A Novel Gel Pad Laryngeal Ultrasound for Vocal Cord Evaluation

Introduction

Vocal cord palsy (VCP) is an important complication of thyroid and parathyroid surgery affecting the patient's quality of life (1). Direct laryngoscopy (DL) is the gold standard for vocal cord (VC) assessment (2). Although laryngoscopy is safe, it is an invasive procedure and can cause patient discomfort and fear (3).

Laryngeal ultrasound (LUS) is a non-invasive method of VC evaluation in patients with risk of VCP (4,5). However, the previously described LUS reportedly has a high failure rate of VC visualization in male patients (6). In male patients, the thyroid cartilage has a tendency for a sharp angle and diffuse calcification. The sharp angle of the thyroid cartilage does not allow close contact of the ultrasound (US) transducer with the skin, and this prevents simultaneous evaluation of both VCs. Diffuse calcification of the thyroid cartilage blocks penetration of US through the thyroid cartilage. A novel gel pad LUS was devised to overcome these limitations. This new method uses a gel pad, mainly composed of water, between the US transducer and the skin to ensure tight contact and to enhance US penetration (7).

Patients and Methods

Methods

The LUS techniques have been described in previous studies (4,5). During the LUS assessment, the patient was positioned flat on the bed with the neck slightly extended. After applying ample gel over the anterior neck, an US transducer was placed transversely over the middle portion of the thyroid cartilage, and the area was scanned until both true and false VCs were visualized. The three laryngeal landmarks-arytenoids (ARs), true cords (TCs), and false cords (FCs) were used for VC evaluation (Fig. 1). A previous study has proved that each LUS landmark had similar reliability and diagnostic accuracy (5). Identifying all three LUS landmarks was not mandatory, and visualizing normal movement in one of the LUS landmarks was sufficient to exclude VCP. Patients were instructed to perform all three maneuvers (breathing, phonation, and Valsalva) (8). The movement of the VCs was assessed during the evaluation. The extent of movement of the VC was graded from I to III: grade I, full or normal symmetric movement; grade II, impaired or decreased movement; and grade III, no movement. After conventional LUS, a gel pad was placed on the patient's neck skin, and an US transducer was positioned on the gel pad to ensure tight contact and to enhance US penetration.

OPEN IN VIEWER

FIG. 1.

(A) Conventional laryngeal ultrasound (LUS) view of normal vocal cords (VCs). All three VC landmarks (TCs, true cords; FCs, false cords; ARs, arytenoids) are visible. (B) Diffuse calcification of the thyroid cartilage blocks the penetration of ultrasound (US). All three VC landmarks are invisible. (C) Gel pad LUS. (D) Gel pad LUS view. FCs are visible through the calcified thyroid cartilage.

Before or after the LUS, the patient was directed to the other center where DL was performed by an experienced surgeon who was unaware of the patient's LUS findings. Using a grading system similar to the LUS, the extent of VC movement on DL was graded from I to III. Grade II or III on DL was defined as VCP. The patients with VCP were followed up at the outpatient department repeatedly until the recovery from VCP.

Student's t-test using IBM SPSS Statistics for Windows v22 (IBM Corp., Armonk, NY) was used to compare the VC visualization rate of each evaluation method. A difference with a p-value of <0.05 was regarded as statistically significant.

Results

A total of 482 consecutive VC examinations were evaluated. Table 1 shows the demographics, indications, extent of the surgery, and the various operative approaches, including 239 (86.0%) conventional open cervical thyroidectomies, and 26 (9.4%) robotic and 13 (4.7%) endoscopic bilateral axillo-breast approach (BABA) thyroidectomies. Suspicion of thyroid cancer was the most common indication (91.7%) for surgery. There were four cases of parathyroidectomy and 13 cases of neck dissection. The median age at operation was 48 years.

The 482 VC examinations revealed that 51 (10.6%; seven male) patients had VCP on DL, and 91 patients had diffuse thyroid cartilage calcification interrupting LUS. On LUS, 27 patients had grade III findings, and 26 patients had grade II findings (Table 1). All VCP cases were unilateral. The conventional LUS and gel pad LUS methods had 93.4% and 99.0% visualization rate, 98.0% and 98.0% sensitivity each, and 99.7% and 99.8% specificity, respectively (Tables 2 and 3). There were three false-positive cases and one false-negative case with both LUS methods.

Table 4 presents the visualization rates of VC landmarks by each LUS method. Visualization rates of laryngeal landmarks by conventional LUS compared with gel pad LUS were 93.4% (450) versus 99.0% (477) for FC, 72.6% (350) versus 83.4% (402) for TC, and 84.6% (408) versus 91.9% (443) for AR, respectively. FC was the most visible landmark and TC the least visible landmark by both conventional LUS and gel pad LUS. Gel pad LUS showed significantly higher visualization rates of all three VC landmarks than conventional LUS (p < 0.001).

Discussion

VCP has the maximal medicolegal and cost implications in thyroid and parathyroid surgery (9). The new 2015 American Thyroid Association Guidelines placed an emphasis on perioperative VC evaluation, even in patients without alterations in their voice (1). Nevertheless, there are still ongoing controversies about routine VC evaluation. DL, which is the gold standard for VC evaluation, frequently causes unnecessary discomfort and gag reflex in patients, especially in the immediate postoperative period. Since the introduction of LUS in 1992 (10), LUS has improved with the advancement in US function. Wong et al. compared the accuracy of LUS and DL in order to investigate the potentiality of LUS as a VC evaluation tool (4,5). LUS can be performed pre- and postoperatively simultaneously during thyroid US. Hence, it has its own unique benefit of reducing the additional medical cost and avoiding patient discomfort caused by DL (11,12).

In the early period, LUS was limited to female patients because VCs in male patients are difficult to visualize appropriately by LUS due to the prominent protrusion and frequent diffuse calcification of the thyroid cartilage, which blocks US penetration (6). Although a novel attempt was made to improve VC visualization in male patients using a lateral approach LUS in a previous study, this lateral approach LUS also had its own limitation of nonsynchronous two-step evaluation of both VCs (12). The novel gel pad LUS described in this study can evaluate VCs in male patients by a one-step procedure using a gel pad to improve US probe contact and to enhance US penetration. The US gel pad was originally developed as an effective medium for scanning irregular surface anatomy such as the musculoskeletal system (7). The US gel pad was applied to LUS for the first time to improve VC visualization. The sonographic view with gel pad LUS was the same as that with conventional LUS, except for the hypoechoic gel pad in the upper screen and a more hyperechoic VC view induced by posterior enhancement of watery content of the gel pad.

The accuracy of LUS is still controversial. Although a previous study and the current study performed in Asia demonstrated high accuracy of LUS (4,5,12), another study performed in the United States reported comparatively poor accuracy of LUS (13). This discrepancy might be due to the difference in body habitus of different races/ethnicities or differences in the skills of the assessor performing LUS. Wong et al. reported that surgeons needed LUS experience of about 40 cases to perform LUS accurately (14). Other well-trained LUS assessors in the United States reported a high accuracy of LUS and demonstrated their LUS skills at the 2014 American Association of Endocrine Surgeons Annual Meeting (15). Therefore, it can be assumed that LUS is a reliable VC evaluation tool based on the currently available evidence.

There were three false-positive cases and one false-negative case with both LUS methods in the current study. All of these false positive and negative cases were of mild grade II VCP (VC paresis), which may cause inter-observer differences in any other VC evaluation method. There was no case of grade III VCP (VC paralysis) among the false positive and negative cases. This mild inter-observer variability in screening tools is considered acceptable.

The proportion of VCP (10.6%) in the current study is different from the complication rate. The patients with VCP were followed repeatedly until resolution. Therefore, VCP findings were counted as multiple observations. There were two cases of permanent VCP after cutting of the recurrent laryngeal nerve invaded by the tumor. Except for these cases, all patients with VCP recovered within three months. All five patients whose VCs were not visualized by LUS were male patients with severe calcification of the thyroid cartilage, and all of them had normal VCs on DL evaluation.

In conclusion, the novel gel pad LUS method significantly enhances the visualization of VCs in patients who have diffuse thyroid cartilage calcification interrupting LUS and, therefore, the overall efficacy of LUS as a perioperative diagnostic tool for VCP.