Arteriovenous Malformations

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1. Epidemiology and Natural History

AVMs are approximately one-seventh to one-tenth as common as cerebral aneurysms with between 2500 and 3000 new cases presenting each year. An estimated 280,000 patients are afflicted with AVMs in the United States. There appears to be a slight male preponderance between just over 1:1 (male to female) to 2:1.

The natural history of AVMs is incompletely understood. The annual risk of hemorrhage is noted to be 4 and 2% for symptomatic and asymptomatic AVMs, respectively. The average time between initial symptoms (related to hemorrhage or seizures) and rehemorrhage is 8 years. Each hemorrhagic episode appears to be associated with a 20% risk of major neurologic morbidity and a 10% mortality. There is an increased risk of rupture with smaller malformations, AVMs with associated aneurysms, a single draining vein, deep venous drainage, or venous stenosis.

2. Pathological Characteristics

AVMs are congenital lesions consisting of rudimentary arteries feeding directly into veins without intervening capillary or venule networks (Fig. 13). They are normally high-pressure, high-flow malformations with a predilection for damage of surrounding parenchyma upon rupture. Involved veins are "arterialized," becoming dilated, elongated, and tortuous under direct arterial pressure. Vasculopathic changes, such as stenosis or thrombosis of feeding arteries and draining veins, are common. AVMs often assume a conical configuration, with the base located near the cortex and the apex extending to the ventricular system. Gliotic brain tissue may be found encompassed within the malformation. As a consequence of arterial shunting, adjacent brain is subject to poor perfusion and ischemia. Flow-related aneurysms are noted in approximately 8-12% of cases, either on feeding vessels or within the nidus (intranidal). Eight-five percent of AVMs are located supratentorially and 15% are found in the posterior fossa.

3. Clinical Manifestations

AVMs most frequently present as a consequence of hemorrhage in patients in their third and fourth decades. Unlike with rupture of cerebral aneurysms, early rebleeding (within 2 weeks) and cerebral vasos-pasm are rare because hemorrhage secondary to

AVMs is predominantly located intraparenchymally rather than within the subarachnoid space. Neurologic improvement is often seen as the clot regresses and is resorbed, and it provides rationale for delayed surgery 1-4 weeks after hemorrhage when the clot is organized and the brain less friable. Seizures are the second most common presentation of patients with AVMs, being noted in 11-33% of cases. Seizures are thought to be related to gliosis in the adjacent brain resulting from hemosiderin deposition and inflammation. Additionally, patients may present with the new onset of headache (from stretching of the dura, elevated venous pressure, or hydrocephalus) or progressive neurological deficit or cognitive decline (from arterial steal or venous hypertension).

4. Diagnosis

Cerebral angiography remains the gold standard for diagnostic evaluation of putative AVMs (Fig. 13). It provides detailed information regarding the configuration and vascular dynamic properties (such as flow rate, arterial steal/venous hypertension, and collateral flow). MRI and CT imaging are also useful adjuncts. Based on lesion size, location, and pattern of venous drainage, operative risk is assigned to a given patient's AVM via the Spetzler-Martin classification scale (Table IX).

5. Treatment

Comprehensive management of patients harboring AVMs involves three main therapeutic modalities: endovascular therapy, microsurgery, and stereotactic radiosurgery. Endovascular therapy, applying catheter-administered materials for embolization, is a useful adjunct to microsurgery and stereotactic radiosurgery to diminish the degree of arterial shunt. Embolization is rarely curative alone, however. Within 1 week following embolization (to prevent recanalization of embolized vessels), microsurgery may be performed to excise the lesion. Surgical risk is ascertained from the Spetzler-Martin scale preoperatively. In younger patients possessing superficial lesion in noneloquent areas, microsurgery is clearly the treatment of choice (Fig. 13). Microsurgery is also more effective than radiosurgery in ameliorating symptoms of intractable epilepsy and headaches. Stereotactic radiosurgery is reserved for patients with small (<3 cm), unruptured AVMs in eloquent brain substance with deep venous drainage. Stereotactic radiosurgery invokes vascular injury and produces delayed thrombosis after months

Figure 13 Arteriovenous malformation. (Left) Lateral view of angiogram after left vertebral injection. Arrow indicates AVM. (Right) Intraoperative photograph shows the same AVM immediately evident after durotomy.

Table IX

Spetzler-Martin Classification Scale for Operative Risk in Patients with AVMs"

Characteristic hypothalamus or thalamus, internal capsule, brain stem, cerebellar peduncles or nuclei) Pattern of venous drainage Superficial only Any deep

Points assigned

Size

Diameter < 3 cm Diameter 3-6 cm Diameter > 6 cm Location

Noneloquent site

Eloquent site (sensorimotor, language, visual,

"Scores of 4 or 5 are associated with the greatest risk of persistent postoperative neurological deficits.

to years. Thus, unlike surgery, protection from hemorrhage imparted from radiosurgery is delayed. Radiosurgery's benefits diminish and its adverse effects (damage to adjacent parenchyma) increase as the size of the lesion increases. Radiosurgery may be a viable option for the treatment of deep, residual AVMs after attempted microsurgical resection.

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