INFRAHYOID NECK: VISCERAL COMPARTMENT AND RETROPHARYNGEAL CONDITIONS PRESENTING AS A NECK MASS
- The visceral compartment and retropharyngeal space can be the source of a palpable neck mass.
- Imaging can most often provide a very likely diagnosis.
- Imaging is critical to proper medical decision making beyond the diagnosis.
- Imaging-guided biopsy may be useful in more difficult diagnostic cases.
The visceral compartment is the central compartment of orientation when dealing with neck pathology and is the logical beginning point for considering the source of a neck mass. A palpable mass arising from the visceral compartment is relatively common. However, the vast majority of palpable neck masses arise from the lateral compartment of the neck. A neck mass arising from the retropharyngeal (retrovisceral) space (RS) accounts for a very a small percentage of patients presenting with palpable neck masses; those are presented here. Otherwise, symptomatic neck masses of the RS are discussed in Chapter 152. Also, masses of the RS are very likely to be transcompartmental if they present as a neck mass. Transcompartmental involvement can occur in deep neck infections, developmental masses such as venolymphatic malformations, and plexiform neurofibromas as well as more aggressive and/or advanced malignant tumors; thus, any strict compartmental or spatial approach is always an imperfect tool from the outset. The method of first considering a space or compartment of origin is meant to serve as a starting point and basic means of organization for the differential diagnosis and more importantly for clinical problem solving and disposition when confronted with a neck mass of uncertain etiology.
Most masses arising from the visceral compartment and RS that may present as a neck mass of uncertain origin and etiology are presented in Table 150.1. The reported imaging experience with many lesions arising within or mainly involving the RS is largely anecdotal; these are discussed more generally in Chapter 152. One must be familiar with the full range of pathology possible because patients with neck masses of uncertain etiology sometimes present confusing clinical problems and/or physical findings that can be greatly simplified by good-quality, well-interpreted images. Such imaging, sometimes combined with imaging-guided biopsy, most often leaves clinicians and patients with little doubt as to the nature of the disease and the safest way to proceed to further histologic diagnosis and/or definitive treatment.
ANATOMIC AND DEVELOPMENTAL CONSIDERATIONS
Venolymphatic malformations arise in conjunction with venous and lymphatic system development. As such, they may originate from any compartment and freely spread between compartments along the pathways of vascular development. By this mechanism, these relatively common malformations are the most ubiquitous of the transcompartmental benign masses encountered in the neck.
Thyroglossal duct remnants extend from the tongue base within the visceral compartment to the thyroid isthmus region. This developmental pathway causes such remnants to present as midline and paramedian neck masses anywhere along that development tract. In the same way, thymic remnants may present in the low neck and thoracic inlet.
Rests of salivary gland tissue are deposited throughout the viscera and spaces of the head and neck during development. These may give rise to both benign and malignant salivary epithelial tumors at virtually any neck level from the visceral compartment and RS. In a similar manner, epidermoid, dermoid, and teratomatous masses may arise from the visceral compartment or RS.
Duplication cysts of the foregut and neuroenteric cysts may be a rare cause of a neck mass. More commonly, cysts and “celes” related to the pharynx such as various diverticuli and pharyngoceles will occur at sites of natural weaknesses at developmental boundaries between muscles or along penetrating neurovascular bundles. Laryngoceles occur as the result of a persistent developmental structure called the laryngeal saccule or appendix of the laryngeal ventricle. Visceral compartment–origin neck masses may be related to branchial cleft or pouch dysgenesis, with a prime example being the infected pyriform sinus tract or third branchial cleft cyst and related tracts that most frequently present as an infection around the thyroid gland on one side.
Neurologic developmental abnormalities can arise from notochord and neural crest remnants and dysraphisms producing a presenting neck mass. The dysraphic abnormalities are beyond the scope of this work. Rests of neural crest cells may be the origin of a cervical neuroblastoma or ganglioneuroma.
Applied Anatomy: Visceral Compartment of the Neck
In the upper neck, the larynx and hypopharynx are the sole occupants of the visceral compartment; this detailed anatomy is discussed in conjunction with those organs.
In the low neck, the anatomy of the visceral compartment becomes very simple, composed essentially of two adjacent tubes, the airway and the esophagus, and the partially attached thyroid and parathyroid glands. The cricoid cartilage is its most prominent landmark and is easy to recognize as it surrounds the airway posteriorly since it is the only complete ring in the airway. The cervical esophagus creates an oval structure posterior to the trachea in the midline. The more detailed anatomy of these structures is also considered in conjunction with these specific organs.
The thyroid and parathyroid glands lie mainly at the level of the cricoid and below. The upper pole of the thyroid is tucked between the infrahyoid strap muscles and margin cricoid cartilage, while the lower pole may extend as far caudally as the sixth tracheal ring. The thyroid gland isthmus may be seen connecting the two lobes anterior to the trachea at the level of its second and third rings. The more detailed anatomy of these structures is considered in conjunction with these specific organs.
The inferior thyroid arteries and veins can be seen on all of these examinations as they course through the fat pad between the posterior aspect of the thyroid and longus colli muscle and lateral to the esophagus. These tiny vessels mark the expected position of the parathyroid glands and the recurrent laryngeal nerve.
The fascia of the visceral compartment is derived from a splitting of the investing layer of deep cervical fascia interwoven with the coverings of the pharyngeal musculature. Laterally and posteriorly, it is not very resistant to the spread of pathologic processes. Numerous gaps and clefts are present in this fascial envelope, many for the passage of neurovascular bundles.
Anteriorly, the fasciae of the visceral compartment are tougher because of the pretracheal fascia (Figs. 149.1 and 149.2 and Chapter 149). The pretracheal fascia lies deep to the infrahyoid strap muscles and splits to enclose the thyroid gland, only adhering to the gland at the thyroid isthmus at about the level of the second through the fourth tracheal ring. The space formed between the pretracheal fascia and trachea allow for the sliding action necessary during swallowing and neck movement; posteriorly, the fatty RS, lying between the visceral compartment and prevertebral fascia, also allows for the movement that makes these functions possible. This makes the visceral compartment and RS together one functional unit.
Techniques and Relevant Aspects
The techniques for computed tomography (CT) studies of the infrahyoid neck for various clinical situations are presented in Appendix A. The rationale for these protocols is presented specifically in Chapter 149 and more generally in Chapter 2.
The techniques for magnetic resonance (MR) studies of the infrahyoid neck for various clinical situations are presented in Appendix B. The rationale for these protocols is presented specifically in Chapter 149 and more generally in Chapter 3.
Standard ultrasound (US) imaging and flow-related techniques are used with transducers appropriate for the depth of penetration required. This is discussed in more detail in Chapter 149 and generally in Chapter 3.
Pros and Cons
The relative value of CT, MR, and US in visceral compartment and RS pathology presenting as a neck mass can be considered in relationship to whether the pathology is likely to be of thyroid origin (Fig. 150.1A,B) and whether the brachial plexus and neural axis is of prime interest at the outset based on clinical information (Fig. 150.1C). Magnetic resonance imaging (MRI) is most appropriate for disorders of the spine and neural axis. This includes patients with brachial plexopathy. CT is the more efficient for study of neurologic deficits related to the vagus, phrenic, and recurrent laryngeal nerves or cervical sympathetic plexus because of the large area of coverage, vascular detail, and bone detail required. Certainly, MRI can be held in close reserve for these latter indications or used primarily if preferred.
This chapter considers patients who present for imaging of the neck and have a palpable and/or visible mass. Imaging studies in this clinical setting must be integrated with proper clinical triage. This produces the most efficacious use of imaging evaluation. Such triage may first include study of pathologic results from needle sampling of the mass. Imaging guidance may be sometimes necessary for such sampling, especially if the mass may be vascular or a nearby vascular structure is at risk during biopsy. If a biopsy is performed prior to imaging, the person performing the biopsy must be sure that the mass is not a vascular lesion and that the chosen method will not injure a major vessel. If a lesion is either pulsatile or potentially related to the carotid artery or jugular vein imaging, perhaps a simple localized US examination should precede biopsy; that same US examination could be used to direct such sampling (Fig. 150.1A,B). US is excellent for prebiopsy screening and biopsy guidance in the hands of an experienced operator. US is otherwise not used for definitive evaluation of visceral compartment and RS masses. Imaging-guided biopsy, if not by US, is by CT for ease, speed, and simplicity.