Common Trunk Arising from Ansa Cervicalis Innervating Strap Muscles along with Sternocleidomastoid: A Case Report

The infrahyoid muscles (omohyoid, sternohyoid, sternothyroid and thyrohyoid) are innervated by Ansa Cervicalis (AC) and are involved in movement of hyoid bone, thyroid cartilage during vocalisation and swallowing. Though variations in the formation and looping pattern of AC are reported in the literature, here we present a rare branching pattern of AC. During routine undergraduate dissection class, we have come across a rare variant of AC which has clinical significance. On the left side of the neck a common trunk was found to be arising from the loop of AC which trifurcated to supply inferior belly of omohyoid, sternohyoid and sternothyroid. This trunk was also providing a branch to sternocleidomastoid. Surgeons should keep such variation in mind while operating in the carotid region. If accidently during surgery or stab wound in the neck, this common trunk is cut, the function of inferior belly of omohyoid, sternohyoid and sternothyroid will be compromised leading to difficulty in deglutition and voice production. In unilateral vocal cord paralysis, artificially created anastomosis between AC and recurrent laryngeal nerve gives excellent to normal result in affected vocal cord. In thyroid surgery, recurrent laryngeal nerve is injured leading to vocal fold paralysis. To restore the nerve supply to the larynx in such cases neurorrhaphy (end to end anastomosis) between AC and recurrent laryngeal nerve is the choice of treatment.