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

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.

Variation of the sternocleidomastoid muscle: a case report of three heads and an accessory head

“…It may confuse the operator during posterior surgical approaches to the neck and occipital. It may be confused with a ganglion or other tumor inducing iatrogenic injury [2]. SCM muscle variants have been subdivided into several small muscle slips [9].…”

“…They segregate into the ventral part of the developing SCM, and a dorsal part for the developing trapezius muscle [9]. Anomalous gene signaling, like the HOX genes, in this developmental interval may result in deviations from the commonly observed morphology and attachments of the SCM [2]. Therefore, the innervation of SCM and trapezius shares the accessory nerve.…”

The sternocleidomastoid muscle variations: a mini literature review

“…This process of separation of the common trapezius/SCM complex could result in different forms of SCM as well as trapezius variations. SCM variations are common where accessory branches from the clavicle or sternum connect to the normally existing SCM or separately either to the mastoid process or attach laterally to the superior nuchal line towards the trapezoid muscle insertion [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27]. The variations at the insertional end of the muscle are less common in comparison to the occurrence of accessory proximal heads [15].…”

“…SCM variations are common where accessory branches from the clavicle or sternum connect to the normally existing SCM or separately either to the mastoid process or attach laterally to the superior nuchal line towards the trapezoid muscle insertion [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27]. The variations at the insertional end of the muscle are less common in comparison to the occurrence of accessory proximal heads [15]. Depending upon the shape and courses of these additional muscular slips, neighboring anatomical structures in the posterior triangle of the neck could be compromised.…”

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“…4 However, several variations can occur, including one or more accessory heads. [5][6][7] Therefore, it is of great importance that the doctors have a clear comprehension of the SCM anatomy and its possible variations, in order to prevent inadvertent complications. The goal of this study was to report the presence of the bilateral three headed sternocleidomastoid muscle.…”