Pectoral Nerves - A Third Nerve and Clinical Implications


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INTRODUCTION. Most anatomical textbooks describe the pectoral nerves as consisting of medial and lateral pectoral
nerve branches originating from the medial and lateral cords of the brachial plexus and innervating the pectoralis major
and minor muscles. However, numerous studies have described significant variation in branching and origin of the
pectoral nerves, including multiple cases suggesting the most common origin of the pectoral nerves is from the anterior
divisions of the brachial plexus rather than the medial and lateral cords. Additionally, the presence of three pectoral
nerves with three distinct origins has been frequently reported, and the lateral pectoral nerve has been reported to give
branches innervating the anterior deltoid in addition to the pectoralis major. Further complicating our understanding of
the pectoral nerves are also reports of variability in the cervical spinal nerve root levels contributing to the pectoral
nerves. OBJECTIVES. Given the frequency of reported variations of the pectoral nerves, reexamining the origin, course
and branching patterns of the pectoral nerves would be useful for clinicians, anatomists, and students. The objective of
this study was to assess the frequency and patterns of common pectoral nerve origins and branching to more accurately
describe their orientation and the possible implications for surgical procedures and injuries involving the brachial plexus
or pectoral nerves. METHODOLOGY. Thirty-one embalmed adult human anatomical donors, previously dissected by
medical students and instructors, were examined to identify the frequency of common variations in the origin and branch
pattern of the pectoral nerves. Observations were done bilaterally, with each unilateral side considered an independent
observation. Fifty brachial plexus dissections were photographed, and the pectoral nerves’ origins, course, and
branching patterns were recorded. Dissection sides with missing or damaged pectoral nerves were excluded.
RESULTS. Of the 50 brachial plexuses that met inclusion criteria, 29 had three distinct pectoral nerve origins, 20 had two
distinct nerve origins, and 1 brachial plexus had only one common pectoral nerve origin. In 20 of the brachial plexus
observations, three distinct pectoral nerves originated from; the superior anterior division (SAD), the middle anterior
division (MAD), and the inferior anterior division (IAD). In all 50 brachial plexuses observed, the pectoral nerves gave
three terminal branches innervating distinct regions: the superior or clavicular pectoralis major muscle (PM), the sternal
or middle PM, and the costal or inferior PM. CONCLUSIONS. In contrast to current descriptions of two pectoral nerve
branches, we most frequently observed three distinct pectoral nerves arising from the SAD, MAD and IAD and coursing
to the superior PM, middle PM, and inferior PM. The three pectoral nerves were also observed to be in a superior to
inferior spatial arrangement. Clinically, knowledge of this arrangement of the pectoral nerves may be particularly
significant in cases such as pectoral nerve transfers, brachial plexus injuries, and surgeries of the breast and axillary
region. Based on variability of the pectoral nerves, further studies may be warranted to fully assess the impact of injuries
to specific locations and branching patterns on function of the pectoralis major muscle.