Anatomical variation of the thyroid gland – Levator glandulae thyroideae

Surgical procedures are usually the treatment of choice when managing severe neck and thyroid-related pathologies; however, they become complicated by damage to the parathyroid glands, the external branch of superior laryngeal nerves, inferior laryngeal nerves, and hematoma due to vascular injury and the chance of residual thyroid tissue being left in case of cancer and Graves’ disease, and the presence of anatomical variations. Therefore, prior knowledge of the incidence of anatomic anomalies would ensure avoidance of such complications. In this study, we describe the presence of a levator glandulae thyroideae that could misguide surgeons during surgical procedures.


INTRODUCTION
The thyroid gland is a butterfly-shaped endocrine organ located in the anterior compartment of the neck at the level C5 to T1 (1). It consists of two lateral lobes connected by a narrow bridge known as the isthmus. The thyroid gland is one of the most important endocrine organs and plays a significant role in regulating metabolism, blood calcium levels, growth, and development (1,2). The size, shape, and structure of the thyroid gland vary among individuals based on gender, race, and age. Organogenesis of the thyroid gland occurs first in comparison to other endocrine organs during the development of the embryo predisposing it to a wide range of anomalies such as the presence of pyramidal lobe and levator glandulae thyroideae (LGT), a fibromuscular band (3,4). Other anomalies involving the thyroid gland development include agenesis/absence of isthmus and persistence of the thyroglossal ducts (1,3). Among these anomalies, the persistence of thyroglossal ducts and the presence of a pyramidal lobe are common, whereas agenesis of isthmus and presence of LGT are rare and only a few cases have been reported of such anomaly in the scientific literature (5).
LGT is a fibromuscular band that extends from the pyramidal lobe in most cases and attaches to the hyoid bone located superiorly.
In the case being discussed, we describe a hyopyramidalis LGT that was discovered during routine cadaveric dissection of head and neck region. In the literature, there have been other researchers that have reported a LGT originating from different sites (Table 1) (1,4,6,7,8,9).

CASE REPORT
During routine dissection of the neck of a 70-yearold male Caucasian cadaver, at the Department of Anatomy of the University of Medicine and Health Sciences, St. Kitts and Nevis, a LGT was identified. It was dissected carefully to look for any further attachments and innervations above and below it. A thin band of muscular fibers stretched from the upper middle border of the isthmus of the thyroid gland to the body of the hyoid bone ( Figure 1). This type of muscle fibers would fall into hyopyramidalis classification of LGT as the fibers were found to be extending from the pyramidal lobe and attached to the hyoid bone located above. No additional abnormalities were found on the left and right lobes of the thyroid gland.

DISCUSSION
Thyroid diseases are a common health disorder among endocrinopathies present in the world population. Most of these diseases such as goiter, thyrotoxicosis, adenoma, and carcinoma are usually associated with enlargement of the gland and require medical and surgical intervention (4).
Our cadaver presented with the LGT muscle arising from the superior border of the isthmus of the thyroid LGT classification according to Mori (1964) (8,9) FIGURE 1. The levator glandulae thyroideae is a fibromuscular band or accessory muscle that originates from the isthmus of the thyroid gland and attaches to the body of the hyoid bone (hyopyramidalis (8,9)), as seen in these images.
LGT gland and attaching to the body of the hyoid bone. The presence of this accessory muscle gains importance in the pathologies which are related to the thyroid gland and their treatment modalities due to its proximity to the gland. Its occurrence is rare and only very few cases have been reported in the medical literature (1). Due to anatomical variations in the muscle tissues encompassing the thyroid organ, it is possible to cause iatrogenic wounds during neck and thyroid surgeries. In a study conducted to observe the thyroid glands of cadavers for the presence of a LGT, approximately 18.75% of the sample population were found to have LGT (10). The variations of the LGT can be associated with embryological disruption or anomalies. One such example of this is the failure of the thyroid gland to descend correctly and regression of the thyroglossal duct (3,11). This type of rare finding is essential to identify and acknowledge during surgical procedures carefully.

Eisler et al. conducted an extensive study regarding the innervation of LGT and concluded that the
LGT must be innervated by either the ansa cervicalis or through the vagus nerve. Loukas et. al reported (2008) that their cadaver's LGT was innervated by a branch of the ansa cervicalis (8). However, according to a recent case report conducted by Chaudhary et al. (2013), LGT may have different origins regarding its innervation. Chaudhary et al. concluded, after a close examination of their cadaver in the study, that the LGT was innervated by branches of the external laryngeal nerve (1).
Loukas et al. discussed the possible embryological origin of the LGT depends on the location it must have derived from. He reported the following possible sites he found in literature: The cricothyroid muscle, the inferior pharyngeal constrictor muscle, mastoid process, hyoid bone, thyroid cartilage, and infrahyoid muscles, the last from which his cadaver's LGT was derived from according to its nerve supply.
Since the thyroid gland is a highly vascularized structure, surgeons must be prepared to find this anatomical variation while carefully exploring to ensure safe surgical procedures, such as thyroidectomies (7,10). Therefore, it is essential to have a thorough understanding of the anterior cervical region and its anatomical anomalies to avoid any major complications in head and neck surgeries.

CONCLUSION
A rare anatomical variation associated with the thyroid gland is the presence of a LGT. We presented a case of LGT during routine cadaveric dissection. We also demonstrate the developmental and surgical implication of the anomaly and the importance of having a thorough knowledge of anatomical variations to handle related clinical interventions with confidence.