Brain Imaging in Venous Vascular Malformations 

Venous vascular malformations, also known as venous angiomas or, more properly, developmental venous anomalies (DVAs), represent congenital anatomically variant pathways in the normal venous drainage of an area of the brain. Once thought to be rare, they are now considered to be the most common vascular malformation in the CNS.^{[1, 2, 3]} They may occur in as many as 2% of individuals. See two examples of venous vascular malformations in the images below.

Although for many years DVAs were commonly called venous angiomas, the newer term DVA has been recommended as more appropriate because the involved vessels are not abnormally formed, but apparently merely dilated. The majority of DVAs are found incidentally and never cause symptoms, although there are isolated reports of patients with syndromes attributed to DVAs (eg, secondary to hemorrhage or thrombosis).

Symptoms from DVAs are thought to be uncommon.^[4] Although headaches, dizziness, and ataxia^[5] have been associated with DVAs, confidently attributing such generalized symptoms to this common lesion is difficult. Symptoms that are directly related to a DVA most often involve DVA thrombosis and/or adjacent hemorrhage.^{[6, 7]}

While some believe that DVAs can hemorrhage on their own, most notably after venous infarction from spontaneous DVA thrombosis, most instances of hemorrhage with DVAs have been in patients with combined vascular malformations. In the vast majority of these cases, the hemorrhage probably originated from the accompanying vascular malformation rather than from the DVA.

Surgical treatment for DVAs has been advocated, but most experts believe that the resulting risk of an iatrogenic venous infarct would far exceed the risk of irreversible damage from the DVA itself during the patient's lifetime. In fact, most patients with DVAs who become symptomatic have an associated cavernous angioma, which suggests that the symptoms are actually caused by the cavernoma. (See the image below.)

DVAs are associated with cavernous angiomas or one of the other types of CNS vascular malformations (ie, arteriovenous malformation [AVM], capillary telangiectasia) in approximately 15-30% of patients. The most frequent conjunction is with cavernous angiomas; indeed, this association is so common that the two may be etiologically related,^[8] and the presence of a DVA on an image should prompt a search for a cavernoma, which is more clinically important.^[2] DVAs are also associated with head and neck venous malformations and hemangiomas. Rarely, DVAs are associated with varices.^[9]

Preferred examination

Although contrast-enhanced computed tomography (CT) scanning and nonenhanced magnetic resonance imaging (MRI) can reveal a DVA, the preferred imaging technique is contrast-enhanced MRI because of its excellent depiction of the small venules and draining vein. The multiplanar capabilities of MRI are especially useful because the typical configuration of a DVA is often best recognized in the coronal plane.^[10]

Limitations of techniques

Although standard contrast-enhanced MRI is excellent in depicting DVAs, adjacent hemosiderin from associated cavernomas may not be appreciated without the use of gradient-echo or echo-planar imaging, especially with fast spin-echo techniques.

Developmental venous anomalies are typically not visible on nonenhanced CT scans but they can be visualized after the administration of contrast medium. They appear as a large vascular structure in the brain parenchyma that drains into the deep or superficial venous system. The smaller surrounding veins are usually arranged in a radial pattern around the central vein. DVAs do not have a surrounding mass effect or edema and the adjacent brain is typically normal.

Degree of confidence

The typical appearance of a DVA on CT scans is often diagnostic but MRI may be needed in atypical cases, particularly those involving the posterior fossa where CT is limited because of streak artifacts.

False positives/negatives

Although arteriovenous malformations can occasionally be mistaken for DVAs on CT scan and vice versa, differentiation between the two is usually not a problem because AVMs have large feeding arteries, tortuous vessels, and abnormal adjacent brain parenchyma that are not observed in DVAs.

On contrast-enhanced MRI, the cluster of veins in developmental venous anomalies has a spoke-wheel appearance (see the first image below); the veins are small at the periphery and gradually enlarge as they approach a central draining vein (see the second and third images below).^{[11, 12]} This appearance has been referred to as caput medusa, or the head of Medusa, because of the serpentine appearance of the curvilinear peripheral draining veins. The intervening brain parenchyma is normal; this is a distinguishing characteristic of a DVA. However, 2 studies reported parenchymal abnormalities within the drainage territory of most DVAs^{[13, 14]}

The draining vein has a fairly straight course toward the deep or superficial venous drainage system, depending on the location of the DVA. When it is adjacent to the lateral ventricles, the draining vein usually merges with a subependymal vein, which may be enlarged. Other DVAs may drain into cortical veins or dural sinuses in the supratentorial brain (see the images below). Infratentorial DVAs have a variety of possible drainage pathways without a clearly dominant one.

On T2- and proton density–weighted images, the draining vein may demonstrate increased signal intensity, particularly on standard spin-echo images. This appearance is caused by gradient moment nulling. If the vessel is obliquely oriented, a yin-yang symbol appearance may occur because the high signal intensity is misregistered and a signal void appears next to a similarly shaped area of increased signal intensity. In the absence of an accompanying vascular malformation, the surrounding brain tissue should have normal characteristics on T2-weighted images, although a case in which gliosis surrounded a DVA has been reported. Nonenhanced T1 images may show the draining vein as a flow void but DVAs are often difficult to visualize without the use of contrast medium. Fluid-attenuated inversion recovery (FLAIR) images may be relatively normal or can show a subtle increased signal. (See the images below.)

Gradient-echo images often show decreased signal intensity in the venous angioma that is not due to hemosiderin but is secondary to the paramagnetic effects of deoxyhemoglobin in the venous blood (see image below). Findings on diffusion images are usually normal.

Magnetic resonance venography is almost never necessary. If obtained, venograms show the draining vein with some of the surrounding radially arranged veins. Because the DVA provides the venous drainage for a section of brain, anatomically normal venous drainage is not present in that area.

Because DVAs are often associated with other CNS vascular lesions (particularly cavernous angiomas), when a DVA is identified, carefully evaluate the brain and obtain gradient-echo (GRE) images. Cavernomas typically appear as focal areas of blood products that often show different stages of evolution (ie, hemosiderin with extracellular methemoglobin). Capillary telangiectasias are small areas of lacelike enhancement that are dark on GRE images without signal intensity abnormality on T2-weighted images. AVMs have enlarged feeding arteries and tortuous vessels with surrounding gliosis.

Degree of confidence

MRI findings are diagnostic in almost all instances. However, in cases with questionable findings, MR venography usually suggests the diagnosis.

Developmental venous anomalies found on angiograms are almost invariably incidental findings, as they are with newer MRIs, and the diagnosis can be made in almost every instance. When observed, the DVA appears as a blush of contrast enhancement during the venous phase of the study and drains into a large anatomically anomalous vein. In its most frequent location (adjacent to the lateral ventricles), the vein usually drains into a subependymal vein, although superficial drainage also occurs.^{[15, 16]}

Degree of confidence

A DVA has a characteristic angiographic appearance and should not be confused with an AVM; no early filling occurs with a DVA.