The approach to the lower anterior cervical spine is technically simple, taking advantage of natural anatomic planes.1 However, it requires dissection of important anatomic structures such as the recurrent laryngeal nerve, superior laryngeal nerve, sympathetic trunk, and occasionally spina] accessory nerve.
The most frequent complication related to the anterior approach is vocal cord paresis due to injury to the recurrent laryngeal nerve and superior laryngeal nerve.211 In most cases, it is a transient event, although occasionally it may be permanent.5,1214 Cloward3 reported a 2% incidence of permanent hoarseness, presumably from vocal paralysis, and Heeneman5 reported an 1 1 % incidence of postoperative voice changes following anterior cervical fusion. Also, dysphagia and dysphonia are not uncommon, especially when approaching the lower cervical spine.
Damage to the sympathetic trunk and the development of Horner's syndrome (ptosis, ipsilateral meiosis, and anhydrosis) resulting from thyroid malignancy, benign thyroid goiter, or following thyroidectomy is well-documented in the literature.1518 The incidence of Homer's syndrome following anterior cervical spine surgery ranges between 0.2% and 4%.8,14,19 Likewise, accidental injury of the spinal accessory nerve with painful loss of shoulder function, protraction, and rotation of the scapula is a recognized complication in neck surgery.20,21
Potential damage to the recurrent laryngeal nerve is minimized by knowledge of the course and location of the recurrent laryngeal nerve. The recurrent laryngeal nerves follow different courses on the right and left sides. On the right side, the nerve leaves the main trunk of the vagus nerve and passes anterior to and under the subclavian artery, while on the left side it passes under and posterior to the aorta at the site of origin of the ligamentum arteriosum. It then runs upward with a variable relation to the inferior thyroid artery.
Figure 1 : The midportion of the right recurrent laryngeal nerve (white arrow) is anterior and lateral to the esophagotracheal groove. The inferior thyroid artery (white arrow head) is labeled (A). The midportion of the left recurrent laryngeal nerve (white arrow) is situated within the esophagotracheal groove on the left side. The inferior thyroid artery (white arrow head) is identified (B).
Figure 2: A schematic illustration of 1) the recurrent laryngeal nerve with the angle of ascent relative to the sagittal plane on the right and its relationship to the inferior thyroid artery, 2) the superior laryngeal nerve and its relationship to the superior thyroid artery, and 3) the spinal accessory nerve anterior to the transverse process of the atlas.
The right nerve either is anterior to (26%-33%) or passes between the branches of the inferior thyroid artery (47%-50%), while the left recurrent laryngeal nerve is more commonly posterior (50%-55%) to the inferior thyroid artery.2224 Anatomic studies of the recurrent laryngeal nerve with regard to the anterior approach to the cervical spine are sparse, and the angle of the recurrent laryngeal nerve relative to the sagittal plane and the location on the cross section are not clearly specified in the literature.
This article focuses on the portion of the recurrent laryngeal nerve located between the clavicle and inferior thyroid artery. Although visualization of the midportion of the recurrent laryngeal nerve through the anterior approach to the lower cervical spine is limited, the recurrent laryngeal nerve may be more vulnerable to damage during anterior cervical spine procedures than during thyroid surgery.
Ebraheim et al25 demonstrated that on the left side of the neck, the recurrent laryngeal nerve has a longer course in the esophagotracheal groove and assumes its relationship to the esophagotracheal groove at the midportion of its course. Meanwhile, more variable (6.5±1.2 mm anterior to and 7.3±0.8 mm lateral to the esophagotracheal groove at the Cl level) is located on the right side. The right-sided approach, therefore, may carry a higher risk than a left-sided approach.
Our anatomic studies25"27 have shown that during an anterior approach to the lower cervical spine on the right side, the recurrent laryngeal nerve can be exposed over 22.8 ±4.3 mm, and it then enters the larynx with an angle of 25° ±4.7° relative to the sagittal plane, while coursing lateral and anterior to the esophagotracheal groove (Figures 1 and 2). Therefore, to avoid injury while performing the anterior approach to the lower cervical spine, the recurrent laryngeal nerve needs to be exposed and a retractor with a blunt tip should be placed beneath the longus colli muscles rather than the esophagus. The ligation of the inferior thyroid artery on the right, if necessary, should be carried out as far lateral as possible.
Another point for consideration is that the nerve may be nonrecurrent on the right side (1% of patients), where it arises from the main trunk of the vagus and then passes directly into the larynx, or the inferior thyroid artery may be absent.2830 Nonrecurrent left inferior laryngeal nerve is rare.31 No structure passing medially to the carotid sheath should be divided until after a normal recurrent laryngeal nerve is identified. If a nonrecurrent inferior laryngeal nerve is identified and cannot be retracted, it may be advisable to close the incision and make a left-sided approach.
The superior laryngeal nerves arise from the vagus and divide into two branches: an internal sensory branch that penetrates the thyrohyoid membrane and an external branch that accompanies the superior thyroid artery (Figure 2). The external branch of the superior laryngeal nerve innervates the cricothyroid muscle that regulates the tension of the vocal cords by rotating the cricoid cartilage.32,33 Injury to the external branch of the superior laryngeal nerve may be caused by: 1) ligation of the thyroid vessels before identification, 2) anatomic variations of the nerve, 3) excessive traction on the midline structures with stretching and pinching of the nerve, 4) excessive suction near the nerve, or 5) postoperative edema of perineural tissues and scar formation.
The external branch of the superior laryngeal nerve runs close to the superior thyroid artery, and attention to the course and position of the nerve is mandatory. Although the consequences of external branch of the superior laryngeal nerve injury often may be relatively mild (change or increased fatigability of voice or loss of ability to sing, especially high notes). it may cause serious problems in professional and social life.33M Careful dissection around the superior thyroid pole may effectively prevent its injury.33-36
Figure 3: The longus colli diverge at the C6 level. The angle between the medial borders of the longus colli is visible (A). The sympathetic trunk ascends laterally relative to the midline and lies over the longus colli and lateral to the medial border of the longus colli (arrow) (B). A schematic illustration of the angle of sympathetic trunk relative to the midline and the angle between medial borders of the longus colli (C).
Since Homer's syndrome is one of the most feared complications in anterior cervical spine surgery, especially when approaching the lower cervical spine.24'644 proper knowledge of the location and course of the sympathetic trunk in relation to the medial border of the longus colli is critical for a desirable surgical outcome. Without accurate identification. section, division, overstretching or compression of the sympathetic trunk are likely to occur. Although patients with Homer's syndrome typically have no functional impairment, there can be significant cosmetic concern (ptosis. meiosis and anhydrosis37) and an associated annoying ipsilateral nasal stuffiness.
Stretching of the sympathetic trunk while retracting the carotid sheath or longus colli muscle laterally to expose the lateral poi-tion of the cervical vertebra! bodies or uncovertebral joints or transverse foramina may produce Iransient or irreversible Homer's syndrome. Symptoms of Homer's syndrome result from damage to the sympathetic fibers anywhere along cervical sympathetic trunk.
The superior cervical ganglion is situated at the C2-C3 level with the internal carotid artery and sheath lying anteriorly and the longus capitis muscle posteriorly. An enlarged superior cervical ganglion can be mistaken for a lymph node of the upper deep jugular chain and may inadvertently be removed during neck dissection. The lower end of the ganglion is united by a connecting trunk to the middle cervical ganglion. The middle cervical ganglion is small and may be absent. It might be replaced by minute ganglia in the sympathetic trunk or may be fused with the superior ganglion. It usually is found at the sixth cervical vertebral level, anterior or superior to the inferior thyroid artery.
The stellate ganglion lies on or lateral to the lateral border of the longus colli muscle between the base of the seventh cervical transverse process and the neck of the first rib.38 Horner's syndrome results from injury to the sympathetic ganglia cephalad. to the inferior half of the stellate ganglion, or from a postganglionic injury. It does not result from a preganglionic (anterior spinal roots, white rami) sympathetic injury. Our anatomic studies have shown the dimensions of the middle cervical ganglion were 9.7±2.1 mm long and 5.2 ± 1 .3 mm wide.
Once the carotid structures are retracted anteriorly, the sympathetic trunk and middle cervical ganglion remain over the longus colli in a loose fascia. The diameter of the sympathetic trunk at the C6 level was 2.7±0.6 mm. These basic data may be helpful in identifying the middle cervical ganglion and sympathetic trunk in the lower cervical approach.
The anterolateral approach offers a direct route to the transverse foramen with less retraction of carotid sheath neurovascular bundle. However, extensive anterolateral cervical dissection or severance of the longus colli muscle to expose the transverse foramina or uncovertebral joint appears to be associated with occurrence of Homer's syndrome. Therefore, the sympathetic trunk is at greater risk using this approach.
The longus colli muscle, the longest and most medial of the prevertebral muscles, extends from the anterior tubercle of the atlas to the bodies of the third cervical to third thoracic vertebrae. It also is attached to the transverse processes of the third to sixth cervical vertebrae (Figure 3A).
The sympathetic trunk lies directly over the longus colli, below the prevertebral fascia, and therefore is at risk during stripping of the longus colli from vertebral bodies or transverse processes. Injury to the sympathetic trunk can be avoided by staying in the midline and by minimizing stripping of the longus colli.
Ebraheim et al25 demonstrated the longus colli diverged laterally from the vertebral body at the lower cervical levels, with an average angle of 12.5° ± 4.7°. Also, the distance between the medial borders of the longus colli gradually increased from upper cervical levels to lower cervical levels (Figure 3A). The distance between the medial borders of the longus colli gradually increased from 7.9±2.2 mm at C3 level to 13.8±2.2 mm at C6 level. However, the sympathetic trunk ascended laterally with an angle of 10.4° ±3.8° relative to the midline.
The upper portion of the sympathetic trunk is situated on the longus capitis (the longus capitis lies just lateral to the longus colli) (Figures 3B and 3C). Therefore, the distance from the medial border of the longus colli to the sympathetic trunk was smaller at the lower cervical levels than at the upper cervical levels. Likewise, the distance from the sympathetic trunk to midline was smaller at the lower cervical levels than at the upper cervical levels. Dissection at the lower cervical levels on the surface of the longus colli jeopardizes the sympathetic trunk; however, when properly identified, the sympathetic trunk can be protected by lateral retraction.
The spinal accessory nerve is composed of a cranial and a spinal root. It enters the neck through the jugular foramen and crosses the transverse process of the atlas and descends obliquely to reach the upper portion of the sternocleidomastoid muscle. After innervating the sternocleidomastoid muscle, the spinal accessory nerve descends obliquely and superficially across the floor of the posterior triangle of the neck to innervate the trapezius approximately 3-5 cm above the clavicle; nevertheless, this distance is variable and may range from 1.5-11 cm.39 It is intimately related to the lymph nodes and embedded in the fibroadipose tissue in the posterior triangle of the neck. The trapezius is the predominant muscle responsible for elevation and medial stabilization of the scapula.
Accidental injury to the spinal accessory nerve from surgical procedures in the neck has been recognized in the literature.20,2',4*42 Denervation of the trapezius caused by surgical dissection in the posterior triangle of the neck leads to painful loss of shoulder function, protraction, and downward or medial rotation of the scapula. 20,40,41
In the upper cervical spine, the spinal accessory nerve usually is anterior to the transverse process of the atlas (Figure 2), where it emerges from the skull base and then courses posteroinferiorly,43"44 and on the medial surface of the sternocleidomastoid muscle in the upper part of the neck.43,45,46 Great care should be taken to prevent nerve injury while executing the anterior approach to the upper cervical spine.
The approach to the lower anterior cervical spine requires dissection in the vicinity of important anatomic structures such as the recurrent laryngeal nerve, superior laryngeal nerve, sympathetic trunk, and occasionally the spinal accessory nerve. The identification and protection of these structures reduces the occurrence of complications and improves the outcome of surgery.
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