carotid artery

Anatomy: The common carotid arteries arise asymmetrically—right from the brachiocephalic trunk, left directly from the aortic arch—and ascend in the neck within the carotid sheath alongside the internal jugular vein and vagus nerve. At the level of C4 (thyroid cartilage), each bifurcates into internal (ICA) and external (ECA) carotid arteries. The carotid sinus (baroreceptor) and carotid body (chemoreceptor) are located at this bifurcation, innervated by CN IX (glossopharyngeal). ICA has no branches in the neck and enters the skull via the carotid canal to form the anterior circulation of the brain (via the Circle of Willis), supplying the eyes, anterior 2/3 of cerebral hemispheres. ECA supplies extracranial structures: face, scalp, thyroid, and meninges via multiple branches (superior thyroid, facial, maxillary, superficial temporal, etc.).

Clinical significance: Carotid atherosclerosis, especially at the bifurcation (a site of turbulent flow), is a major cause of ischemic stroke and TIA. Auscultation may reveal a carotid bruit. Carotid duplex ultrasound is used to assess stenosis; significant stenosis (>70%) may warrant carotid endarterectomy or stenting to prevent stroke, per NASCET/ACAS trial data. Amaurosis fugax (transient monocular vision loss) suggests emboli from ICA/ophthalmic artery. Carotid dissection (often traumatic or spontaneous, associated with connective tissue disorders) can cause stroke in younger patients, presenting with neck pain, Horner syndrome, and cerebral ischemia. Carotid sinus massage or hypersensitivity can cause reflex bradycardia/syncope (carotid sinus syndrome) due to baroreceptor stimulation, mediated by CN IX afferents and vagal efferents. Carotid body tumors (paragangliomas) present as painless neck masses at the bifurcation, classically causing splaying of ICA/ECA on imaging ("lyre sign").