29 - A Constellation of Anatomical Variations: A Case Study of Altered Development Within Cervical Myotomes

Poster Board Number: 29
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Abstract Body : Our group set out to perform a routine human body prosection over a two-month period for the purpose of medical education. During dissection, we noted several anatomical variations, which were documented for research purposes. Firstly, we observed a variant muscle bilaterally with vertically oriented fibers in the superior anterolateral aspect of the thoracic wall, superficial to the serratus anterior muscle. Upon further dissection, we found that this muscle shared fibers with the middle scalene. Its proximal attachment was traced to the transverse process of C1, while the distal attachment extended inferiorly to the 4th - 5th rib, which included a pronounced fascial slip. We refer to this muscle as a supernumerary scalene muscle. Subsequently, we observed several other variations in the cervical region, including a slightly high insertion of the serratus anterior at the superior angle of the scapula as well as segmental innervation of the serratus anterior from several cervical spinal nerves, in addition to its typical innervation of the long thoracic nerve. We also observed in the cervical region that the levator scapulae muscle had two distinct muscular slips bilaterally. Distally in the forearm, we observed an additional radial extensor of the wrist bilaterally, which we refer to as extensor carpi radialis intermedius. In addition to these muscular variations in the cervical region, we observed an aberrant left vertebral artery that branched directly from the aortic arch. Collectively, we deem that these four muscular anomalies (supernumerary scalene, serratus anterior, levator scapulae, and extensor carpi radialis intermedius muscles), in addition to the aberrant left vertebral artery, represent a constellation of variations that likely are interrelated through altered cervical somite segmentation and myotome development (C5-C7 levels). We posit that these variations in conjunction suggest anomalous mesodermal differentiation and likely shares an embryological link. An embryological model is proposed. This case study represents a fascinating example of anatomical variation, how anatomical variations may cluster in constellations, and highlights the complexity of embryonic development. Given the nature of these variations, this work has immediate clinical implications in the context of conditions such as thoracic outlet syndrome or procedures such as brachial plexus nerve blocks and vascular surgery.