Cerebrovascular Diseases

John Ruge and Christopher Neumann Arteriovenous Malformations

The management of arteriovenous malformations (AVM) continues to evolve. Multimodality treatment including surgery, endovascular therapy and radiosurgery is the current standard of care.

Epidemiology

Hemorrhage from an AVM is 4 times more likely to cause an intracerebral or intraventricular hematoma than aneurysmal rupture. Although the AVM is the most common cause of hemorrhagic stroke in children, most AVMs become symptomatic between 20 to 40 years of age.

Pathology

An AVM is a congenital defect in the formation of the capillary network that normally intervenes between cerebral arteries and veins. This defect is believed to arise between the fourth and eighth embryonic weeks, at a time before arterial walls fully develop. Abnormal arteriovenous communications in AVMs low-resistance shunts and redirect blood into the venous system, often at high pressurs and flow rates. The center of the malformation where the actual arterial to venous communications are located is termed the nidus. In larger AVMs, tortuous and dilated vessels covered by thickened arachnoid are seen on the surface of the brain. The nearby brain tissue is often atrophic and discolored from previous hemorrhage. Most AVMs have an inverted wedge shape with the 'point' directed towards the ventricle. Venous drainage from AVMs can be either superficial or deep. Often several veins lead away from the nidus, eventually leading to normal venous channels. Almost all cases when examined microscopically demonstrate evidence of prior hemorrhage by the abundance of hemosiderin-laden macrophages. Thinning of some vessels occurs and can result in aneurysmal dilation, but these vessels are usually venous in origin.

Clinical Features

AVMs can present in many ways including an atypical chronic headache, high-output heart failure in infants and seizures. Intracranial/intraventricular hemorrhage is the most common cause of symptoms experienced by children. More children than adults display hemorrhage as a first symptom; 79% of pediatric cases present with hemorrhage. In comparison, more adults than children experience is-chemic symptoms or seizures. Spontaneous intracranial hemorrhage is heralded by a sudden headache and confirmed by the presence of intraventricular or intracerebral

Pediatric Neurosurgery, edited by David Frim and Nalin Gupta. ©2006 Landes Bioscience.

hemorrhage on imaging studies. Altered consciousness, meningeal irritation and later-alized motor and sensory signs also are typically seen. Epilepsy (presumably arising from areas of surrounding gliosis) occurs in 20% to 67% of adult patients with AVM.

Evaluation

An initial computed tomography (CT) or magnetic resonance imaging (MRI) scan is performed to identify intracranial hemorrhage, cerebral swelling and/or hy-drocephalus. Lumbar puncture is rarely necessary unless one is trying to exclude a subarachnoid hemorrhage (SAH) with a negative imaging study. MRI also is useful in assisting the neurosurgeon in determining the location of the nidus (Chapter 2, Fig. 7) with respect to the hematoma. If possible, a magnetic resonance angiogram (MRA) performed at the same time allows better resolution of the cerebral vascula-ture. The gold standard, however, remains 4-vessel cerebral angiography. Fine resolution of the cerebral vessels and an appreciation of the dynamic flow within an AVM is not possible without this procedure. Embolization of significant feeding vessels can be done through an endovascular route. Usually, the timing of this procedure is planned in conjunction with a definitive surgical resection.

Treatment

Surgical Resection

Though any intracranial vessel can supply an AVM, 90% of malformations are found supratentorially in the middle cerebral artery (MCA) distribution. The optimal treatment for intracranial AVM is complete surgical excision. Several principles guide surgical management. First, feeding and draining vessels run a straight course to and from the malformation, becoming coiled within it, and therefore can be followed toward the nidus. Second, AVMs are less likely to bleed spontaneously during surgery than aneurysms, and thus can be more easily manipulated. Since AVMs are usually wedge-shaped and project toward the ventricle, the ventricular wall should be visualized to increase confidence of total resection. Finally, the brain tissue adjacent to the AVM is gliotic and provides a plane of dissection. A post-treatment angiogram is required to confirm complete obliteration. Anticonvulsants are typically required for 6 to 24 months following excision.

Embolization

Occasionally used as the sole treatment, but risk of recurrence remains. Best utilized as an adjunct to surgery, either to reduce high flow AVMs or to devascularize AVMs to reduce surgical risk.

Radiosurgery

Although the treatment is associated with minimal risk, there is no change in the risk of hemorrhage until the AVM has been completely obliterated. However, the process of nidus obliteration following radiosurgery can take up to 2-3 years. Approximately 80% of small AVMs will be obliterated 2 years after treatment. This modality is most useful for small AVMs in surgically inaccessible areas of the brain, or to treat residual portions of the nidus after surgical resection.

Table 1. Spetzler system for grading AVMs

Graded Feature

Points Assigned

Size of AVM

Eloquence of adjacent brain Noneloquent Eloquent Pattern of venous drainage Superficial Deep

Outcome

The prognosis for symptomatic AVMs is less favorable for children than adults. The higher mortality rate in younger patients is due to the increased incidence of hemorrhage. In addition, there is evidence that smaller AVMs have a greater propensity for hemorrhage than larger ones. The primary hemorrhage is fatal in 5% of cases. Rebleeding occurs in 28% of pediatric AVMs and is associated with a poorer prognosis. The mortality rate with rebleeding is 25%, regardless of treatment method. Postoperative motor, speech and cerebellar deficits can improve greatly, and 70% of children with AVMs are expected to be neurologically intact after surgery. Spetzler has developed a grading system for AVMs that correlates with outcome (Table 1).

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