Report of a non-looped variant of ansa cervicalis with omohyoid innervation from accessory nerve branch and omohyoid attachment to mastoid process

A variant of the innervation of the infrahyoid neck musculature is reported in which the typical looped ansa cervicalis structure is absent. In this variant, the infrahyoid muscles (sternohyoid, sternothyroid omohyoid and thyrohyoid) were innervated by a presumptive superior root of “ansa cervicalis” traveling with vagus nerve (CN X) and not branching from hypoglossal nerve (CN XII). The omohyoid muscle, typically innervated by the inferior root of ansa cervicalis, is instead innervated by nerve fibers branching from the accessory nerve (CN XI). This formation created a non-looping variant of ansa cervicalis. Furthermore, the omohyoid muscle did not attach to the hyoid bone but instead attached to the mastoid process of the temporal bone by merging its fibers superiorly and posteriorly with the clavicular portion of the sternocleidomastoid muscle, creating a “sternocleidoomomastoid” muscle innervated by a branch of accessory nerve.

This variation was found in one black male cadaver from a cohort of 25 male and female cadavers.

Only one variation of ansa cervicalis was observed.

As neck dissections and surgical procedures of this region are performed for a variety of conditions—including coronary artery bypass grafting and metastatic neck disease—variations of this type are of broad clinical surgical importance.