Case report: Bilateral and asymmetric anatomical variation extending between the trapezius and sternocleidomastoid muscles
Among the muscles of the posterolateral portion of the neck, the upper bundles of the trapezius muscle, or upper trapezius (TS), and the sternocleidooccipitomastoid or sternocleidomastoid muscle (ECOM) are the most superficial. Its protrusion allows to superficially delimit between the anterior margins of the TS muscle and the posterior of the ECOM muscle, the lateral or posterior cervical triangle, which contains the large nerve bundles that descend from the cervical spine and extend towards the upper limb and also, the large blood vessels in the arm. The trapezius and ECOM muscles are often the target of anatomical variations, including these, supernumerary insertion heads. Here we present a case report that evidences not yet reported muscle bundles running between the two muscles of the posterior cervical triangle. As exposing the musculature of the nape of a male, black and middle-aged man, we observed spindle-shaped muscle bundles approximately 8 cm long, extending transversely in the nuchal region in a latero-anterior direction, from the upper third of the trapezius muscle to the upper third of the ECOM muscle. We hypothesized that the common embryonic origin of the two muscles might explain the here described bilateral and asymmetric anatomical variation of the posterior cervical triangle. Eventual functionality ou clinical significance of such morphological variation will require further investigation.
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