The main components of the infrahyoid neck include the hypopharynx, larynx, spaces that continue from the suprahyoid neck (carotid, retropharyngeal/danger, prevertebral spaces), brachial plexus, thyroid gland, and the parathyroid glands.

The hypopharynx extends from the hyoid bone and vallecula to the inferior cricoid margin/cricopharyngeus muscle. There are three subsites: the pyriform sinus, postcricoid region (posterior to the arytenoid and cricoid cartilage), and posterior pharyngeal wall. The medial wall of the pyriform sinus extends to become the aryepiglottic fold. The main pathology encountered in radiology includes squamous cell carcinoma and Zenker's diverticulum.

The larynx consists of supraglottic, glottic, and subglottic regions built around a cartilaginous frame (cricoid, arytenoid, epiglottic/aryepiglottic, and thyroid cartilage). Thyroid and cricoid cartilage invasion is important to assess for cancer staging but assessment can be challenging due to a mixture of ossified and chondrified regions. Intrinsic cartilage also accounts for the occasional development of chondrosarcoma in this region.

The main components of the supraglottic region are the epiglottis, aryepiglottic folds (laryngeal side as the folds also form the medial wall of the pyriform sinus), false vocal folds (inferior extent of the aryepiglottic folds), and laryngeal ventricles. The pre-epiglottic space anterior to the epiglottis is predominantly fat-filled. The false vocal folds are superior to the true vocal cords, have a strip of paraglottic fat in the paraglottic space laterally, do not arise from the cricoarytenoid joint, and do not come to a sharp anterior point. An important concept for cancer assessment and spread is that the paraglottic and pre-epiglottic spaces contain lymphatic and blood vessels (and upstages tumors to at least pT4a (moderately advanced local disease). A laryngocele arises from the ventricle and may be due to chronic pressurization or obstructive tumor

The glottis is comprised of the true vocal cords and the related anterior and posterior commissures (and area 0.5 cm-1.0cm inferior to the undersurface); all 4 of these are important to assess for cancer staging. The true vocal folds can be identified on axial images as those arising posteriorly from the cricoarytenoid joint region and converging to a sharp point at the anterior commissure. There is no paraglottic fat laterally at this level. The true vocal cords lack intrinsic lymphatics.

Beyond squamous cell carcinoma, key pathology includes vocal cord paralysis. All intrinsic laryngeal muscles are innervated by the recurrent laryngeal nerve (see section below) with the exception of the cricothyroid muscle. Imaging evidence of vocal cord paralysis reflects the anatomy, function, and innervation. Paralysis of the posterior cricoarytenoid muscle (the vocal cord abductor) leads to medialization of the arytenoid cartilage and posterior vocal cord. Posterior cricoarytenoid atrophy may also be present. Other findings include enlargement of the ipsilateral ventricle (sail sign), medialization of the aryepiglottic fold, and enlargement of the pyriform sinus.

The subglottic region is identified by the rounded configuration and extends from 1 cm inferior to the apex of the ventricle inferiorly to the inferior border of the cricoid.

epiglottis, SCC, paralysis, larnyngocele (internal, external, mixed, thyrohyoid membrane)..

As mentioned previously, the thyroid cartilage is actually related to the larynx is the thyroid gland is situated inferior to it. The thyroid gland proper consists of two lateral lobes connected by a midline isthmus. The thyroid gland wraps around the cricoid cartilage and upper tracheal. The configuration of the thyroid gland is variable, may be asymmetric right to left and a posterior extension called the tubercle of Zuckerkandl may be present. It is important to recognize this tubercle so it is not confused with pathology such as a parathyroid adenoma or pathologic lymph node.

The embryologic origin at the foramen cecum at the posterior tongue is important to know since ectopic thyroid tissue may be found at that location (lingual thyroid) or along a midline migratory tract to the thyroid gland. Ectopic thyroid tissue is often incidental but can present clinically, for example as a midline mass or dysphagia.

Arterial supply comes from the superior thyroidal artery (via the ECA) and the inferior thyroidal artery (via the thyrocervical trunk). The bilateral recurrent laryngeal nerves often pass along the lateral aspect of the gland and tubercles of Zuckerkandl if present.

The parathyroid glands are vascular structures typically located posterior to the thyroid gland. Usually, these are not readily identifiable on CT or MRI. A parathyroid adenoma can be seen and differentiated by kinetics of contrast enhancement with 4D CT (enhances and washes out to a greater degree than lymph nodes or normal thyroid tissue).

It is important to be able to trace the expected course of the vagus and recurrent laryngeal nerves as pathology along these tracts can lead to vocal cord paralysis. These span both the suprahyoid and infrahyoid neck, but are included in this section as this is where much of the action often is. The vagus nerve arises from the nucleus ambiguus in the medulla (butterfly level), exits the pars vascularis of the jugular foramen, to descend within the posterior carotid sheath. The left recurrent laryngeal nerve wraps around the aortic arch while the right courses posterior to the brachicephalic artery. Both recurrent laryngeal nerves then ascend in the tracheoesophageal groove to the level of the cricoarytenoid joint. The courses vary in the setting of aortic arch variant anatomy however (e.g. the right recurrent laryngeal nerve arises near the level of the cricoid with an aberrant right subclavian artery).

Reference

Paquette CM et al. Unilateral vocal cord paralysis: a review of CT findings, mediastinal findings, and the course of the recurrent laryngeal nerves. Radiographics 2012;32(3)

Trunks, Division, Cords. Between anterior and middle scalene muscles. C5-T1..Can see on CT or MRI, best evaluated with dedicated brachial plexus protocols.