Symptomatic Vasogenic Edema in Arteriovenous Malformations
Symptomatic Vasogenic Edema in Arteriovenous Malformations provides Clues regarding AVM Pathogenesis
Cerebral arteriovenous malformations (AVMs) can cause neurologic symptoms by bleeding or by inducing seizures. Less common but more dramatic is the syndrome of progressive neurologic deterioration caused by an AVM. This syndrome is usually associated with large AVMs and may sometimes be caused by "vascular steal" whereby blood is shunted preferentially through the low resistance channels of the AVM with consequent ischemia in surrounding brain. The vascular steal theory of the etiology of progressive neurologic deterioration finds support in the observation that occluding feeders to the AVM can ameliorate the symptoms. Other mechanisms, however,are likely to play a role in some of these progressive deficits. Obstructive hydrocephalus from ventricular compression by dilated deep veins is one such mechanism that can be readily identified by CT and MRI scans. Venous hypertension from arterialization of the venous system is also a likely cause of symptoms in many patients, as demonstrated in the abiltity of dural AVMs-which draw no blood from the intracranial vasculature-to produce an identical syndrome. Rarely, AVMs may become symptomatic due to mass effect, which has been demonstrated in some large AVMs with no history of hemorrhage but with regions of hypodensity (edema) around the lesion (Fig. 1).
We have recently had experience at the MGH Brain Aneurysm/AVM Center with a number of such patients who presented with progressive neurologic deficits without angiographic evidence of steal or computed tomographic evidence of hemorrhage. In each case, magnetic resonance imaging revealed evidence of mass effect and edema surrounding the AVM nidus. Interestingly, all of the affected patients had radiographic evidence of venous thrombosis effecting the AVM's drainage. Pathophysiologically, venous outflow obstruction appeared to be a prominent feature in these cases as there was angiographic evidence of venous thrombosis in the AVM drainage system in each patient. In addition, the majority of patients presenting with this syndrome had venous ectasias.
In all of the treated cases the edema resolved after successful arterial embolization. Patients were then treated by surgical removal of their AVM or in one case by successful proton beam therapy. Several patients were also treated acutely with anticoagulation because of evidence that venous thrombosis had precipitated their peri-nidal edema and neurologic deterioration.
Implications of venous-thrombosis-induced perinidal-edema regarding AVM pathogenesis
A number of observations suggest that obstruction of the venous drainage of AVMs may be an important cause of hemorrhage, which is the most common cause of AVM-related neurologic symptoms.
First, a small number of draining veins, venous stenosis, and deep venous drainage have all been identified as risk factors for hemorrhage from an AVM. In addition, within gross specimens of AVMs evidence for subacute thrombosis and previous thrombosis of vascular channels may be observed. Finally, AVMs may spontaneously thrombose or transiently decrease in size consistent with partial thrombosis. This usually occurs in association with hemorrhage. All of these factors argue that acute thrombosis of part of an AVM may be a frequent cause of hemorrhage from these lesions.
Alternatively, increase in venous resistance due to progressive thickening might cause an increase in the pressure in the AVM. The absence of elasticity of the AVM coupled with the inability of the feeding vessels to regulate flow (because of the absence of innervation) presumably causes a pressure increase throughout the lesion in response to outflow obstruction (Fig. 2). Such obstruction may then lead to one of three outcomes: edema, hemorrhageif a weak point in the vascular channels cannot withstand the increased pressure, or complete thrombosis of the lesion.
Our recent experience with AVMs presenting with symptomatic cerebral edama and venous occlussion illustrates the importance of acute obstruction of AVM drainage in causing AVM symptoms. It also demonstrates that special considerations and therapies are necessary to successfully treat patients with symptomatic vasogenic edema caused by arteriovenous malformations.
Further discussion of this topic and references to the original medical literature may be found in:
Tatter SB, Ogilvy CS. Vascular malformations. General considerations. In Ojemann RG, Ogilvy CS, Heros RC, Crowell RM, eds. Surgical Management of Cerebrovascular Disease, Third edition. Williams & Wilkins, Baltimore, in press.