69 Aerodigestive Anatomy & Embryology wuth Radiologic Correlates Flashcards Preview

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Flashcards in 69 Aerodigestive Anatomy & Embryology wuth Radiologic Correlates Deck (18):
1

What are the three primary subunits of the larynx?

What are the three primary subunits of the larynx?

The larynx serves three main and incredibly important functions: airway protection, phonation, and swallowing. Given the complexity of tasks it must complete, it stands to reason that is has equally complex anatomy. It is sometimes easier to compartmentalize the larynx to help better focus discussion and study. Laryngeal cancer staging has helped do that by breaking the larynx into three subsections: the supraglottis, the glottis, and the subglottis.

The supraglottis extends from the rostral edge of the epiglottis to the middle of the laryngeal ventricle. This subsection of the larynx includes the laryngeal surface of the epiglottis, the false vocal folds (also known as the vestibular folds), the aryepiglottic folds, and the most superior portions of the arytenoid cartilages. The glottis extends superiorly from the midventricle to 1 cm below the true vocal folds. This subsection contains the true vocal folds and the majority of the arytenoid cartilages. The subglottis extends from 1 cm below the true vocal folds to the most caudal edge of the cricoid cartilage.

2

What cartilages compose the larynx?

What cartilages compose the larynx?

The larynx is composed of two unpaired cartilages and three paired cartilages (Figure 69-1). The unpaired cartilages—the thyroid and cricoid cartilages—are palpable externally and serve as excellent surgical landmarks. The paired cartilages—the arytenoid, cuneiform, and corniculate cartilages—are internal to the larynx. The cuneiform and corniculate cartilages are sometimes jointly referred to as sesamoid cartilages and serve as minor structural elements. The hyoid bone is the most superior aspect of the larynx and serves as an important insertion site of several extrinsic laryngeal muscles. These cartilages are all seen on modified barium swallow studies used to assess dysphagia (Figure 69-2).

The thyroid cartilage is formed by two ala that fuse in the midline. Each ala contains both superior and inferior cornua, the former tethering to the hyoid via the lateral thyrohyoid ligament and the latter serving as an articulating joint with cricoid cartilage. At the midline, there is a small notch that helps define the Adam’s apple; this serves as a second connection to the hyoid via the median thyrohyoid ligament.

The cricoid cartilage is the only complete cartilage ring of the trachea. It has a characteristic three-dimensional shape often referred to as a signet ring, meaning the cricoid cartilage has the shape of many common high school class rings. The ring is oriented such that the larger plate sits posteriorly, allowing a relatively narrow craniocaudal profile anteriorly, while offering additional height posteriorly upon which the arytenoid cartilages sit. The cricoid articulates with the arytenoid cartilages at ball-and-socket joints along the superior surface of the ring.

3

What muscles are found within the larynx?

What muscles are found within the larynx?

The larynx utilizes two separate groups of muscles—the extrinsic and intrinsic laryngeal muscles—to perform tasks related to phonation, airway protection, and swallowing. The extrinsic musculature connects one structural element of the larynx to another structural element outside of the larynx; conversely, the intrinsic musculature connects one structural element inside of the larynx to another.

4

Describe the extrinsic musculature.

Describe the extrinsic musculature.

The extrinsic musculature includes muscles that elevate and depress the laryngeal suprastructure within the neck, and while they have a small role in phonation they are primarily utilized in swallowing. A combination of the activation of the geniohyoid, digastric, mylohyoid, thyrohyoid or stylohyoid muscles will result in laryngeal elevation while laryngeal depression can be achieved with activation of the strap muscles (sternohyoid, sternothyroid, omohyoid). The extrinsic musculature has various innervations including cervical rootlets and cranial nerves V and VII. The final extrinsic muscle group to discuss is the pharyngeal constrictors. The constrictors, all innervated by the pharyngeal plexus, help advance food bolus distally. The superior constrictor does not insert on the larynx but the middle and inferior constrictors do, resulting in elevation and posterior translation with swallows.

5

Describe the intrinsic musculature.

Describe the intrinsic musculature.

The intrinsic musculature is primarily associated with airway protection and phonation. They are all paired muscles with the exception of the interarytenoid muscle (see Figure 69-1). These muscles can be classified as abductors or adductors based on how they move the vocal folds with activation. The adductors include the thyroarytenoid, the lateral cricoarytenoid, and the interarytenoid muscle. The lone abductor is the posterior cricoarytenoid muscle. The cricothyroid muscle is considered an intrinsic laryngeal muscle, but it does not directly cause adduction or abduction of the vocal folds. The cricothyroid muscle works to pivot the thyroid cartilage anteriorly along the axis created by the cricothyroid joint, lengthening and tensing the vocal fold to allow a higher register of phonation.

6

What type of mucosa is found in the larynx?

What type of mucosa is found in the larynx?

The majority of the mucosa is columnar respiratory epithelium. However, stratified squamous epithelium overlies the vibratory portions of the true vocal fold. The transition points from respiratory epithelium to squamous epithelium occur at the superior and inferior arcuate lines; the former is within the laryngeal ventricle while the latter rests just below the true vocal fold.

This stratified squamous epithelium is thought to be protective against trauma caused by high-frequency collisions of the vocal folds against each other during phonation. Despite this protective quality, this existence of different epithelia and transitions between epithelial types is important in understanding both malignant and benign pathologies. Understanding epithelial types in the larynx can help understand likely locations for malignancies (squamous cell carcinoma of the glottis) and benign pathologies (recurrent respiratory papillomatosis usually only occurs at the transition points between respiratory and squamous epithelia).

7

What is the laryngeal ventricle?

What is the laryngeal ventricle?

The ventricle is a unique anatomic outpouching split between the supraglottic and glottic larynx. The inferior extent is the true vocal folds. The superior limit is not straightforward given the three-dimensional nature of the ventricle. When looking down the airway from above, the superior extent appears to be the false vocal fold. However, on anatomic dissection, one finds that the ventricle extends laterally and then superiorly beyond the false fold, allowing a potential space to form that is not easily examined on nasolaryngeal endoscopy. This potential space can collapse on itself and is known as the laryngeal saccule. The saccule can become important in discussions of saccular cysts and must be examined thoroughly when performing direct laryngoscopy for cancer diagnosis/surveillance (Figure Endoscopic view of anterior commissure with 70-degree rigid endoscope showing saccular cyst.).

8

What are the quadrangular membrane and the conus elasticus?

What are the quadrangular membrane and the conus elasticus?

The quadrangular membrane is a fibroelastic membrane extending from the epiglottis to the false vocal fold, terminating in the ventricular ligament. It roughly comprises the aryepiglottic folds but has no definitive structural integrity. The conus elasticus is also a fibroelastic membrane but it extends from the superior aspect of the cricoid cartilage and spans superiorly to interdigitate with the vocal ligament within the true vocal folds.

Both the quadrangular membrane and the conus elasticus serve as potential barriers for the spread of malignancy.

9

What is the significance of the cricoarytenoid joint? How does the arytenoid cartilage’s shape help specialize the joint?

What is the significance of the cricoarytenoid joint? How does the arytenoid cartilage’s shape help specialize the joint?

The cricoarytenoid joint is a specialized ball-and-socket joint that is integral in allowing the larynx to perform its vital functions. The joint allows the arytenoids to both rotate on a vertical axis while also allowing them the freedom to glide and tip anteromedially.

The arytenoid has a pyramidal shape with the base serving as the socket for the cricoarytenoid joint. The arytenoid, however, has two important appendages known as the vocal process and the muscular process. The vocal process serves as the insertion point of the vocal ligament while the muscular process, as the name implies, serves as the insertion point for several intrinsic laryngeal muscles. The muscles insert in several vectors, allowing the arytenoid to rotate on the cricoarytenoid joint facet, in turn abducting or adducting the vocal ligament and vocal folds. This coordinated, highly specialized activation of laryngeal musculature allows for airway protection and phonation.

10

What laryngeal muscles are used for respiration?

What laryngeal muscles are used for respiration?

Any muscle that helps to adduct or abduct the vocal folds is technically utilized during respiration. For optimal airflow, the vocal folds should be in an abducted position for respiration. Therefore the most important muscle is the posterior cricoarytenoid muscle, the lone abductor of the vocal folds. It is innervated by the recurrent laryngeal nerve (RLN).

11

What muscles are used for phonation?

What muscles are used for phonation?

As with respiration, any muscle that helps to adduct or abduct the vocal folds is technically utilized during phonation. Converse to respiration, optimal phonation occurs when the vocal folds are adducted to the midline. The muscles responsible for this motion are the thyroarytenoid, the interarytenoid, and the lateral cricoarytenoid muscles. These muscles are primarily innervated by the RLN.

While optimal phonation occurs with adducted vocal folds, as stated above, the laryngeal framework can pivot along the cricothyroid joint. This movement allows for tensing of the vocal ligament as it is stretched between the vocal process of the arytenoid cartilage and the insertion of the ligament on the thyroid cartilage. The cricothyroid muscle is responsible for this anterior pivoting and its activation will lead to higher pitched phonation. This muscle is the lone intrinsic muscle innervated by the superior laryngeal nerve (SLN).

12

Can phonation occur through alternative neurolaryngeal pathways?

Can phonation occur through alternative neurolaryngeal pathways?

The innervation detailed in the previous question is the dominant pathway; however, there are other variations. While most of the intrinsic musculature responsible for adduction of the vocal folds (a key element necessary for phonation) is innervated by the RLN, there is a named anastamosis between the RLN and SLN. This anastamosis is the nerve of Galen and can lead to some phonatory activation from the SLN.

13

What muscles are used for swallowing?

What muscles are used for swallowing?

Swallowing is an incredibly complex task that involves multiple muscle groups inside and outside of the larynx. From a laryngeal standpoint, airway protection must occur to minimize risk of aspiration. This involves two separate tasks: first, the glottis must be closed to protect from frank aspiration so the vocal folds must be adducted to the midline (see above). Simultaneously, the extrinsic musculature is activated. This results in laryngeal elevation, inversion of the epiglottis, and proximal pharyngeal constriction to help direct the food bolus towards the esophagus.

14

Do any of the intrinsic laryngeal muscles have bilateral innervation?

Do any of the intrinsic laryngeal muscles have bilateral innervation?

Only one—the interarytenoid muscle receives bilateral innervation. This can result in an interesting clinical finding when there is known denervation of one side of the larynx (from a RLN injury during thyroidectomy, for example). With such an injury, one would expect immobility of the ipsilateral hemilarynx; however, one may see a slight adducting motion of the ipsilateral arytenoid cartilage. This can sometimes be confused for residual ipsilateral innervation but is more likely innervation of the interarytenoid muscle from the contralateral RLN.

15

What are the layers of the vibratory vocal fold?

What are the layers of the vibratory vocal fold?

From histologic studies, we now have a great understanding of vocal composition. While once thought to be a solid layer of muscle with a thin overlying epithelial layer, we now understand there are several integral layers all of which contribute to the unique vibratory function of the vocal folds (Figure 69-4).

The deepest portion of the vocal fold is the thyroarytenoid muscle and the most superficial is the squamous epithelium. Between them is a tri-layered level known as the lamina propria. The intermediate and deep layers fuse together and integrate to form the vocal ligament. The superficial layer of the lamina propria is a gelatinous matrix that is incredibly important for the unique vibratory qualities necessary for phonation.

16

How does the larynx form in utero?

How does the larynx form in utero?

The larynx begins to form during the fourth week of gestation with the emergence of the laryngotracheal groove. This groove begins just caudal to the fourth branchial arch and forms the beginning of the unified upper aerodigestive tract. As the groove deepens, it begins to separate from the primitive esophagus in a coronal plane with the formation of the esophagotracheal septum. Lung buds will eventually descend from the resultant laryngotracheal diverticulum while the most cranial aspect is destined to form the larynx.

The larynx forms with contributions from the third, fourth, and sixth branchial arches as detailed in Table 69-1 with a few exceptions. The hyoid bone forms with contributions from both the second and third branchial arches. The epiglottis originates from the hypobranchial eminence with contributions from both the third and fourth branchial arches.

Understanding the branchial origins of these structures also helps give insight to laryngeal sensory innervation. As the SLN derives from the same branchial arch as the epiglottis and thyroid cartilages, it follows that the supraglottis and superior aspect of the glottis contain afferent SLN innervation. A similar relationship is found with the RLN carrying sensory innervation from the inferior glottis and subglottis.

The laryngeal framework begins to form around week five of gestation and continues through week nine. Early within that timeframe the epithelium of the larynx fuses and obliterates the previously present lumen. This lumen must reform to allow for proper tracheal and lung development; it does so between weeks seven and ten of gestation. The laryngeal ventricles form as the lumen recanalizes but manages to leave the tissue that ultimately becomes the false and true vocal folds.

17

What in vivo pathologies exist because of derangements of laryngeal embryologic formations?

What in vivo pathologies exist because of derangements of laryngeal embryologic formations?

Two relatively common pathologies can occur secondary to abnormal embryologic development. Laryngeal webbing occurs with failure to recanalize the lumen of the airway. Laryngeal clefts form because of incomplete cleavage of the laryngotracheal diverticulum from the primitive esophagus. As the esophagotracheal septum forms in a cranial direction, the progression may stall. The earlier the stall occurs during embryologic development, the more severe the laryngeal cleft.

While they are explained in depth elsewhere, branchial cleft abnormalities can also involve the hypopharynx. Both third and fourth branchial cleft cysts can originate in the piriform sinus and, in select cases, cautery of the orifice within the sinus or excision can be curative.

18

Are any portions of the laryngeal framework ossified at birth?

Are any portions of the laryngeal framework ossified at birth?

No. The hyoid is the earliest structural aspect of the larynx to ossify, occurring around the child’s second birthday. The thyroid cartilage begins to ossify in the teenage years and the cricoid cartilage starts to ossify in the third or fourth decade of life. Complete ossification may not occur in all portions of the laryngeal framework but almost always occurs in the hyoid. This lack of ossification may serve as a protective mechanism against laryngeal fracture during childhood.

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