Surgical Approaches and Nuances for Lobar Arteriovenous Malformations

14 Surgical Approaches and Nuances for Lobar Arteriovenous Malformations

Cameron G. McDougall, Jonathan White, and H. Hunt Batjer

Abstract

Frontal, temporal, parietal, and occipital arteriovenous malformations (AVMs) make up a significant proportion of cerebral AVMs. Many of these AVMs are favorably located in noneloquent areas of the brain with good cortical presentation. Because of this, if done well, it is possible to surgically remove these AVMs with a treatment risk that is lower than their untreated natural history. The first key to delivering good outcomes is proper patient selection. A detailed history, a careful review of the imaging, and an honest appraisal of the surgeon’s ability are paramount. A decision then needs to be made about preoperative embolization. The risk of embolization must be balanced against the potential benefits of decreased blood loss, shorter operative times, and the clarity embolization brings to the angioarchitecture of the AVM. Once in the operating room, the patient must be positioned carefully to allow for full exposure of the AVM. When the microscope is finally brought into use, the pial surface must be broken circumferentially around the presenting surface of the AVM and then deepened evenly around the lesion. Careful hemostasis should be achieved as each margin is dissected to avoid having bleeding from multiple parts of the AVM at once. Once resection is thought to be complete, postoperative angiography is needed to make sure all early venous drainage has been eliminated. Patient care does not end when the procedure is over. A successful outcome is also dependent on attentive care in the intensive care unit during the first few days postresection.

Keywords: cerebral arteriovenous malformation, surgical technique, complication avoidance, microsurgery, endovascular embolization

Key Points

With proper patient selection and good microsurgical dissection, a significant number of lobar arteriovenous malformations can be treated with a risk much lower than their untreated natural history.
Excellent preoperative evaluation including a thorough neurological examination, detailed anatomic and functional imaging, and judicious use of preoperative embolization are important in achieving the best possible outcomes.
Proper patient positioning, selection of the appropriate surgical approach, and meticulous intraoperative dissection will maximize technical success.
Patient care does not end with the skin closure. Aggressive and attentive postoperative ICU care and diligent outpatient follow-up are needed.

14.1 Introduction

14.2 Patient Selection

This represents the cornerstone of all AVM management and its importance cannot be overstated. There is significant controversy surrounding the optimal management of AVMs. The ARUBA study (a randomized study of unruptured brain AVMs) concluded that unruptured AVMs should not be treated and found an annual event (death or new stroke) rate of 2.2% in those managed medically and a 30.7% event rate in patients undergoing treatment.¹ This study has been criticized because a significant number of patients who were screened were never randomized. Furthermore, many authors would consider the event rate in the treatment arm of ARUBA to be on the extreme end of the spectrum.² If nothing else, this study suggests that if unruptured AVMs are to be treated then it must be done with a very low risk of complications.

The key to achieving this goal is in careful patient selection (in particular for the unruptured population). It can be helpful to categorize the process into three parts: (1) patient factors such as patient age, comorbidities, expectations, and concerns (e.g., hemorrhage anxiety); (2) presentation details such as pervious hemorrhage, associated seizure disorder, and current neurological deficits can influence the decision-making process; (3) AVM features, such as anatomical location and the presence of high-risk features (e.g., nidal/flow-related aneurysms), and the configuration of the AVM itself such as nidal compactness necessitate careful consideration.

A helpful scale for patient management is the Spetzler–Martin supplementary grading scale (► Table 14.1). It assigns the patient a score based on several factors found to be significant predictors of outcome following surgical treatment. Increasing scores are associated with an increased likelihood of a poor outcome. Scores > 6 were associated with a 55% chance of an adverse surgical outcome or death.³ However, scales such as this serve only as a guide, and the decision to treat any patient with any modality must be made on an individualized basis and measured against the projected natural history of that specific AVM.

Table 14.1 The Spetzler–Martin supplementary grading system

Category	Points
Size
< 3 cm	1
3–6 cm	2
> 6 cm	3
Venous drainage
Superficial	0
Deep	1
Eloquence
No	0
Yes	1
Age
0–20 years	1
21–40 years	2
> 40 years	3
Nidus
Compact	0
Diffuse	1
Hemorrhage
Yes	0
No	1

Patients who are younger and healthy with cortical AVMs that have good superficial representation in relatively noneloquent areas of the brain favor resection. Older, sicker, patients with deeply located AVMs or AVMs in eloquent locations favor observation or radiosurgery.

14.3 Patient Positioning

14.3.1 General Considerations

Proper position is crucial in all of cranial neurosurgery but is especially true of AVMs. The cases can be long, there is potential for significant blood loss, and, unlike a tumor, it can be extremely difficult to take out an AVM if the entire lesion is not well visualized under the craniotomy flap. When choosing a patient position, several factors must be taken into account. First, it is best if the lesion is in the center of the brain exposed by the bone flap. Next, the position must facilitate access to the proximal feeding vessels and allow visualization of the draining veins. The veins, which in general cannot be sacrificed until the majority or all of the arterial input has been taken, can tether the mass of the nidus. This limitation must be taken into account when positioning. Finally, the patient must be positioned in such a way that the surgeon is reasonably comfortable during the procedure. Long surgeries with the surgeon in some contorted position make for a long, painful day and lead to rushing and careless mistakes.

14.3.2 Frontal Lobe AVMs

Obviously, patient positioning will be dictated by AVM location. In the superficial frontal lobe, especially in the superior, middle, and inferior frontal gyrus, arterial supply is from anterior cerebral arteries and the distal middle cerebral arteries. The decision about positioning is usually straightforward with the head turned in such a way that lesion is in the most superior part of the exposure. Lesions on the orbital surface generally warrant a lateral exposure if they are far from the midline, allowing for a lateral subfrontal view. Orbital surface lesions with a more medial presentation are usually best accessed from the anterior subfrontal view, making a head neutral with significant extension the position of choice.

Lesions of the frontal lobe presenting to the interhemispheric fissure present several reasonable choices for positioning. One is a head neutral position with an interhemispheric approach and gentle retraction of the frontal lobe ipsilateral to the lesion from medial to lateral. This works well, particularly if the lesion is relatively superficial in the interhemispheric fissure. Another clever approach to a lesion presenting to the interhemispheric fissure is to place the patient lateral with the lesion side down. In this position, gravity naturally pulls the lesion-side brain down, giving a more natural medial-to-lateral retraction of the brain. Another benefit of this position is that excess bleeding tends to run out of the interhemispheric fissure, thus avoiding pooling of blood in the surgeon’s line of sight.

14.3.3 Temporal Lobe AVMs

Like frontal lobe AVMs, temporal lobe AVMs are fed by distal middle cerebral artery (MCA) branches. Depending on how anterior and medially they present, they also have feeding from the anterior temporal artery and the anterior choroidal artery. Lesions that are more posterior can also recruit posterior cerebral artery feeding. It is important to keep this vasculature in mind when deciding what position to use.

AVMs of the anterior temporal lobe, whether presenting medially or laterally, can be exposed through a pterional exposure or one of its numerous modifications. A true lateral position or supine with the head rotated the appropriate amount usually works well for this approach. Lesions of the posterior temporal lobe that present to the lateral cortex require a more posterior exposure

14.3.4 Parietal Lobe AVMs

AVMs of the parietal lobe often extend anteriorly into the posterior temporal lobe or posteriorly into the occipital lobe. The details of this cortical presentation drive the patient position. Lesions with more anterior extension can be positioned supine with the head turned. For progressively more posterior lesions, it may be more comfortable to have the patient either in a semi-sitting position or prone. There is a “no man’s land” between the posterior parietal lobe and the anterior occipital lobe that can sometimes make it difficult to decide which exposure is best.

14.3.5 Occipital Lobe AVMs

Like their frontal and temporal lobe counterparts, AVMs of the occipital lobe can present medially or posteriorly to the cortical surface, or they may present to the interhemispheric fissure. Lateral and posterior presenting AVMs may be positioned laterally, supine with the head turned, sitting, semi-sitting, or prone. Each of these positions has its relative benefits and shortcoming. Sitting and semi-sitting have the advantage of allowing pooled blood to run off during surgery, facilitating visualization and maximizing the effectiveness of the bipolar. The disadvantage is the added time it takes to position properly and the added risk of hypotension and air embolism. The prone position is a good compromise depending on the AVM’s presentation to the surface. The surgeon then has the choice of operating at the patient’s head with the brain essentially upside down or working over the patient’s tucked arm looking superiorly.

14.4 Exposure

14.4.1 General Considerations

Maximizing the AVM exposure and creating a safe working corridor is an important detail which is often overlooked. In general, it is best for the entire AVM to be visualized after exposure, including the draining vein if it presents to the surface. Working under the bone edge, especially if there is any problem in controlling intraoperative bleeding, can lead to serious problems. Exposure should also take into account the arterial feeders to the AVM. When possible, it is reassuring to have proximal control of the feeding vessel away from the AVM in the event of difficulty in controlling bleeding while working around the nidus. When possible, it is preferable to expose an adequate margin of normal brain around the AVM. This normal brain on the AVM’s edge is vulnerable to injury, either by feeding vessels retracting into it during the AVM resection or from hemorrhage from normal perfusion pressure breakthrough bleeding. In many AVMs, it is not clear where the precise margin is during resection and so erring on the side of some extra exposure is wise.

After the skin and muscle have been reflected, the bone flap is elevated. The skin exposure and elevation of the bone flap can be bloody if there is external carotid or muscular collateral feeding of the AVM. The transosseous venous channels can be a source of important bleeding while turning the craniotomy as they can be arterialized.

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