Risk factors for vocal cord palsy in low anterior cervical spine surgery: a retrospective cohort study

Introduction

Anterior cervical spine surgery is a common spinal operation performed for the management of degenerative, traumatic or oncological spinal conditions (1). Vocal cord palsy (VCP) or recurrent laryngeal nerve palsy (RLNP) is a known complication following this surgical procedure (2-4). This complication typically manifests in symptoms including hoarseness, persistent cough, dysphagia and aspiration (3). The reported incidence of symptomatic VCP following anterior cervical discectomy and fusion (ACDF) or anterior cervical corpectomy and fusion (ACCF) has varied in the literature, ranging from 0.07% to 11% (5-9). When routine laryngoscopy is performed following ACDF, including for asymptomatic patients, the incidence of VCP has been reported as high as 24.2%, which is especially interesting given degree of voice hoarseness does not necessarily indicate RLNP (2,3). This difference between symptomatic cases and overall incidence suggests many cases may be missed, highlighting the importance of detection and appropriate management as required. Studies have shown that VCP is typically temporary, with most symptoms resolving within days to weeks, although permanent VCP has been reported in less than 1% of cases (10-12).

The pathogenesis of RLNP has been postulated to be multi-factorial including direct ligation, neuropraxia secondary to compression or traction of the nerve, and post-operative oedema (13,14). The risk factors for VCP following ACDF are not entirely understood. Injury to the RLN related to endotracheal intubation has also been characterised by several studies and is the most well accept risk factor (15,16). One study found that lower endotracheal cuff pressures reduced the incidence of VCP from 6.5% to 1.3% (14,15). A systematic review by Tan et al. found that the evidence for other risk factors, such as reoperation or laterality, is limited (11).

Studies of the anatomical course of the recurrent laryngeal nerve (RLN) have suggested significance in the laterality and level of the incision, although this hypothesis has been met with contention. Cadaveric studies have found that the left RLN has a more predictable vertical course within the tracheoesophageal groove, and therefore may be at a lower risk of injury, while the right RLN has a shorter and more oblique course (16-18). In comparing both laterality and level of approach, it was found that the right RLN at the C7 level was exposed to the greatest level of stretch during retraction in a right sided approach (14). Currently no studies exist examining the risk of RLNP in patients operated on these specific levels. It is imperative to minimize the risk of this disabling postoperative complication given the individual patient impact as well as the overall healthcare burden. Our study aims to identify and stratify risk factors amongst this higher risk group, including the anatomic factor of distance between lowest operated disc level to sternum, which we used as a ’surrogate measure’ of anatomical course length of the RLN. We present this article in accordance with the STROBE reporting checklist (available at https://asj.amegroups.com/article/view/10.21037/asj-25-79/rc).

Methods

A retrospective cohort study of all consecutive patients who underwent an anterior cervical approach operation at a major tertiary centre (The Alfred Hospital) was conducted between January 2010 and December 2019. Patients were included if they met the following inclusion criteria: (I) age 18 or over; (II) underwent anterior approach cervical operations including both ACDF or ACCF for any indication; (III) lowest operative level C6/7 and/or C7/T1. The study centre is a level 1 trauma and tertiary spine referral institution in which both orthopaedic and neurosurgical spinal teams perform spinal operations in elective as well as emergent capacities. Therefore, case characteristics such as admission type (elective versus emergency), operative unit and surgeon, operative time (≥3 or <3 hours) and diagnosis type (trauma, degenerative, tumour or infective) were recorded. There were no exclusion criteria for these consecutively enrolled patients.

Patient demographics [age, gender, body mass index (BMI)], clinical variables (operation type, surgical levels, incision laterality, operation time), and surgical or medical complications were noted. A BMI threshold of 30 kg/m² was chosen to dichotomize the cohort in keeping with existing definitions of obesity (19). Surgical complications included surgical site infection, wound complication, instrumentation-related complication, return to theatre and vocal cord palsy. Vocal cord palsy was assessed through presence of new post-operative symptoms including dysphagia and dysphonia with or without confirmation through laryngoscopy by otolaryngology specialists.

Radiological review of pre-operative imaging was completed by two authors (M.H. and H.Q.L.) to quantify anatomical course length and hence potentially the degree of traction on the RLN. Anatomical course length was calculated by identifying midline sagittal imaging assessed using the tip of the C7 spinous process. The number of vertebral bodies was calculated between the sternal notch as seen on pre-operative computed tomography (CT) or magnetic resonance imaging (MRI) imaging, to the centre of disc of interest (lowest operative level). This distance from surgical disc to the sternum was thus measured in number of vertebral bodies. Figure 1 demonstrates this surrogate measure. No data was missing in this study.

Figure 1 Surrogate measure of anatomical course, calculated the number of vertebral bodies between the sternal notch as seen on pre-operative CT or MRI imaging, to the centre of disc of interest (lowest operative level), on midline sagittal imaging. CT, computed tomography; MRI, magnetic resonance imaging.

Statistical analysis

Univariate, bivariate and multivariate analysis was conducted examining demographic data, clinical variables and radiological variables. The Fisher exact test was used to analyse univariate categorical data. Comparison of groups with and without imaging was examined using the Mann-Whitney U tests. Binary logistic regression analysis was performed to identify independent predictors of vocal cord palsy. Stepwise multivariate analysis and logistical regression was conducted on selected variables based on its P value (<0.20, two-tailed) as well as prognostic factors defined in existing literature. A P value (two-tailed) of less than 0.05 was considered statistically significant. All statistical analyses were performed using STATA/IC version 14.2 (StataCorp, College Station, TX).

Ethical considerations

This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by Alfred Health Institutional Human Research Ethics Committee (No. 595/18) and individual consent for this retrospective analysis was waived.

Results

A total of 488 patients were identified who met inclusion criteria between January 2010 and December 2019. They had an average age of 50.2±15.1 years, with 62.6% (n=306) male patients. A proportion of 53.9% (n=263) were elective cases, compared to 46.1% (n=225) emergency cases. Symptomatic vocal cord palsy, defined as dysphagia and/or dysphonia and/or odynophagia and/or airway concerns, such as stridor occurred in 7.6% of patients (n=37), with 31 of these confirmed on laryngoscopy (P=0.12). The majority of operations were performed at the lowest operative level of C6/C7 (85.5%, n=417), whilst 14.5% were performed at a lowest operative level of C7/T1 (n=71) (Figure 2). Right sided incisions were more common than left sided approaches but there was no significant difference in the rate of vocal cord palsy (8.1% vs. 2.2%, P=0.12). There were 400 patients (82.0%) who underwent an ACDF, compared to 88 (18.0%) who underwent a corpectomy (P=0.14).

Figure 2 Flowchart demonstrating the number of included patients, and the incidence of vocal cord palsy depending on lowest operative level.

Demographic variables showed no statistically significant relation with vocal cord palsy rates (Table 1). In terms of clinical variables, bivariate analysis using Fisher’s exact test did not find statistical significance for elective or emergency admission type; degenerative, trauma, tumour or infection diagnosis leading to operation; operation type of ACDF or corpectomy, number of levels operated on, laterality of incision and reoperation (Table 1). Operation length ≥3 hours was associated with a higher rate of RLN palsy at 11.9% (20/168) compared to 5.3% (17/320) for operation length <3 hours (P=0.01). Bivariate analysis of operative level found significant higher rates of vocal cord palsy in patients operated at level C7/T1 compared to C6/C7 (21.1% vs. 5.3%, P<0.001).

Radiological analysis with surrogate assessment of anatomical course was not possible on 45 patients (9.2%) because pre-operative imaging did not capture the sternal notch (n=20) or imaging was not accessible (n=25). Unavailability of CT and/or MRI was due to imaging not being performed or not being accessible because of external radiology providers. Radiological measurements were performed on 443 patients (90.8%) of which 36 (8.1%) had postoperative symptomatic RLN palsy.

A greater proportion of patients with no imaging available were elective admissions with degenerative pathology. This patient group was more likely to have pre-operative imaging external to the health service (Table 2). Subgroup analysis in the group with identifiable radiological characteristics demonstrated similar findings to the total cohort analysis. Increased length of operation (≥3 hours) and lower operative level (C7/T1 compared to C6/7) were the only statistically significant risk factors for VCP (Table 3).

A higher risk of vocal cord palsy exists when the lowest operative level was closer to the sternal notch (Table 4). Where there was no vertebral body between sternal notch and lowest operative disc level, vocal cord palsy occurred in 100.0% (1/1). With one vertebral body distance, this became 28.6% (4/14) and with two vertebral bodies distance 13.9% (15/108). Three vertebral bodies gave a percentage reduction to 4.9% (11/224) and four vertebral bodies distance had a vocal cord palsy percentage of 5.4% (5/92). None had a VCP when there are five vertebral bodies between sternal notch and lowest operative level (P=0.88).

Stepwise multivariate analysis was conducted on all variables (Table 5). Odds ratios (ORs) with a significant 95% confidence interval (CI) are shown for increased length of operation, lower operative level and higher number of vertebral bodies between sternal notch and lowest operative level. All other variables including reoperation and incision laterality did not show a significant correlation to RLN palsy.

Discussion

Background

Vocal cord palsy secondary to RLN injury during an anterior cervical spine surgical approach represents a debilitating complication (5,6). The majority of symptoms resolved within 1 week post-operatively as a transient phenomenon and Morpeth et al. noted that 80% of VCP following ACDF improve after 12 months (7). Unlike thyroid surgery in which there is direct exposure of the nerve beneath Zuckerkandl’s tubercle and its associated ligament of Berry, the purported mechanism of RLN injury during the archetypal Smith-Robinson approach remains less distinctive (8). Initially, Bulger et al. attempted to explain injury during these classic anterior cervical spine approaches with the concept that in 1% of cases the RLN arises directly from the vagus nerve as a non-RLN passing directly to the cricothyroid joint without caudal or mediastinal descent (8). This is without the expected recurrent course usually beneath the subclavian artery on the right or the ligamentum arteriosum on the left (9). Indeed, in these patients the right sided non-RLN is associated with an aberrant retro-oesophageal subclavian artery (8). For this reason, placement of the retractor blades in the plane between trachea and oesophagus medially and carotid sheath laterally leads to undue stretch on the non-RLN. This is reasonably supported by pathophysiological studies conducted by Wall et al. that stretch of a nerve beyond 15% of its base length leads to irreversible damage in animal models (10).

However, Tan et al. and Jung et al. noted after analysis of 34 studies that the incidence of VCP ranged from 2.3–24.2% which is significantly higher than the incidence of non-RLNs alone (11,20). As such, dos Santos Menezes Siqueira et al. identified other possible anatomical factors which may increase the rate of RLN injury (12). This included extra laryngeal divergence, thickening in the elderly and a more tortuous ascending recurrent course (12). Importantly, these are patient specific factors which are difficult to identify preoperatively in order to caution surgeons in taking greater care during surgery. Hence, the hypothesis for our study was that a shorter surrogate anatomical course length increases the risk of vocal cord palsy under the proposition that the closer and shorter the surgical approach is to the hook of the right RLN around the subclavian artery, the greater traction would likely be applied. Our assessment showed significant higher rates of vocal cord palsy with a smaller number of vertebral body levels from the sternal notch. A greater distance between the lowest operative level and the sternal notch was a protective factor against postoperative incidence of VCP (OR 0.55, 95% CI: 0.32–0.92, P=0.02). This suggests that surgeons should carefully examine individual patient anatomy particularly in relation to the neck length as this is a new identified risk factor.

Innovation of study

Furthermore, this study also specifically examines the subgroup of patients undergoing lower anterior cervical spine surgery, defined as having a lowest operative level of C6/7 or C7/T1 and found that this led to a higher rate of postoperative VCP (P<0.05). Chen et al. noted that the level of C6/7 was the only risk factor for VCP and similarly proposed that the right RLN is shorter with a more oblique course accounting for this finding (13). Weisberg et al. investigated this asymmetry of the RLNs in a cadaver model between the C4 and C7 vertebral bodies (14). Whilst the left RLN had sufficient redundancy as a consequence of its looping course, the right RLN had minimal slack and was not protected in the trachea-oesophageal groove (14). In fact, a right sided approach to the C7 vertebral body results in 12% of stretch with 3 cm and 24% with 4 cm of Cloward retraction respectively (14). It is evident that the C6/7 level or lower subaxial cervical spine levels carry greater risk, with Nikandish et al. even describing a case of bilateral VCP due to surgery at this level raising caution for surgeons (15-18,21-23). Ultimately, the level of surgery is critical with Razfar et al. concluding that C6/7 was a risk factor and Strohl et al. finding C7/T1 also a significant hazard for post=operative VCP (OR 5.5, 95% CI: 1.3–23, P=0.02) (24,25). Conversely, higher levels of surgery in the cervical spine were protective (25). Our novel study confirms these two levels as risk factors and provides a surrogate measure for predicting rate of post-operative VCP not only reliant upon absolute vertebral level but relative length relevant to individual patient anatomy.

Pathophysiology

Given the longer recurrent and thus more lax course of the left than the right RLN, the laterality of side of approach has been investigated. Jung et al. drew attention to this by highlighting a reduction in the incidence of VCP from 13.3% to 6.5% when a left-sided approach was used (26). However, these authors are in the minority and contradicted by Chen et al. who conducted a retrospective cohort study of 1,895 undergoing ACDF all from the right-side with only 9 patients suffering a prolonged VCP lasting over 3 months (13). In support of this, Beutler et al. and Kilburg et al. found no correlation in side of approach to rates of VCAP palsy (5,27). Moreover, Gokaslan et al. conducted a large multicentre cohort study concluding that 79% of VCP actually occurred after a left-sided approach (2). This was consistent with Heeneman et al. who found that there was a higher rate of permanent paralysis for left sided approaches (28). In our study, the majority of surgeons continue to employ a right sided approach, including in 90.7% (402/443) of our patients in this study given we determined incision laterality was not found to have a statistically significant impact on rates of post-operative VCP.

RLNP—other risk factors

Beyond preoperative consideration of level involved, there has been increasing awareness on indirect compression of the RLN within the trachea-oesophageal groove from endotracheal tube (ETT) pressure. Oh et al. conducted a meta-analysis and found that the number of levels and indirectly presumably the length of the operation during which the ETT was inflated itself was not a predictor of RLN palsy rates (OR 1.04, 95% CI: 0.56–1.95, P=0.89) (29). Apfelbaum et al. examined 900 consecutive patients over a 12-year interval with 30 cases of RLN palsy identified (30). After the initial 250 cases, ETT cuff pressure and release was used which resulted in a lower rate of RLN temporary paralysis (1.9% vs. 6.4%, P<0.01) (30). Importantly, release of cuff pressure was associated with a significantly more central position of the larynx and thus reduced stretch with a clinical correlation and reduction in incidence of VCP. In a holistic sense, the robustness or frailty of the individual patient as well as the burden of the surgery being considered need to be carefully evaluated in a delicate equilibrium (31-38).

Reoperation did not prove to be an independent risk factor in our study. Vocal cord palsy occurred in 13.6% (3/22) of the total cohort of patients examined who had revision surgery (15.8% in the cohort with imaging, 3/19). This is in concordance to the risk found by Beutler et al. (9.5%) and Coric et al. (10.5%) (5,39). There is still likely much that is understood regarding the exact pathophysiology of RLNP palsy. Yerneni et al. reported an unusual case of delayed VCP in which a patient awoke completely intact, but developed RLN palsy day 3 post-operatively (40). This potently refutes that traction or stretch injury is the entire mechanism and raises suspicion that there may be small vessel ischaemia, vasospasm of vasa vasorum or even activation or viral resurgence as other hypotheses that require further investigation (40).

Strengths

We conducted a robust retrospective cohort study at a high-volume tertiary centre with specific strict inclusion criteria which lends our statistically significant findings a sense of internal validity. Our findings that lower cervical levels of C6/7 and C7/T1 being risk factors for VCP palsy are consistent with contemporary anatomical, pathophysiological and existing clinical findings. What is unique is that we propose a novel indirect preoperative measure of this, namely distance between lowest operative level to the sternal notch, as a surrogate measure of predicting post-operative risk of RLN injury and VCP palsy risk.

Limitations

Limitations of this study include its retrospective study design which poses the risk of selection bias. Furthermore, this study also only analyzes a single centre’s large dataset with a strong sense of internal validity but not external validity. A larger randomized prospective study performed across multiple centres would enable readers to draw a greater sense of external validity to the conclusions.

Conclusions

Vocal cord palsy is a significant cause of morbidity in anterior cervical spine surgery. Our study identified that a lower operative level with subsequent increased traction on the more oblique and less recurrent right sided laryngeal nerve increases the risk of postoperative vocal cord palsy. A novel preoperative risk factor easily identified on preoperative imaging as distance between lowest operative disc level and the sternal notch is a critical measure that clinicians and utilize to risk stratify patients.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://asj.amegroups.com/article/view/10.21037/asj-25-79/rc

Data Sharing Statement: Available at https://asj.amegroups.com/article/view/10.21037/asj-25-79/dss

Peer Review File: Available at https://asj.amegroups.com/article/view/10.21037/asj-25-79/prf

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://asj.amegroups.com/article/view/10.21037/asj-25-79/coif). B.T.S.K. serves as an unpaid editorial board member of AME Surgical Journal from May 2024 to June 2026. The other authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by Alfred Health Institutional Human Research Ethics Committee (No. 595/18) and individual consent for this retrospective analysis was waived.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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