Role of Stroboscope in Injury of External Branch of Superior Laryngeal Nerve During Thyroid Surgery

Abstract

Role of stroboscope in injury of External branch of superior laryngeal nerve during thyroid surgery. Study conducted in a tertiary care hospital. 40 patients were selected and divided into two groups as Group A and B. Benign thyroid swellings were included, excluding preoperative vocal cord pathology by performing videolaryngoscope and videostroboscope. After thyroidectomy procedure stroboscopic fundamental frequency, symmetry, periodicity, glottic closure and mucosal wave pattern were assessed in both the groups and compared. The mean difference in fundamental frequency between pre operative and postoperative in Group A was 37.85 (SD = 33.69) and in Group B was 58.40 (SD = 77.55) with p-value of 0.287. Symmetry of cords in Group A was 90% and asymmetry was 10%, while in Group B symmetry of cords was 75% and asymmetry of 25% with p-value of 0.407. Other parameters like regular periodicity, complete glottic closure, normal mucosal pattern was 95% in Group A and 75% in Group B. Irregular periodicity, incomplete glottis closure, decreased mucosal wave pattern was 5% in Group A and 25% in Group B. Stroboscope is an essential and useful tool in diagnosing EBSLN injury. It is useful in assessing pre operative voice assessment before and after thyroid surgery than a video laryngoscope.

Introduction

Thyroidectomy is most common surgical procedure performed in Otorhinolaryngoloy. Major concern in thyroid surgery is morbidity due to injury to anatomical structures [1]. The important structures that should be preserved during thyroid surgery are the Recurrent laryngeal nerve (RLN), Parathyroid glands and the External branch of superior laryngeal nerve (EBSLN) [2]. Numerous research works has been done on ways to preserve the RLN and parathyroid glands, less attention were given to EBSLN [3]. In fact it is described as a ‘neglected’ nerve in thyroid surgery; despite the fact that injury to this nerve can cause significant disability [4]. Unilateral injury to EBSLN may result in mild huskiness; however bilateral injury can result in devastating consequences. In most individuals, these changes tend to be a nuisance, but it can be catastrophic in voice professionals [5]. So voice after thyroidectomy surgeries is gaining importance recent days. A Stroboscope is a great diagnostic tool in diagnosing voice change in these patients. In recent days since the incidence of invasive thyroid disease is increasing, we need to assess the pre operative voice in all patients who undergo thyroid surgeries.

Aims and Objectives

1. To outline the importance of videostroboscopy in diagnosis of External branch of superior laryngeal nerve injury during thyroid surgery.

2. Also to outline that videostroboscopy as an important diagnostic tool for vocal cord status before taking up for thyroid surgery.

Materials and Methods

This is a prospective study conducted on 40 patients who underwent thyroidectomy procedures from October 2013 to October 2015 in ENT Department at a tertiary care centre in Central India. Institutional ethical committee clearance was obtained. Patients of either sex who presented with complaints of neck swelling, confirmed to be benign thyroid swelling on investigation and subjected to thyroid surgery were included in this study. Patients with malignancy of thyroid, preexisting abnormal vocal cord function, previous neck irradiation and past surgery involving cervical exploration were excluded from this study. Patients were included in the study after obtaining informed consent. All these patients underwent a detailed examination including videolaryngoscopic examination and videostroboscopic analysis for preoperative vocal cord assessment. Pre operative fundamental frequency (f0) of vocal cord was noted in all these patients. We divided the 40 patients into two groups as Group A where the EBSLN was identified and preserved by conducting a meticulous superior pole dissection. Jolls’s triangle was identified and nerve located according to Cernea’s classification [5]. Group B includes those patients where the EBSLN could not be identified by not carrying out the superior pole dissection in a meticulous way or by traditional method of clamping the superior pole vessels close to the superior pole of thyroid. Recurrent laryngeal nerve was preserved in both the groups by using a nerve monitor. All patients who underwent the procedure were followed up on day 3 of post operative period and 1 month after the surgery. Both videolaryngoscopy and videostroboscopy were carried in all these patients.

A Rigid stroboscopy was used in our study. After topical anesthesia with 10% xylocaine spray to the posterior tongue, the patients asked to lean forward with the neck flexed and the head extension (Kirstein position). With mouth open and tongue protruded, tongue was retracted anteriorly and rigid telescope was carefully inserted. Patient was asked to produce “ee” sound. This was carried out in low, mid and high frequency pitch. Parameters assessed using stroboscope includes fundamental frequency using a strobe unit. Other parameters like symmetry of the vocal cords, periodicity, glottic closure and mucosal wave pattern were studied. The results were tabulated, statistical analysis was done using SPSS software. Chi square test was applied and p-value less than 0.05 is considered as a significant result.

Results

Among the 40 patients in our series, female population was 88%. Mean age group was between 31 to 40 years. The mean difference in fundamental frequency between pre operative and postoperative in Group A was 37.85 (SD = 33.69) and in Group B was 58.40 (SD = 77.55) with p-value of 0.287 (Table 1). Symmetry of cords in Group A was 90% and asymmetry was 10%, while in Group B symmetry of cords was 75% and asymmetry of 25% with p-value of 0.407 (Table 2). Periodicity of vocal cords among Group A was regular in 95% and irregular in 5%.In Group B cords were regular in 75% and irregular in 25%. Glottic closure (GC) of vocal cords among Group A was complete in 95% and incomplete in 5%. In Group B, GC was complete in 75% and incomplete in 25% with p-value of 0.182. Mucosal wave pattern of vocal cords among Group A was normal in 95% and decreased in 5%. In Group B mucosal pattern was normal in 75% and decreased in 25% with p-value of 0.182 (Table 3).

Table 1.

Comparison of pre and post operative fundamental frequency (f0) among EBSLN identified and not identified group

Group	Number of patients (N)	Pre-op f0		Post-op f0		Mean difference of f0		p-value
		Mean	SD	Mean	SD	Mean	SD
A	20	174.30	47.57	212.15	60.26	37.85	33.69	0.287
B	20	214.25	52.42	272.65	84.64	58.40	77.55

Table 2.

Association between EBSLN identification and symmetry of vocal cords in stroboscope

Group	Symmetry of vocal chords		p-value
	Symmetry (%)	Asymmetry (%)
A	18 (90%)	2 (10%)	0.407
B	15 (75%)	5 (25%)

Table 3.

Association between EBSLN identification and periodicity, glottic closure and mucosal wave pattern of vocal cords in stroboscope

Group	Periodicity of vocal cords		Glottic closure		Mucosal wave pattern		p-value
	Regular (%)	Irregular (%)	Complete (%)	Incomplete (%)	Normal (%)	Decreased (%)
A	19 (95%)	1 (5%)	19 (95%)	1 (5%)	19 (95%)	1 (5%)	0.182
B	15 (75%)	5 (25%)	15 (75%)	5 (25%)	15 (75%)	5 (25%)

Discussion

The human eye can see only five images per second. However, stroboscope allows the eye to see the high frequency vibration of the vocal folds by intermittent flash synchronized with the phonation frequency (Talbot’s law). It is therefore an optical phenomenon that captures the fragments of complete movement [6].

Injuries to the nerve can be common during thyroidectomy causing cord paralysis. A complete paralysis is one in which all the motor units of the nerve are affected. In incomplete paralysis, some of the motor units in the nerve are affected due to mechanical pressure, straining of the nerve, or cauterization during the procedure. In complete paralysis the vocal fold is immobile and has no muscle tone. Glottic closure is incomplete. The movements of the vocal folds are asymmetrical and mucosal wave is absent or reduced in the paralytic vocal fold. In incomplete paralysis, the vocal fold is slightly mobile and has some muscular tone. Glottic closure is better. The mucosal wave is reduced in the affected fold and the cords are asymmetrical. Abnormalities in the vibratory pattern become more apparent in Superior Laryngeal Nerve palsy, anterior glottis moves to the affected side because of the tone of the unaffected cricothyroid muscle. The mucosal wave will be asymmetrical as the wave is late in the affected fold with changes in mucosal wavepattern [7, 8]. In our series, preoperative videolaryngoscopy and videostroboscopy were normal in all patients.

We found that the mean difference in fundamental frequency was significant in Group A and Group B. 4 of our patients had typical voice change symptoms and rest of them did not have any significant voice change. These patients had a significant change in fundamental frequency. 2 of our patients in Group A had asymmetry in vocal cords and 5 among the Group B had asymmetry in cords. Results of periodicity, glottic closure and mucosal wave pattern were affected in 1 patient among the Group A and 5 patients in the Group B. Results shows that 5% chance of injury to the Group A, possibly an incomplete injury. 25% of patients showed injury to the nerve in Group B. Videostroboscopy was repeated at the end the 1 month and all of the patients had an improvement in voice except 2 patients. Videostroboscopy after 1 month in Group B showed signs of EBSLN injury like anterior bowing of cords. These patients had anterior bowing and asymmetry of cords as a significant feature in stroboscope and rest of the parameters like periodicity, glottic closure and mucosal wave pattern had an improvement. Results of our study were found similar to a study conducted by Kondamudi et al. [9].

Our study mainly focuses on the importance of videostroboscope as a sensitive tool in early diagnosing of EBSLN palsy. Most of our patients in the nerve injured group showed normal findings in videolaryngoscopy, showing video stroboscope efficacy.

We infer that the chance of injury to the EBSLN is more if the nerve is not properly identified and preserved in the superior pole. The reason for injury could be ligating the superior pole vessels close to the superior pole of thyroid as practiced as a traditional method, using of electrocautery in superior pole and inadequate exposure of superior pole. In our series out of the 5 patients who had EBSLN injury, 3 of them showed improvement in voice over a period of 3 months but 2 patients continued to have features of nerve impairment. Though there is an improvement in voice even after the nerve is injured, most of these patients cannot produce high pitched voice. This can be catastrophic in professional voice users like singers, lawyers etc. EBSLN injury can be best avoided by following some simple techniques like wide exposure of superior pole by identifying Joll’s triangle [10] and classifying the nerve according to Cernea et al. [5]. Individual ligation of superior pole vessels should be done without using any electrocautery.

Conclusion

Superior laryngeal nerve preservation is important in all cases of thyroid surgery. Pre operative voice assessment using stroboscope should be carried out in all patients who undergo thyroid surgery. Stroboscope helps in identifying early EBSLN palsy. The mucosal wave has a greater velocity and further travels along the mucosa on the normal vocal fold. The probable cause for these vibratory findings is the reduced stiffness in the paralyzed cord, which reduces the velocity and extent of the travelling mucosal wave. Stroboscope helps in identifying this mucosal abnormality which cannot be assed using a videolarygoscope. We conclude from our study that stroboscope is an essential tool for diagnosing EBSLN injury. It should be identified in all cases of thyroid surgery and preserved to prevent post operative voice change in thyroidectomy patients. Limitations of our study are small sample size; p-values obtained were not significant possibly because of the small sample size. Still we conclude our study saying that Stroboscope is an essential tool in diagnosing EBSLN injury in thyroidectomy patients.

Funding

No external funding was obtained for this study.

Compliance with ethical standards

Conflict of interest

The authors have no conflict of interest.

Ethics Approval

Due Institutional Ethics Committee approval was obtained for this study.