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INTRODUCTION
The omohyoid muscle is an infrahyoid muscle and consists of two bellies united by an intermediate tendon. This muscle is an important landmark in the neck because it divides the anterior and posterior cervical triangles into smaller triangles.1 It passes behind the sternocleidomastoid muscle and lies superficial to the internal jugular vein, the brachial plexus, phrenic nerve, and transverse cervical artery and vein.1-3 Many studies reported variations in the omohyoid muscle and referred to the duplicated omohyoid muscle as a rare variation of the omohyoid muscle.1,2,4-6
The levator glandulae thyroideae muscle has been reported to occur infrequently and is described as an accessory muscle.6 This muscle has been reported by some authors and the function and embryological origin of this muscle have been the subject of debate since the first report by Winslow in 1743.7 In 1964, Mori classified this muscle into five types based on their origin and insertion.8,9
Although there is a high frequency of variation in the cervical region, the duplicated omohyoid muscle combined with the occurrence of the levator glandulae thyroideae muscle is rare.2,5 This study reported rare variations in the infrahyoid muscle group and classified the variations according to previous studies.
CASE REPORT
This variation was observed a during routine dissection of a 51-year-old Korean female. We observed a duplicated omohyoid muscle on the right side and the levator glandulae thyroideae muscle on the left side (Fig. 1). The classification of the omohyoid muscle by Loth is based on forms, intermediate tendon, attachment, and innervation.6 Mori classified the levator glandulae thyroideae muscle into five types according to their origin and insertion.8,9
The superiorly positioned omohoyoid muscle showed a typical appearance of the normal superior and inferior bellies with the intermediate tendon in between. The inferiorly positioned muscle was similar to the superiorly positioned muscle which was united with the sternohyoid muscle at the medial part of the clavicle. The classification of the superior omohyoid muscle by Loth was as follows: 1) omohyoid muscle is present and has an intermediate tendon (Type V); 2) incomplete intermediate tendon but with no constriction of the muscle belly at the site of the tendon (Type II); 3) attachment to the superior border of the scapula (Type IV); and 4) to be supplied by the ansa cervicalis. The location of the attachment of the inferior omohyoid muscle to the scapula was the same as in the case of the superior omohyoid muscle. The inferior omohyoid muscle did not have an intermediate tendon and the innervated nerve was the ansa cervicalis.
The levator glandulae thyroideae muscle originated superiorly from the inferior border of the thyroid cartilage and was inserted inferiorly on the sheath of the thyroid gland, particularly the isthmus. This finding would be best categorized as thyreoglandularis according to Mori's classification. This muscle measures one square centimeter on each side.
DISCUSSION
Anderson has theorized that the origin of the superior belly of the omohyoid muscle differs from that of the inferior belly.1,2,10 For this reason, several authors have reported variations in the omohyoid muscle.1,2,5,10 The variations in the superior belly are more common than those in the inferior belly. The action of the omohyoid muscle depresses the hyoid bone after it has been elevated, and it has been suggested that this muscle tenses the lower part of the deep cervical fascia in prolonged inspiratory efforts, reducing the tendency for soft parts to be sucked inward.3 Some reported that the omohyoid muscle is activated during depression of the mandible and head rotation. In addition, the fibrosis and contracture of a unilateral omohyoid muscle has been found to cause torticollis.11 Others described the omohyoid muscle as involved in the dilation of the external and internal jugular veins during the opening of the mouth12 and would then presumably facilitate venous return to the heart.10 Recently, this muscle has been used as a landmark for a feasible study that used an endoscope for exploration of the brachial plexus,13 for the treatment of a bowed vocal fold using the transposition of this muscle, and for the restoration of vocal cord abduction.14,15 In this case, superior and inferior omohyoid muscles may play a role in the depression of the hyoid bone with the sternohyoid muscle.
The levator glandulae thyroideae muscle is described as an occasionally unpaired or paired muscle that extends from the hyoid bone to the isthmus of the thyroid gland, more frequently on the left side.8 After the first report of the levator glandulae thyroideae muscle in 1743,4 Lehr indicated that the prevalence of this muscle is one in 203 cases (0.49%),9 and Watanabe and Suda16 reported a prevalence of this muscle in six out of seven cadavers (85.7%). Harjeet, et al.17 determined the prevalence of the levator glandulae thyroideae muscle to be 19.8%. It is believed that the function of this muscle is to elevate the thyroid gland.18 The embryologic origin of this muscle has been greatly debated. Some authors suggested that the elevator glandulae thyroideae muscle is an embryologic derivation of the cricothyroid muscle.7 Eisler reported that this muscle is derived from the cricothyroid muscle, thyrohyoid muscle, and inferior constrictor of the pharynx.19 Watanabe and Suda17 proposed that the presence of this muscle is entirely genetic and it seems that the levator glandulae thyroideae muscle presented here is a derivative of the infrahyoid muscles. In this case, we think that the embryologic origin of the levator glandulae thyroideae muscle may be similar to that of the omohyoid muscle.
Overall, the variations of the omohyoid muscle and the levator glandulae thryoideae muscle are derived from the infrahyoid muscle group and are clinically significant.8,11 Hence, correct information about muscular variation is important for avoiding iatrogenic injuries when performing surgery or examining the neck region. The need to understand muscular variation is of greater importance than before because of the increased number of endoscopic surgeries and the development of diagnosis using images.
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