Professor of Radiology and Neurology Chairman, Department of Radiology and Nuclear Medicine Uniformed Services University of the Health Sciences 4301 Jones Bridge Road Bethesda, Maryland 20814-4799
The opinions expressed herein are those of the author, and are not to be construed as representative of the Uniformed Services University of the Health Sciences or the Department of Defense.
arterio-venous malformations
- AVM (shunt lesions)
cavernous hemangioma (slow flow)
telangiectasia
mixed malformations
Gross Brain. Shows dilated arteries and veins. 41886554.jpg |
Lateral, selective ICA angiogram. The Ica is dilated, feeding a "ball of vessels" with a short "transit time" and an early draining vein. 42186531.jpg |
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T1W MRI Multiple serpentine flow voids. 43c94564.jpg |
T2W MRI Multiple serpentine flow voids. 43d94564.jpg |
The most familiar of the vascular malformations is the arteriovenous malformation. This lesion typically presents in young adult patients usually with a seizure or a hemorrhage. This is the classic lesion recognized on the angiographic studies as a large tangle of dilated blood vessels with rapid flow and early draining veins. This lesion forms early during embryonic life through the direct communication of an artery with a vein, without an intervening capillary bed. Because this is a low resistance shunt pathway, blood may selectively drain through this fistulous communication. There is, during embryonic as well as postnatal life, a progressive enlargement and dilatation of the feeding arteries, and a concomitant dilatation of the draining veins. Very commonly the "nidus", which is the actual site of the abnormal communication, may be difficult to identify either radiologically or pathologically. Most of the lesion consists of the dilated feeding arteries and the dilated draining veins. Because the lesion develops simultaneously with the brain, there is usually neural tissue in between the dilated arteries and veins. The veins are usually of a much larger caliber than the arteries, because their walls are not supported by connective tissue and smooth muscle. Often times the brain tissue in between the vascular channels becomes atrophic, gliotic, and even calcified. There may be associated atrophy in the brain tissue adjacent to the mass, because the mass represents a sump and low resistance pathway that "steals" blood away from the normal tissue.
On CT scans, AVM's usually appear has tangles of hyperdense serpentine or "tubular" structures ("white worms"). AVM's that are not complicated by hemorrhage or infarction may have remarkably little (if any) mass effect and no vasogenic edema. However, variceal enlargement of draining veins can displace brain. Calcification in gliotic brain tissue surrounding the AVM is common. If an AVM is complicated by bleeding, the primary lesion may be partially obscured or "buried" in the hematoma. Excluding thrombosed AVM's, contrast enhancement is universally intense. On MR images, because of the brisk flow through the shunt, AVM's present as a tangle of serpentine curvilinear hypointensities on routine spin-echo sequences. The high-velocity flow also masks gadolinium enhancement. Angiography demonstrates: dilated feeding arteries; multiple arterial feeders from various vascular territories; rapid opacification and rapid washout (short "transit-time"); and large "early draining veins".
The treatment for AVM's varies with location, size, and presence of
complicating factors such as hemorrhage. Endovascular treatment, flow-directed
embolization, surgical resection, and radiation (including particle beam
and stereotaxic radiosurgery) have all been used.
Photomicrograph. Multiple vascular spaces of varying size. The vessels are stacked upon each other, without intervening brain. 43ea6984.jpg |
T2W MRI Near the left frontal horn, there is a nodular hypointense lesion, without mass effect. 52436168.jpg |
Plain CT The left frontal lobe has an amorphous calcified lesion, without obvious mass effect. 62436168.jpg |
T2W MRI There is a nodular hypointense lesion adjacent to the right ventricle, without mass effect. Two small lesions are present 72436168.jpg |
Plain CT Near the right ventricle, there is a large calcified lesion, without mass effect. 82436168.jpg |
The second kind of vascular malformation is the cavernous malformation (cavernoma or cavernous hemangioma). These lesions come in two different modes, they may be inherited, and, therefore, typically multiple and bilateral. However, there may be solitary and sporadic cavernous hemangiomas as well. There have been numerous articles showing a high incidence of autosomal dominant inheritance for cavernous hemangiomas among Hispanic Americans of Mexican ancestry. A recent article from the New England Journal documents that the reported association is not only valid and true, but it may be the result of a "founder mutation". The molecular biology of multiple cavernous hemangiomas suggests that the abnormal chromosome is number 7, and genetic markers suggest that most affected families probably have a single common ancestor - the "founder" of cavernous hemangiomas.
The cavernous hemangioma is basically a "blood sponge". It is a slow flow lesion, and not a shunt. Like the arterial venous malformation, it may present with either hemorrhage or seizures. This "blood sponge" consists of variable sized vascular spaces that vary between capillaries, sinusoids, and larger cavernous spaces. However, unlike the arterial venous malformation, there is no intervening brain tissue between the vascular spaces - hence, the name "blood sponge". Classically, these cavernous malformations, because of their slow flow, were not identified on routine angiographic investigations. They were typically called "occult" or "cryptic" malformations.
On imaging studies, these lesions oftentimes have remarkably minimal
mass effect. They may be slightly hyperattenuating or calcified on non-contrast
CT. Contrast enhancement may be minimal or marked. They typically have
minimal, if any, mass Effect (if they are not complicated by hemorrhage).
In an analogous fashion, surrounding vasogenic edema does not occur, unless
there has been a complication with hemorrhage. On magnetic resonance imaging,
which is far more sensitive in their detection, they oftentimes have a
very characteristic appearance. The cavernous hemangioma, may have internal
areas of thrombosis and/or hemorrhage. This internal bleeding is typically
of multiple different ages. There may be conversion of hemoglobin to methemoglobin
which produces foci of hyperintensity on the T1 weighted image. Hemosiderin
may be cleared from the central area of the lesion, and can be deposited
around the periphery of the cavernous hemangioma. The peripheral hemosiderin
produces significant T2 shortening, producing a "black halo" surrounding
the lesion. The hemosiderin rim around the outside is usually continuous
(rather than discontinuous) and may be associated with a "blooming" effect
with progressively greater degrees of T2 weighting.
Vertebral angiogram - Lateral Venous angioma 9g96009.jpg |
Vertebral angiogram - Frontal Venous angioma 9e96009.jpg |
Tile Mosaic The Medusa, with snakes for hair. 9f96009.jpg |
Honolulu palm trees. The trunk resembles the "transcortical vein" the fronds mimic the radiating veins. 9island.jpg |
Enhanced CT Linear density, with "crown of vessels" on one end. 9d96009.jpg |
T2W MRI Tubular flow void, with radiating "crown of voids". 9c96009.jpg |
Venous angiomas are the third type of vascular malformation. These may be isolated anomalies, but have reported in association with cavernous malformations. (Some authors have reported that the cavernous hemangioma may be a secondary lesion produced by the venous varix.) The venous angioma is a post-capillary malformation. The classic description of the venous angioma or varix includes a crown of multiple small venules that converge on a larger venous trunk. The venous trunk usually drains into a dural sinus. One theory suggests that the primary problem is a lack of "bridging veins" connecting the cortex to the dural sinuses. Any vein that does make the connection to the dural sinus (the so-called "transcortical vein") will now drain an unusually large volume of brain, and therefore enlarges. The "crown" of veins that converge onto the connecting trunk are "collecting veins" that drain the capillaries from the affected volume. Because a large volume of brain, and a corresponding large volume of blood drain into a single vessel, the venous pressure within the varix can be elevated. This elevated pressure may help produce the secondary cavernous hemangiomas. Hemorrhage from a varix is unusual, and many cases are either incidental findings, or present in association with a cavernous hemangioma.
On CT, the varix is a tubular structure, often oriented to point toward
the cortical surface and to the nearest dural sinus. The varix has the
attenuation of blood and enhances after contrast infusion. On MR imaging,
there is usually a tubular flow-void in the varix on routine spin-echo
sequences. Angiographically, the varix has been described as a "medusa
head". It really looks more like a hydra or a palm tree - the dominant
transcortical vein is the trunk; and the radiating crown of feeding veins
are the leaves.
Vein of Galen malformations (VGM) usually involve combination of lesions. The great vein of Galen receive flow from the internal cerebral veins and the basal veins of Rosenthal. It is a large vein, that in turn drains into the straight sinus. However, unlike the dural sinuses ( and unlike systemic veins elsewhere in the body) the vein of Galen is unsupported by surrounding tissue, lacks a fibrous wall, and is instead free within the fluid of the quadrigeminal plate cistern. Because the vein of Galen is such a large vascular channel, unsupported by surrounding tissue, any increase in venous pressure results in a dilatation of the vein - often converting its normal cylindrical shape into a sphere - hence the nickname "vein of Galen Aneurysm". Increased pressure within the deep venous system also interferes with normal venous development, usually producing persistence of embryologic channels that normally regress (e.g. the "falcine vein"). Thus, VGM is a better name, including both the enlarged central vein, as well as other venous anomalies.
Many etiologies have been suggested for the creation of a VGM. Clinical material is available to support several different mechanisms, all of them are potential causes for increased pressure in the deep central veins of the fetus including:
Dural fistula (high flow)
Parenchymal AVM (high flow)
Choroidal shunts (high flow)
Straight sinus thrombosis (obstruction)
Hypoplasia of the straight sinus (relative obstruction)
The clinical presentation of the VGM is variable, many patients present at birth with high-output cardiac failure, persistent patent ductus arteriosus, an audible cerebral bruit, a palpable thrill, and various degrees of hydrocephalus. Hydrocephalus is caused either by direct mechanical effects (compression of the aqueduct by mass effect from the dilated vein) or secondarily through decreased CSF reabsorption from central venous hypertension.
The radiologic diagnosis is often straight-forward. A rounded mass is
identified in the appropriate location of the quadrigeminal plate cistern
(QP cistern). The mass is usually hyperdense on plain CT, and enhances
brightly. Fetal and even obstetric US will show the mass is vascular, with
pulsatile flow on Doppler. MR may show a pulsation artifact in the phase-encoding
direction, as well as a flow-void on routine SE pulse sequences. Treatment
and outcome are variable. Significant factors influencing morbidity include
clinical status, degree of shunt (if a high-flow lesion), degree of hydrocephalus,
and the nature of the primary cause of the VGM.
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