Bilateral sternocleidomastoid variant with six distinct insertions along the superior nuchal line.

Anat Cell Biol. 2018 Dec;51(4):305-308. doi: 10.5115/acb.2018.51.4.305. Epub 2018 Dec 29.

Dupont G, Iwanaga J, Altafulla JJ, Lachkar S, Oskouian RJ, Tubbs RS.

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Abstract

Anatomical variations of the sternocleidomastoid muscle (SCM) have been observed to occupy multiple origins and insertion points and have supernumerary heads, sometimes varying in thickness. During routine dissection, a SCM was observed to have six distinct insertions that interface with the course of the superior nuchal line, ending at the midline, bilaterally. This variation was also seen to receive innervation from the accessory nerve as well as the great auricular nerve. To our knowledge, this variant of supernumerary insertions and nerve innervations has not yet been reported. These variants may pose as problematic during surgical approaches to the upper neck and occiput, and should thus be appreciated by the clinician. Herein we discuss the case report, possible embryological origins, and the clinical significance of the observed variant SCM.

KEYWORDS:

Anatomy; Mastoid; Neck surgery; Sternocleidomastoid; Variant

Fig. 1

Bilateral variant sternocleidomastoid muscles at their attachments onto the skull. (A) Left posterior neck. (B) Right posterior neck. GAN, great auricular nerve; GON, greater occipital nerve; LON, lesser occipital nerve; OA, occipital artery; PAM, posterior auricular muscle; SCM, sternocleidomastoid muscle; 3rd ON, third occipital nerve.

Fig. 2

Seven tendons of the variant muscle on left side. Note that the seventh tendon of the variant muscle reaches the greater occipital nerve (※). GAN, great auricular nerve; LON, lesser occipital nerve; PAM, posterior auricular muscle; SCM, sternocleidomastoid muscle.