Unusual variant of an accessory thoracic muscle

The anterior thoracic wall musculature presents significant morphological variability. The literature describes a few accessory thoracic muscles (ATMs) and discusses possible clinical implications. A dissection report describes an unusual ATM variant.

The variant case was identified during the routine dissection of a 78-year-old female donated cadaver.

An accessory, and previously unknown, muscle was identified on the right anterior thoracic wall in the intermediate plane of the pectoralis major and minor muscles (PM and Pm). It originated from the external oblique muscle (at the level of the fifth rib), and in its distal attachment, the muscular belly became aponeurotic and fused with the axillary fascia. The insertion was located anteriorly to the axillary neurovascular structures, but was not closely related. On the left anterior thoracic wall, the musculature was presented as typical.

The variant case could not be classified as a previously reported variant due to its origin in the external oblique muscle. The proposed name for the accessory muscle is the 'abdomino-fascialis' muscle. Surgeons operating in the axilla and the anterior thoracic wall should be aware of these variants, which can significantly complicate several procedures.