Hypogenic Pectoralis Major Muscle Associated With Complete Agenesis of the Pectoralis Minor Muscle: A Cadaveric Case Report

The pectoralis major (PM) and pectoralis minor (PMi) are muscles located in the anterior chest wall. The PM is a fan-shaped muscle composed of the clavicular and sternocostal heads. Typically, the clavicular head originates from the anterior surface of the medial half of the clavicle. The sternocostal head, located just inferior to the clavicular head, originates from the anterior surface of the sternum, superior six costal cartilages, and aponeurosis of the external oblique muscle. The PMi lies deep into the PM, positioned on top of the rib cage. Innervation of these muscles is provided by the medial and lateral pectoral nerves. Pectoralis muscle anomalies can occur due to congenital reasons, genetic factors, or developmental changes. This paper explores one such anomaly. During a routine educational cadaveric dissection at the University of Toledo College of Medicine, asymmetrical PM muscles were identified in a 98-year-old male. The dissection revealed that the sternocostal head of the left PM was not fully developed. Additionally, the left PMi muscle was missing, and the left medial pectoral nerve was absent. Anomalous development of the PM is often associated with other musculoskeletal developmental defects, and the clinical presentation can vary depending on the involvement of structures. The PMi acts as a surgical landmark and can also be used as a myo-cutaneous flap in reconstructive surgeries. The PMi tendon is often used in the rotator cuff and acromioclavicular joint repairs. Understanding the anomaly presented in this case report will help physicians manage future cases of anomalous PM and PMi in their patients.