Bilateral accessory cleidohyoid in a human cadaver

Reports of infrahyoid muscle variations abound in the literature. Specifically, anatomical reports mentioning the cleidohyoideus accessorius muscle date back to the 19th century. Several authors have attempted to classify the anatomical variation that constitutes a muscle inserting from the clavicle into the hyoid bone in the presence of an intact omohyoid. In 1897 Le Double described variations of the omohyoid muscle. One such variation was the cleidohyoideus that presented as a double or additional muscle belly inserting into the clavicle and the hyoid bone [1]. In 1912, Loth described cases of infrahyoid muscles originating from the clavicle and inserting into the hyoid bone, classifying them as cleidohyoid muscles [2]. In 1923 Steinbach reviewed the existing literature and classified infrahyoid variations into four categories. One of these categories was that of the “cleidohyoideus accessorius muscle” defined as a “muscle inserting in the clavicle and the hyoid bone when the omohyoid is present” [3]. A more recent literature review by Sasagawa includes ten reports with mention of the cleidohyoideus accessorius muscle [4]. Four of those ten reports state cleidohyoideus accessorius prevalence figures ranging from 0.60 % to 1.04 %. However no details in terms of the unilaterality or bilaterality of the muscle are mentioned. In 1996 in his encyclopedia of human anatomical variation, Bergman illustrated numerous variations of accessory muscles to the omohyoid. One of these variations consists of a fascicle with insertions in the medial clavicular head and the hyoid bone. No data on the prevalence or the characteristics of this variation were reported [5]. Fukuda [6], and Hatipoglu [7] document this variation found unilaterally. This paper describes a case of the cleidohyoideus accessorius muscle found bilaterally during routine dissection of a human cadaver. A brief discussion of the clinical significance of this abnormality is included. To our knowledge, this is the first definitive case of a bilateral cledohyoideus accessorius muscle in the medical literature.