Arteriovenous Malformations

would be managed differently from a similar lesion in a 75-year-old patient with seizures. In patients where surgical risk exceeds expected natural history, alternative treatments such as stereotactic radiosurgery should be considered, if appropriate. Alternatively, staged embolization may reduce AVM size and complexity, creating an amenable lesion for surgical excision.

There are several general principles and techniques that are useful during AVM surgery. The techniques used to resect AVMs in specific locations, such as perisylvian, parasagittal, medial temporal and parahippocampal, trigonal, intra-ventricular, basal ganglia or posterior fossa, are beyond the scope of this chapter but are reviewed in detail by other authors. Surgical AVM excision should be an elective procedure, even in patients with ruptured AVMs. In these patients, waiting at least 3 or 4 weeks allows the patient to recover and the hematoma to liquefy, which can greatly facilitate surgery. We routinely administer steroids, anticonvulsants and prophylactic antibiotics before and during AVM resection, particularly for supratentorial AVMs. The use of lumbar CSF drainage or mannitol for brain relaxation is individualized after careful study of all imaging studies. Frameless stereo-taxy or intraoperative ultrasound is useful to help localize deep AVMs, whereas electrophys-iologic monitoring, such as motor mapping, may be useful when the lesion is located near or within eloquent cortex.

The location of the AVM nidus dictates the operative approach. The relative location of the AVM nidus can be predicted from the pre-operative cerebral angiogram. In general, the patient should be positioned with the AVM uppermost and avoiding any vascular obstruction in the neck from head positioning. The craniotomy and dural opening should be wide enough to identify completely the AVM's vascular anatomy. In particular, the surgeon should have enough room to gain access to and differentiate feeding vessels, draining veins, terminal vessels and "en passant" vessels. In addition, the surgical approach should be planned so that the angle of approach is as perpendicular to the major feeding arteries or the dominant nidal plane as possible. A wide opening, however, also allows modification of the angle of view, which is particularly useful when using the microscope or when hemorrhage occurs beyond the immediate surgical site.

The excision of an AVM requires several basic steps. First, the AVM needs to be identified. This is relatively straightforward when the AVM nidus is superficial. When the nidus is below the surface, an arterialized draining vein can be followed to the AVM. Also, a deep AVM nidus may be localized by careful comparison of operative and angiographic vascular anatomy, ultrasound or frameless stereotaxy. Any superficial feeders to the AVM should be secured with bipolar cautery or small clips and divided as close to the nidus as possible. Each vessel, however, must be carefully followed to ensure that it does not supply normal brain.

Second, a circumferential incision is made around the nidus, sparing the normal, adjacent cortex. This dissection is facilitated by use of cisternal and sulcal anatomy to reduce injury to normal brain. During this dissection, it is important to preserve the draining veins until the entire AVM and its feeders is defined because occlusion of venous outflow may increase intra-nidal pressure, causing sudden intraoperative hemorrhage. When there is both superficial and deep venous drainage, it may be possible to eliminate the superficial vein, provided there is adequate deep venous drainage.

Third, any deep feeders should be secured and divided. These vessels lack a muscular coating and tend to retract and continue to bleed into the brain; careful use of the suction and bipolar generally allow these vessels to be identified and secured with micro-clips, when needed. Once all the AVM feeders have been divided, the AVM should collapse and the draining vein should no longer appear "red" and arterialized. The last remaining vascular pedicle (i.e. the draining vein) can then be secured and divided. The resection cavity requires careful inspection to ensure there are no AVM remnants or leaking vessels. The surgical cavity is then lined with a hemostatic agent, such as Surgicel, and a brief period of normotension or a Valsalva maneuver is performed to check hemostasis before closure. Complete AVM excision should be documented by post-operative angiography. In some patients with complex AVMs, intraoperative angiography may be useful. However, intraoperative angiography does not replace the need for post-operative angiography.

Post-operatively, any factors that may increase intracranial, venous or arterial pressure should be avoided. In some patients, particularly those undergoing resection of large and complex AVMs, relative hypotension (90-110 mmHg systolic) during the first 24-48 hours post-operatively may be useful. Similarly, patients at significant risk for normal perfusion pressure breakthrough may benefit from sedation or barbiturate coma. Normal perfusion pressure breakthrough is a rare complication that is postulated to result from impaired autoregulation after excision of a high-flow AVM, which then leads to severe brain edema and hemorrhage, without evidence of residual AVM [19]. Patients at risk for normal perfusion pressure breakthrough generally present with progressive neurologic deficits, suggesting a steal syndrome, have an AVM nidus greater than 4 cm in diameter, have long and large-caliber arterial feeders and lack filling of adjacent normal vasculature on angiography secondary to shunting. The risk of normal perfusion pressure breakthrough may be reduced in these patients, particularly those with large AVMs, by staged embolization prior to surgical resection.

There are several other complications that can occur after AVM surgery. Patients are at high risk for seizures, in part because cortical venous drainage may have been altered by surgery. Generalized seizures can increase the risk of post-operative intracranial hemorrhage; therapeutic levels of anti-seizure medication should therefore be ensured in the immediate perioperative period. The risk of long-term seizures, however, is less than 10%. Prophylactic anticon-

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