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The Anatomy Of The Larynx And Its Role In Phonation

Why are hoarseness and change to the pitch of the speech possible issues of thyroid surgery?

Complications of thyroid surgery leading to hoarseness and changes to the pitch of the speech are due to problems for the laryngeal nerves. Laryngeal nerve harm can have a detrimental influence on the muscles that move the vocal folds (cords), hence influencing the quality of voice. Although incidence of nerve damage during thyroid businesses has decreased during the last two decades(*), appropriate safety measures are routinely carried out during thyroid surgery, to minimise nerve harm. The thyroid vessels are strongly from the laryngeal nerves. Therefore, an important thought in reaching successful thyroid surgery is the precise knowledge of the anatomy and physiology (*) of the larynx and thyroid, in conjunction with the surgical strategy itself.

Anatomy of the larynx and its role in phonation

The larynx is a highly intricate body organ (Figure 1) which contains a network of cartilage, intrinsic and extrinsic muscles, and connective cells. It is situated in the anterior side of the neck of the guitar based on the third and 6th cervical vertebrae1. The cartilages found in the laryngeal wall membrane are the solo thyroid, epiglottis and cricoid cartilages, which the thyroid is the largest cartilage; and the combined arytenoid, cuneiform and corniculate cartilages. The arytenoid cartilages have vocal process to supply the sites of attachment for the vocal ligament, in doing so permitting opening and closing movements of the vocal folds in phonation. Thus the larynx performs an important role in phonation in speech.

The vocal folds (true vocal cords) will be the wedge-shaped buildings that protrude from the lateral surfaces of the larynx, creating a small aperture across their air passage known as the rima glottidis. Audio is produced when the adductor muscles including the cricoarytenoid muscle contract. The vocal folds are then helped bring closer together which closes the rima glottidis thus providing high amount of resistance to exhaled air from the lungs. Air is then forced through the bottom advantage of the vocal cord followed by the top edge. As air passes through the vocal folds the air pressure decreases producing a Bernouli Result* which in turn causes the lower advantage to close accompanied by the upper edge and finally concluding the vocal folds again alongside one another. Closure of the vocal folds produces simple, regular vibrations that induce sound. Sound is then converted to tone of voice by vocal tract resonators which condition the sound to create various resonances.

Changes to the pitch of tone of voice depend on the degree of pressure and amount of the vocal folds. Alterations to pitch are typically due to activities of intrinsic laryngeal muscles, specifically the cricothyroid muscle. The cricothyroid muscle stretches the vocal ligaments by increasing the arch of the cricoid cartilage and tilting the lamina backwards (*) and in so doing increasing the strain and length of the vocal folds (*). Therefore, contraction of the cricothyroid muscle abducts the vocal folds so that when they vibrate, high pitched audio is produced.

By contrast, production of lower pitch noises require the vocal folds to adduct, narrowing the rima glottidis and therefore reducing vocal fold muscular anxiety.

Nerve supply to the larynx - relations to thyroid gland

Below the larynx lays the thyroid gland which is one of the bigger endocrine glands with an abundant vascular system. The thyroid gland descends to the anterior region of the neck located deep to the sternothyroid and sternohyoid muscles from the amount of C5-T1 (*). It contains two lateral lobes connected by the isthmus that lay on the still left and right part of the trachea. There can also be a small pyramidal lobe which has its superior end ascending from the isthmus to the oblique series. A thin fibrous capsule encompasses the thyroid gland which expands into deeper parenchyma (*) of the gland. The capsule combines with the cricoid cartilage by dense connective tissue creating the suspensory ligament of Berry. Once the ligament of Berry is developed, the RLN can gain gain access to in to the larynx (*).

Innervation of the larynx is important in the transmission of nerve impulses to and from the brain. The vagus nerve innervates the larynx via the external and inside laryngeal nerves, which are collectively terminal branches of the superior laryngeal nerve (SLN), and via the repeated laryngeal nerve (RLN). Most intrinsic laryngeal muscles are innervated by the RLN except for cricothyroid muscle which comes by the external LN.

The exterior LN descends on the outside fascia of the substandard pharyngeal constrictor muscle (*) in close relationship with the superior thyroid artery. The external LN can in uncommon situations (*) run beneath the sternothyroid muscle, in its course, to the oblique line of the thyroid cartilage to innervate the cricothyroid muscle. Interestingly, the bond of the artery and nerve is highly variable and new variations have been documented(*): for example, Type 1 anatomy shows the nerve crosses the superior thyroid vessels add up to, or greater than one centimetre superior to the thyroid pole. The internal LN provides sensory fibres to the larynx and the laryngopharynx after penetrating the thyrohyoid membrane. The inner LN further divides into the superior, middle and substandard branches before getting into the larynx.

The RLN has close contact posterior laterally (*) with the lateral lobes of the thyroid gland. It really is termed recurrent because it retrogrades in the torso and continues superiorly back to the neck (*). A close association exists between the superior region of the RLN and the poor thyroid artery. Just like the SLN and the superior artery, this romantic relationship is highly changing. The RLN can be found anterior, posterior, or through the branches of the second-rate thyroid artery. The span of the still left and right recurrent laryngeal nerves of the RLN grows to the larynx through just a little different routes. The right recurrent LN extends to the larynx after looping about the right subclavian artery and ascending at an oblique perspective in the tracheosophageal groove (*). The nerve pierces the second-rate constrictor muscle of the pharynx before coming into the larynx. The still left RLN road differs by looping posteriorly across the aortic arch, but like the right RLN, it ascends in the tracheosophageal groove (*) until reaches the larynx via the second-rate constrictor muscle. Both main divisions of the RLN will be the anterior and posterior branches. The anterior branch equipment motor fibres to all the intrinsic laryngeal muscles with the exception of cricothyroid( which is supplied by the exterior LN). The posterior branch is mostly sensory and transports the sensory fibres from the larynx and laryngopharynx(*).

Injury to laryngeal nerves during thyroid surgery

The laryngeal nerves carry a higher risk for damage during thyroid surgery which manifests tone problems such as changes in pitch and hoarseness. It is well noted that thyroidectomy is the most common cause of injury to the external LN (*). The close relationship that exists between the external LN and the superior thyroid artery predisposes the exterior LN to injury when the artery is clamped during thyroid surgery. Problems for the exterior LN leads to paralysis of the cricothyroid muscle, planner of the vocal folds. Patients with exterior LN harm lose the capability to forcefully task their voice and also lose their higher voice register. That is due to lack of function of the cricothyroid muscle to alter the strain of the vocal folds. Sometimes, the speech becomes monotonous in personality. The consequences of problems for the external LN are generally understated and unnoticeable in patients aside from those whose professions largely be based upon the use of the normal speech such as professional singers and orators(*). Words function returns on track after a few months after surgery unless injury to the exterior LN is permanent.

Recurrent laryngeal nerve accident is still of major concern in thyroid surgery, as it is the most typical post-thyroidectomy problem (*). Injury to the RLN frequently results from common operative techniques such as suturing, crushing and ligating the nerve and its neighbouring branches. The clamping of the RLN alongside the substandard thyroid artery during surgery can seriously ruin the RLN due to the close closeness of the RLN to the inferior thyroid artery. Since the key functions of the RLN is to innervate the laryngeal muscles and invite abduction and adduction of the vocal folds, injury of the nerve results paralysis of the vocal fold. In unilateral RLN paralysis, where for example the RLN is totally transacted (*), the tone of voice immediately becomes hoarse because the paralysed vocal collapse assumes a paramedian position. The lack of nerve supply results in cord flaccidty where the paralysed vocal collapse gradually atrophies. Furthermore, the vocal collapse is unable to adduct for phonation, and abduct for deep breathing causing limited closure of the rima glottidis. Therefore, air will evade during phonation in so doing leading to dysphonia (hoarseness). Dysphonia may either stay permanent or decline over time (*). Bilateral lesion of the RLN, however, has much more serious complications. The patient with bilateral paralysis have both vocal cords in a paramedian position (*) and can't be abducted upon ideas resulting in airway obstruction. As a result the patient displays biphasic stridor (*) which causes a high-pitched voice and noisy respiration. In the uncommon events the vocal collapse will remain completely paralysed after thyroid surgery and patients may experience a full loss of tone of voice.

Possible ways to minimise nerve damage

Preventing inadvertent problems for the SLN and RLN is crucial in achieving successful thyroid operations which retains the patient's quality of tone. Intraoperative neuromonitoring of the laryngeal nerves is a way of identifying and monitoring the course of the RLN and SLN and thus, avoiding its iatrogenic personal injury during surgery. Patients with unilateral paralysis of the RLN can experience ansa-RLN reinnervation. Reinnervation restores tone and bulk to the intrinsic laryngeal muscles and therefore restores a comparatively normal speech, without interfering with the vocal fold function or composition.

Conclusion

The laryngeal nerves are branches of the vagus nerve. With the exception of the cricothryoid muscle, the RLN innervates the laryngeal muscles which organize the vocal folds for phonation. Evidently, the relationship between your RLN and the SLN and the laryngeal muscles are of great importance because harm to the laryngeal nerves lead to changes in words quality. The close marriage between your thyroid vessels and the laryngeal nerves is an initial reason why careful techniques are essential in thyroid surgery, to minimise injury to the laryngeal nerves preventing voice complications. Medical management of the issues in thyroid surgery have lately experienced great improvements(*) which easily regain the patient's normal voice bettering their everday life.

Fig. 1 Anterior and posterior view of the larynx6

Fig2. (Remaining) anterior view of thyroid gland

http://www. trifoundation. com/gfx/

Thyroid_Diagram. jpg

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