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K. Orhan (ed.)Ultrasonography in Dentomaxillofacial Diagnosticshttps://doi.org/10.1007/978-3-030-62179-7_7
7. Sonographic Anatomy and Pathology Extracranial Nerves
7.1 General Sonographic Anatomy
The extracranial nerves are only partly available by ultrasonography. The cervical part of the vagal nerve is the only part of the cranial nerves that can be easily identified between the carotid arteries and the internal jugular vein .
The vagal nerve exits from the skull base along the lateral wall of the internal carotid artery. The division of the common carotid artery is the landmark after which it runs between the internal jugular vein and the common carotid artery.
The brachial plexus can be seen in level V though it is seldom readily visible by US. It is located laterally to the anterior scalene muscle and often appears as hypoechoic oval structures when displayed obliquely.
Besides the vagus nerve, there are no specific anatomical landmarks for the other cranial nerves, such as the hypoglossal, the accessory, and the facial nerve [2–4].
7.2 Inflammatory Changes
The most common clinical picture of inflammatory changes is the presence of carotidynia, a painful syndrome with pain and tenderness over the carotid areas [5, 6].
7.3 Benign Tumors
Paragangliomas of the autonomous system are rare neuroendocrine tumors that occur due to glomus body hyperplasia or hamartomatous development, and they appear to originate from modified smooth muscle cells [7, 8]. They have a female prevalence of about 3:1 and mainly present in middle-aged people [9, 10]. The initial clinical presentations are insidious even in cases when the tumors reach a large size. Accurate and comprehensive imaging evaluation is the key to their diagnosis. Because they are located in the skull base and are not readily viewed by ultrasound, it is most commonly found by other imaging techniques. The preoperative diagnosis of glomus tumors remains challenging. Inaccurate diagnoses are largely attributed to this tumor’s rarity and the lack of distinguishing clinicomorphologic characteristics [11, 12].
Furthermore, such lesions have heterogeneous appearances on radiologic images. A glomus tumor may initially be diagnosed as a salivary tumor, sebaceous cyst, neurofibromatosis, dermoid cyst, developmental tumor, vascular malformation, or another type of mesenchymal neoplasm. Although vascular malformations and cystic soft tissue lesions can usually be ruled out using color doppler ultrasonography, the differential diagnosis of solid tumors remains challenging. Recently, fluorodopa (F-DOPA) positron emission tomography was used for detecting glomus tumors [12, 13]; however, the validity and specificity of this technique for tumors in the head and neck region requires verification. As formal diagnostic guidelines are absent, a thorough radiographic set of examinations and studies may be needed to set the diagnosis but histologic examination and immunohistochemical analysis remain the gold standards [14, 15].