Anatomical Variations of the Ansa Cervicalis: A Systematic Review and Meta-analysis of Cadaveric Studies.

Background: The ansa cervicalis is an important cervical plexus neural loop supplying the infrahyoid muscles and frequently used in laryngeal reinnervation and reconstructive neck surgeries, despite considerable anatomical variation with important surgical implications. Aim: To systematically evaluate the prevalence and anatomical variations of the ansa cervicalis and analyze its positional relationship with clinically important landmarks. Materials and Methods: A systematic review and meta-analysis were conducted according to the PRISMA guidelines. Electronic databases, including PubMed, Scopus, Web of Science, and Google Scholar, were searched for observational anatomical studies on the ansa cervicalis. Data on prevalence, position relative to the carotid sheath and central tendon of omohyoid, and relationship with the internal jugular vein were extracted and quantitatively analyzed using Jamovi software. Results: Twenty studies comprising 1294 cadaveric specimens were included. The ansa cervicalis was present in nearly all cases, with its absence observed in only 1%. The AC was significantly more likely to be anterior to the carotid sheath than posterior to it. The CTOM was mostly located superior to the tendon, followed by at the same level, and inferior positioning was the least common. Based on its relationship with the IJV, the external (lateral) type was the predominant configuration, followed by the internal (medial) type, whereas the mixed type was rare. Conclusion: The ansa cervicalis demonstrates substantial anatomical variability, despite its near-universal presence. Awareness of these variations is essential for safe surgical navigation during neck dissection, carotid procedures, and laryngeal reinnervation.