Introduction

The anterior neck region is made up of many structures that are crucial in maintaining essential functions for life. The skin is the most superficial organ in the neck region, and it functions as a barrier to the underlying structures. The fat in the neck region acts as cushioning for the surrounding structures. As for the anterior neck muscles, they manifest movements to the head and neck. The muscles in the anterior neck region can further separate into the suprahyoid muscles and the infrahyoid muscles. The hyoid bone demarcates these muscles into a superior group and an inferior group. Contraction of the suprahyoid muscles will elevate the hyoid bone while contraction of the infrahyoid muscles will depress the hyoid bone. These muscles contract in the different phases of speaking, chewing, and swallowing. But both the suprahyoid muscles and the infrahyoid muscles also act as a barrier to protect the trachea, thyroid gland, and the esophagus.

One of the muscles grouped into the infrahyoid muscles is the sternohyoid muscle. The first root word of sternohyoid is "sterno," this equates to the sternum while the last root word is "hyoid," which references the hyoid bone. The simplicity of the name of this muscle is that its name explains the regions that it originates from and to which it attaches. As for the actions of the sternohyoid muscle, it contributes to the depression of the hyoid bone.

Structure and Function

In the neck region, there is a group of muscles located inferior to the hyoid bone called the infrahyoid muscles. This group of muscles contains the omohyoid muscle, the sternohyoid muscle, the sternothyroid muscle, and the thyrohyoid muscle. Due to the origins and attachments of the infrahyoid muscles, they function to depress the hyoid bone when they contract. This group of muscles also contribute to the protection of the trachea, esophagus, blood vessels, and thyroid gland.

As for the sternohyoid muscle, it is a flat muscle located on both sides of the neck. This muscle originated from the medial edge of the clavicle bone, sternoclavicular ligament, and posterior side of the manubrium. The sternohyoid muscle then ascends the neck and attaches to the body of the hyoid bone. The sternohyoid muscle also has a continuation muscle that attaches the thyroid to the hyoid bone via the thyrohyoid muscle. The primary function of the sternohyoid muscle is to work in sync with the other infrahyoid muscles to depress the hyoid bone. The hyoid bone is depressed during jaw opening for speak, mastication, and swallowing. The sternohyoid muscle also participates as a minor contributor to head motions.[1] 

Embryology

Most muscle tissue in the human body derives from the mesenchymal tissue of the mesodermal germ layer. The mesenchymal tissue will differentiate into muscles, blood vessels, and connective tissue. The sternohyoid muscle will form from the mesenchymal tissue in the neck region during fetal development.

Muscle embryology is a process called myogenesis. This process involves the myoblast cells derived from the mesodermal germ layer. These myoblast cells will elongate and fuse to form muscle fibers called myotubes. This induction of this process occurs via fibroblast growth factor (FGF). The myotubes will grow and proliferate until they terminally differentiate into myocytes. The myocytes will be the muscle cell that is present in all types of muscles.

Blood Supply and Lymphatics

The external carotid artery is the vessel that perfuses the structures in the face and neck. The external carotid artery adequately perfumes the head, face, and neck through several arterial branches. One of the branches formed is the superior thyroid artery. After the superior thyroid artery branches from the external carotid artery. It descends towards the thyroid gland. As the superior thyroid artery makes its way to the thyroid gland, it will perfuse the structures with which it comes in contact. One of the structures that the superior thyroid artery passes is the sternohyoid muscle. The sternohyoid muscle will receive its arterial blood supply from the superior thyroid artery.[1][2][3]

The lymphatic drainage of the sternohyoid muscles divides into two separate paths since its a paired muscle. The right sternohyoid muscle will drain lymph fluid into the supraclavicular or cervical lymph nodes while the left sternohyoid muscle drains its lymph into the left supraclavicular or cervical lymph nodes. The right supraclavicular and cervical lymph nodes will then drain the lymph into the right lymphatic duct. The left supraclavicular and cervical lymph nodes will drain the lymph into the thoracic duct. Both the right lymphatic duct and the thoracic duct will drain the lymph back into the central circulation.[4]

Nerves

The nerve innervation to the sternohyoid muscle derives from the first, second, and third cervical nerve roots (C1-C3). These nerve roots form a plexus called the ansa cervicalis. The ansa cervicalis provides motor innervation to the sternohyoid muscle. The ansa cervicalis nerve also innervates the omohyoid muscle, the sternohyoid muscle, and the thyrohyoid muscle.[5]

Muscles

The sternohyoid muscle is a part of a group of muscles called the infrahyoid muscles. This group of muscles contains four main muscles: the omohyoid muscle, the sternohyoid muscle, the sternothyroid muscle, and the thyrohyoid muscle. The primary function of this group of muscles is to depress the hyoid bone during speech and swallowing. 

Physiologic Variants

The sternohyoid muscle may vary in size, length, and thickness in different individuals. The origins and attachments of the sternohyoid may also vary slightly. In some cases, the sternohyoid was found to be one muscle instead of a paired muscle. The sternohyoid muscle has also been found to originate from the omohyoid muscle's tendon instead of the sternum. Interestingly, some individuals completely lack the presence of the sternohyoid muscle. 

Surgical Considerations

Since the sternohyoid muscle is in the anterior neck region, neck surgery involving the region below the hyoid bone will also involve the sternohyoid muscle. Since the sternothyroid muscle and the other infrahyoid muscles provide a muscular barrier to the trachea, esophagus, thyroid gland, and parathyroid glands.[6] The sternohyoid muscle is retracted or dissected away in laryngectomies, tracheostomies, thyroidectomies, and parathyroidectomies. In thyroid and parathyroid surgeries, the sternohyoid muscle has to be retracted or dissected away. The thyroid and parathyroids are beneath the infrahyoid muscles. The sternohyoid muscle has been used as a muscle flap for reconstructive surgeries also. Using the sternohyoid muscle as a flap for reconstruction in tracheostomies and laryngectomies have shown to improve postoperative outcomes. The sternohyoid muscle flap can transfer to a different region.[7][8][9][10][11]

Clinical Significance

The sternohyoid muscle has implications in pathologies affecting the neck region. It has classically been difficult for health care provides to differentiate the causes of anterior neck pain or swellings. Since the sternohyoid muscle is one of the structures located in the anterior neck region, it could be a possible etiology to neck pain or swelling. In sternohyoid muscle syndrome, there is a mass present in the lateral neck region that disappears with swallowing. Classically this condition is caused by omohyoid muscle syndrome, but research has found that the sternohyoid muscle can also be the cause of the swelling.[12]

Other Issues

The sternohyoid muscle contributes to depression of the hyoid bone in speaking, chewing, and swallowing. If the sternohyoid muscle were to become injured, these mechanisms would become impaired. Also, if the nerve or blood perfusion to the sternohyoid muscle was to be compromised, the sternohyoid and the infrahyoid muscles could atrophy or even undergo necrosis.[13] 

The close relationship between the sternohyoid muscle and the thyroid gland makes the sternohyoid muscle a site of possible metastasis in thyroid cancers. If thyroid cancer spreads, it could affect the sternothyroid gland and manifest as a nodule in the muscle.[4]