Anatomical variations of the lower cranial nerves in the neck

ABSTRACT Objective: The routes of the lower cranial nerves in the neck, their distance to some anatomic points and their neighborhoods are variables. As these unknown variables increases the risk of a surgical complication, clarification of these variants will provide easy recognition and protection of the cranial nerves and decrease the risk of complication. So, this study aims to reach descriptive and complementary anatomic information by investigating the routes of the lower cranial nerves in the neck and measuring the distances to anatomic points. Materials & Methods: To determine the distances of the lower cranial nerves (vagus n., accessory n., and hypoglossal n.) to distinct anatomic points, 70 dissections were performed on 39 fresh cadavers between the ages 18-50. The relations of the spinal accessory nerve with the internal jugular vein were investigated around the jugular foramen, at the level of posterior belly of digastric muscle and at a lower level. The distances of hypoglossal nerve and vagus nerve from the cricoid cartilage, hyoid bone and the posterior belly of the digastric muscle were measured. The software SPSS 22.0 was used to perform the analysis. Results: According to the results of the hypoglossal nerve dissections, the mean distance of the hypoglossal nerve is 27.7 mm from the carotid bifurcation, 9.3 mm from the hyoid bone and 54.7 mm from the cricoid cartilage. In the 70 dissections for vagus nerve, vagus nerve was at the posterolateral of the carotid artery in 1 dissection (1.43%), at posterior of the carotid artery in 1 (1.43%) and at medial in 2 dissections (2.9%). The mean distance of the vagus nerve is 23.7 mm from the lateral border of the cricoid cartilage, and 20.8 mm from the lateral surface of the hyoid bone. The mean depth oF the vagus nerve in the posterior belly of the digastric muscle was measured as 8.4 mm. At the level of the jugular foramen, the accessory nerve was at the lateral of the internal jugular vein in 6 dissections (8.57%), at the posterior in 10 (14.29%), at the anterolateral in 7 (10%), at the anterior in 36 (51.43%), at the posterolateral in 1 (1.43%), at the anteromedial in 7 (10%) and at the medial in 3 (4.29%). At the level of the posterior belly of the digastric muscle, the accessory nerve was at the lateral of the internal jugular vein in 39 dissections (55.7%), at the posterior in 8 (11.4%), at the anterolateral in 1 (1.43%), at the anterior in 21 (30%) and at the posterolateral in 1 (1.43%). Below the level of the digastric muscle, the spinal accessory nerve was at the lateral of the internal jugular vein in 63 dissections (90%), at the posterior in 3 (4.3%), at the anterolateral in 1 (1.43%), at the anterior in 2 (2.86%) and at the posterolateral in 1 (1.43%). Conclusion: The routes and neighbourhoods of the lower cranial nerves (10th, 11th, 12th cranial nerves) at the neck were investigated and their distances to anatomic points were measured. It was seen that he spinal accessory nerve is mostly at the anterior of internal jugular vein at the level of the jugular foramen whereas it was at the lateral of internal jugular vein at the level of the posterior belly of the digastric muscle or below. It was showed that the vagus nerve could be at the medial of the carotid artery although it is a rare condition. Also it was concluded that the distance between the hypoglossal nerve and hyoid bone is less than 1 cm. These properties should be kept in mind while performing neck dissections and the hypoglossal nerve should be specially thought in laryngectomy operations. Along with these, it should be remembered that this study was a cadaver study in which the anatomy may show differences compared to the alive neck anatomy. Keywords: Spinal accessory nerve, hypoglossal nerve, vagus, cadaver study, lower cranial nerves, jugularforamen.