Abstract
Voice related complications are a major cause of morbidity in patients undergoing thyroidectomy in the postoperative period in the modern world. EBSLN being a primary tensor of vocal cords via cricothyroid muscle is important to produce high pitched sound above 150 Hz and it is essential for professional voice users. This nerve is prone to injury during surgery because of its proximity to the upper pole of the thyroid and complex anatomical relationship with the superior thyroid vessels. We aimed to study the occurrence of injury to EBSLN during surgery via acoustic voice analysis and comparing it with the pre-operative values. Patients of benign thyroid swelling with Bethesda type I to III and normal thyroid function tests were taken up for surgery. Pre- and post-operatively acoustic voice analysis was done in a soundproof room. Fundamental frequency (F0), amplitude perturbation (shimmer), fundamental frequency perturbation (jitter), and harmonic/noise ratio (HNR) were calculated and analysed. Intraoperatively, EBSLN was identified in Joll’s triangle and preserved. The acoustic voice analysis was performed on the postoperative day one and seven, and compared with the pre-operative values. The mean age of patients was 35.73 ± 9.13 years. Most common age group 36–45 with male to female ratio being 1:9. Cystic colloid goitre was seen in 85% patients. F0 decreased from the preoperative value (221.447 ± 45.981 Hz) on postoperative day one (217.283 ± 46.323 Hz), p-value < 0.01 and on day 7, F0 (219.872 ± 45.096 Hz) p = 0.46. Pitch sigma increase (p < 0.01) from pre-operative (16.791 ± 14.716 Hz) to postoperative day 1 (18.454 ± 15.088 Hz) and on day 7 (17.79 ± 15.315 Hz), p = 0.09. Jitter (%) pre-operatively (0.46%±0.17%), post-operative day 1 (0.45%±0.21%, p = 0.05) and postoperative day 7 (0.44%±0.19%, p = 0.11). Shimmer (dB) the pre-operative (0.582 ± 0.317 dB), post-operative day 1 (0.552 ± 0.301 dB), p = 0.08 and postoperative day 7 (0.566 ± 0.302 dB) p = 0.90 on day 7. HNR pre-operatively (19.009 ± 3.939 dB), postoperative day 1 (18.803 ± 4.14 dB, p = 0.58) and postoperative day 7 (18.740 ± 3.630 dB, p = 0.46). Acoustic voice analysis showed decreased pitch and increase in pitch sigma on postoperative day one which improved on day seven, and it shows that there was no permanent injury to ESBLN during surgery. Acoustic voice analysis being a non-invasive objective procedure, can be used to diagnose EBSLN injury post-thyroidectomy. However, research with large sample size is required to conclusively study the diagnostic efficacy of acoustic voice analysis to diagnose injury to EBSLN.
Keywords: External branch of superior laryngeal nerve (EBSLN), Acoustic analysis, Thyroidectomy
Introduction
Thyroidectomy is a commonly performed surgery. Voice-related complications remain a cause of significant post-operative morbidity making a meticulous dissection and thorough knowledge about the various nerves associated with the thyroid gland, essential for the surgeon. Cricothyroid muscle supplied by the external branch of superior laryngeal nerve (EBSLN) is a primary tensor of vocal cords and is essential to produce high pitched voices usually at frequencies above 150 Hz. The symptoms associated with injury to EBSLN are mild to moderate breathy voice, reduction in average pitch, and reduced voice range with difficulty attaining high pitch which is important for professional singers. EBSLN is at risk of injury during surgery due to the proximity of the nerve to the superior pole of the thyroid gland and its variable course and relationship to the superior thyroid artery. Various classifications have been proposed describing the anatomy of EBSLN with relation to the superior thyroid artery but the most widely accepted classification was given by Cernea et al. [1].
There is a paucity of data studying injury to EBSLN via acoustic voice analysis in thyroidectomy. Hence, we planned to perform preoperative and postoperative voice analysis of a patient undergoing thyroidectomy using computer software.
Materials and methods
The observational study with a sample size of forty was conducted in the Departments of Otorhinolaryngology and Radio-diagnosis in Maulana Azad Medical College and associated hospitals, New Delhi over a span of one year, after approval from the Institutional Review Board. Patients presenting to the OPD with benign thyroid swelling were taken up for surgery as per American Thyroid Association guidelines and having normal thyroid function tests, along with pre-operative FNAC proven cases as per Bethesda classification type I to III. Patients with history of previous thyroid surgery, head & neck malignancy, radiotherapy and chemotherapy to the neck, history of previous head and neck surgery, pre-operative FNAC proven cases as per Bethesda IV to VI, and patients with injury to recurrent laryngeal nerve post-operatively as it would manifest as cord paralysis were excluded from the study.
Written and informed consent was obtained from the patients. Detailed relevant history was taken. General physical examination and otorhinolaryngological examination was performed.
Preoperatively patients underwent an ultrasound neck using Philips iU22 ultrasound machine. The volume of each lobe of the thyroid was calculated using the rotational ellipsoid formula (Vellipsoid = π/6 length of the lobe in cm X breadth of the lobe in cm X the width of the lobe in cm). Based on the volume in milliliters, lobes were categorized into Grade 1 (< 20 mL) and Grade 2 (> 20 mL). Preoperative thyroid function tests (Serum TSH, Free T3, and Free T4). FNAC Thyroid was done. Preoperative voice recording was done using Samson Meteor Mic USB Studio Condenser Microphone-Chrome (Model number B004MF39YS) as shown in Fig. 1. Sound recording was done in a soundproof room.
Fig. 1.
Samson meteor mic USB studio condenser microphone-chrome (Model number B004MF39YS)
The patient assumed a natural and comfortable position and the mouth positioned 1 cm away from the microphone. The vowel /i/ was produced sustainably at a comfortable pitch and intensity for 3 to 5 s. The signal was digitized, analyzed, and stored using PRAAT software. The analysis indicators include average fundamental frequency (F0), the amplitude perturbation (shimmer), the fundamental frequency perturbation (jitter), and the harmonic/noise ratio (HNR) and analysis using PRAAT software. The spectrograph obtained using PRAAT is given in Fig. 2.
Fig. 2.
Spectrograph obtained using PRAAT
General anesthesia was given and orotracheal intubation was done. The skin incision was given and subplatysmal flaps were raised. The strap muscles were divided partially using electrocautery to gain access to the superior pole of the thyroid gland followed by gentle traction to the gland in a caudal and lateral direction. At the superior pole of the thyroid gland, careful blunt dissection was carried in the avascular plane, medial to the superior pole of the gland, and cricothyroid muscle, to identify EBSLN in the sternothyroid-laryngeal or Joll’s triangle. EBSLN was identified as it usually descends parallel to superior thyroid artery and lies on inferior constrictor muscle before innervating cricothyroid muscle as shown in Figs. 3 and 4.
Fig. 3.
ESBLN can be identified intraoperatively in Joll’s triangle which is bounded by cricothyroid and inferior constrictor medially, sternothyroid muscle anteriorly, and upper pole of thyroid laterally. CTM: Cricothyroid muscle, EBSLN: External branch of the superior laryngeal nerve, ICM: Inferior constrictor muscle, SPT: Superior pole of the thyroid gland, STA: Superior thyroid artery, STM: Sternothyroid muscle, Joll’s: Joll’s triangle
Fig. 4.
RLN can be identified in the Beahrs triangle formed by the common carotid artery, inferior thyroid artery, and RLN as shown in the figure. CCA: Common carotid artery, ITA: Inferior thyroid artery, RLN: Recurrent laryngeal nerve, TE groove: Tracheoesophageal groove, Thy: Thyroid gland, TRH: Trachea
The distance of EBSLN from the upper pole of the thyroid where it crosses superior thyroid artery was measured using Vernier calipers.
Acoustic voice analysis was done again on postoperative day 1 and 7 using the method described above.
Results
The mean age of patients in our study was 35.73 ± 9.13 years. The most common age group of patients was 36–45 years. In the present study consisting of 40 patients, 90% of patients were females with male to female ratio of 1:9. The right lobe was involved in 55% patients, 30% in the left lobe and 15% had bilateral swelling. The thyroid nodules after USG were classified into cystic and solid variants. The swelling was cystic in 90% of cases and solid in 10% cases.
In the current study, the most common pathology obtained in the post-operative histopathological analysis was colloid goiter (85%), followed by adenomatous goiter (5%), benign follicular nodule (5%), follicular adenoma (2.5%), and multinodular goiter (2.5%).
The mean volume of thyroid in study subjects obtained via ultrasound was 24.187 mL with a minimum volume of 1.44 mL and a maximum volume of 202.73 mL.
Type 2b EBSLN was most commonly (44/7%) found in the current study as shown in the table. The nerve could not be identified in 2 subjects as given in Table 1.
Table 1.
Relation of the external branch of superior laryngeal nerve distance to the superior Pole in study subjects (n = 38)
| Relation of EBSLN to superior pole | No. | % |
|---|---|---|
| Type 1 | 11 | 28.9 |
| Type 2a | 10 | 26.3 |
| Type 2b | 17 | 44.7 |
Type 2b EBSLN as per Cernea classification is associated with thyroid glands with large volume as given in Table 2; Fig. 5.
Table 2.
Association of pre-operative thyroid volume with different types of EBSLN as per cernea classification
| Type of EBSLN | P-value | |||
|---|---|---|---|---|
| Type 1 | Type 2a | Type 2b | ||
| Thyroid | ||||
| Volume (mL) | 5.983 ± 3.245 | 13.687 ± 2.496 | 43.976 ± 45.303 | < 0.001 |
Fig. 5.
Association of pre-operative thyroid volume with different types of EBSLN as per Cernea classification
Mean pitch (F0), pitch sigma, jitter, shimmer, HNR obtained pre-operatively and postoperative day one and day seven are below in Tables 3, 4 and 5.
Table 3.
Preoperative voice parameters in study subjects
| Mean F0 (Hz) | Pitch sigma (Hz) | Jitter | Shimmer (dB) | HNR (dB) | |
|---|---|---|---|---|---|
| Mean | 221.447 | 16.791 | 0.46% | 0.582 | 19.009 |
| Median | 230.494 | 13.531 | 0.45% | 0.527 | 18.939 |
| SD | 45.981 | 14.716 | 0.177% | 0.317 | 3.939 |
| Minimum | 108.213 | 1.313 | 0.19% | 0.135 | 8.530 |
| Maximum | 282.014 | 52.513 | 0.98% | 1.882 | 27.697 |
Table 4.
Immediate post-operative (day 1) voice parameters in study subjects
| Mean F0 (Hz) | Pitch sigma (Hz) | Jitter | Shimmer (dB) | HNR (dB) | |
|---|---|---|---|---|---|
| Mean | 217.283 | 18.454 | 0.45% | 0.552 | 18.803 |
| Median | 232.900 | 15.254 | 0.39% | 0.506 | 19.304 |
| SD | 46.323 | 15.088 | 0.213% | 0.301 | 4.14 |
| Minimum | 103.055 | 1.374 | 0.15% | 0.137 | 6.220 |
| Maximum | 284.287 | 57.363 | 1.03% | 1.552 | 27.390 |
Table 5.
Post-operative voice parameters at day 7 in study subjects
| Mean F0 (Hz) | Pitch sigma (Hz) | Jitter | Shimmer (dB) | HNR (dB) | |
|---|---|---|---|---|---|
| Mean | 219.872 | 17.797 | 0.44% | 0.566 | 18.740 |
| Median | 234.594 | 14.995 | 0.42% | 0.534 | 18.724 |
| SD | 45.096 | 15.315 | 0.199% | 0.302 | 3.630 |
| Minimum | 110.088 | 1.518 | 0.17% | 0.144 | 7.846 |
| Maximum | 283.922 | 56.450 | 1.06% | 1.680 | 26.808 |
F0 decreased significantly from the preoperative value (221.447 ± 45.981 Hz) on postoperative day one (217.283 ± 46.323 Hz) with a p-value < 0.01. However, it recovered on acoustic voice analysis done on postoperative day seven (219.872 ± 45.096 Hz) with a p-value of 0.46 but it was still lower than the pre-operative value. Pitch sigma increased from a value of 16.791 ± 14.716 Hz to 18.454 ± 15.088 Hz on postoperative day one which was statistically significant (p-value < 0.01) and it improved on postoperative day seven to 17.79 ± 15.315 Hz. The change in pitch sigma on day seven when compared with preoperative value was statistically not significant (p-value = 0.09). The comparison between various parameters obtained on acoustic voice analysis is given in Table 6.
Table 6.
Comparison of pre-operative and post-operative voice parameters in study subjects
| Preop | 1-day post-operative |
P-value With preop |
1-week post-operative |
P-value With preop |
|
|---|---|---|---|---|---|
| Mean F0 (Hz) | 221.447 ± 45.981 | 217.283 ± 46.323 | < 0.01 | 219.872 ± 45.096 | 0.46 |
| Pitch sigma (Hz) | 16.791 ± 14.716 | 18.454 ± 15.088 | < 0.01 | 17.79 ± 15.315 | 0.09 |
| Jitter | 0.46%±0.17% | 0.45%±0.21% | 0.05 | 0.44%±0.19% | 0.11 |
| Shimmer (dB) | 0.582 ± 0.317 | 0.552 ± 0.301 | 0.08 | 0.566 ± 0.302 | 0.90 |
| HNR (dB) | 19.009 ± 3.939 | 18.803 ± 4.14 | 0.58 | 18.740 ± 3.630 | 0.46 |
The rest of the acoustic voice analysis parameters showed no statistically significant change in the post-operative period.
Change in F0 and pitch sigma was present in patients having Type2b EBSLN as per Cernea classification which was statistically significant (p < 0.01). Change in F0 and pitch sigma in patients having Type 1 and Type 2a EBSLN anatomy was not statistically significant. It was also observed that in patients with Type 2b EBSLN anatomy HNR decreased on post-operative day one which was statistically significant (p = 0.04). However, change in HNR on post-operative day seven when compared with pre-operative value was statistically significant (p = 0.35). The data is given in Table 7.
Table 7.
Comparison of change in voice parameters in study subjects with different relation of EBSLN to the superior Pole
| Preop | 1 day after the operation |
P-value With preop |
1 week after the operation |
P-value with preop |
|
|---|---|---|---|---|---|
| Type 1 EBSLN | |||||
| Mean F0 (Hz) | 221.798 ± 41.011 | 219.543 ± 44.598 | 0.13 | 221.495 ± 43.555 | 0.85 |
|
Pitch → sigma (Hz) |
16.689 ± 17.246 | 17.599 ± 16.307 | 0.24 | 16.047 ± 16.524 | 0.21 |
| Jitter | 0.45%±0.14% | 0.47%±0.22% | 0.78 | 0.43%±0.14% | 0.23 |
|
Shimmer (dB) |
0.544 ± 0.213 | 0.547 ± 0.221 | 0.99 | 0.569 ± 0.193 | 0.18 |
| HNR (dB) | 19.899 ± 2.813 | 20.092 ± 3.535 | 0.47 | 19.280 ± 2.310 | 0.32 |
| Type 2a EBSLN | |||||
| Mean F0 (Hz) | 217.091 ± 52.281 | 213.899 ± 52.656 | 0.16 | 217.135 ± 51.343 | 0.87 |
|
Pitch → sigma (Hz) |
14.968 ± 12.350 | 16.037 ± 13.873 | 0.38 | 16.883 ± 13.402 | 0.05 |
| Jitter | 0.47%±0.24% | 0.44%±0.22% | 0.07 | 0.45%±0.23% | 0.08 |
|
Shimmer (dB) |
0.774 ± 0.488 | 0.741 ± 0.445 | 0.20 | 0.764 ± 0.448 | 0.95 |
| HNR (dB) | 17.589 ± 4.109 | 17.996 ± 5.188 | 0.50 | 17.782 ± 4.781 | 0.87 |
| Type 2b EBSLN | |||||
| Mean F0 (Hz) | 224.382 ± 50.325 | 218.374 ± 49.188 | < 0.01 | 220.714 ± 47.636 | 0.30 |
|
Pitch → sigma (Hz) |
16.642 ± 15.413 | 18.850 ± 15.895 | < 0.01 | 17.875 ± 16.561 | 0.21 |
| Jitter | 0.47%±0.17% | 0.44%±0.22% | 0.23 | 0.45%±0.23% | 0.60 |
|
Shimmer (dB) |
0.495 ± 0.216 | 0.456 ± 0.209 | 0.38 | 0.453 ± 0.209 | 0.47 |
| HNR (dB) | 19.710 ± 4.376 | 18.792 ± 4.044 | 0.04 | 19.142 ± 3.826 | 0.35 |
Discussion
In the present study which was done from October 2018 to November 2019 in 40 patients, EBSLN was identified in 95% (38/40) of the cases. The nerve was not visualized in 5% (2/40) cases. These findings are thus similar to the systemic meta-analysis of EBSLN identification rates by Cheruiyot [2]. According to a systematic meta-analysis study of 56 studies done by Cheruiyot et al., a total of 13,444 hemi larynx were studied and the overall pooled EBSLN identification rate was found out to be 89.24%. The rate was higher for cadaveric studies (95%) followed by intra-operative studies (86.99%). Intraoperative identification rates were higher when intraoperative nerve monitoring was used (95.9%) as compared to direct visual identification [2].
Numerous studies have been reported in the literature studying the incidence of EBSLN as per Cernea classification as given below in Table 8.
Table 8.
Correlation as per Cernea classification with other studies
| Study | Type 1 (%) | Type 2a (%) | Type 2b (%) | Not seen (%) |
|---|---|---|---|---|
| Cernea et al. 1992 [1] | 63.0 | 17.0 | 20.0 | 0 |
| Cernea et al. 1995 [3] | 23.0 | 15.0 | 54.0 | 8.0 |
| Aina et al. 2001 [4] | 16.1 | 51.8 | 24.8 | 7.3 |
| Bellantone et al. 2001 [5] | 58.6 | 19.6 | 10.2 | 11.6 |
| Ozlugedik et al. 2007 [6] | 22.5 | 60.0 | 17.5 | 0 |
| Mishra et al. 2007 [7] | 28.2 | 53.8 | 10.3 | 7.7 |
| Chuang et al. 2010 [8] | 16.2 | 39.5 | 38.3 | 6.0 |
| Pradeep et al. 2012 [9] | 71.5 | 16.8 | 7.2 | 4.5 |
| Athvale et al. 2013 [10] | 38.2 | 60.0 | 1.8 | 0 |
| Ravikumar et al. 2016 [11] | 17.1 | 57.2 | 18.8 | 6.9 |
| Menon et al. 2017 [12] | 29 | 61 | 10 | 0 |
| Ekhar et al. 2017 [13] | 34.88 | 52.33 | 12.79 | 0 |
| Present study | 27.5 | 25 | 42.5 | 5 |
In the present study of 40 cases, a total of 38 EBSLN were identified during surgery. The incidence of type 1 nerves was 27.5 which is similar to the findings of Mishra et al. (28.2%) and Menon et al. (29%) [7, 12].
Type 2a nerve was seen in 25% of cases in the current study which is similar to findings of Cernea et al. 17% (1992) and 15% (1995), Bellantone et al. 19.6%, and Pradeep et al. 16.8% [1, 3, 5, 9]. However, a few studies have a reported a higher incidence of type 2a nerves, Ozlugedik et al. 60%, Mishra et al. 53.8%, Chuang et al. 39.5%, Ravikumar et al. 57.2%, Menon et al. 61% and Ekhar et al. 52.33% [6, 7, 8, 11, 12, 13]. In the meta-analysis by Cheruiyot et al., the incidence of type 2a was highest (41.84%) followed by type 2b (19.18%) [2].
“ The incidence of Type 2b nerves was observed in 42.5% of cases, which aligns with findings from other studies that report prevalence ranging from 7.2–54%. Overall, the incidence of Type 2 or at-risk nerves was found to be 67.5%, consistent with several studies.” [2, 7, 8, 10, 12, 13].
Most of the studies described in the literature have described EBSLN anatomy in either cadaveric or patients with small-volume goiters. There have been few studies that described EBSLN anatomy as per Cernea classification in large goiters.
In a study by Ravikumar et al. of 456 cases, the incidence of type 2b nerves in large goiters (> 50mL) was 161/180 (89.4%) [11]. Cernea et al. in their study of 15 large goiters documented a higher incidence of type 2b nerves (54%) [3]. In a study of 112 cases by Pagedar et al., type 2b nerve was seen in 48.3% of large goiters [14]. In another study of 100 patients by Menon et al., the incidence of type 2a nerve in large goiters (> 20mL) was 28/39 (72%) and that of type 2b nerve was 6/39 (15%) [12]. Aina et al. in their study of 202 EBSLN documented an incidence of 51.3% of type 2b nerves in large goiters (> 100 g) [4]. In a study by Ekhar et al., type 2b nerve was present in 90.90% of large goiters (> 100 g), type 2a in 84.44% of large goiters, and type 1 in 63.33% of large goiters [13]. In a study of 72 neck dissections by Furlan et al., the incidence of ‘at-risk’ nerve was higher in large goiters [15].
In present study, in small goiters (< 20 mL), Type 1 EBSLN was seen in 11/24 patients (45.8%), type 2a in 10/24 patients (41.67%), type 2b in 3/24 patients (1.25%) and not seen in 2/24 patients (8.33%). However, in large goiters (> 20 mL), type 2b nerve was seen in 14/14 patients (100%).
In the present study, in patients who had type 1 anatomy of EBSLN the mean volume of thyroid gland 5.983 ± 3.245 mL, in patients with type 2a, was 13.687 ± 2.496 mL and in patients with type 2b anatomy of EBSLN, the mean volume of the thyroid gland was 43.976 ± 45.303 mL. This association was statistically significant (p < 0.001). These findings corroborate with the finding of a higher incidence of type 2b nerves in large goiters [2, 11–14, 16–18].
In the current study of 40 cases, the acoustic analysis parameters analyzed were mean pitch (Hz), pitch sigma (Hz), jitter (%), shimmer (dB), and HNR (dB). The mean pitch (F0) recorded pre-operatively (221.447 ± 45.981 Hz) when compared with F0 obtained on postoperative day 1 (217.283 ± 46.323 Hz) showed a statistically significant decrease (p < 0.01) which correlates with findings of Gour et al. and Lang et al. [19, 20]. F0 recovered on day 7 but it was still lower than pre-operative F0 (219.872 ± 45.096 Hz) and showed no statistically decrease when compared with preoperative value (p = 0.46) which correlates with findings of Santosh et al. [21] The comparison with different studies is given in Table 9 below.
Table 9.
Comparison of mean pitch (F0) among various studies
| Study | Pre-operative mean (Hz) | Post-operative mean (Hz) | p-value |
|---|---|---|---|
| Santosh et al. 2011 [21] |
Male- 120.01 ± 8.99 Female- 184.99 ± 38.29 |
Male- 117.45 ± 15. Female- 186.05 ± 26.93 |
> 0.05 |
| Gour et al. 2013 [19] | 425.76 ± 68.14 | 402.79 ± 65.27 | < 0.05 |
| Lang et al. 2015 [20] | 194.9 ± 34.9 | 188.0 ± 34.0 | 0.001 |
| Present study | 221.447 ± 45.981 |
217.283 ± 46.323 Hz Day 1 219.872 ± 45.096 Day 7 |
< 0.01 0.46 |
Pitch sigma increased significantly (p < 0.01) from pre-operative (16.791 ± 14.716 Hz) to postoperative day 1 recording (18.454 ± 15.088 Hz) and on day 7 recording (17.79 ± 15.315 Hz), pitch sigma improved and showed no statistical significance (p = 0.09).
These findings are consistent with the findings observed by Gour et al. and Lang et al. [19, 20].
In the present study, the pre-operative and post-operative acoustic voice parameters namely, mean pitch (F0), pitch sigma, jitter, shimmer, HNR were compared with pre-operative USG volume of the thyroid gland and it showed no statistically significant change and association with the postoperative values of the acoustic voice parameters. Thus, implying that the function of both EBSLN and cricothyroid muscle is not dependent on the volume of the thyroid gland. However, the voice of the patient will be affected if the volume of the thyroid gland is causing compression on the neck and occluding the airway by the mass effect when the patient lies down. Voice can also be affected if any malignancy of the thyroid gland invades the larynx and strap muscles.
No patient in the present study had complaints of voice change on postoperative day seven and the change in mean pitch (F0) and pitch sigma observed on day one returned to normal on day seven along with other acoustic voice analysis parameters. This implies that no patient in the present study had any injury to EBSLN intraoperatively. The possible reasons for the change in F0 and pitch sigma in the current study include the following:
Psychogenic causes: Emotional state of the patient, cooperation levels, postoperative pain, and behavioral patterns can affect voice and acoustic parameters. The patient in the postoperative period could be unwilling to exert to produce high pitch due to a fear of impacting the wound thus, decreasing the recorded pitch (F0) [22, 23].
Orotracheal injury: The most common method of securing the airway is orotracheal intubation. Success in achieving a secure airway for surgery requires a thorough understanding of various anatomical and pathological variations to prevent any inadvertent injury to the upper airway. Excessive manipulation of the airway during intubation, damage to vocal cords and arytenoids, and prolonged duration of surgery are known to affect the voice, especially by decreasing F0 [14, 22, 23, 24, 25].
Removal of thyroid gland drastically modifies the blood supply of neck including larynx which might lead to mucosal congestion and edema of cords thus lowering the pitch (F0) [5, 19, 26].
Strap muscles affect the glottic function and act as accessory muscles during inspiration. There is downward traction on the trachea with deep inspiration which causes traction over the cords leading to the abduction of vocal folds. The strap muscles are usually divided during thyroidectomy and their improper approximation during closure and removal of thyroid can lead to fixation of strap muscles to the larynx which hampers the traction provided by the strap muscles on the vocal folds thus, decreasing the overall tension in cords. This explains the lowered pitch obtained during acoustic voice analysis [23, 26–28].
In the reported literature it has been documented that as many as 30% suffer from temporary and around 14% suffer from a permanent change in voice due to the causes mentioned above [29–32].
Intra-operative damage to EBSLN can occur due to accidental nerve transection, nerve entrapment in the ligature applied away from the superior pole, cautery burn, ischemia & nerve stretching due to excessive traction to thyroid gland [25, 33]. To prevent this, the surgeon must be well versed with the anatomy of EBSLN and its variations along with the effect of the increased size of the thyroid gland on the nerve. However, in the literature various surgical techniques have been described as follows:
-
I.
The entire course of the nerve should be identified along with dissection of individual branches of the superior thyroid artery and vein at the upper pole of the thyroid. The vessels should then be ligated individually and as close to the upper pole of the gland [33, 34].
-
II.
In case the EBSLN is not identified during surgery, the ligature must be applied as close to the upper pole as possible [33].
-
III.
Intra-operative nerve monitoring should be used which is the best method to avoid injury to EBSLN [25, 33].
Limitations of the current study:
Low sample size.
Visual identification of EBSLN.
No use of intra-operative nerve monitoring.
Unavoidable patient factors during acoustic voice analysis like pain after surgery, patient’s apprehension, poor attention span in the post-operative period which could have led to a change in the voice of the patient while recording.
Conclusion
Acoustic voice analysis showed decreased pitch and increase in pitch sigma on postoperative day one which improved on day seven, which shows that there was no permanent injury to ESBLN during surgery. Acoustic voice analysis being a non-invasive objective procedure, can be used to diagnose EBSLN injury post-thyroidectomy. However, research with large sample size is required to conclusively study the diagnostic efficacy of acoustic voice analysis to diagnose injury to EBSLN.
Declarations
Ethical Approval
Authors also certify that the Institution ethical committee was taken prior to the study. “Compliance with Ethical Standards” is there in the present study.
Conflict of interest
No disclosure of potential conflicts of interest.
Consent to Participate
Research involving human participants but not the animals.
Informed Consent
Informed consent was taken from all the participants.
Footnotes
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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