The Anatomy Of The Larynx And Its Role In Phonation

The Anatomy Of The Larynx And Its Role In Phonation Why are hoarseness and change to the pitch of the voice possible complications of thyroid surgery? Complications of thyroid surgery resulting in hoarseness and changes to the pitch of the voice are a result of injury to the laryngeal nerves. Laryngeal nerve damage can have an adverse effect on the muscles that move the vocal folds (cords), hence affecting the quality of voice. Although incidence of nerve injury during thyroid operations has decreased over the last two decades(*), appropriate precautions are routinely implemented during thyroid surgery, to minimise nerve damage. The thyroid vessels are closely associated with the laryngeal nerves. Therefore, an important consideration in achieving successful thyroid surgery is the detailed understanding of the anatomy and physiology (*) of the larynx and thyroid, coupled with the surgical approach itself. Anatomy of the larynx and its role in phonation The larynx is a highly intricate organ (Figure 1) which consists of a network of cartilage, intrinsic and extrinsic muscles, and connective tissue. It is situated in the anterior side of the neck in line with the third and sixth cervical vertebrae1. The cartilages found in the laryngeal wall are the single thyroid, epiglottis and cricoid cartilages, of which the thyroid is the largest cartilage; and the paired arytenoid, cuneiform and corniculate cartilages. The arytenoid cartilages have vocal process to provide the sites of attachment for the vocal ligament, thereby permitting opening and closing movement of the vocal folds in phonation. Thus the larynx plays an important role in phonation in speech. The vocal folds (true vocal cords) are the wedge-shaped structures that protrude from the lateral surfaces of the larynx, creating a narrow aperture across their air passage known as the rima glottidis. Sound is produced when the adductor muscles such as the cricoarytenoid muscle contract. The vocal folds are then brought closer together which closes the rima glottidis thereby providing high resistance to exhaled air from the lungs. Air is then forced through the bottom edge of the vocal cord followed by the upper edge. As air passes through the vocal folds the air pressure decreases producing a Bernouli Effect* which causes the lower edge to close followed by the upper edge and finally closing the vocal folds back together. Closure of the vocal folds produces smooth, regular vibrations that create sound. Sound is then converted to voice by vocal tract resonators which shape the sound to produce various resonances. Changes to the pitch of voice depend on the degree of tension and length of the vocal folds. Alterations to pitch are largely due to actions of intrinsic laryngeal muscles, namely the cricothyroid muscle. The cricothyroid muscle stretches the vocal ligaments by raising the arch of the cricoid cartilage and tilting the lamina backwards (*) and thereby increasing the tension and length of the vocal folds (*). Therefore, contraction of the cricothyroid muscle abducts the vocal folds so that when they vibrate, high pitched sound is produced. By contrast, production of lower pitch sounds require the vocal folds to adduct, narrowing the rima glottidis and thus decreasing vocal fold muscular tension. Nerve supply to the larynx relations to thyroid gland Below the larynx lies the thyroid gland which is one of the larger 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 level of C5-T1 (*). It consists of two lateral lobes connected by the isthmus that lie on the left and right side of the trachea. There may also be a small pyramidal lobe which has its superior end ascending from the isthmus towards the oblique line. 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 forming the suspensory ligament of Berry. Once the ligament of Berry is formed, the RLN can gain access into 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 internal laryngeal nerves, which are collectively terminal branches of the superior laryngeal nerve (SLN), and via the recurrent laryngeal nerve (RLN). Most intrinsic laryngeal muscles are innervated by the RLN except for cricothyroid muscle which is supplied by the external LN. The external LN descends on the outer fascia of the inferior pharyngeal constrictor muscle (*) in close association with the superior thyroid artery. The external LN can in rare occasions (*) run beneath the sternothyroid muscle, in its course, towards the oblique line of the thyroid cartilage to innervate the cricothyroid muscle. Interestingly, the connection 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 equal 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 internal LN further divides into the superior, middle and inferior branches before entering the larynx. The RLN has close contact posterior laterally (*) with the lateral lobes of the thyroid gland. It is termed recurrent because it retrogrades in the chest and continues superiorly back into the neck (*). A close association exists between the superior region of the RLN and the inferior thyroid artery. Like the SLN and the superior artery, this relationship is highly variable. The RLN can be found anterior, posterior, or through the branches of the inferior thyroid artery. The course of the left and right recurrent laryngeal nerves of the RLN reaches the larynx through slightly different routes. The right recurrent LN reaches the larynx after looping around the right subclavian artery and ascending at an oblique angle in the tracheosophageal groove (*). The nerve pierces the inferior constrictor muscle of the pharynx before entering the larynx. The left RLN route differs by looping posteriorly around the aortic arch, but like the right RLN, it ascends in the tracheosophageal groove (*) until reaches the larynx via the inferior constrictor muscle. The two main divisions of the RLN are the anterior and posterior branches. The anterior branch supplies motor fibres to all the intrinsic laryngeal muscles with the exception of cricothyroid( which is supplied by the external LN). The posterior branch is predominantly sensory and transports the sensory fibres from the larynx and laryngopharynx(*). Injury to laryngeal nerves during thyroid surgery The laryngeal nerves carry a high risk for injury during thyroid surgery which manifests voice problems such as changes in pitch and hoarseness. It is well documented 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 external LN to injury when the artery is clamped during thyroid surgery. Injury to the external LN results in paralysis of the cricothyroid muscle, coordinator of the vocal folds. Patients with external LN damage lose the ability to forcefully project their voice and additionally lose their upper voice register. This is due to loss of function of the cricothyroid muscle to alter the tension of the vocal folds. Occasionally, the voice becomes monotonous in character. The effects of injury to the external LN are generally subtle and unnoticeable in patients except for those whose careers largely depend on the use of their normal voice such as professional singers and orators(*). Voice function returns to normal after a few months after surgery unless injury to the external LN is permanent. Recurrent laryngeal nerve injury is still of major concern in thyroid surgery, as it is the most frequent post-thyroidectomy complication (*). Injury to the RLN frequently results from common surgical techniques such as suturing, crushing and ligating the nerve and its neighbouring branches. The clamping of the RLN together with the inferior thyroid artery during surgery can severely damage the RLN due to the close proximity of the RLN to the inferior thyroid artery. Seeing as the main functions of the RLN is to innervate the laryngeal muscles and permit abduction and adduction of the vocal folds, injury of the nerve results in paralysis of the vocal fold. In unilateral RLN paralysis, where for example the RLN is completely transacted (*), the voice immediately becomes hoarse because the paralysed vocal fold assumes a paramedian position. The lack of nerve supply results in cord flaccidty in which the paralysed vocal fold gradually atrophies. In addition, the vocal fold is unable to adduct for phonation, and abduct for deep breathing causing inadequate closure of the rima glottidis. Therefore, air will escape during phonation thereby leading to dysphonia (hoarseness). Dysphonia may either stay permanent or decline over time (*).Bilateral lesion of the RLN, however, has more serious complications. The patient with bilateral paralysis have both vocal cords in a paramedian position (*) and cannot be abducted upon inspiration leading to airway obstruction. As a result the patient exhibits biphasic stridor (*) which causes a high-pitched voice and noisy breathing. In the rare occasions the vocal fold will remain permanently paralysed after thyroid surgery and patients may experience a complete loss of voice. Possible techniques to minimise nerve damage Preventing inadvertent injury to the SLN and RLN is crucial in achieving successful thyroid operations which maintains the patients quality of voice. Intraoperative neuromonitoring of the laryngeal nerves is a way of identifying and monitoring the course of the RLN and SLN and thus, preventing its iatrogenic injury during surgery. Patients with unilateral paralysis of the RLN can undergo ansa-RLN reinnervation. Reinnervation restores tone and bulk to the intrinsic laryngeal muscles and hence restores a relatively normal voice,without interfering with the vocal fold function or structure. Conclusion The laryngeal nerves are branches of the vagus nerve. With the exception of the cricothryoid muscle, the RLN innervates the laryngeal muscles which coordinate the vocal folds for phonation. Evidently, the relationship between the RLN and the SLN and the laryngeal muscles are of great importance because damage to the laryngeal nerves result in changes in voice quality. The close relationship between the thyroid vessels and the laryngeal nerves is a primary reason why meticulous techniques are essential in thyroid surgery, to minimise injury to the laryngeal nerves and prevent voice complications. Surgical management of the complications in thyroid surgery have recently experienced great improvements(*) which safely restore the patients normal voice improving their everday life. Fig.1 Anterior and posterior view of the larynx6 Fig2. (Left) anterior view of thyroid gland http://www.trifoundation.com/gfx/ Thyroid_Diagram.jpg REFERENCES Drake,R.L.,Vogl,W and Mitchell, A W M Grays Anatomy for medical students.Elsevier, Churchill, Livingstone 2005 Tortora, G.J., Derrickson,B, Prezbindowski,K.S. Learning guide: Principles of anatomy and physiology, eleventh edition , Wiley 2006 Bliss, R.D., Gauger, P.G and Delbridge, L.W. Surgeons approach to the thyroid gland: surgical anatomy and the importance of technique. World journal of Surgery 24 (8) 891-897, 2000 Chan,W.F.,Lang,B.H.H. and Lo,C.Y.The role of intraoperative neuromonitoring of recurrent laryngeal nerve during thyroidectomy: a comparative study on 1000 nerves at risk. 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