What is the anatomy, innervation, function, and clinical significance of the omohyoid muscle?

Omohyoid Muscle: Anatomy, Innervation, Function, and Clinical Significance

The omohyoid is a two-bellied infrahyoid muscle that depresses and retracts the hyoid bone during swallowing, is innervated by the ansa cervicalis, and serves as a critical anatomical landmark during thyroid and neck surgery.

Anatomical Structure

The omohyoid consists of two distinct bellies connected by an intermediate tendon:

• Inferior belly: Originates from the upper border of the scapula, medial to the scapular notch, and courses forward and upward in the lower neck 1
• Superior belly: Extends from the intermediate tendon to insert on the lower border of the hyoid bone
• The intermediate tendon is typically bound to the clavicle and first rib by a fascial sling

Anatomical Variations

Significant variations occur in approximately 3% of cases 2:

• The inferior belly may originate from the clavicle instead of the scapula 2
• Accessory omohyoid muscles have been documented 1
• Rare variants include attachment to the mastoid process rather than hyoid bone 3
• A new classification system integrating 4 types of consistency and 4 types of morphometric variations has been proposed to document these variations 1

Innervation

The omohyoid is primarily innervated by the ansa cervicalis (C1-C3 nerve roots) 4:

• The superior belly receives branches from the ansa cervicalis below the loop in 64.3% of cases 4
• Some evidence suggests dual innervation, with the inferior belly potentially receiving supply from the accessory nerve (CN XI) in certain individuals 5
• The nerve typically enters the superior portion of the muscle 4

Critical caveat: Non-looped variants of the ansa cervicalis exist where the typical looped structure is absent, with innervation patterns differing significantly from standard anatomy 3.

Function

Primary Functions

• Depression and retraction of the hyoid bone during swallowing 6
• Hyoid stabilization to facilitate tongue movement during deglutition 6

Secondary Biomechanical Role

Recent evidence demonstrates the omohyoid activates during shoulder abduction:

• Muscle thickness and cross-sectional area increase significantly during isometric shoulder abduction at 90° compared to rest (p < 0.01) 6
• No activation occurs at 0° abduction 6
• Scapular position changes may influence omohyoid function, suggesting a biomechanical link between shoulder mechanics and neck muscle activity 6

Clinical Significance

Thyroid Surgery Landmark

The omohyoid serves as a crucial anatomical reference during thyroidectomy 7:

• The thyroid gland sits deep to the cervical strap muscles (including omohyoid) in the lower anterior neck 7
• The muscle helps define surgical planes and boundaries during neck dissection
• Understanding its relationship to the recurrent laryngeal nerve (RLN) and superior laryngeal nerve (SLN) is mandatory for thyroid surgeons 7

Reconstructive Surgery Applications

The omohyoid is utilized in infrahyoid myocutaneous flap reconstruction:

• Used for tongue reconstruction in lingual carcinoma cases 2
• Requires preservation of its neurovascular supply during flap preparation 4
• Arterial supply: Typically two pedicles from the superior thyroid artery supply the superior belly in 57.1% of cases 4

Surgical Risks Related to Variations

Variations in inferior belly attachment pose specific risks 2:

• Clavicular attachment variants place the muscle in close proximity to the internal jugular vein and brachial plexus
• Risk of inadvertent injury during neck surgeries or trauma
• Particularly relevant during coronary artery bypass grafting (internal jugular vein access) and neck dissections for metastatic disease 3

Laryngeal Reinnervation Procedures

The omohyoid's relationship to the ansa cervicalis makes it relevant during vocal fold paralysis treatment:

• Ansa cervicalis to RLN anastomosis is performed for laryngeal reinnervation after thyroid surgery complications 7
• The ansa cervicalis serves as the donor nerve for these procedures
• Understanding omohyoid innervation patterns helps preserve function during these reconstructive procedures 7

Key Clinical Pitfalls

1. Assuming standard anatomy: With 3% variation rate and multiple documented anomalies, surgeons must be prepared for unexpected attachments and innervation patterns 1, 2

2. Overlooking dual innervation: Some individuals have accessory nerve contribution to the inferior belly, which may be relevant during neck dissections involving CN XI 5

3. Ignoring vascular supply: The superior thyroid artery provides the primary blood supply; inadvertent injury during thyroid pole dissection can compromise muscle viability if used for reconstruction 4

4. Neglecting shoulder-neck biomechanics: The omohyoid's activation during shoulder abduction suggests functional connections that may be relevant in patients with combined neck and shoulder pathology 6