Identification Of The Spinal Accessory Nerve

The most common complaint after radical neck dissection is the discomfort of shoulder droop resulting from spinal accessory nerve transection. Functional neck dissection preserves the spinal accessory nerve. However, shoulder function after functional neck dissection is not always normal. The explanation to this apparently paradoxical fact must be sought in the variable innervation of the shoulder, especially with respect to the participation of the cervical plexus in shoulder motility. Injury to the motor branches of the cervical plexus that supply the deep muscles of the neck may explain the variation in the degree of disability of the shoulder after preservation of the spinal accessory nerve. The possibility of motor supply to the trapezius from the cervical plexus in human beings is still controversial and rests upon indirect embryological, surgical, and clinical evidence.

The spinal accessory nerve must be identified as it enters the sternocleidomastoid muscle (Fig. 5-14). This point is usually located at the junction of the upper one third and lower two thirds of the muscle. During the dissection of the medial aspect of the fascia of the sternocleidomastoid

Figure 5-13 Relations between the hypoglossal nerve and the lingual vessels in the right submandibular fossa. hn, hypoglossal nerve; lv, lingual vein; la, lingual artery.
Figure 5-14 The spinal accessory nerve is identified as it enters the sternocleidomastoid muscle on the right side. sc, sternocleidomastoid muscle; IJ, internal jugular vein; sa, spinal accessory nerve.

muscle, the entrance of the spinal accessory nerve into the muscle is easily identified. The nerve is usually accompanied by a satellite vessel that must be carefully cauterized to avoid excessive nerve stimulation.

Once identified at its entrance in the sternocleidomastoid muscle, the nerve is followed superiorly toward the internal jugular vein (Fig. 5-15). The spinal accessory nerve usually comes obliquely in a posterior and inferior direction from the jugular foramen. The relations between the spinal accessory nerve and the internal jugular vein are variable. In approximately two thirds of cases, the nerve crosses external to the vein. In the remaining cases, the nerve passes behind the vein (Fig. 5-16) or even across it. The surgeon must keep this important information in mind while dissecting the spinal accessory nerve toward the internal jugular vein. When the vein is approached, precise identification of its wall is mandatory before complete isolation of the nerve is accomplished. Otherwise, the internal jugular vein may be easily injured.

The isolation of the spinal accessory nerve in this region takes place through the fibrofatty tissue of the upper jugular area where the scalpel is not very effective. Thus, the scissors is recommended for this step of the operation. For a satisfactory removal of all fibrofatty tissue in this area it is important to completely isolate the spinal accessory nerve from the surrounding tissue. This will facilitate the delivery of the tissue beneath the nerve by means of the spinal accessory maneuver.

On a complete functional approach, the spinal accessory nerve may also be found in the posterior triangle of the neck. At this level, the surgical position of the patient and the traction applied to the dissected tissue may displace the nerve from its original course. Usually, a slight anterior curvature is created through the neural anastomosis of the spinal accessory nerve with the second, third, and fourth cervical nerves. To avoid injuring the spinal accessory nerve in the posterior triangle, a thorough knowledge of its anatomy is essential.

The spinal accessory nerve enters the supraclavicular triangle at its upper angle, deep to Erb's point, and descends obliquely in a posterior and inferior direction toward the trapezius muscle (Fig. 5-17). Its course is usually associated with the posterior border of the levator scapulae muscle. The spinal accessory nerve should not be confused with several supraclavicular branches of the

Figure 5-15 The nerve is completely exposed between the muscle and the internal jugular vein (right side). SC, sternocleidomastoid muscle; IJ, internal jugular vein; sa, spinal accessory nerve; hn, hypoglassal nerve; SS, specimen from the submandibular area.
Figure 5-16 Lateral view of the right spinal accessory nerve crossing posterior to the internal jugular vein. IJ, internal jugular vein; sa, spinal accessory nerve.
Figure 5-17 Relations between the spinal accessory nerve and the branches of the cervical plexus within the posterior triangle of the neck. sa, spinal accessory nerve; sc, sternocleidomastoid muscle; tm, trapezius muscle; ♦, Erb's point; *, branches of the cervical plexus.

cervical plexus that follow a similar but more superficial course (Fig. 4-37). Although it is usually not necessary, the novice surgeon may find electric stimulation useful to confirm the location of the spinal accessory nerve in the posterior triangle.

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