Aneurysm & AVM Center
Vasogenic Edema in Arteriovenous
Malformations provides Clues regarding AVM
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
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. Conclusions
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.
Figure 1. Edema in association with
a large, unruptured, AVM. Top: MRIs from a patient with acute neurologic
deterioration and acute edema surrounding this temporo-occipital AVM. Note the
obliteration of the right lateral ventricle in the coronal image (left).
Bottom: Angiogram demonstrates the AVM nidus (right) and a venous
ectasia (sometimes called a venous "aneurysm") in the draining veins
(left, arrows). The sagittal sinus fills minimally and late in the right
carotid angiogram suggesting that obstruction of the venous outflow system is
the likely cause of edema in this patient.
Figure 2. Elasticity of normal pial
arteries and feeding vessels from AVMs and the response of pressure profiles in
AVM vs. normal cerebral vasculature to changes in venous resistance. The elasticity
of AVM feeding arteries is much smaller than that of the normal cerebral vasculature.
Elasticity is expressed as the response of vessel diameter to increasing pressure.
As the resistance increases the arterial circuit has little ability to regulate
flow into the malformation and therefore pressure increases throughout the lesion.
The comparison is to a hypothetical profile of normal brain blood vessels. Error
bars represent the standard error of the group means for values obtained from
the literature. Modified from Ogilvy CS, Klausen A, Wellman T, Bevan RD, Bevan
JA: Feeding artery rigidity and hemodynamics of cerebral arteriovenous malformations.
In The Human Brain Circulation. Functional Changes in Disease. Bevan RD,
Bevan JA, eds. Totowa, NJ, Humana Press, 1994, pp 405-12.