Discussion

Evaluation of vocal fold mobility before and after thyroid and parathyroid surgery is often the primary reason that endocrine surgeons refer patients; yet, as this study showed, other causes of dysphonia can be present, including structural (14.7 per cent pre-operative and 11.2 per cent post-operative) and functional (35.7 per cent pre-operative and 22.5 per cent post-operative) aetiologies. On routine laryngoscopy, 2 per cent of asymptomatic patients^{Reference Brodnitz5} and more than 36 per cent of dysphonic patients will have a structural vocal fold lesion.^{Reference Brodnitz5,Reference Lancer, Syder, Jones and Le Boutillier6} Furthermore, more than 25 per cent of symptomatic patients will have some form of functional dysphonia.^{Reference Morrison, Nichol and Rammage7}

The American Thyroid Association guidelines recommend laryngoscopy if clinical suspicion exists.^{Reference Haugen, Alexander, Bible, Doherty, Mandel and Nikiforov8} Surgeons often rely on the presence or absence of voice changes to decide if enough clinical suspicion exists to refer a specific patient, but the symptom of voice changes may not correlate well with laryngoscopic findings. In a study of 285 pre-operative patients diagnosed with vocal fold paralysis, Lorenz et al. found only 25 per cent were hoarse, and 13 per cent had no symptoms whatsoever.^{Reference Lorenz, Abuazab, Sekulla, Schneider, Nguyen Thanh and Dralle9} Similarly, Sittel et al. found that nearly one-third of patients with unilateral vocal fold immobility were asymptomatic or eventually compensated to become asymptomatic.^{Reference Sittel, Stennert, Thumfart, Dapunt and Eckel10}

The AAO-HNS 2013 clinical practice guideline for improving voice outcomes after thyroid surgery recommends laryngeal examination in: patients with pre-operative voice changes, those who have thyroid cancer with suspected extra-thyroidal extension, or those who have had prior neck surgery.^{Reference Chandrasekhar, Randolph, Seidman, Rosenfeld, Angelos and Barkmeier-Kraemer1} Vocal pathology other than immobility was found in 116 patients in this study (75.8 per cent) who underwent laryngoscopy for these specific clinical practice guideline guided reasons. Patients referred for non-AAO-HNS clinical practice guideline indications were just as likely to have vocal pathology identified, however.

The AAO-HNS advocated for all patients to undergo laryngoscopy prior to thyroid surgery; however, there was not enough evidence to make this a key action statement.^{Reference Chandrasekhar, Randolph, Seidman, Rosenfeld, Angelos and Barkmeier-Kraemer1} The British Association of Endocrine and Thyroid Surgeons, and the German Association of Endocrine Surgeons, both recommend that all patients undergo pre- and post-operative examination before and after thyroid surgery, regardless of symptoms and history.^{Reference Chandrasekhar, Randolph, Seidman, Rosenfeld, Angelos and Barkmeier-Kraemer1,Reference Musholt, Clerici, Dralle, Frilling, Goretzki and Hermann11} The National Comprehensive Cancer Center and British Thyroid Association recommend laryngoscopy for any patient undergoing thyroid cancer, regardless of suspicion for extra-capsular extension.^{Reference Chandrasekhar, Randolph, Seidman, Rosenfeld, Angelos and Barkmeier-Kraemer1}

In this study, only nine thyroid cancer patients who did not also have a history of prior surgery or significant dysphonia were referred for pre-operative endoscopy. This indicates that recommendations from the practice guideline to refer any patient undergoing total thyroidectomy for cancer were not routinely followed at our institution. Only 2.3 per cent of all patients undergoing thyroid or parathyroid surgery at our institution during the study period were referred, although it is not known whether patients were referred to other institutions for laryngoscopy, but this is not likely.

Even though the number of referrals increased significantly after publication of the AAO-HNS clinical practice guideline in 2013, this study does not establish the publication of this guideline as the reason for the referrals. In fact, it is just as likely that referrals increased as a result of greater familiarity with the referral process and increased efforts to boost referrals by the laryngeal surgery service at our institution. Nonetheless, more cases of treatable vocal fold pathology have been diagnosed as the number of referrals has increased.

Recently, transcutaneous laryngeal ultrasonography has gained favour in the assessment of pre-operative vocal fold mobility.^{Reference Heller12,Reference Wong, Au, Lam and Lang13} It has a reported sensitivity of 93–100 per cent in post-operative and 88.9 per cent in pre-operative patients with vocal fold paralysis.^{Reference Wong, Au, Lam and Lang13} Flexible laryngoscopy served as a ‘gold standard’ for diagnosis in these studies. Transcutaneous laryngeal ultrasonography has many advantages, including the ability to rapidly and non-invasively assess neurological vocal fold function, and to integrate easily within the normal diagnostic workflow (most patients will be undergoing a diagnostic ultrasound with fine needle aspiration or surveillance ultrasound prior to thyroid surgery). The major limitation of transcutaneous laryngeal ultrasonography is that this procedure does not diagnose structural and functional laryngeal disorders which may be contributing to dysphonia. Patients with these findings would therefore not have their dysphonia explained or treated effectively.

Patients undergoing thyroid and parathyroid surgery can benefit from a diagnosis of structural and functional vocal fold pathology other than mobility problems. For example, post-operative patients with normally mobile vocal folds may benefit from voice therapy if they have a functional dysphonia such as muscle tension dysphonia, which was the second most common pathology identified in our post-operative cohort. In addition, laryngeal surgery may be warranted to remove structural lesions, which can be benign or malignant. Two patients (one with leukoplakia and one with a subglottic mass) had potentially malignant lesions that required further investigation and definitive treatment unrelated to their thyroid problem.

This study is limited by its retrospective design. Moreover, the amount of laryngeal structural and functional pathology may be underestimated; this further underscores the importance of routine laryngoscopy in patients undergoing thyroid surgery. An additional limitation of the study is the fact that most of the patients were referred from the Partners Healthcare International system, which tends to pull patients from Northeastern USA, and consists of mainly insured patients from the middle to upper socioeconomic classes. As such, these findings may not be generalisable to all populations.

Earlier diagnosis and treatment for patients with voice disorders may help to expedite recovery and lead to an improved quality of life.^{Reference Kavookjian, Holcomb, Garnett and Kraft14} By referring a patient for laryngoscopy, endocrine surgeons can have pre-operative understanding of vocal fold mobility for surgical planning. Knowledge of pre-operative laryngoscopy findings can allow the surgeon to proceed confidently, knowing that a patient with dysphonia has an accurate diagnosis for their pre-operative symptoms, for which further surgery is not likely to lead to airway compromise. Finally, serious conditions such as vocal fold carcinoma may be identified early and treated accordingly.^{Reference Friedman, Hillman, Landau-Zemer, Burns and Zeitels15} The utility of pre-operative examination extends beyond the simple evaluation of vocal fold mobility; it should also include a thorough assessment of the function and anatomy of the larynx, to guide surgical planning and manage post-operative voice expectations.