The use of superior parathyroid gland as an anatomical landmark in identifying recurrent laryngeal nerve during total thyroidectomy: a prospective single-surgeon study

Injury to a recurrent laryngeal nerve (RLN) is a serious complication after thyroid surgery leading to ipsilateral vocal fold paralysis, hoarseness of voice, and swallowing problems, and is a main cause of malpractice associated with thyroid surgery [1, 2, 3, 4].

In this regard, the proper identification of RLN during thyroid surgery through sufficient knowledge of the surgical anatomy and the assistance of reliable anatomical landmarks along the course of the nerve is considered of critical importance, given that it minimizes the risk of injury [3, 5, 6, 7, 8]. Hence, the direct inspection and complete exposure of RLN during thyroidectomy using crucial anatomical landmarks have become the gold standard method for the preservation of RLN and to prevent inadvertent injury to the nerve [2, 5, 6, 7, 9].

Besides injury to the RLN, thyroidectomy is also potentially associated with complications related to injury to the superior laryngeal nerve, or the parathyroid glands, and thus a surgeon must have an in-depth understanding of the anatomy of both the thyroid and parathyroid glands to minimize the morbidity of the operation [10].

The superior parathyroid gland (SPG) has a relatively constant location and a more consistent relationship with RLN compared to the inferior parathyroid gland while sparing the parathyroid gland is a routine procedure during thyroidectomy [7, 10, 11, 12]. However, despite these advantages, the use of SPG as a potential landmark in localizing RLN during thyroidectomy has only been addressed by a few studies [11, 12].

This prospective single-surgeon study aimed to determine the anatomical position of RLN relative to SPG in total thyroidectomy patients and thereby to evaluate the utility of SPG as a landmark to locate RLN during thyroid surgery.

This study was conducted in accordance with the ethical principles stated in the Declaration of Helsinki and approved by the Clinical Research Ethics Committee of Uludag University (No. 2019-15/1). Written informed consent was obtained from all patients.

Assessments

Patient demographics (age, sex) and thyroidectomy indications (benign and malignant) were recorded in each patient. All thyroidectomy operations were carried out by the same surgeon (EG). Following the identification of the SPG, the RLN was exposed in the vicinity of the gland via careful dissection until its visualization, and then the natural distance between the RLN and SPG was preserved. Technically, the SPG was identified prior to the RLN. After SPG was identified, the RLN was carefully exposed in its vicinity and dissection was ceased to preserve the natural distance between the RLN and SPG. The relation of RLN to SPG was assessed based on the measurement of the natural distance (in millimeters) between the 2 structures (inferior border of the SPG and the intersection point of inferior thyroid artery and RLN) in a state of medial traction of the thyroid. The distance was categorized as 0–5 mm (category A), 6–10 mm (category B), and ≥11 mm (category C) (Fig. 1).

Fig. 1
The categories used for the assessment of the distance recorded between the recurrent laryngeal nerve and superior parathyroid gland.

• Click for larger image

This prospective series of total thyroidectomies revealed the identification of RLN in close proximity to SPG in the majority of patients. The RLN was identified within 5 mm of the SPG in 90.7% of cases, and within 1 mm of the SPG in 65.1% of cases. The RLN-to-SPG distance was 6–10 mm in 8.5 % of patients, and ≥11 mm in 0.7% of cases.

Similarly, in a prospective thyroidectomy series in 59 patients, the average distance between the RLN and SPG during thyroid surgery was found to be 5.03 mm (standard deviation, 1.79 mm), while the RLN was reported to lie within 5 mm of SPG in 83.1% of cases and within 6–10 mm in 15.3 % of cases [12].

Also, in a study assessing 103 consecutive thyroid lobectomies in 73 patients, the RLN was found in close proximity to the SPG, within 5 mm and 1 mm of the gland in 88.9% and 62.6% of cases, respectively [11]. The rates of permanent postoperative vocal fold paralysis were 2.5%, 12.5%, and 14.3% for the distance categories of 0–5 mm, 6–10 mm, and ≥11 mm, respectively [11].

Additionally, the height of the thyroid lobe and the presence of thyroid malignancy were reported to be positively associated with the distance between the 2 structures, with greater distance between RLN and SPG in cases of longer lobes (in thesuperior–inferior dimension) and cancerous lobes [11]. Hence, the preoperative assessment of lobe size and pathology is considered likely to provide patient-specific information to the surgeon regarding the appropriateness of using this approach in RLN identification [11].

In fact, in an analysis of a thyroidectomy-specific database involving 11,370 patients, the risk of RLN injury was found to be independently associated with total thyroidectomy (vs. lobectomy) and diagnosis of thyroid malignancy (vs. benign indications) [13].

In our series, all patients had total thyroidectomy which was performed due to diagnosis of malignancy in majority of cases, emphasizing the potentially increased risk of RLN injury and thus the further importance of accurate intraoperative RLN identification through reliable anatomical landmarks. The identification of RLN in close proximity to SPG in the majority of our patients, despite the presence of thyroid malignancy in almost 70%, seems notable in this regard, emphasizing the potential utility of SPG as a reliable landmark for RLN identification even in thyroidectomies due to malignant indications.

Given the likelihood of permanent injury to the nerve in up to 1.4% of thyroidectomies, even in expert hands, along with temporary paralysis occurring in up to 5.4% of cases, the proper identification of the nerve during surgery is considered to be of critical importance regarding its association with at least a fivefold decreased risk of RLN injury [13, 14, 15, 16, 17, 18, 19, 20]. Hence, our findings support that a consistent relationship exists between the RLN and the SPG, increasing the reliability of using SPG as a landmark to visualize and preserve RLN during thyroidectomy operations [11, 12]. This approach, which is already part of routine procedures for sparing the parathyroid gland in thyroidectomies, enables RLN identification without increasing operation time or morbidity risk [11]. Nonetheless, identification of the SPG is a prerequisite for this approach, which can be challenging in some cases and is possible only when the patient has an intact SPG [11].

In conclusion, this prospective single-surgeon thyroidectomy series study indicates the likelihood of localizing the RLN in close proximity to SPG during total thyroidectomy operations. Hence, the SPG can be used as a landmark to identify RLN, and as part of routine parathyroid-sparing thyroidectomy operations, it may represent a convenient complementary approach to minimize the risk of iatrogenic injury to RLN in patients with an intact SPG. The relevance of this approach in reducing the risk of RLN injury should be further investigated in large-scale prospective studies in the setting of thyroidectomy, particularly for malignant indications.