Clinical meaning
Carotid artery dissection (CAD) is a potentially catastrophic vascular condition in which a tear develops in the tunica intima (innermost layer) of the internal carotid artery, allowing blood to enter the arterial wall and create a false lumen (intramural hematoma) between the intimal and medial layers. This intramural hematoma expands within the vessel wall, progressively narrowing the true lumen and reducing cerebral blood flow, or may extend subadventitially to form a pseudoaneurysm. Carotid artery dissection is responsible for approximately 10-25% of ischemic strokes in young and middle-aged adults (under 50 years), making it a critical diagnosis that requires rapid recognition and intervention. The arterial wall consists of three concentric layers: the tunica intima (endothelial cells resting on a basement membrane and internal elastic lamina), the tunica media (smooth muscle cells embedded in an extracellular matrix of elastin and collagen fibers that provide structural integrity and vasomotor tone), and the tunica adventitia (the outermost connective tissue layer containing the vasa vasorum that supply the outer wall). In carotid dissection, the initiating event is a breach of the intimal layer, which can occur spontaneously or from trauma. Once the intima is breached, arterial blood under systemic pressure dissects into the media, cleaving the vessel wall along the plane between the intima and media or between the media and adventitia. The resulting intramural hematoma creates two distinct pathological consequences depending on its location within the wall. Subintimal dissection (between intima and media) causes the intramural hematoma to expand inward, compressing and narrowing the true arterial lumen, producing stenosis and reducing distal cerebral perfusion. This luminal narrowing activates platelet aggregation and the coagulation cascade on the disrupted endothelial surface, forming thrombus within the residual true lumen. These thrombi can either progressively occlude the vessel (causing hemodynamic stroke from hypoperfusion) or embolize distally to occlude intracranial arteries (causing thromboembolic stroke, which is the more common mechanism). Subadventitial dissection (between media and adventitia) causes the hematoma to expand outward, forming a pseudoaneurysm -- a contained outpouching of the vessel wall that can compress adjacent structures, most importantly the sympathetic nerve fibers running along the carotid artery (producing ipsilateral Horner syndrome: miosis, ptosis, and anhidrosis) and cranial nerves IX, X, XI, and XII as they exit the jugular foramen in close proximity to the internal carotid artery (causing lower cranial nerve palsies with dysphagia, hoarseness, tongue deviation, and shoulder weakness). The pathophysiology of spontaneous carotid dissection involves intrinsic weakness of the arterial wall. Histological examination of dissected carotid arteries frequently reveals ultrastructural abnormalities of collagen and elastin fibers in the media, suggesting an underlying connective tissue vulnerability. This is supported by the strong association between CAD and heritable connective tissue disorders: Ehlers-Danlos syndrome type IV (vascular type, caused by COL3A1 mutations affecting type III collagen), Marfan syndrome (FBN1 mutations affecting fibrillin-1 and elastic fiber formation), fibromuscular dysplasia (present in 15-20% of CAD patients, characterized by segmental non-atherosclerotic arterial wall irregularity), and osteogenesis imperfecta. Even in the absence of a defined connective tissue disorder, many CAD patients show subtle signs of generalized arteriopathy. Traumatic carotid dissection results from mechanical forces applied to the neck that stretch, compress, or hyperextend the carotid artery over the lateral masses of the upper cervical vertebrae (C1-C3), where the internal carotid artery is relatively fixed and vulnerable to injury. Common mechanisms include motor vehicle accidents with hyperextension-rotation injuries, chiropractic cervical manipulation, sports injuries (particularly martial arts, wrestling, and contact sports), and even minor trauma such as vigorous coughing, sneezing, or roller coaster rides in individuals with underlying arterial wall fragility. The internal carotid artery is most vulnerable to dissection at two anatomical locations: the pharyngeal segment (C1-C2 level, where the artery curves medially and is relatively mobile) and the petrous segment (where it enters the bony carotid canal and becomes fixed). The inflammatory response following dissection contributes to ongoing vessel wall damage. The intramural hematoma triggers an inflammatory cascade with infiltration of macrophages, neutrophils, and T-lymphocytes into the vessel wall. These inflammatory cells release matrix metalloproteinases (MMPs) that degrade the extracellular matrix, potentially weakening the wall further and promoting pseudoaneurysm expansion. Additionally, activated platelets on the disrupted endothelial surface release platelet-derived growth factor (PDGF) and thromboxane A2, promoting local thrombosis and vasoconstriction that worsen ischemia. The clinical significance of carotid artery dissection extends beyond the acute stroke risk. Approximately 60-80% of patients develop ischemic stroke or TIA as their presenting symptom, typically within the first two weeks after dissection onset. The remaining patients present with local signs including ipsilateral headache or neck pain (which may precede stroke by hours to days, providing a critical warning window), Horner syndrome, pulsatile tinnitus, or cranial nerve palsies. Recognition of these warning signs is essential for preventing stroke through early anticoagulation or antiplatelet therapy.