The thyroid gland is a butterfly shaped structure that lies on the windpipe below the Adam's Apple. The Adam’s apple is formed by the thyroid cartilage of the larynx (voice box), which joins the trachea (windpipe) to the back of the oral cavity.
The larynx comprises the thyroid cartilage, the cricoid ring and the arytenoid cartilages. The vocal cords run from the thyroid cartilage at the front to the arytenoid cartilages, which sit on the back of the cricoid ring. Rotation of the arytenoids allows the cords to be brought together or apart, and movement between the thyroid and cricoid cartilages allows them to be stretched or relaxed. The larynx is innervated by two nerves on each side. The dominant nerves are the recurrent laryngeal nerves (RLN’s), additional innervation is supplied by the superior laryngeal nerves. The recurrent laryngeal nerves run down through the neck and turn back up to reach the larynx, and hence run past the thyroid gland below the larynx. The left RLN runs down into the chest before turning up, the right runs to the level of the base of the neck. The superior laryngeal nerves enter the larynx through the cricothyroid membrane (between the cricoid and thyroid), and hence are close to the superior poles of the thyroid gland.
The larynx has three functions:
Disorders of the thyroid gland may affect any of these functions of the larynx.
A normal larynx
The epiglottis is visible at the top of the picture; the vocal cords are seen above the trachea running back to the arytenoid cartilages, at the bottom of the picture.
Underproduction of thyroid hormone is characterised by, among other things, a deepening of the voice. This occurs through the thickening of the vocal folds as a result of the deposition of mucopolysaccharide, which lowers their fundamental frequency and hence the note produced. This will return to normal with appropriate thyroxine replacement.
Enlargement of the thyroid gland, whether benign or malignant, may stretch the adjacent recurrent laryngeal nerve. In the majority of cases this happens slowly and does not affect the function of the nerve. However, should the growth be rapid or the thyroid expand as a result of a bleed into a cyst the function of the nerve may be affected. The risk of nerve damage during surgery is also increased if the nerve is stretched over an exceptionally large goitre.
Some thyroid cancers, particularly anaplastic carcinoma, may directly invade local structures. This may lead to paralysis of the larynx if the recurrent laryngeal nerves are involved, or direct invasion of the thyroid cartilage may lead to fixation of the vocal folds.
During surgery the recurrent laryngeal or the superior laryngeal nerves may be damaged. Damage to the recurrent laryngeal nerves is a rare occurrence and is usually the result of the nerve being stretched as the thyroid gland is removed, and therefore recovers. On occasion, in difficult cases such as revision operations, or in association with malignant thyroid disease, the nerve may be cut inadvertently or require to be sacrificed in the interest of disease clearance. Injury to one nerve will result in paralysis of the affected vocal cord affecting the voice which becomes weaker, harsh and breathy, and may be associated with cough and spluttering on drinking liquids initially. Injury to both nerves is very rare and results in inability to open the vocal cords adequately, and therefore difficulty breathing. These effects may be temporary or permanent and treatment should it be required is directed accordingly.
The incidence of recurrent laryngeal nerve injury is in the region of 0-2% and will vary between surgeons. In revision cases this figure can increase to 25-30%. The incidence of injury to the less important superior laryngeal nerves is harder to establish, lying between 0-25%.
Prior to thyroid surgery it is important to establish that the vocal cords are functioning and moving normally and that the primary thyroid problem, or previous thyroid surgery has not affected the vocal cord function and mobility. This is normally done by an ENT surgeon who will carry out a laryngoscopy in the clinic. Using a little local anaesthetic a fine bore flexible endoscope is passed through the nose to allow an excellent view of the larynx. The procedure is painless and takes only a minute or so. Occasionally a surgeon will opt to ask his anaesthetist to check the cord movement on induction of anaesthesia instead.
The optimal function of the larynx requires the glottis (space between the vocal cords) to be able to close for speech and protection of the airway, and open for breathing, particularly inspiration. The symptoms caused by paralysis of one vocal cord depend initially on the position in which that vocal cord is resting. The cord will usually rest in the paramedian position, in which the normal movement of the other side of the larynx will provide an adequate airway and may be capable, in time, of closing the glottis to allow reasonably normal speech and a cough. Initially the movement of the good side will not bring it far enough across to close the glottis, the voice is breathy and the cough lacks force (as this requires the rapid opening of a closed glottis).
? Unilateral vocal cord palsy
A photograph of the larynx, the patient’s right vocal cord is paralysed by damage to the recurrent laryngeal nerve; the right arytenoid has prolapsed forward.
A course of speech and language therapy may help the mobile vocal fold compensate.
Should the mobile side of the larynx fail to compensate adequately the immobile vocal fold may be surgically repositioned closer to the midline. This is usually done either endoscopically via an injection technique (using temporary or permanent bulking materials) lateral to the immobile cord, or through a neck incision with the insertion of a silastic block to push the vocal cord towards the midline (Ishiki thyroplasty).
Paralysed left vocal cord lies away from midline and may become slack and bowed.
The appearance of the same left vocal cord following injection of material to bulk up and bring the cord towards the midline to allow it to meet the right cord on phonation resulting in voice improvement.
If the both recurrent laryngeal nerves are paralysed, the larynx is unable to open sufficiently to allow inspiration. If this is due to thyroid pathology the patient will typically present acutely short of breath. If the paralysis is a complication of surgery it will become apparent when the endotracheal anaesthetic tube is removed, and is likely to require immediate re-intubation.Unless there is already a paralysed vocal fold due to disease or previous surgery it is very difficult to justify bilateral recurrent nerve damage in the "average thyroidectomy". With the use of intra-operative nerve monitoring and meticulous identification of the nerves bilateral permanent bilateral damage should not occur. In all the years of Mr Lynn's career he never personally caused a bilateral nerve palsy. (At Hammersmith Hospital some 15 years ago a redo thyroidectomy was performed against Mr Lynn's advice and both recurrent nerves were damaged and the patient has a permanent tracheostomy). It is his view except in the most difficult cases that removal of the thyroid resulting in bilateral nerve paralysis must be regarded negligent. Some surgeons justify their actions by suggesting it is a rare but well recognised complication .I believe that this view relates to technically difficult operations but not a routine thyroidectomy for Grave's disease.The thyroid surgeon at St Georges Hospital had this view 50 years ago!
The formation of a surgical tracheostomy (an opening between the trachea and the skin below the level of the larynx) will relieve the immediate obstruction to breathing. The removal of the tracheostomy tube will depend upon either the recovery of recurrent laryngeal nerve function, or surgery to the larynx to improve the airway. Surgery would typically involve a laser resection of part of the larynx to allow both breathing and voice. Because this does not restore the normal movement of the larynx it involves a degree of compromise of voice quality, which may deteriorate at the expense of improvement of the airway.
A bilateral vocal cord palsy. The cords are in the midline with a narrow (black) airway between them. The paralysed vocal folds will typically rest further apart than this but will move paradoxically, coming together passively on inspiration instead of actively moving apart. This means that the obstruction is worse when breathing harder.
Laser posterior cordectomy improves the airway, but the voice may deteriorate.
Damage to the superior laryngeal nerve may cause a relatively slight deterioration in vocal quality, manifest usually as reduced ability to project the voice, and is usually only significant in professioisnal voice users or singers.The most famous example is that of Amelita Galli-Curci the famous Italian soprano whose exquisite voice quality was ruined by a thyroidectomy performed under local anaesthetic in the 1930's.The presumed damage to the external branch of her superior laryngeal nerve resulted in weakness of the cricothyroid muscle. It is now thought that her superior laryngeal nerve was not damaged and her singing voice deteriorated naturally Except for patients with special voice requirements external branch of the superior laryngeal nerve damage usually requires no treatment but occasionally patients may need to be helped by speech therapy.