Dorsoscapularis triangularis: embryological and phylogenetic characterization of a rare variation of trapezius

Abstract

The muscle trapezius shows considerable morphological diversity. Variations include an anomalous origin and complete or partial absence of the muscle. The present study reported, a hitherto undocumented complete bilateral absence of the cervical part of trapezius. Based on its peculiar origin and insertion, it was named dorsoscapularis triangularis. The embryological, phylogenetic and molecular basis of the anomaly was elucidated. Failure of cranial migration of the trapezius component of the branchial musculature anlage to gain attachment on the occipital bone, cervical spinous processes, ligamentum nuchae between 11 mm and 16 mm stage of the embryo, resulted in this anomaly. A surgeon operating on the head and neck region or a radiologist analyzing a magnetic resonance imaging of the cervical region would find the knowledge of this morphological variation of trapezius useful in making clinical decisions.

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

“…Developmentally, this variant may have occurred due to aberrancies in the signaling of the Hox gene in the development of the myotome that gives rise to the SCM and trapezius, both seen to fuse at their respective insertions, and on occasion make contact with one another along their medial parts [4715]. Embryologically, these structures arise together as an aggregation of cells readily observed at Carnegie stage 15 (CS15), and are referred to as the trapezius/SCM muscle complex.…”

Review of the Literature on Anatomical Variations of the Trapezius Muscle

“…These variations include an abnormal origin or the partial or complete absence of the muscle. Mehra et al 10 reported a rare case of a dorsoscapular triangle, due to the bilateral and complete absence of the cervical part of the trapezius muscle. In this case, each side of the muscle was presented as a right angle triangle extending from the 5 th cervical vertebrae to the 10 th thoracic vertebrae.…”

“…10 In the case of the trapezius muscle, causes may be inflammatory, vascular, neuropathic and myopathic. The congenital absence of the trapezius muscle maybe part of a Poland syndrome or Klippel-Feil syndrome.…”