Imaging of Carotid Canal Sympathetic Plexus Schwannoma

Discussion

Schwannomas may present at any age but most commonly occur between the fourth and sixth decades.¹¹ Our first patient was younger than the average age of reporting for head and neck schwannomas, but the patient had no additional findings to suggest neurofibromatosis type II, which more commonly presents in the second decade.⁹

Diplopia was experienced by all patients with this type of schwannoma. Ture et al⁴ reported a 23-year-old man who also presented with diplopia and was found to have the only other reported case of a carotid canal sympathetic plexus schwannoma. The abducens nerve (cranial nerve VI) is in proximity to the ICA, more medially located than the other cranial nerves¹² in the center of the cavernous sinus. This anatomy may place the abducens nerve at particular risk for injury with cavernous sinus pathologies and result in the presenting complaint of diplopia experienced by these patients from unrestrained medial rectus muscle activity.

On surgical investigation of the index case by Ture et al,⁴ the schwannoma was inferomedial to the ICA and thus probably involving the cavernous sympathetic plexus. In patient 2, the mass was anterior medial to the right carotid artery and therefore also probably arose from the cavernous sympathetic plexus. However, in patient 1 the lateral projection to the cochlear promontory was likely the tympanic branch of the caroticotympanic nerve. These findings are suggestive of the first schwannoma in the literature to arise from the internal carotid sympathetic plexus. Because this is also the first case of internal carotid occlusion secondary to a carotid canal sympathetic plexus schwannoma, the position of the tumor lateral to the ICA cannot be confirmed. Although not histologically confirmed in patient 2, the second mass surrounding the left ICA is suggestive of the first reported case of bilateral carotid canal sympathetic plexus schwannoma.

The imaging characteristics of a carotid canal sympathetic plexus schwannoma are much like those of schwannomas in other portions of the body. They are well circumscribed, slow-growing masses that smoothly expand canals and foramina. It is these classic imaging features that make the authors confident in the diagnosis of patient 2. Hyperintense T2 signal intensity is noted in schwannomas, predominately from the Antoni type B myxoid material with areas of lower signal intensity collagen and cellular components (Antoni type A).⁹ Schwannomas are often homogeneous on T1 and noncontrast CT but may undergo cystic degeneration, calcification, hemorrhage, or fibrosis that gives a heterogeneous appearance.⁹ There is usually avid enhancement, which is more commonly homogeneous than heterogeneous. In the rare case of malignant degeneration, the mass may become less defined and exhibit rapid growth, with bony or brain invasion, rather than remodeling.

As discussed, the most common mass in the cavernous sinus is a meningioma, and it could have similar signal intensity characteristics to a schwannoma. It would be unusual for a meningioma to have an elongated course that smoothly expands the carotid canal. There was neither reactive hyperostosis nor dural tail. Other schwannomas also could be considered but would not be expected to course along the petrous carotid canal. The signal intensity and enhancement characteristics of a schwannoma are different from those expected for an epidermoid, aneurysm, or a blood vessel neoplasm. Lastly, the benign-appearing expansion of the canal and no history of primary malignancy make more aggressive neoplasms unlikely.