Saturday, March 13, 2021

Recurrent Laryngeal Nerve Injury Case File

Posted By: Medical Group - 3/13/2021 Post Author : Medical Group Post Date : Saturday, March 13, 2021 Post Time : 3/13/2021
Recurrent Laryngeal Nerve Injury Case File
Eugene C. Toy, MD, Lawrence M. Ross, MD, PhD, Han Zhang, MD, Cristo Papasakelariou, MD, FACOG

CASE 36
A 35-year-old woman complains of a 2-month history of hoarseness and some choking while drinking liquids. She denies viral illnesses. She underwent surgery for a cold nodule of the thyroid gland 9 weeks ago. Her only medication is acetaminophen with codeine.

 What is the most likely diagnosis?
 What is the anatomical explanation for her symptoms?


ANSWER TO CASE 36:

Recurrent Laryngeal Nerve Injury
Summary: A 35-year-old woman has a 2-month history of voice hoarseness and choking after undergoing surgery for a cold (nonfunctioning) thyroid nodule

• Most likely diagnosis: Injury to the recurrent laryngeal nerve

• Anatomical explanation for her symptoms: Vocal cord paralysis


CLINICAL CORRELATION
This woman underwent surgery for a thyroid nodule. A cold nodule is defined as a mass that does not take up radioactive (i.e., “hot”) iodine isotope. Surgery of the thyroid gland can sometimes injure the recurrent laryngeal nerve, which runs through the posterior superior suspensory ligament of the thyroid gland. The recurrent laryngeal nerve provides motor innervation to the larynx and sensory innervation to the laryngeal mucosa. A traction injury or inadvertent severing of the nerve leads to vocal cord paralysis. With injury to just one nerve, the vocal cord on the same side bows into a paramedian position instead of closing straight to the midline, leading to hoarseness. When drinking, the patient may choke if the liquid is aspirated into the trachea. When vocal cord function does not return after 6 months to 1 year, then injection of the affected vocal cord with Teflon can be helpful.

There are four small parathyroid glands within the thyroid tissue, usually two in the left lobe and two in the right lobe of the thyroid gland. These tiny parathyroid glands secrete parathyroid hormone to maintain calcium balance. Inadvertent injury due to excision of the parathyroid glands can lead to hypocalcemia, manifested by fatigue, dyspnea (shortness of breath), brittle skin and nails, tetanic muscle contractions, seizures, or difficulty swallowing.


APPROACH TO:
The Neck: Thyroid Gland

OBJECTIVES
1. Be able to identify the parts of the thyroid gland
2. Be able to draw branches of the arteries and veins that supply the thyroid gland
3. Be able to identify the main features of the larynx and list features that assist in respiration (phonation) or protect the laryngeal inlet during swallowing
4. Be able to identify the course of the different branches of the vagus nerve cranial nerve (CN) X that innervate the larynx
5. Be able to describe the consequences of injury to the recurrent laryngeal nerve and contrast with the consequences of injury to the superior laryngeal nerve


DEFINITIONS
COLD NODULE: A region of the thyroid gland that does not take up hot iodine radioisotope (as visualized with thyroid scintigraphy) because the tissue does not contain follicular thyroid cells

ECTOPIC: Tissue that resides in an unexpected or abnormal location

ASPIRATE: To suck food or liquid into the bronchial tree of the lungs, possibly resulting in inflammation or pneumonia


DISCUSSION
The thyroid gland is located at the base of the neck. It consists of left and right lobes connected by a narrow isthmus. During development, the gland forms at the base of the tongue at the foramen cecum and descends into the neck along the thyroglossal duct, reaching its final position inferior to the cricoid cartilage (vertebral levels C5 through T1). Occasionally, ectopic thyroid tissue will deposit along the duct. This sometimes manifests as a pyramidal lobe arising from the midline along the remnants of the duct.

As an endocrine gland, the thyroid receives a rich vascular supply. The superior thyroid artery is the first anterior branch of the external carotid artery. It descends laterally to the hyoid bone, giving off the superior laryngeal artery, which pierces the thyrohyoid membrane. The superior thyroid artery continues toward the gland lateral to the thyroid and cricoid cartilages. It crosses along the superior border of the thyroid and usually anastomoses with the contralateral superior thyroid artery. The inferior thyroid artery is a branch of the thyrocervical trunk, which arises from the first part of the subclavian artery. The artery ascends, giving off an ascending cervical artery, and then curves inferiorly to enter the thyroid gland from the posterior surface. There are many anastomoses between branches of the superior and inferior thyroid arteries. Rarely, an artery arising directly from the brachiocephalic trunk or the aortic arch, called the thyroidea ima artery, will ascend to supply the thyroid. The gland is drained by three pairs of veins. The superior and middle thyroid veins drain to the internal jugulars, and the inferior thyroid veins drain to the brachiocephalics.

The thyroid lies anterior to the trachea (Figure 36-1), a hollow tube that conducts air to the lungs. It forms from the inferior pharynx, anterior to the esophagus. The wall of the trachea is supported by a series of cartilaginous rings. All of the unnamed rings have a C shape, leaving the posterior wall flexible to accommodate expansion of the esophagus during swallowing. Superior to the thyroid gland are the cricoid and thyroid cartilages. These are specialized structures that protect the underlying structures of the larynx.

The structures of the larynx serve two functions: to modulate expelled air to make sounds used in the production of speech and to protect the airway from food and drink passing to the esophagus. The larynx consists of three single cartilages (epiglottis, thyroid, and cricoid), and three paired cartilages (arytenoid, corniculate, and cuneiform), for a total of nine. The thyroid cartilage resembles a partially open book, with the union of its two plates forming a laryngeal prominence (Adam’s apple) anterosuperiorly. Inferior to the thyroid cartilage is the cricoid cartilage

thyroid gland anatomy

Figure 36-1. Nerves related to the thyroid gland. (Reproduced, with permission, from Lindner HH. Clinical Anatomy. East Norwalk, CT: Appleton & Lange, 1989:138.)

which resembles a signet ring. It’s large lamina is posterior. Posterior to the thyroid cartilage is the epiglottis, a cartilaginous structure that is bound to the thyroid cartilage by the thyroepiglottic ligament. The arytenoid cartilages rest on the superior margin of the cricoid cartilage and are held in place by capsules that surround the cricoarytenoid joint. The epiglottis attaches to the arytenoid cartilages through the quadrangular membrane. The free superior border forms the aryepiglottic fold, and the free inferior border forms the vestibular ligament (false vocal fold). The other major structure of the larynx is the conus elasticus, another broad ligament inferior to the quadrangular membrane. This ligament is the fusion of the lateral and median cricothyroid ligaments. The free superior border also attaches to the arytenoid cartilage and forms the vocal ligament (true vocal fold). The space between the two vocal folds is the rima glottidis. When the rima glottidis is wide (i.e., the folds are abducted), maximal air flow is permitted through the trachea. When the rima glottidis is closed (i.e., the folds are adducted), no air flows. When the rima glottidis is narrow, the expelled air will vibrate the vocal folds and produce a sound.

The intrinsic musculature of the larynx is devoted mostly to fine-motor control of the vocal folds to modulate pitch and intonation during speech. Perhaps the most important muscles are the posterior cricoarytenoids, which are the only muscles used to abduct the vocal folds and are necessary to widen the rima glottidis for breathing. All of the other muscles function to adduct the rima glottidis or modulate the tension of the vocal chords. The lateral cricoarytenoids adduct the vocal folds. The transverse and oblique arytenoid muscles bring the two arytenoid cartilages together, which indirectly act to close the posterior portion of the rima glottidis. The cricothyroids lengthen and tighten the vocal fold, whereas the thyroarytenoid relaxes it. The vocalis muscle runs under the vocal fold and produces local modulations in tightness (e.g., relaxing posteriorly while tightening anteriorly;
Figure 36-2).

Larynx anatomy

Figure 36-2. Larynx showing vocal cords.

Several structures protect the trachea from food or liquid traveling to the esophagus. The first of these is the epiglottis, which deflects food laterally around the quadrangular membrane to the piriform recess and into the esophagus. The epiglottis itself does not apply sufficient force to completely close off the laryngeal inlet. During swallowing, the suprahyoid muscles contract and, through the thyrohyoid membrane, lift the larynx up against the epiglottis. The infrahyoid muscles attached to the external face of the thyroid cartilage help to return the larynx to its resting position.

Most of these intrinsic laryngeal muscles are innervated by the recurrent laryngeal nerve, a branch of the vagus nerve (CN X). The only exception is the cricothyroid, which is innervated by the external branch of the superior laryngeal nerve, also a branch of the vagus. Thus, damage to the superior laryngeal nerve will affect voice quality, particularly the ability to reach high tones. More significantly, damage to the recurrent laryngeal nerve will impair the ability to abduct the vocal folds, possibly leading to respiratory distress if the injury is bilateral. Unilateral damage to the recurrent laryngeal nerve will result in inability to tightly adduct the two vocal folds, resulting in hoarseness. In addition, the protective function of the rima glottidis may be lost, and food or liquid that does not go down the esophagus may flow into the trachea and cause a choking response. In extreme cases, aspiration pneumonia may result.

Sensory innervation of the larynx is also mediated by the vagus nerve. In the supraglottic region (above the vocal fold), the mucosa is innervated by the internal branch of the superior laryngeal nerve. In the infraglottic region (below the fold), the mucosa is innervated by the recurrent laryngeal nerve. Thus, damage to the superior and recurrent laryngeal nerves may also have deficits in reflex behaviors that depend on sensory input from the larynx.


COMPREHENSION QUESTIONS

36.1 Which of the following muscles is most important to allow air movement through the larynx?
    A. Posterior cricoarytenoids
    B. Lateral cricoarytenoids
    C. Cricothyroid muscle
    D. Infrahyoid muscles

36.2 A 33-year-old woman underwent partial thyroidectomy for hyperthyroidism in which the thyroid failed to take up radioactive iodine. She is noted to have some hoarseness of voice 1 month later. Which of the following is the most likely explanation?
    A. Endotracheal tube trauma to the vocal cords
    B. Injury to the cricoid cartilage
    C. Injury to the thyroid cartilage
    D. Injury to the recurrent laryngeal nerve

36.3 A 15-year-old boy is eating a fish dinner and inadvertently has a bone “caught in his throat.” He complains of significant pain above the vocal cords. Which of the following nerves is responsible for carrying the sensation for this pain?
    A. Superior laryngeal nerve
    B. Recurrent laryngeal nerve
    C. Spinal accessory nerve
    D. Hypoglossal nerve

36.4 A 25-year-old woman underwent surgery for a thyroid nodule. Two months later, she complains of dryness of skin and muscle spasms. Which of the following is the most likely explanation?
    A. Low serum magnesium
    B. Low serum calcium
    C. Low serum potassium
    D. Low serum sodium
    E. Low serum glucose


ANSWERS
36.1 A. The posterior cricoarytenoid muscles are the only muscles that abduct the vocal folds and are necessary to widen the rima glottidis for breathing.
36.2 D. Injury to the recurrent laryngeal nerve is common during thyroid surgery and may lead to the inability to tightly adduct the two vocal folds, resulting in hoarseness. In addition, the protective function of the rima glottidis may be lost, and food or liquid that does not go down the esophagus may flow into the trachea and cause a choking response.
36.3 A. The laryngeal mucosa above the vocal cords is innervated by the superior laryngeal nerve, whereas mucosa below the vocal cords is innervated by the recurrent laryngeal nerve.
36.4 B. This patient likely has hypocalcemia due to excision of the parathyroid glands.


ANATOMY PEARL
 The right recurrent nerve is located more laterally than the left recurrent nerve because of the course of the right subclavian artery.
 The posterior cricoarytenoids are the only muscles to abduct the vocal folds and are necessary to widen the rima glottidis for breathing.
 Most of the intrinsic laryngeal muscles are innervated by the recurrent laryngeal nerve, a branch of the vagus nerve (CN X).
 Bilateral injury to the recurrent laryngeal nerves may lead to respiratory distress, whereas unilateral injury results in hoarseness.
 Injury to the recurrent laryngeal nerve may affect the protective function of the rima glottidis, increasing the opportunity for a choking response.

References

Gilroy AM, MacPherson BR, Ross LM. Atlas of Anatomy, 2nd ed. New York, NY: Thieme Medical Publishers; 2012:106, 600, 603, 607. 

Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy, 7th ed. Baltimore, MD: Lippincott Williams & Wilkins; 2014:1016−1017, 1030, 1076. 

Netter FH. Atlas of Human Anatomy, 6th ed. Philadelphia, PA: Saunders; 2014: plates 76−78.

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