Foundational knowledge of anatomy is essential for physicians. Resources utilized to impart this knowledge during training focus on typical anatomy. Variation extant in humans is largely ignored, which may compromise patient outcomes. Knowledge of variant anatomy requires physicians to regularly review the literature. Physical examination and ultrasound imaging were used to characterize and identify the muscle, employing a Mindray MX7 ultrasound machine and curvilinear transducer.
The muscle was identified as thoracobrachialis, a variant of pectoralis major. The muscle caused an asymmetry visible on gross examination at the axilla. Palpation revealed it to be tendinous in the axilla, gaining muscle mass as it descended in the brachium. This observation was confirmed by ultrasound. The thoracobrachialis appeared to terminate by merging with the triceps midshaft of the humerus. The muscle described in this report most closely resembles a family of variants that arise in the thorax in association with the pectoralis major, and insert either midshaft of the humerus or on the medial epicondyle. These variants were initially discovered during cadaveric dissection and this is the first report of such a muscle discovered in a healthy living individual without attendant clinical symptoms. Physical examination and ultrasound imaging identified the muscle as thoracobrachialis, a variant of pectoralis major. This report identifies and characterizes a supernumerary muscle of the thorax in a living individual.
