wide arachnoid dissection and gravity retraction to facilitate optimal working angles for the more tedious steps of excision.
- Feeding arteries must be located and appropriately sacrificed using microclips, bipolar cautery, or both.
- The exposure of the arteriovenous malformation nidus should proceed in a circumferential manner until the apex is visible, augmented by intraoperative navigation to ensure correct pial incision and transparenchymal trajectory.
- The surgeon must protect the dominant draining vein until the feeding arteries are sacrificed; if the vein is injured prior to sacrifice of the feeding arteries, the surgeon must control the venous bleeding without occluding the vein and focus on eliminating the feeding arteries.
- Timely removal of the arteriovenous malformation is the best method to control the bleeding.
Arteriovenous malformations (AVMs) are an aberrant dysplastic connection between arteries and veins without intervening capillaries, which form a nidus and facilitate high-flow arteriovenous shunting. AVMs are an important cause of morbidity and mortality among neurosurgical patients and most commonly present with hemorrhage and less frequently with refractory seizures, chronic headaches, or focal neurologic deficits.1 The average annual risk for AVM rupture is 2.2% per year and increases to 4.5% per year for rerupture.2 The risk is increased with identification of an intranidal aneurysm(s) and for deep parenchymal AVMs.2 With the increased utilization of magnetic resonance imaging (MRI) evaluation of the brain for unrelated reasons, AVMs are increasingly identified incidentally. Therefore, neurosurgeons must be aware of the indications for therapy and selection of an appropriate mode of treatment.
Management options for cerebral AVMs include microsurgical excision, angiographic embolization, and stereotactic radiosurgery, or a combination approach.3,4,5,6,7 Goals for successful intervention for AVMs include prevention of (re)hemorrhage, hinder progression of mass effect-induced preintervention neurologic deficits, and/or provide seizure control. Large and/or complex lesions are often best managed with observation,8,9 but large malformations can be made more amendable to radiotherapy or microsurgical excision by the use of preoperative embolization. The Spetzler–Martin grading scale has been adopted to guide recommendations for surgical management of AVMs, based on predicting prognosis from nidus size, eloquence of adjacent parenchyma, and the venous drainage pattern.10 The grading system can be utilized to optimize the recommendation for microsurgical management based on characteristics of the AVM predictive of morbidity.10,11
In decisions regarding the use of radiosurgery, the Pollock Flickinger score can be utilized to predict patient outcomes.12 Endovascular embolization as a stand-alone technique for AVM obliteration is rarely utilized and generally reserved for small single pedicle AVMs, although with the advance of endovascular instrumentation and use of Onyx embolization medium, the stand-alone use of this modality is being studied for more complex lesions to determine its future applicability.13,14,15 Embolization is more commonly utilized to devascularize feeders that will be difficult to surgically access, followed by definitive microsurgical excision.
13.2 Materials and Methods
This step-by-step approach to the microsurgical management of AVMs is based on literature review of foundational books discussing the principles of AVM operative management, primary clinical research articles describing the recent trends in management practices, and personal surgical experience.
Successful AVM excision requires meticulous perioperative planning and diagnostic evaluation. A complete history and neurologic exam should be performed, followed by a thorough radiographic evaluation including computed tomography (CT), MRI, and catheter angiogram evaluation (► Fig. 13.1). Out of these modalities, the catheter angiogram provides the most valuable information regarding nidus hemodynamics and angioarchitecture. The patient’s unique AVM should be scored based on the scoring scales discussed in the introduction to appropriately recommend the optimal modality for therapy. The remaining discussion within this section will guide the reader through the technical principles for microsurgical AVM excision.
Fig. 13.1 Illustrative case demonstrating a large right-sided temporoparietal convexity AVM. Left upper is a lateral internal carotid artery (ICA) arteriogram demonstrating the primary feeding arteries coursing along the anterior pole of the nidus and derive from middle cerebral artery (MCA) branches. The primary draining vein is observed superoposteriorly along the nidus. The superior and posterior margins of the nidus are obscured by the presence of embolic material. Right upper is an MR image providing proximity information of the nidus relative to the adjacent ventricle and eloquent cortex, and discloses the presence of parenchymal feeders in at the ventricular trigone. Lower left and right are CTA images providing enhanced vascular anatomy relative to the surrounding parenchyma and cranium.
Routine preoperative arterial pedicle embolization for augmentation of feeder devascularization can be performed based on surgeon preference. We believe aggressive overembolization of cortical pedicles, which are commonly targeted endovascularly, may result in an increase in the flow within parenchymal white matter perforators (► Fig. 13.2). This anomaly is likely explained on the principle that a cerebral AVM will endure and facilitate continued arteriovenous shunting, because of the AVM serving as the path of least resistance for any sources of hemodynamic flow. The enhanced flow through deep parenchymal feeders poses a greater intraoperative risk for hemorrhage during disconnection and these feeders are commonly inaccessible for embolization. Therefore, selective embolization of those feeders that are relatively inaccessible early in microsurgical dissection is reasonable. A preoperative discussion between the microsurgeon and interventionalist is warranted before embolization is undertaken.
Fig. 13.2 Surgeon preference dictates use of preoperative arterial pedicle embolization. By endovascularly obliterating the major cortical pedicles, increased flow within parenchymal perforators is encouraged due to being the path of least resistance for hemodynamic flow. This enhanced flow increases the risk of hemorrhage from these vessels, which already pose a significant surgical challenge.
13.3.1 Patient Positioning
Patient positioning begins with attention toward head position, which should involve consideration of cranial venous return and the anticipated operative corridor. The patient’s head should be kept above the level of the heart, with the neck in slight extension, and avoiding severe unilateral rotation at the neck. These measures prevent intracranial venous hypertension, which is particularly important to avoid in the setting of AVM surgery. When planning patient positioning concerning the operative trajectory, the need for optimal working angles, maximizing exposure of feeding arteries, and minimizing risk to the major draining veins should be considered. The high risk for intraoperative bleeding and unique technical challenges associated with AVM surgery necessitate a generous operative corridor that provides numerous and flexible working angles to efficiently manage subcortical bleeding.
The major pitfalls encountered with patient positioning include failure to maximize gravity retraction and forgoing the use of free surfaces while attempting to access the lesion. The failure to maximize gravity retraction on the brain results in the need for use of aggressive fixed retraction, which increases cortical injury risk and, potentially, morbidity.
Following appropriate patient positioning, the craniotomy can be planned with the assistance of neuronavigation based on MRI or preferably CT angiogram (CTA) data. The goal is to achieve a wide craniotomy with exposure of the AVM nidus, feeding arteries, draining veins, and a small region of normal parenchyma surrounding the AVM (► Fig. 13.3). Craniotomies for AVMs should be made generously and not adherent to principles of minimally invasive surgery. The large craniotomy will permit optimal management of unforeseen bleeding, at which time it would not be feasible to enlarge the craniotomy timely for improved exposure.
Fig. 13.3 Optimal head positioning, incision, and craniotomy are illustrated for a left frontal AVM. This craniotomy has been performed in a liberal manner to permit a generous durotomy and exposure of normal perinidal parenchyma. The craniotomy outline also demonstrates the utility of a wide craniotomy in avoiding inadvertent nidus transgression during drilling. Injury to a primary draining vein during this stage of the operation can be disastrous due to the difficulty in controlling the bleeding without completely occluding the vein.
To permit ideal manipulation of the nidus and parenchyma, the craniotomy can also be planned to provide early exposure and opening of cerebrospinal fluid (CSF) cisterns. If this is impractical, such as in the case of an interhemispheric craniotomy, a lumbar drain can be used to gradually drain CSF during the craniotomy, providing relaxation during parenchymal manipulation.
It is paramount to avoid penetrating the dura while drilling to create the craniotomy; therefore, a greater number of burr holes as well as making short passes with the craniotome decreases the risk for injury to the underlying engorged draining veins. The avoidance of this complication can be challenging because the draining veins within the dural leaves can often be large enough to erode the inner table of the calvarium. This complication can also be avoided by use of lumbar CSF drainage to facilitate thorough dissection of the decompressed dura away from the inner calvarial table before the footplate is employed.
13.3.3 General Steps and Nuances of Technique in AVM Resection
The universal mandatory steps for microsurgical excision of every AVM should be adhered to for each lesion. Violating any of these principles or their specific order increases the risk for adverse outcomes or complications.
Step 1: Three-Dimensional Understanding of the Malformation
Careful analysis of each sequence of the MRI, CTA, and preoperative angiogram should be undertaken, and planning of a well-thought-out strategy for approach undertaken. MRI is particularly helpful to map the functional cortices and their anatomical relationship to the nidus and the hematoma if the patient has suffered from a hemorrhage preoperatively. Distinction between feeding vessel aneurysms and nidal aneurysms should be made at this step in the operation. CTA should be used for intraoperative navigation due to its ability to provide a high-resolution depiction of the vascular anatomy in relation to adjacent parenchymal landmarks. The main draining veins and embolization material are useful as superficial landmarks to transpose the preoperative angiogram onto the operative field, which can be quite challenging with complex AVMs.
It is imperative to thoroughly understand and “memorize” the location, morphology, and serpentigenous routes demonstrated by the angioarchitecture. Feeding arteries can be identified by the surface landmarks, large draining veins, and embolic material. The location of the main marginal feeding arteries should serve as the target for the initial dissection and disconnection. These large feeding arteries can nest themselves within the sulci surrounding the AVM, requiring meticulous subarachnoid dissection to expose the vessels. Preoperative identification of these vessels enhances the efficiency of the disconnection (► Fig. 13.4). Large deep white matter feeders should also be identified using CTA prior to nidus dissection to appropriately plan for adequate hemostatic maneuvers. These vessels lack the durability of the tunica media demonstrated in the superficial feeding vessels of the AVM, enhancing their friability and risk for hemorrhage when manipulated intraoperatively.8
Fig. 13.4 Common depth of nidus angioarchitecture for a superficial convexity AVM is illustrated. The role the perinidal sulci have in hiding the feeding arteries is illustrated. This nidus also demonstrates how the primary draining vein can intermingle with the other cerebrovascular components of the aneurysm in the deeper segments near the apex necessitating a meticulous descent during dissection to avoid injuring this critical structure. Deep parenchymal arterial feeders are illustrated and highlight the challenge encountered by these structures due to their anatomical complexity and potential for significant hemorrhage.