An Anomalous Infrahyoid Muscle

During the dissection of a cadaver an anomalous infrahyoid muscle was discovered. This was unlike those variations of the infrahyoid muscles described previously.`~The anomalous muscle is classified as a cleidohyoid muscle because it arose from the clavicle and inserted on the hyoid bone. The previous reports2"'5 of anomalous cleidohyoid-type muscles are general descriptions which lack diagrams and measurements. Therefore, the detailed anatomy and location of these muscles is difficult to determine, especially in regard to their point of origin from the clavicle. This is significant because the inferior portion of the muscle described in this report was in the direct line of surgical approach used to perform a scalene node biopsy, a phrenic nerve crush or avulsion, a cervicodorsal sympathectomy, the removal of a cervical rib, or a transection of the anterior scalene muscle for correction of the scalenus anticus syndrome. The cadaver was that of a 76-year-old white male who died of pulmonary tuberculosis. The anomalous muscle was located inferior and medial to a normal right omohyoid muscle (Figure 1). Muscle fibers along its lateral edge interdigitated with those on the deep surface of the superior belly of the normal omohyoid for a distance of 4.1 cm. inferior to the lower border of the hyoid bone. The muscle fibers along its medial edge inter-digitated with those of the superior portion of a normal right sternohyoid muscle for a distance of 4.0 cm. The combined insertion of the anomalous muscle and the superior belly of the omohyoid occupied an area 1.3 cm. in width on the hyoid bone lateral to the insertion of the sternohyoid muscle. After emerging from the interdigitations described above, the anomalous muscle continued as a broad fasciculus in an infero-lateral direction dorsal to the sternocleido-mastoid muscle to its origin from an area 2.1 cm. in width on the dorsal and inferior surface of the clavicle, 6.0 cm. lateral to the clavicular head. The anomalous muscle was innervated by three small nerves from the ansa cervicalis. These nerves also supplied branches to the superior belly of the normal omohyoid muscle.