HYPOPHARYNX: INTRODUCTION, NORMAL ANATOMY, AND FUNCTION
Techniques and Relevant Aspects
The hypopharynx is studied in essentially the same manner as the larynx; therefore, the principles related to imaging of the hypopharynx with computed tomography (CT) and magnetic resonance (MR) should be reviewed in Chapter 201. Specific problem-driven protocols for CT and MR are presented in Appendixes A and B.
There is little or no use for ultrasound in studying the hypopharynx except in practices that use this imaging to assess lymph node disease in patients with hypopharyngeal cancer.
The approach with radionuclide studies depends on the aim of the examination. Most of the current usage is limited to cancer evaluation with fluorine-18 2-fluoro-2-deoxy-D-glucose positron emission tomography (FDG-PET). This is discussed in more detail in conjunction with the larynx in Chapters 201 and 206.
Pros and Cons
Indications for Study
Evaluating the Extent of Known Hypopharyngeal Cancer and Related Regional Lymph Node Metastases and Posttreatment Surveillance
This is discussed more fully in conjunction with laryngeal cancer in Chapter 206.
Evaluation of Submucosal or Peripharyngeal Masses of Uncertain Etiology
Both CT and MR can be used to demonstrate the presence, nature, and extent of a submucosal mass lesion. In most cases, a CT study is adequate for all of these tasks. A variety of sometimes unusual lesions, either benign or malignant, can be encountered in this clinical context. As the initial biopsy results may be inconclusive or negative, MR may be used in an attempt to further characterize the lesion. In young patients, MR might be preferred because of the absence of ionizing radiation.
Throat Pain, Referred Otalgia, Dysphonia, and Vocal Cord Dysfunction
CT is the preferred modality for these studies since it not only provides a good evaluation of the hypopharynx and larynx but also can easily be extended to include the upper thoracic cavity or intracranial structures in cases of potential vagus nerve dysfunction. Adjunctive magnetic resonance imaging (MRI) may be used selectively to evaluate the cisternal course of the vagus nerve and its brain stem nuclei.
CT is the preferred modality for the study of pharyngeal trauma, as it can be performed more simply than MRI in the acute or immediate subacute clinical setting and provides superior cartilage detail and multiplanar images. It simultaneously allows a thorough evaluation of the other neck structures, including soft tissue injury that might be associated with pharyngeal tears, vascular injury, and cervical spine injuries, among others. It is also better for identifying gas and retained foreign bodies; this is especially useful in the evaluation of a potentially swallowed foreign body.
Water-soluble contrast and barium studies remain the preferred studies for gross screening for perforations or false passages, functional disorders, and mucosal lesions.
There is some debate with regard to whether CT or MR should be the primary imaging tool for evaluating patients with hypopharyngeal cancer. This discussion often emphasizes which study is best at showing cartilage involvement and is discussed in detail in conjunction with laryngeal cancer evaluation in Chapter 206.
The hypopharynx is an inferior continuation of the pharynx. The pharynx is essentially a muscular tube suspended from the skull base, and it is useful to recall the general anatomy of the upper pharynx when thinking about its lowermost segment, the hypopharynx. The three components of the hypopharynx are the posterior pharyngeal wall, the pyriform sinuses, and the postcricoid region (Fig. 215.1).
The epithelium of the oropharyngeal and hypopharyngeal mucosa is squamous. It is continuous above with the mucosa of the nasopharynx with no visible line of transition (Fig. 215.1). There are thin constrictor muscles beneath the mucosa of the posterior and lateral pharyngeal walls. The retropharyngeal space lies between the constrictor muscles and the prevertebral fascia that covers the prevertebral muscles. That space contains a thin layer of loose fibrofatty tissue so that the entire pharynx can distend and move freely during swallowing. The constrictor muscles are thin. Beyond that inherent weakness as a barrier, there is an even more potential weak spot in the lateral pharyngeal wall just below the hyoid bone level where the middle and inferior constrictor muscles do not overlap. The lateral wall in this area is composed of the thin, fibrous thyrohyoid membrane that is penetrated by the vessels, nerves, and lymphatics of the larynx and hypopharynx; this is discussed in more detail in Chapter 201 (Fig. 215.2A,H).
The hypopharyngeal walls are continuous with the cervical esophagus below (Figs. 215.1 and 215.2). The transition to the cervical esophagus is below the arytenoids and about 3 to 4 cm in length (Figs. 215.2A,E–F; 215.3; and 215.4). This is the postcricoid portion of the hypopharynx. The lower 1 to 2 cm of this is often referred to as the cricopharyngeus muscle (Figs. 215.1, 215.2A,E–F; 215.3; and 215.4). Like the Passavant ridge, this is really only a specialized sphincterlike area of the inferior as opposed to the superior constrictor, and the “cricopharyngeus muscle” is not really a separate muscle. It is a functional zone of the inferior constrictor.