The most common of the lesions in the carotid space are discussed in relation to their radiographic diagnosis and clinical implications. The appearance of tumors and lesions on computed tomography or magnetic resonance imaging is presented, and their differential diagnosis is discussed. The image of each carotid disease is presented, and the discussion concludes with treatment recommendations and considerations.
Key Points
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The carotid space is a complex fascial space spanning the neck from the skull base to the thoracic inlet, encompassing many of the major neurovascular structures of the neck.
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Lesions of the carotid space may arise from asymmetry of normal vasculature, inflammatory or infectious processes, and benign or malignant tumors including metastatic disease processes.
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Cross-sectional imaging plays a key role in identifying particular abnormalities and in planning surgical and nonsurgical therapies.
Introduction
The carotid space is encircled by the 3 layers of the deep cervical fascia (superficial, middle, and deep), which is often referred to as the carotid sheath and extends from the skull base to the aortic arch. The carotid space is a paired space bordered by the visceral space medially, the prevertebral space posteriorly, and the sternocleidomastoid muscle anterolaterally. In the suprahyoid portion of the neck, the carotid space is often synonymous with the poststyloid compartment of the parapharyngeal space.
The carotid space contains several neurovascular elements, including the carotid artery, internal jugular vein (IJV), vagus nerve (CN X), ansa cervicalis, and sympathetic plexus. Superiorly, the carotid space also contains the glossopharyngeal nerve (IX), the accessory nerve (XI), and the hypoglossal nerve (XII), which all pierce the fascia of the carotid sheath. Within the carotid space, the carotid artery lies medial to the IJV, and the vagus nerve is typically found on the posterior aspect of the 2 vessels. The cervical sympathetic plexus is found embedded in the posterior portion of the carotid sheath and the ansa cervicalis is embedded in the anterior portion of the carotid sheath.
Lesions involving the carotid space may arise from any of these constituents. Therefore, radiographic imaging often helps in the diagnosis of lesions involving the carotid space. Lesions of the carotid space may arise from asymmetry of normal vasculature, inflammatory or infectious processes, and benign or malignant tumors, including metastatic disease processes. The most common of these are discussed in relation to their radiographic diagnosis and clinical implications.
Pseudotumor
The anatomy and radiologic imaging appearance of the carotid artery and IJV can vary significantly, and this can lead to confusion when evaluating lesions within the carotid space. An ectatic carotid artery and more commonly an asymmetrically enlarged IJV are most commonly an incidental finding on computed tomography (CT) or magnetic resonance imaging (MRI). However, either can also present as a lateral neck mass originating in the carotid space. The right IJV is usually larger than the left IJV and can be several times larger when compared with the left. In addition to being an ectatic vessel, the internal or common carotid artery can have a retropharyngeal course, which may present as a pulsatile retropharyngeal or retrotonsillar mass ( Fig. 1 ). This has clear clinical significance when the artery itself is thought to be a mass and may prompt a life-threatening biopsy. In addition, knowledge of a retropharyngeal carotid artery may be crucial to surgical planning for resection of tonsillar or hypopharyngeal tumors. A thorough understanding of the normal arterial and venous anatomy and common variants is important to differentiate a focally dilated artery or vein properly from lymphadenopathy or other disease process. CT, MRI, and ultrasonography can all be helpful for identifying vascular structures in the lateral neck, but ultrasonography becomes less helpful for retropharyngeal anomalies. Multiplanar reformatted CT imaging, in conjunction with rapid scrolling of images while viewing, can clearly define the nature of retropharyngeal structure if it is an aberrantly located carotid artery.
Vascular lesions
Vascular lesions, including jugular vein thrombophlebitis, venous and arterial thrombosis, arterial dissection, and arterial aneurysms, present as processes involving the carotid space.
Thrombosis, either arterial or venous in nature, may be diagnosed with contrast-enhanced CT, MRI, or ultrasonography. On contrast-enhanced CT, venous thrombosis appears as a tubular intraluminal filling defect with or without associated mass effect with enhancing vessel wall from the dilated vasa vasorum. In addition, there is usually a distended IJV proximal to the thrombus. MRI can be used when exposure to intravenous contrast needs to be avoided. MRI produces greater soft-tissue contrast and can delineate altered rates of blood flow more sensitively than CT. On T2-weighted MR, acute thrombus of the IJV will have a bright lumen, whereas subacute IJV thrombus will have a low signal. Ultrasonography is an easy, safe, noninvasive, and widely available test. Findings on ultrasonography include a dilated and incompressible vein, intraluminal clot, and no response to the Valsalva maneuver. IJV thrombosis is thought to be an underdiagnosed condition that usually occurs as an associated complication of head and neck infections or surgery, central venous access, malignancy, polycythemia, or intravenous drug abuse. Signs and symptoms of IJV thrombosis can often be subtle but usually include cervical pain and/or neck swelling. A more specific finding is a palpable cord beneath the sternocleidomastoid muscle. Anticoagulant therapy is typically the initial treatment for isolated IJV thrombosis, assuming there is no contraindication for anticoagulation.
IJV may become thrombosed from surrounding soft tissue inflammation or lymphadenopathy. Alternatively, an IJV thrombosis may become secondarily infected, resulting in an IJV thrombophlebitis. IJV thrombophlebitis is radiographically diagnosed in the same manner as a thrombosis with similar findings. There may be significant soft-tissue swelling or other inflammatory changes surrounding the IJV on neck imaging in these cases ( Fig. 2 ). An IJV thrombophlebitis caused by an extension of oropharyngeal or odontogenic infection is referred to as Lemierre syndrome. This is classically an anaerobic suppurative process that can produce septic emboli ( Fig. 3 ). The treatment of IJV thrombophlebitis requires treating the infectious process with an appropriate antibiotic. Although anticoagulation is not always necessary in the treatment of IJV thrombophlebitis, systemic anticoagulation is usually recommended if there is any sign of septic emboli or clot propagation.
Arterial dissection
Carotid artery dissection and aneurysms begin as a tear in the intimal lining of the artery, allowing blood under arterial pressure to enter the wall of the artery. This can result in an intramural hematoma leading to a thrombus, which causes narrowing of the lumen. If the tear extends beyond the intima of the vessel, a pseudoaneurysm develops. In either case, the dissected intima or pseudoaneurysm can become a source of distal emboli. Carotid artery dissection and/or pseudoaneurysm can present with symptoms ranging from headache to hemiparesis. A pseudoaneurysm can present as a pulsatile neck mass or cause mass effect on adjacent structures, causing neurologic symptoms such as a Horner syndrome ( Fig. 4 ). Carotid artery dissection most commonly presents with ipsilateral face or neck pain. Although rare, the cause of carotid artery dissection or aneurysm is usually from either mechanical trauma or an underlying connective tissue disorder, such as Ehlers-Danlos syndrome. CT angiography is considered the study of choice when carotid artery dissection or aneurysm is suspected, and will classically reveal a change in the caliber of the blood vessel. Other findings on CT angiography indicative of a dissection include an oval, irregular, or slit-like cross section of the vessel lumen. Treatment of carotid artery dissection with anticoagulation is usually performed to prevent thromboembolic complications. More invasive treatment is considered when cerebral ischemic symptoms persist despite anticoagulation, when there is a contraindication to anticoagulation, or when clinically significant narrowing of lumen diameter occurs ( Fig. 5 ). Surgery has a limited role in the management of carotid artery dissection. However, carotid artery aneurysms can be surgically repaired with a vein bypass graft or endovascular stent if needed.
Infectious processes
Infectious processes from the surrounding parapharyngeal space can spread directly into the carotid space. The parapharyngeal space communicates with all of the deep neck fascial spaces, therefore there is the potential for all deep neck infections to spread to the carotid space. The most common source of parapharyngeal infections with subsequent carotid space involvement is from odontogenic source, but infections may originate from numerous sources, including the tonsils/oropharynx, nasopharynx, parotid or submandibular glands, trauma, or intravenous drug use. Inflammatory processes involving the carotid space typically present with neck pain, swelling, fever, trismus, or torticollis. More ominous signs include neural deficits such as a Horner sign or palsies of CN IX to XII. Regular spiking fevers may be suggestive of IJV thrombophlebitis and septic embolization (discussed previously). CT scanning with contrast is the imaging modality of choice and is helpful in identifying which fascial compartments are involved. Surgical drainage is usually required, and the approach to drainage depends on the fascial compartments involved as well as the relationship of the abscess and carotid space. In selected cases where the abscess lies medial to the carotid space, the approach to drainage may be through a transoral approach. If there is any concern for damage to the carotid artery of IJV, an external transcervical approach should be used.
Benign tumors
Benign tumors arising within the carotid space are either neurogenic in origin or associated with the paraganglia of the carotid body, jugulotympanic region, or vagus nerve. At their institution, the authors have also found paragangliomas of both the hypoglossal nerve and the cervical sympathetic trunk within the carotid space.
Paraganglia are collections of cells of neuroectoderm origin that secrete catecholamines and help the autonomic nervous system respond to physiologic stressors. Tumors of the paraganglia, or “paragangliomas,” are richly vascular, slow-growing tumors that are typically benign, with fewer than 10% being malignant. These tumors themselves are rarely secretory. Although most paragangliomas are sporadic in nature, roughly 10% are associated with an autosomal-dominant inheritance pattern or are part of a multiple endocrine neoplasia syndrome (MEN-II or MEN-III). Synchronous or metachronous paragangliomas are fairly common in inherited cases, but rare in patients with sporadic tumor development. These synchronous or metachronous paragangliomas result in roughly 10% of patients overall having multicentric lesions, so all areas for potential paraganglioma development should be closely examined on MRI, particularly in patients with a family history. Paragangliomas are typically diagnosed based on radiologic features and clinical examination. Although CT scan and angiography can aid in the diagnosis of paragangliomas, MRI is considered to be the most helpful imaging modality. Larger paragangliomas may have a characteristic “salt-and-pepper” appearance on T1-weighted and T2-weighted MR images. These hyperintense and hypointense foci are related to slow-flowing and fast-flowing intravascular blood and to intratumoral hemorrhage. The T1-hyperintense foci of slow moving blood were more commonly visible with older MR techniques. Paragangliomas typically have avid enhancement on both MR and CT images. Contrast-enhanced MR angiography (MRA) has been shown to aid in the diagnosis of these hypervascular lesions, and may also be used to identify additional subtle paragangliomas ( Fig. 6 ).
