Arterial Dissection

Arterial dissection involves intimal disruption or tearing of the intima with subsequent dissection of blood into the arterial vessel wall. This intramural hemorrhage may result in luminal narrowing or total occlusion of the vessel lumen [42]. Vessel dissection can be divided into two major categories. The first is traumatic in origin usually following blunt injury to the head and neck or injury from chiropractic manipulation. The second major category is that of spontaneous vessel dissection and occurs without predisposing trauma. In the second type of dissection there is usually underlying disease involving the vessel wall that predisposes these patients to dissection. Such predisposing factors include hypertension, fibromuscular dysplasia, genetic diseases such as Marfan syndrome with cystic medial necrosis (Fig. 23), Ehlers Danlos syndrome, alpha-1 anti-trypsin deficiency, or other etiologies including drug abuse with sym-pathomimetics and infection [2,43,44]. Dissection of the carotid or vertebral arteries has a high association with ischemic injuries of the brain either due to vessel occlusion at the dissection site or to embolization. Early recognition of a vessel dissection is important to initiate treatment, often directed to the use anticoagulants or antiplatelet therapy in an attempt to prevent or minimize cerebral ischemic complications [43]. In some cases, invasive treatment may be indicated with either trapping of the dissected segment of the vessel (therapeutic occlusion of the vessel proximal and distal to the level of dissection) or with stenting of the dissected vessel segment to prevent severe lu-minal stenosis or occlusion. Posttraumatic dissection may not be amenable to anticoagulant therapy if there are accompanying brain contusions or systemic injuries that may result in hemorrhage.

Traumatic carotid dissection most often occurs at the level of C1 - C2 just inferior to the skull base. It is due to a hyperextension and rotation injury that leads to shearing injury of the vessel wall as it is pulled across and stretched over the interior arch of C1. This type of injury is often associated with mandibular fractures (Fig. 24a-c).

Spontaneous carotid artery dissection often is accompanied by a Horners syndrome with ipsilat-eral ptosis and miosis. Dissection of the carotid or vertebral arteries is often accompanied by neck pain and ipsilateral headache. Chiropractic manipulation more often results in vertebral artery dissection and is less commonly responsible for carotid artery dissection [45].

The diagnosis of vessel dissection is best made with the combination of MRI and MRA. The MRA findings include vessel lumen narrowing that usually occurs over a relatively long segment compared with atherosclerotic disease. The vessel wall

Fig. 23. Dissection of the aortic arch with extension into the right innominate artery in a patient with Marfan's Syndrome. Note the prominent dilatation of the innominate artery (white arrow) and the intimal flap that extends from the aorta into the innominate artery (black arrows)

is thickened due to the presence of intramural hematoma and in some cases aneurysmal dilatation due to false aneurysm formation in the area of dissection may be seen rather than luminal narrowing. The MRI findings are generally best shown on Tl-weighted images with fat saturation. In the case of acute dissection, intramural hematoma can be seen as high intensity (Fig. 24c) or intermediate signal intensity (Fig. 25) depending on the age of the hematoma within the vessel wall. This finding is highly specific for vessel dissection [46,47].

MRI and MRA have been shown to be more sensitive for carotid vessel dissection then for vertebral artery dissection (Fig. 26a-c). The sensitivity for carotid artery dissection using a combination of MRI and MRA [43,48-50] is very high and comparable with carotid catheter angiography. The sensitivity for detection of vertebral artery dissection has been variable with most reports indicating that it is significantly less sensitive compared with catheter vertebral angiography missing up to 30% of vertebral artery dissections. On the other hand, the specificity of MRI/MRA combination in carotid or vertebral artery dissection was higher when compared with catheter angiography [43]. Thus, although less sensitive MRA is very specific for vertebral artery dissection compared with an-giography [48].

Vertebral Artery Dissection Mri

Fig. 23. Dissection of the aortic arch with extension into the right innominate artery in a patient with Marfan's Syndrome. Note the prominent dilatation of the innominate artery (white arrow) and the intimal flap that extends from the aorta into the innominate artery (black arrows)

Right Innominate Artery Dissection

Fig. 24a-c. Acute traumatic dissection of the right internal carotid artery suffered in an automobile accident. a Coronal CE MRA shows good filling of all of the neck vessels. Note that there is a subtle, abrupt change in caliber of the distal right internal carotid artery just before it enters the skull base (arroW). b Rotation of the MRA in a different plane confirms the caliber change (arroW). c Axial Tl-weighted fat saturated image just below the skull base shows hyperintense signal within the wall of the right internal carotid artery indicating intramural hemorrhage from the traumatic dissection (arrow). The change in vessel caliber, while highly suggestive of an acute dissection in the appropriate clinical setting, is a nonspecific imaging finding. However, the presence of intramural hemorrhage in the wall of the vessel is highly specific for vessel dissection. It is the presence of the intramural hemorrhage that makes MRA more specific than catheter angiography for the diagnosis of vessel dissection

Intracranial Carotid Dissection Mri

Fig. 25. Axial Tl-weighted fat saturated image in a patient with an acute left internal carotid artery dissection that extends into the in-trapetrous portion of the internal carotid artery. Note that the wall of the carotid artery is thickened (arrows) but is not hyperintense since this is a very acute carotid dissection that is less than six hours old. It generally takes 24 to 36 hours for the intramural hemorrhage to appear hyperintense on the Tl-weighted MR images. Note also the importance of fat saturation so that the thickened wall is visualized without interference from hyperintense perivascular fat

Dissection Fat Sat

Fig. 26a-c. Right vertebral artery dissection post-chiropractic manipulation. a Oblique view of the CE MRA shows only very subtle, minimal narrowing of the right vertebral artery (arrow) beginning just below its turn around the C2 level. b Edited view of the right carotid and right vertebral arteries again shows minimal narrowing (arrow) which is non-specific and not definite for dissection. c Axial T1-weighted fat saturated image at the level of C3 shows hyperintense intramural hematoma partially surrounding the wall of the right vertebral artery (arrow). Note that the underlying vessel lumen is not significantly compromised by the dissection, which accounts for the near normal appearance on the MRA

Fig. 26a-c. Right vertebral artery dissection post-chiropractic manipulation. a Oblique view of the CE MRA shows only very subtle, minimal narrowing of the right vertebral artery (arrow) beginning just below its turn around the C2 level. b Edited view of the right carotid and right vertebral arteries again shows minimal narrowing (arrow) which is non-specific and not definite for dissection. c Axial T1-weighted fat saturated image at the level of C3 shows hyperintense intramural hematoma partially surrounding the wall of the right vertebral artery (arrow). Note that the underlying vessel lumen is not significantly compromised by the dissection, which accounts for the near normal appearance on the MRA

The recommended protocol for diagnosis and detection of suspected cervical neck vessel dissection is axial T1-weighted spin echo images with fat saturation and an inferior saturation pulse to suppress flow through the neck to demonstrate the presence of intramural hemorrhage in combination with contrast enhanced 3D MRA. The fat saturated Tl-weighted anatomic images should be obtained prior to the contrast enhanced MRA since the presence of circulating gadolinium may cause perivascular enhancement, which can be confused with intramural hemorrhage.

MRI and MRA are also indicated for follow-up evaluation of patients with known cervical vessel dissection. This is valuable to detect possible pro gression of stenosis or occlusion of the dissected vessel. Such progression of disease may indicate the need for invasive therapy such as surgery or stenting to prevent ischemic cerebral complications. In addition, evaluation of response to therapy, which usually consists of anticoagulant, and/or antiplatelet therapy is needed. Signs of possible healing with resolution of intramural hemorrhage and return of the vessel lumen to normal or near normal caliber following treatment will help to determine the end point of anticoagulant therapy [5, 42]. Thus, it is recommended to follow the patient at periodic intervals until complete vessel wall healing is determined.

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Responses

  • Bercilac
    Can a hypoplasia be confused with a hemorrhage or hematoma on mra confused?
    8 years ago

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