Arteriovenous Malformations

Microsurgery

Surgery for the treatment of AVMs yielded dismal results until the introduction of angiography and microsurgical techniques in the 1940s and the 1960s, respectively. The goal of surgical treatment for most AVMs is to cure the malformation and eliminate the risk of hemorrhage. An important advantage of surgery over other treatment options such as radiosurgery and endovascular techniques is the immediate protection from hemorrhage. Despite the various advances in endovascular technology and radiosurgery, microsurgical resection remains the treatment of choice for most parenchymal AVMs (23-25).

Various surgical treatment concepts have evolved depending on the size and natural history of the lesions. These include staged resections or combination with preoperative embolization or radiosurgery (26-28). The tenet of microsurgery for AVMs is the resection of the nidus with minimal hemorrhage and brain retraction. There is no place for minimally invasive, small craniotomy flaps in AVM resection. A wide exposure is important to accommodate brain swelling, identify the surface and vascular anatomy, and allow for safe resection of the AVM in the event of catastrophic bleeding.

Stereotactically guided resection of AVMs was one of the first applications of this modality (29). Although its value for the volumetric resection of tumors is widely accepted, its use in AVM resection has been slow to take off. AVMs, similar to tumors, can be defined by volumes in stereotactic space based on imaging. It seems logical that image guidance can be used to identify the margins of the nidus and the major feeding arteries. Recently, a novel method using image guidance for the resection of AVMs in 22 patients was described (30). Preoperative helical CTA with 3D reconstruction was obtained, and intraoperative neuronavigation was used. Temporary clips were placed on all identifiable feeding arteries greater than 3 mm in diameter to decompress the nidus. The dissection was then performed along the main draining veins based on image guidance. The morbidity and mortality rates were 14 and 0%, respectively. The 4-mo follow-up was too short to make any meaningful conclusions compared with the literature. Larger published series with longer follow-ups are needed to determine whether surgical outcomes can be improved with these evolving techniques.

Endovascular Treatment

The endovascular treatment of AVMs has made significant progress since the obliteration of a carotid-cavernous fistula (CCF) with a muscle embolus reported by Barney Brooks in 1931 (31). This led to the search for an ideal embolic agent in the following years, including silastic spheres, spheres with silk sutures to increase thrombogenicity, porcelain beads, Gelfoam, steel balls, and Teflon-coated spheres (32-34). The major drawback of these early attempts was the lack of microcatheters and control over the emboli, which resulted in unacceptable morbidity and neurological deficits. Advances in imaging techniques and delivery catheters and development of a solidifying liquid embolic agent (35) have revolutionized modern-day embolization (Fig. 2). Road-mapping techniques allow the interventionalist to advance the catheter safely in tortuous arteries using a negative map of the vascular tree with a superimposed image of the catheter tip. Despite these major advances, endovascular treatment of AVMs remains primarily an adjunct to microsurgery or radio-

Fig. 2. Embolization of pial arteriovenous malformation. (A) Axial MRI shows small AVM on lateral edge of brainstem (arrow). (B) AP vertebral angiogram reveals the AVM fed by a single branch of the anterior inferior cerebellar artery. (C) AP film shows "glue" in nidus of AVM after embolization. (D) AP vertebral angiogram after embolization documents complete obliteration of AVM.

Fig. 2. Embolization of pial arteriovenous malformation. (A) Axial MRI shows small AVM on lateral edge of brainstem (arrow). (B) AP vertebral angiogram reveals the AVM fed by a single branch of the anterior inferior cerebellar artery. (C) AP film shows "glue" in nidus of AVM after embolization. (D) AP vertebral angiogram after embolization documents complete obliteration of AVM.

surgery. Complete obliteration of AVMs is possible only in rare cases, and the morbidity associated with it is not insignificant.

Radiosurgery

Radiosurgery is a viable minimally invasive treatment alternative for AVMs that are not suitable for surgical resection. Radiosurgery appears to induce a pathological process in the nidus that leads to gradual thickening of the vessels, leading to thrombosis (36,37). Obliteration rates from 70 to 98% have been reported, depending on the nidus volume and the dose delivered (38). Larger AVMs pose a greater problem with lower obliteration rates owing to the diffi culty of defining the complete nidus volume and using reduced doses for safety. Incomplete obliteration fails to reduce the risk of hemorrhage, thereby requiring other treatment modalities such as surgery, embolization, or alternate strategies of radiosurgery. Repetitive radiosurgery is an evolving concept. A 2-yr obliteration rate of 62% (62 of 101 patients) was reported by Karlson et al. (39) after repetitive radiosurgery to treat AVMs. Fourteen patients developed radiation-induced deficits and six experienced additional hemorrhage. This concept will continue to evolve as more AVMs in critical areas continue to be treated with this modality. Another strategy for large AVMs is prospectively staged radiosurgery, in which two adjacent volumes of the AVM nidus are treated at intervals of 3-6 mo. This allows for the delivery of a larger total dose and minimizes radiation to adjacent normal tissue (40).

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