Vascular relations: A guide to differentiating common carotid space lesions

CASE HISTORY

A 33-year-old male patient with no known comorbidities presented to our hospital with a painless swelling in the left side of the neck, which gradually increased in size over the past 1 year. On local examination, a mobile, non-tender, and compressible mass was palpated in the left upper neck at the level of the angle of the mandible. No palpable cervical lymphadenopathy was found. Magnetic resonance imaging (MRI) was performed for further evaluation, which revealed a T1-weighted isointense and T2-weighted heterogeneously hyperintense mass arising from the left carotid space. The mass showed slow progressive enhancement on dynamic post-contrast MRI sequences. A contrast-enhanced magnetic resonance angiogram was performed, which demonstrated lateral displacement and mild splaying of the left internal and external carotid arteries [Figure 1].

Figure 1: (a and b) Axial T2-and T1-weighted images demonstrate a well-defined, lobulated mass arising from the left carotid sheath superior to the carotid bifurcation. (c) Splaying and lateral displacement of the left internal and external carotid arteries (arrow). (d) Lesion showing progressive heterogenous post-contrast enhancement

What is the diagnosis, and what should be done next? Once you have finalized your answer, turn to the next page to read on.

DIFFERENTIAL DIAGNOSIS AND FURTHER MANAGEMENT

The differential diagnoses considered were carotid body tumor, cervical sympathetic chain schwannoma, and vagal schwannoma.^[1] Vagal schwannomas displace the carotid arteries anteriorly rather than splaying them. Cervical sympathetic chain schwannomas displace the carotid arteries anteriorly and laterally but occasionally splay the internal and external carotid arteries without encasement. Carotid body tumors are usually pulsatile in nature, show multiple internal flow voids, splay and encase the carotid arteries, and show intense early enhancement followed by rapid washout. However, in our case, the mass showed progressive enhancement. Hence, in the given clinical setting and after reviewing the radiological features, a probable diagnosis of schwannoma arising from the cervical sympathetic chain was made. Given its benign appearance on imaging, surgical excision was advised, and the patient underwent an uneventful surgery. Intraoperative notes suggested a possible sympathetic chain origin of the tumor, and post-operative histopathology confirmed the tumor to be a schwannoma.

FINAL DIAGNOSIS

Schwannoma of the cervical sympathetic chain.

DISCUSSION

Schwannomas are benign, well-encapsulated, and slow-growing nerve sheath tumors. They constitute 5% of all benign soft tissue tumors.^[2] They are composed of Schwann cells in a collagenous matrix and can originate from any cranial, peripheral, or autonomic nerve in the body. Approximately 45% of all schwannomas occur in the head-and-neck region.^[2]

On MRI, schwannomas generally produce hypointense to isointense signals on T1-weighted images and hyperintense signals on T2-weighted images. The signal depends on the cellularity. Classically, a central hypointensity and surrounding hyperintensity on T2-weighted axial images is observed in schwannomas; this is known as the target sign which occurs due to the concentration of Antoni type A cells at the center and Antoni type B cells at the periphery. Antoni type A areas are hypercellular and hence produce less bright signals as compared to Antoni B areas which are fluid rich. This is because the T2 relaxation time diminishes with increasing cellularity.^[3] Schwannomas show the T2 shine-through effect, which is observed as a higher signal on both diffusion-weighted images and apparent diffusion coefficient maps.^[4] On gadolinium-enhanced studies, marked heterogeneous enhancement is common. In general, schwannomas are hypovascular tumors, but if images are acquired beyond the first 60 s after contrast injection, the lesion might mimic a hypervascular tumor because of dense enhancement due to contrast pooling from slow venous drainage. Hence, a dynamic contrast-enhanced study is required to accurately assess the enhancement characteristics of a suspected schwannoma. This explains the progressive pattern of contrast enhancement seen in our case.

Neurofibroma is a close differential diagnosis of schwannoma anywhere in the body, especially in the head-and-neck. Unlike schwannoma, it is an unencapsulated nerve sheath tumor that shows no clear demarcation between the hypercellular Antoni type A and hypocellular Antoni type B areas. Although indistinguishable radiologically in most instances, a major histological difference between a neurofibroma and schwannoma is that the nerve fibers course through a neurofibroma while they tend to travel around the margin of the tumor in case of a schwannoma.^[5] This makes precise excision of a neurofibroma without causing nerve damage more challenging.^[6]

Most extracranial schwannomas are found in the parapharyngeal space and originate from the vagus nerve. However, a schwannoma arising from the cervical sympathetic chain is a rare entity. The cervical sympathetic trunk courses over the longus capitis and longus colli muscles lying deep beneath the prevertebral fascia. During this course, the cervical sympathetic chain is situated posteromedial to the carotid vessels. As a schwannoma arising from the cervical sympathetic chain is located in the carotid space, the parapharyngeal fat is displaced anteriorly, and the visceral space is displaced medially. In practice, lesions arising from the sympathetic chain are included in carotid space pathologies, even though in strict anatomical terms, the sympathetic chain is located outside the carotid sheath.

Since the sympathetic chain is located posteromedial to the carotid sheath, the internal and common carotid arteries are displaced in the anterior and lateral directions, respectively, by schwannomas arising from the cervical sympathetic chain [Figure 1].

Schwannomas arising from the cervical sympathetic chain can splay the internal and external carotid arteries, thereby mimicking the “Lyre” sign which is a characteristic imaging feature of carotid body tumors[⁷]. Since carotid body tumors arise from the wall of the common carotid artery, they tend to completely involve the angle of the carotid bifurcation and occasionally encase the internal and external carotid arteries. Carotid space schwannomas do not fill this angle of the carotid bifurcation but merely separate the internal and external carotid arteries as they grow in size.^[8]

Moreover, the angle of contact between the mass and arteries is not more than 180° of the vessel circumference. This means that schwannomas arising from the cervical sympathetic chain can splay the bifurcation without encasing the arteries or involving the angle of the carotid bifurcation.

Schwannomas arising from the cervical sympathetic chain should be distinguished from other masses in the neck originating from the carotid space [Figure 2]. Differential diagnoses of cervical sympathetic chain schwannomas include:

Figure 2: Diagram depicting the vascular relations of common carotid space lesions

1. Vagal schwannomas: The vagus nerve is located between the internal jugular vein and internal carotid artery in the carotid sheath; hence, vagal tumors tend to separate these vessels and anteriorly displace the internal carotid artery [Figure 3]. Such a separation is not usually seen in case of schwannomas arising from the cervical sympathetic chain, as the sympathetic chain runs posteromedial to both the internal jugular vein and internal carotid artery.^[9]
2. Paragangliomas: Carotid body tumors show intense early arterial enhancement with rapid washout and characteristic flow voids (“salt and pepper” appearance) on MRI. Since carotid body tumors arise from the wall of the carotid bulb, they tend to fill the crotch of the carotid bifurcation and also encase the internal and external carotid arteries, unlike schwannomas arising from the cervical sympathetic chain that merely separate them as they grow in size but do not fill the crotch of bifurcation.[Figure 4]
3. Glomus vagale tumors, on the other hand, are located superior to the carotid bifurcation, displace the internal jugular vein and internal carotid artery anteriorly, and do not involve the carotid bifurcation.

Figure 3: (a) Axial T2 weighted (T2W) magnetic resonance (MR) showing a well circumscribed ovoid T2 hyperintense lesion in the right carotid space displacing the internal jugular vein and the internal carotid artery laterally (arrow). (b) The lesion appears isointense on T1W images. (c) The direction of vessel displacement is more conspicuous on the Axial Steady state free precession image. (d) The lesion shows heterogeneous enhancement on axial T1 post contrast sequence

Figure 4: (a and b) Axial T2-weighted and post-contrast dynamic contrast-enhanced magnetic resonance images showing an intense, homogeneously enhancing lesion completely encasing the carotid bifurcation. (c and d) Sagittal maximum intensity projection magnetic resonance angiogram and T2-weighted images demonstrate the classic “Lyre Sign” due to splaying of the internal and external carotid arteries

Other parapharyngeal space lesions mimicking carotid space schwannomas are described in Table 1.

Table 1: Different parapharyngeal space lesions mimicking carotid space schwannomas and their imaging features

CONCLUSION

Schwannoma of the cervical sympathetic chain is a rare entity. Knowledge of the vascular relations and the direction of displacement of carotid vessels can be useful in making an accurate preoperative diagnosis and differentiating between various carotid space lesions.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient has given his consent for his images and other clinical information to be reported in the journal. The patient understands that his name and initials will not be published and due efforts will be made to conceal his identity, but anonymity cannot be guaranteed.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.