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INTRACRANIAL VASCULAR MALFORMATIONS

James G. Smirniotopoulos, M.D.

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

Visit our Website - http://rad.usuhs.mil/

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.


INTRODUCTION

There are many different types of lesions which are called vascular malformations. It is important to distinguish "vascular malformation" as a category of disease, and AVM as a specific type of malformation. The classic pathologic subtyping of vascular malformations include:

Arterio-venous malformations - AVM (shunt lesions)

Cavernous hemangioma (slow flow)

Venous malformation (varix)

Telangiectasia

Vein of Galen Malformations

Mixed malformations

BIBLIOGRAPHY























ARTERIOVENOUS MALFORMATION

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.
 
 

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Gross Brain.
Shows dilated arteries and veins on the surface, in the subarachnoid space.
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Lateral, selective ICA angiogram.
The ICA is dilated, feeding a "ball of vessels" with a short "transit time" and an early draining vein.
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T1W MRI
Multiple serpentine flow voids.
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T2W MRI
Multiple serpentine flow voids.
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CAVERNOUS MALFORMATION

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 1996 article from the New England Journal [REF 3]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.  Note: Florence Griffith Joyner (Flo-Jo) died at age 38 from an epileptic seizure caused by a cavernous angioma.
 
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Photomicrograph.
Multiple vascular spaces of varying size.
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T2W MRI.
Near the left frontal horn, there is a nodular hypointense lesion, without mass effect.
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Plain CT.
The left frontal lobe has an amorphous calcified lesion, without obvious mass effect.
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T2W MRI.
There is a nodular hypointense lesion adjacent to the right ventricle, without mass effect. Two small lesions are present
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Plain CT.
Near the right vent
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VENOUS ANGIOMAS

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.
 
 
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Lat Vertebral Angiogram.
 

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AP Vert Angio.

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Mosaic of Medusa

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Gross Photo of Palm Tree

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Contrast Enahanced CT.

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T2W MRI.

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VEIN OF GALEN MALFORMATIONS

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.
 
 

BIBLIOGRAPHY

1. Coffey RJ, Nichols DA, Shaw EG, and the Gamma Unit Radiosurgery Study Group. Stereotactic Radiosurgical Treatment of Cerebral Arteriovenous Malformations. Mayo Clinic Proceedings 1995;70(3):214-21.

2. Griffin C, Delapaz R, Enzmann D. Magnetic Resonance Appearance Of Slow Flow Vascular Malformations Of The Brainstem. Neuroradiology 1987;29:506-11.

3. Günel M, Awad IA, Finberg K, Anson JA, Steinberg GK, Batjer HH, Kopitnik TA, Morrison L, Giannotta SL, Nelson-Williams C et al. A founder mutation as a cause of cerebral cavernous malformation in Hispanic Americans. N Engl J med 1996;334:946-51.

4. Harrison MJ, Eisenberg MB, Ullman JS, Oppenheim JS, Camins MB, Post KD. Symptomatic cavernous malformations affecting the spine and spinal cord. Neurosurg 1995;37:195-204; discussion 204-.

5. Hasso AN, Bell SA, Tadmor R. Intracranial vascular tumors. Neuroimaging Clin N Am 1994;4:849-70.

6. Horowitz MB, Jungreis CA, Quisling RG, and Pollack I. Vein of Galen Aneurysms: A Review and Current Perspective. AJNR Am J Neuroradiol 1994;151486-96.

7. McGrath FP, Givney RG, Owen DA, Erb SR. Case Report: Multiple Hepatic and Pulmonary Haemangioblastomas - A New Manifestation of von Hippel-Lindau Disease. Clinical Radiology 1992;45:37-39.

8. Murphey MD, Fairbairn KJ, Parman LM, Baxter KG, Parsa MB, Smith WS. From the archives of the AFIP. Musculoskeletal angiomatous lesions: radiologic-pathologic correlation. Radiographics 1995;15:893-917.

9. Pollock B, Flickinger JC, Lunsford LD, Bissonette DJ, and Kondziolka D. Factors That Predict the Bleeding Risk of Cerebral Arteriovenous Malformations. Stroke 1996;27(1):1-6.

10. Segall HD, Ahmadi J, McComb G, Zee C, Becker TS, and Han JS. Computed Tomographic Observations Pertinent to Intracranial Venous Thrombotic and Occlusive Disease in Childhood. State of the Art, Some New Data, and Hypotheses. Radiology 1982;143:441-9.

11. Wolf HK, Campos MG, Zentner J, Hufnagel A, Schramm J, Elger CE, and et.al. Surgical pathology of temporal lobe epilepsy. Experience with 216 cases. J Neuropathol Exp Neurol 1993;52:499-506.