Approximately 5 percent of dissections are associated with connective tissue disorders, including Marfan syndrome, Ehlers Danlos syndrome Type IV, polycystic kidney disease, fibromuscular dysplasia, alpha-1 antitrypsin deficiency and osteogenesis imperfecta Type I.
Pathophysiology of Internal Carotid Artery Dissection
A dissection occurs when a partial-thickness tear develops in one or more layers of the arterial wall, allowing blood to enter and accumulate between the layers. A dissection can result in luminal stenosis, vessel occlusion, aneurismal dilatation, thrombus formation or hemorrhage
Dissections are usually classified by the anatomic pattern of injury and the layers of the involved vessel. Subintimal dissections occur between the intima and media, and can result in luminal stenosis due to accumulation of blood under the dissection flap. Subadventitial dissections occur between the media and adventitia, often resulting in aneurysmal dilatation due to weakness of the vessel. Cervical dissections are most commonly seen in the mobile segments of the affected artery. In the ICA, dissections most often occur 2 to 3cm distal to the carotid bulb. The most common site of a vertebral artery (VA) dissection is at the V2 segment where the artery travels within the transverse foramen, and the V3 segment where the vessel travels around the lateral masses of the C1 and C2 vertebrae.
The most common presenting symptoms are head or neck pain and cerebral ischemic symptoms. Pain is the most common presenting symptom, present in about 70 percent of patients. Horner syndrome—present in 30 to 60 percent of patients—can be due to ischemia of the vasa nervorum or compression of the pericarotid sympathetic fibers, and seems to be more common in subadventitial dissections. However, Horner syndrome can also be present in a VA dissection secondary to brainstem ischemia.
Transient ischemic attack or stroke occurs in 67 percent of dissection patients, usually due to artery-to-artery embolism rather than hemodynamic compromise due to luminal stenosis.
Imaging Tools for Differential Diagnosis
Catheter angiography remains the gold standard for the radiographic diagnosis of dissections, usually demonstrating a tapered, or “flame-shaped” stenosis or occlusion. While angiography is more invasive than other diagnostic modalities, it allows identification of intraluminal thrombus and evaluation of collateral circulation. It may also confirm complete occlusion of the vessel or the presence of a “string sign” indicating incomplete occlusion, which is important information for the management of the dissection.
Magnetic resonance imaging (MRI) and magnetic resonance angiography (MRA) are alternate, less invasive methods of imaging. While they yield more information regarding the extent of ischemic injury and the sensitivity rivals catheter angiography for ICA dissections, the sensitivity for detecting VA dissections is much less. Other limitations include difficulty distinguishing intramural hematoma from intraluminal thrombus, and a tendency to overestimate the degree of vessel stenosis.
Fat-suppressed T1 images of the neck can help to distinguish intramural hematoma from surrounding adipose tissue. Computed tomography (CT) and angiography (CTA) are also less invasive than catheter angiography. A non-contrast head CT can provide information about ischemic damage, and the sensitivity and specificity of CTA rivals those of both catheter angiography and MRA.
Duplex ultrasonography (US) is the least sensitive and specific modality, with limitations including operator variability, difficulty distinguishing atherosclerotic lesions from dissections and a tendency to overestimate severe stenosis and occlusion.
Treatment and Management
The management of dissections depends on a number of factors, including the presence and amount of cerebral ischemia or infarction, vessel stenosis or occlusion, and symptom stability versus progression.
Patients presenting to the emergency department with ischemic symptoms, whether or not a dissection is suspected, should still be treated with intravenous thrombolytics if possible. For those patients with stable symptoms and/or stenosis without hemodynamic compromise, antithrombotic treatment is sufficient. It is estimated that approximately 85 percent of dissections will heal with antithrombotic treatment alone. Medical treatment is physician-dependent, with antiplatelet medications, including aspirin, or anticoagulation both being acceptable therapies. There have been no randomized controlled trials comparing antiplatelet medications and anticoagulation in the treatment of cervical dissections. In 2010, a review by The Cochrane Collaboration compared 36 observational studies that included 1,285 patients with dissections to determine a benefit of antiplatelet medications versus anticoagulation. They found no difference in stroke or death when comparing the two treatment options.
The Cervical Arterial Dissection in Stroke Study, a randomized controlled trial comparing antiplatelet medications and anticoagulation, is currently ongoing.
For those patients with unstable symptoms or severe stenosis, the management can be more complicated. If a patient has a large infarct in the territory of the dissected artery, permanent occlusion of the artery may be indicated to prevent hemorrhagic complications associated with possible reperfusion or artery-to-artery emboli. For those patients with severe stenosis and unstable symptoms without significant infarction, or those with unstable aneurysmal changes, endovascular therapy with stenting may be the treatment of choice.
The management of intracranial dissections differs due to anatomical differences in the arteries. Intracranial vessels have thinner adventitia and media, fewer elastic fibers in the media, lack an external elastic membrane, and have diminished vasa vasorum. Due to increased risk of subarachnoid hemorrhage, management of intracranial dissections is usually limited to antiplatelet medications or surgical repair.
To meet the specialists at Santa Barbara Neuroscience Institute at Cottage Health System, visit www.sbni.org. To refer a patient, please contact the transfer center at 1-888-MY-CAL-NEURO.