A prevalence study of the atlantomastoid muscle.

Purpose: To contribute to the documentation and consideration of the prevalence of the atlantomastoid muscle in humans.

Methods: Sixty formalin-fixed cadaveric sides were dissected for the presence of the atlantomastoid muscle (2 sides per donor). Laterality, origin, insertion, and other observable characteristics of the variant were recorded along with donor sex for comparative analyses.

Results: The atlantomastoid muscle was observed in nine sides (15%): two left side only, three right side only, and 2 bilaterally. The origin of the muscle was consistent from the transverse process of the atlas, lateral to both the insertion of obliquus capitis inferior and the origin of obliquus capitis superior. In one instance, the origin of atlantomastoid was continuous with the most superior tendinous slip of levator scapulae. The insertion of each atlantomastoid variant was the mastoid process, however, the precise location was variable.

Conclusion: The atlantomastoid muscle is a common accessory muscle of the suboccipital region; the muscle was present in 15% of the cadaveric sides dissected in this study. Despite the notable prevalence of atlantomastoid muscles, there is little current literature dedicated to its study. Here, we document the first new information related to the prevalence of atlantomastoid since 1964 and discuss the development and potential clinical significance of the muscle.