Introduction

Radical neck dissection, first described by Crile in 1906 [1], is the standard against which all surgical approaches to neck metastases is compared. Consideration was subsequently given to more conservative surgical techniques, but they did not gain widespread acceptance and were condemned by Martin et al in the 1950s [2].

Surgical management of the neck has continued to evolve toward more conservative approaches, however. Even selective neck dissection techniques are now widely accepted. Current controversies center around the appropriate use of modified and selective neck dissections, indications for elective neck treatment, and selection of the best therapeutic modality for elective neck treatment. This article will address these as well as other surgical considerations, and will review the surgical techniques currently available for neck treatment.

Anatomic Considerations

A neck dissection is, in fact, an anatomic dissection of the neck. Therefore, a clear grasp of the anatomy of the cervical lymph node groups and cervical aponeurotic system is essential for proper understanding of neck dissection.

Cervical Lymph Node Groups

There are approximately 150 to 350 lymph nodes above the clavicles. Since the lymphatic system is embryologically related to the venous system, the lymph nodes are closely related to veins. The cervical lymph nodes have commonly been divided into surgical levels to facilitate discussions of patterns of metastases and types of neck dissection (Figures 1 and 2).

The submandibular and submental nodes constitute the level I group. The submental lymph nodes are located in the midline within the triangle defined by the anterior bellies of the digastric muscles and the hyoid bone. The submandibular nodes are situated within a triangle bounded by the anterior and posterior bellies of the digastric muscle and the mandibular body. These nodes are in close relationship to the submandibular gland and the posterior facial vein. Notably, the gland itself does not include any lymph nodes.

The internal jugular vein lymph nodes are limited anteriorly by the lateral margin of the sternohyoid muscle and posteriorly by the posterior border of the sternocleidomastoid muscle, and are subdivided into three groups. The level II group, or upper internal jugular nodes, extend from the skull base superiorly to the hyoid bone or carotid bifurcation inferiorly. These nodes , also commonly referred to as jugulodigastric nodes, are closely related to the superior portion of the spinal accessory nerve as well. The middle jugular group (level III group) continues inferiorly to the omohyoid muscle. The lower jugular group (level IV) nodes extend from the omohyoid muscle to the level of the clavicle.

The level V group, or posterior triangle lymph nodes, are bounded by the posterior edge of the sternocleidomastoid muscle, anterior border of the trapezius muscle, and clavicle. Superiorly, these nodes closely approximate the spinal accessory nerve, and are often referred to as the spinal accessory chain. The nodes that are located near the confluence of the posterior triangle and upper jugular groups are sometimes termed junctional nodes. The supraclavicular nodes are part of the posterior triangle group.

The anterior compartment group (level VI) is bounded superiorly by the hyoid bone, inferiorly by the sternum, and laterally by the common carotid arteries. Included in this group are the paratracheal, pretracheal, precricoid (Delphian), and tracheoesophageal groove nodes.

Additional groups that are sometimes dissected outside the normally described boundaries of the neck include the facial, parotid, preauricular, retroauricular, suboccipital, and retropharyngeal nodes.

Cervical Aponeurotic System

The cervical aponeurotic system divides the neck into compartments, including the lateral and paravisceral spaces, that are removed with a neck dissection (Figure 3). The superficial cervical fascia is a thin layer that invests the platysma muscle. The deep cervical fascia is subdivided into three layers: investing (superficial), pretracheal (middle), and prevertebral (deep).

The investing layer completely encircles the neck, splitting to enclose the sternocleidomastoid, omohyoid, and trapezius muscles. Superiorly, the investing fascia splits to enclose the subman- dibular and parotid glands, and attaches to the lower border of the mandible. The pretracheal layer encompasses the trachea, thyroid gland, and esophagus.

The prevertebral layer covers the prevertebral, scalene, levator scapulae, splenius, and semispinalis capitis muscles, and constitutes the floor of the lateral neck. The phrenic nerve, brachial plexus, sympathetic trunk, and dorsal scapular nerve all lie deep to the prevertebral fascia.

The carotid sheath consists of all three layers of deep cervical fascia and contains the carotid artery and vagus nerve. The jugular vein is located within an extension of the carotid sheath as well.

Critical to an understanding of modified and selective neck dissections is the concept that the lymph nodes lie within the lateral and paravisceral spaces, not within the fascial wrappings of the muscles or the carotid artery. The lymph nodes may, however, be in very close association with these fascial layers. The spinal accessory nerve must be uniquely considered in neck dissection, as it actually travels through the lateral space of the neck just deep to the investing fascia and superficial to the prevertebral fascia.

Tumor Biology

Squamous cell carcinoma of the upper aerodigestive tract is the most common tumor to metastasize to the cervical nodes. Less commonly, neck dissection is performed for malignant neoplasms of the thyroid gland, parotid gland, or skin. Distant metastases from below the clavicle occur, but neck dissection is generally of no benefit in the management of these neoplasms.

Once a metastasis reaches a lymph node, tumor growth results in an indurated and more rounded node. Increasing size of the lymph node is directly associated with increased risk of tumor invasion through the capsule, and eventually, fixation to adjacent structures.

The risk of cervical metastases from different primary sites and stages has been estimated by examining the percentage of patients with similar stage tumors who present with clinically positive node [3], the distribution of pathologically positive nodes in an electively dissected, clinically negative neck, and failure rates in untreated necks. In this fashion, the relative risk of cervical metastases may be approximated for specific sites. For example, the glottis has a low risk of metastasizing to the neck(< 20%), whereas the nasopharynx has a high risk (> 30%). Finally, lesions such as early supraglottic laryngeal tumors are considered to be of intermediate risk (20% to 30%) [4].

In many cases, these fairly consistent patterns of metastases allow for the application of selective removal of nodal groups for specified lesions. There are, however, situations in which the normal pattern of nodal metastases do not hold. For example, the right and left sides of the cervical lymphatic system normally do not shunt. However, previous neck surgery, prior irradiation, or multiple positive nodes obstructing the lymphatic system may alter the patterns of cervical metastases such that shunting across the submental region occurs, or unusual nodal groups are involved for a given primary. Furthermore, recurrent lesions are associated with a higher risk of cervical metastasis.

Diagnosis

Palpation of the neck for metastases, by itself, may be associated with significant error rates. Therefore, CT is used extensively to image both the primary site and neck, and may increase the accuracy of neck staging from 70% to 93% [5]. Computed tomography is especially useful in detecting occult nodal disease, determining the presence or absence of extranodal disease, demonstrating the resectability of extensive nodal disease, and evaluating inaccessible areas, such as the parapharyngeal space, retropharyngeal space, and tracheoesophageal groove (Figure 4).

Although MRI can detect nodal disease as well [6], we currently limit its use to selected primary sites, such as the nasopharynx, or to those cases in which a specific issue, such as perineural infiltration, needs to be addressed. Ultrasound is used more extensively in Europe than in the United States to evaluate nodal disease. Magnetic resonance imaging and ultrasound may be helpful in determining nodal fixation to the carotid artery, but no imaging method is definitive in this regard.