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 systematic review aims to evaluate the range of the anatomical variations in these muscles and their relationship to innervation patterns. A systematic search was conducted using PubMed and Google Scholar databases. Articles reporting variations in infrahyoid muscles and/or ansa cervicalis were independently evaluated following the PICOTS framework. The anatomical quality 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 the thyrohyoid, with no accessory variation observed in the latter. Atypical infrahyoid muscles were identified in 11 cases, 9 of which presented as levator glandulae thyroideae, and 2 as single cases. Variations in the ansa cervicalis were documented in 29 cases, only 1 case involved both ansa cervicalis and infrahyoid muscle variations. The extensive variability of the omohyoid muscle led to the development of a new classification system that integrates 4 types of consistency and 4 types of morphometric variations, providing valuable insights for clinical practice. The specialized use of the infrahyoid muscles in cancer staging, reconstruction after neck cancer surgery, and thyroid surgery underscores the need for a new framework to document their variations, particularly in the omohyoid muscle.