Arteriovenous Malformations of the Cerebral Convexities

Preoperative Planning

23.1.1 Patient Selection

All arteriovenous malformation (AVM) therapies should be directed toward prevention of hemorrhage, neurologic decline, epilepsy, and death. As a general rule, the AVM must be completely obliterated with no evidence of arteriovenous shunting on an angiographic study sometime after the completion of treatment before the AVM can be said to be cured. Thus, pretreatment imaging almost always will include catheter angiography with injection of most, if not all, of the main arteries supplying the head, as well as cross-sectional imaging such as computed tomography (CT) or magnetic resonance imaging (MRI). Additional studies to delineate eloquent brain regions and white matter tracts, such as functional MRI and diffusion tensor imaging, may be indicated.

Approximately 65% of intracranial AVMs are supratentorial; these most commonly present with intraparenchymal hemorrhage and seizures. ¹^, ² The ideal treatment strategy may involve a combination of observation with medical therapy, surgical resection, embolization, and stereotactic radiosurgery (SRNS), and each or all of these therapies are tailored specifically to each individual lesion.

SRNS has allowed for treatment of lesions previously felt to be too eloquently located for surgical excision. SRNS is most effective in treating AVMs less than 3 cm in diameter, with at best an 80% obliteration in 2 years after treatment. ³ Catheter-directed embolization is rarely curative and is typically used as an adjunct to surgery or SRNS to occlude vascular pedicles or reduce blood flow. ⁴

ARUBA (A Randomized Trial of Unruptured Brain Arteriovenous Malformations) supports medical management for unruptured AVMs although the results of this study have been highly controversial for many reasons, including that the highly selected population of unruptured aneurysms probably has a more benign natural history than AVMs that have ruptured, the preponderance of embolization treatment in the treated group, and the short duration of follow-up that will tend to emphasize short-term complications of treatment more than the natural history. ⁵ Several studies discuss the importance of meticulous patient selection in order to improve the natural history of AVM hemorrhage. ⁶^, ⁷ According to one recently published meta-analysis, unruptured and ruptured AVMs carry a 2.2 and 4.5% annual risk of hemorrhage, respectively. ⁸ Therefore, it should be emphasized that the primary indication for surgery is prevention of further hemorrhage. Other indications for surgery include: (1) evacuation of the hematoma and (2) control of seizures.

Other considerations for AVM surgery are to have adequate blood available before the surgery, good venous access in case of major bleeding, and good microsurgical instrumentation especially bipolar coagulation forceps such as irrigating, cooled, or specially coated ones to minimize sticking during coagulation.

23.2 Anatomy

Most AVMs have an inverted conical shape, with the apex pointing toward the ventricle, a critical morphological feature of AVMs, which can make surgical excision difficult. Most often, the AVM is separated from the surrounding parenchyma by a gliotic border, hematoma, or ventricular fluid. At the apex, small vascular “tufts” make dissection of the border extremely tedious since these tufts are very fragile, resist coagulation, and bleed extensively when entered. Blood loss can be minimized by dissection around these vascular tufts with control of the small vessels at the apex of the AVM. Draining veins may be single or multiple and may travel to the cortical surface or deep into the subependymal venous system. Preservation of a major draining vein or veins can be critical to smooth excision. If these veins are taken too early, the arterial feeders end in a blind pouch leading to severe AVM expansion, turgidity, and excessive bleeding. ⁹

Concomitant feeding vessel aneurysms need to be addressed with open surgery if at all possible. All feeding vessels usually follow normal anatomical patterns, and therefore familiarity of such patterns is paramount with excision strategies. In addition, vessels “en passage” may exist and need to be preserved in order to prevent neurologic deficits.

23.2.1 Intracerebral Hemorrhage and Timing of Surgery

An intracerebral hematoma can create surgical advantages that facilitate AVM resection. The hematoma can create a cavity within the brain that minimizes brain retraction, decreases mass effect, and allows for enhanced visualization of the AVM. Early surgery in these cases can be problematic because the surrounding brain may be edematous and the AVM nidus may be obscured by the hematoma. Although craniotomy for evacuation of the hematoma in the acute phase after hemorrhage may be necessary in patients with a large hemorrhage and significant mass effect, in most cases it is advantageous to delay surgery until the clot liquefies. This process of liquefaction can be followed very easily with CT and/or MRI.

23.3 Operative Procedure

23.3.1 Positioning and Craniotomy

The patient’s head should be elevated above the right atrium of the heart to enhance venous return. The head is placed in rigid fixation avoiding extreme rotation and neck extension to prevent jugular vein compromise. The head should be positioned so that the cortical surface of the AVM is parallel to the floor to permit a perpendicular approach to the lesion with minimal brain retraction.

Stereotactic image guidance can help with scalp and bone flap planning. A general principle of AVM surgery is to create a somewhat larger-than-needed bone flap so that there is assurance that one has access to all feeding vessels. When the skin incision is made, patients receive mannitol (0.25 g/kg) and furosemide (10 mg) intravenously to enhance brain relaxation.

23.3.2 Microsurgical Procedure

A circumferential retractor system with flexible, tapered blades is used. A video display linked to the microscope is important to allow the neurosurgical nurses and adjunct staff to view the operation and permit rapid responses during critical times. Intraoperative ultrasound or stereotactic image guidance may be useful. At least three suction devices should be available, and of these, one is assigned for emergency backup.

A complete assortment of vascular microneurosurgical instruments is necessary. Both aneurysm clips and AVM clips should be available. Temporary vessel occlusion can be done with small aneurysm clips, and smaller vessels can be occluded with “Sundt-type” AVM clips. Larger vessels can be permanently occluded with aneurysm clips. To avoid injury to “en passage” arteries, no final commitment or permanent coagulation or ligation is made until the vessel is seen to enter the AVM. Clips allow for reversibility of actions until final commitment is made.

Other required instruments include short and long microscissors and a high-quality nonstick bipolar electrocautery device. The authors prefer platinum tips over irrigating forceps, although both are acceptable. Variable-length instruments allow for initial short depth dissection and, later, deeper dissection.

Opening the dura is not necessarily routine since the feeding arteries, draining veins, or the AVM itself may be adherent to the dura and be at risk for injury, potentially associated with troublesome bleeding in the absence of definition of the AVM anatomy. The dura is opened under loupe or microscope magnification. A circumferential dural incision is made some distance away from the cortical aspect of the AVM to avoid injury to AVM cortical feeding vessels. Under the operating microscope, the cortical surface is inspected for the location of the AVM, feeding arteries, and other superficial vessels. These vessels are dissected and isolated with establishment of the gliotic border between the AVM and cortex ( ▶ Fig. 23.1). Feeding vessels are followed distally toward the AVM, allowing for placement of temporary vascular clips on feeding arteries, and for permanent occlusion only when the vessel is confirmed to enter the AVM. It may be necessary to open sulci and fissures to identify feeding arteries and to establish proximal control of normal vascular structures. Occasionally, the only part of the AVM visible on the surface of the cortex may be an arterialized cortical vein; this may be followed subcortically to the nidus of the AVM below.

Fig. 23.1 Initial dissection involves proximal control of feeding arteries to the arteriovenous malformation (AVM). (a) The arachnoid is opened for exposure. (b) Each artery is followed to the AVM, and only when it is seen to enter the AVM is it coagulated and cut. Insets show that each vessel is initially coagulated with bipolar forceps over a length of the vessel and then cut partway to enhance control of bleeding if coagulation attempts have failed. Only after it is found to be fully coagulated is it completely divided.

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