Anatomical variations of the infrahyoid muscles and ansa cervicalis: a systematic review and an updated classification system for the omohyoid muscle

The four infrahyoid muscles of the anterior neck are primarily innervated by the ansa cervicalis. This systematicreview aims to evaluate the range of the anatomical variations in these muscles and their relationship to innervationpatterns. A systematic search was conducted using PubMed and Google Scholar databases. Articles reporting variations ininfrahyoid muscles and/or ansa cervicalis were independently evaluated following the PICOTS framework. The anatomicalquality assessment tool was used to assess the quality of publications reporting anatomical variants. Seventy-seven studies,encompassing eighty-four cases, were included in the analysis. Of the 56-publication reporting infrahyoid muscle variations,44 pertained to the omohyoid muscle (main or accessory), 3 to the sternohyoid, 4 to the sternothyroid, and 1 to thethyrohyoid, with no accessory variation observed in the latter. Atypical infrahyoid muscles were identified in 11 cases, 9 ofwhich presented as levator glandulae thyroideae, and 2 as single cases. Variations in the ansa cervicalis were documentedin 29 cases, only 1 case involved both ansa cervicalis and infrahyoid muscle variations. The extensive variability of theomohyoid muscle led to the development of a new classification system that integrates 4 types of consistency and 4 types ofmorphometric variations, providing valuable insights for clinical practice. The specialized use of the infrahyoid muscles incancer staging, reconstruction after neck cancer surgery, and thyroid surgery underscores the need for a new framework todocument their variations, particularly in the omohyoid muscle.