As the extent of thyroid surgical resection increases, so do the surgical complications. Vocal fold palsy results in voice changes that can mostly be compensated by the contralateral vocal folds, a complication that is often overlooked. This article reviews the progress of research on thyroid surgery and vocal cord palsy.
1. Anatomy of the larynx
The thyroid surgeon should be thoroughly familiar with the anatomy of the larynx. The larynx is composed of thyroid cartilage and cricoid cartilage connected by the cricothyroid joint. The vocal cords are located in the middle part of the thyroid cartilage in the laryngeal cavity, with the anterior edge attached to the middle part of the inner front edge of the thyroid cartilage and the posterior part attached to the arytenoid cartilage at the posterior edge of the cricoid cartilage. When the arytenoid cartilage is rotated laterally, the vocal folds are abducted to open the vocal folds, and when it is rotated in the middle, the vocal folds are retracted to close the vocal folds. The arytenoid muscle is the main component of the vocal folds and is the main muscle that regulates the adduction of the vocal folds, while the cricoarytenoid muscle behind it is the main abductor. The cricoarytenoid muscle is located on the lower surface of the larynx and when contracted causes the thyroid cartilage to tilt forward on the surface of the cricoid cartilage, thus causing tension in the vocal folds.
The motor fibers of the recurrentl aryngealnerve (RLN) innervate all intrinsic laryngeal muscles except the cricothyroid muscle, and the sensory fibers innervate the laryngeal mucosa below the glottis, the upper esophagus, and the trachea. The internal branch of the superior laryngeal nerve innervates the sensation of the laryngeal mucosa above the glottis, and the external branch of the superior laryngeal nerve innervates the movement of the cricothyroid muscle. Sound is produced by the exhalation of airflow through a partially closed and tense vocal cord. The vocal range is achieved by the contraction of the cricothyroid muscle that tenses the vocal folds. Therefore, any condition that can cause vocal fold dyskinesia can result in altered voice.
2. Incidence of RLN injury or vocal cord paralysis
The incidence of RLN injury or vocal cord palsy after thyroid surgery varies widely. a review by Jeannon et al. found that the rates of temporary and permanent RLN injury were 9.8% and 2.3%, respectively, while the rate of permanent RLN injury ranged from 0% to 18.6%. data by Bergenfel et al. showed that the incidence of immediate postoperative vocal cord palsy was 4.3%. a review of the literature by Dralle et al. found that the rates of temporary and permanent RLN palsy ranged from 0% to 7.1% and 0% to 11%, respectively, and that there was no difference in the rate of intraoperative nerve injury with or without visual recognition, and that the use of intraoperative nerve monitoring reduced the rate of RLN injury. The rate of RLN injury was 2.5% according to the British BTA.
sevim found that the extent of thyroid cancer surgery was closely related to RLN injury, with temporary and permanent RLN injury rates of 5%/3%, 7%/3%, and 24/17% for subtotal or total thyroidectomy, total thyroid reoperation, and cervical lymph node dissection, respectively. Qin Qian et al. found a strong correlation between the rate of RLN injury and whether or not the RLN was dissected, with a temporary and permanent RLN injury rate of 1.49%/0% in the dissected group and a temporary and permanent RLN injury rate of 6.03%/2.16% in the undissected group, respectively. However, these reports did not routinely perform preoperative and postoperative vocal fold function tests, so they do not truly reflect the rate of RLN injury.
3. Causes of voice changes after thyroid surgery
Hartl et al. summarized the causes of postoperative voice changes in the thyroid gland as follows
(1) Nerve injury. This includes injuries to the RLN and the external branches of the superior laryngeal nerve, resulting in nerve dysfunction, axonal rupture, and nerve dissection.
(ii) Tracheal intubation-related injury. Vocal cord injury or edema; displacement of the cricothyroid joint, etc.
(iii) Local non-neurological factors. Injury caused by muscle ligation or denervation; injury to non-RLN, non-supraglottic nerve plexus around the larynx; local scar or fixation of the larynx; injury to the cricothyroid muscle caused by surgery, etc.
④ Other coincidental factors of voice alteration. For example, viral infection, etc.
4.Pathological changes after nerve injury and vocal folds
(1) Pathological changes after nerve injury
The pathological changes of nerve injury are related to the degree of injury. Squeeze and other minor damage to the nerve caused by nerve demyelination changes, there is a transient impairment of conduction function, called nerve disuse, 6 to 8 weeks later can recover on its own. More severe injuries.
① compression, pulling or ischemia can cause myelin injury and nerve axon fracture, this injury relies on the regeneration of nerve fibers to repair, but the repair is often disordered, resulting in uncoordinated movement of the vocal cord muscle contraction and the vocal folds;
Chi et al. found that tumor-infiltrated paralyzed RLN still had residual function existed, and further deterioration of hoarseness and atrophy of the vocal folds occurred after its excision.
(2) Pathological changes of the vocal folds
In the early stage of RLN injury, the vocal folds are located in the paramedian position, and the effect of the paralyzed vocal folds on the voice can be offset by vocal fold edema, and then the vocal folds begin to atrophy, causing the voice to be affected. Over time, the vocal folds are bowed by atrophy and may be located laterally, failing to close the vocal folds and worsening hoarseness. The ultimate position of the vocal cords and their ability to recover is determined by the severity of the injury. After bilateral vocal cord paralysis both vocal cords are in a paramedian position, but movement is impaired, resulting in partial obstruction of airflow, while exhibiting wheezing sounds and respiratory distress in both directions of breathing. The angle of rotation of the posterior angle of the vocal folds toward the injured side is small after injury to the superior laryngeal nerve, and the arch of the vocal folds on the injured side changes very little, which is easily overlooked during laryngoscopy.
5.Separation of voice change and vocal fold function
(1) Voice change without vocal fold paralysis
There may be subjective or objective voice changes after thyroid surgery, while the vocal fold movement is normal. The most typical subjective manifestations are speech fatigue and difficulty in producing high notes. Studies have found that even without vocal cord paralysis, 30% to 80% of patients complain of voice changes. Objective changes include reduced basal audio and tone breadth, which can occur in patients without postoperative vocal cord paralysis. pereira et al. found that 28% of patients still had non-specific voice changes 4 years after uncomplicated thyroid surgery.
Why do voice changes occur despite the normal gross morphology of the RLN? The possible mechanism is subclinical RLN, extralaryngeal branch dysfunction of the superior laryngeal nerve. It may also be associated with injury to the vocal cords or larynx due to tracheal intubation, direct cricothyroid muscle injury, local muscle denervation due to surgery, scar compression, and possibly voice changes due to concurrent non-thyroidal lesions.
(2) Vocal fold palsy without voice changes
Vocal fold palsy may not have significant voice changes. In the early postoperative period, vocal cord edema due to intubation may counteract the effects of vocal cord palsy on the voice. In the later stages, patients may still be asymptomatic for a variety of reasons, including residual function of the paralyzed vocal folds, location, and compensation of the contralateral vocal fold function. Symptoms of permanent vocal fold paralysis may also improve due to compensatory contralateral vocal fold, which is interpreted as recovery of vocal fold function from paralysis due to the absence of a vocal fold examination. sittel et al. studied 98 cases of vocal fold paralysis and found that 20% of patients had normal voice and the remaining 8% recovered normal.
6. Assessment of vocal fold and nerve function
(1) Preoperative vocal fold examination
All patients undergoing thyroid surgery should have a preoperative vocal fold function examination. Since 1/3 of patients with unilateral vocal cord palsy are asymptomatic, some studies have found that the sensitivity of preoperative voice changes to predict vocal cord palsy ranges from 33% to 68%; therefore, the presence or absence of vocal cord palsy cannot be determined by voice changes. Preoperative vocal cord palsy is mostly due to tumor infiltration or surgical injury. randolph and Kamanj found that preoperative RLN palsy has a sensitivity of 76% and specificity of 100% for predicting thyroid cancer. In addition, preoperative vocal cord examination can detect congenital vocal cord palsy, and if preoperative examination is not performed, postoperative voice changes are often mistakenly attributed to surgery.
The NCCN guidelines point to preoperative vocal fold palsy as a high risk factor for thyroid nodules to thyroid cancer and recommend preoperative vocal fold screening for patients with papillary, follicular, Schottel cell and medullary thyroid carcinomas, but not for undifferentiated carcinomas. The British Thyroid Association (BTA) guidelines, on the other hand, recommend preoperative vocal cord screening for patients with preoperative voice abnormalities and for patients with thyroid cancer.
The 2009 revised edition of the American Thyroid Association (ATA) guidelines does not recommend preoperative or postoperative vocal cord evaluation for all patients with thyroid nodules. As can be seen, the BTA and ATA hold different views on whether or not to perform preoperative vocal cord testing for thyroid surgery. The author’s unit has routinely performed preoperative vocal cord function testing on all thyroid surgery patients for more than 10 years.
(2) Intraoperative nerve monitoring
Traditionally, intraoperative direct visualization and protection of the RLN is considered to be an effective means of avoiding postoperative vocal cord paralysis. Recent studies have found that intraoperative RLN monitoring is important for nerve protection and prediction of postoperative vocal cord paralysis. Cluang et al. used intraoperative nerve monitoring to detect the cause of nerve injury in a timely manner and significantly reduce the incidence of postoperative vocal cord paralysis, and proposed a standard operation for intraoperative nerve monitoring in the thyroid gland.
The intraoperative study of RLN by Serell et al. found 25.7% on the right side, 22.9% on the left side, and 8.9% bilaterally with simultaneous extralaryngeal branches, and found that motor fibers of RLN were present in the anterior rather than posterior branches of the extralaryngeal branches of RLN. Casella et al. also found that the extralaryngeal branches of RLN were a risk factor for the development of postoperative vocal cord paralysis, and 1% of patients had trigeminal extralaryngeal branches. Autopsy revealed that the superior laryngeal nerve was also branching. Therefore, the extra-laryngeal branches of the RLN and the branches of the external branches of the superior laryngeal nerve must also be carefully identified and protected intraoperatively.
(3) Postoperative vocal fold assessment
Vocal fold function and voice separation are determined by several factors. Postoperative assessment of the vocal folds is essential if the surgeon is to make an objective assessment of the surgical outcome. Some data show that the incidence of postoperative vocal fold palsy doubles when vocal fold examination is routinely performed postoperatively compared to examination of symptomatic patients only. The current rate of clinical use of postoperative vocal fold examination is quite low, with multicenter data showing only 21.5%. Only the BTA recommends postoperative vocal cord examination for patients with postoperative voice changes of more than 2 weeks.
7. Treatment of vocal cord paralysis
Medical RLN injury is the main cause of postoperative vocal fold palsy, and any vocal fold palsy that occurs after thyroid surgery should be treated aggressively. Surgeons should combine pre- and post-operative vocal fold function tests and intraoperative nerve monitoring to make precise judgments about the causes and mechanisms of postoperative voice changes. If a transverse injury to the RLN (cut, ligature or suture, etc.) is found intraoperatively or judged postoperatively, timely intraoperative repair or timely postoperative reoperative exploration to repair the nerve or decompression treatment should be performed.
If it is determined that the RLN is intact, has the potential to recover on its own, and the patient can tolerate misaspiration and choking, a one-year waiting period is recommended to allow the function to recover on its own, while the recovery of nerve function is observed every 3 to 6 months. Dysphonia that severely affects quality of life is also an indication for surgery. permanent RLN injuries that do not recover after 12 months also require surgical treatment. Laryngoplasty and simultaneous arytenoid cartilage internal transfer are the current standard procedures for the treatment of post-thyroidectomy vocal cord paralysis.
8. Summary
Voice changes are more common after thyroid surgery and are reported in the literature at a much lower rate than the actual incidence because vocal cord examination is not routinely performed preoperatively or postoperatively. To more comprehensively assess the quality of thyroid surgery and to provide patients with a more complete and safe standardized model of thyroid surgery should be the direction of development in this discipline. Therefore, only through preoperative and postoperative vocal cord assessment and neuromonitoring analysis during surgery can the causes and mechanisms of postoperative vocal cord paralysis be clarified and effective measures and remedies be taken to avoid vocal cord paralysis.