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32 Lower Trunk Variation and Histologic Evidence of Brachial Plexus Compression Sites at the Thoracic Inlet
Rafael Felix P. Tiongco, BA1; Dellon A., MD, PhD2,3
1Johns Hopkins University School of Medicine, Baltimore, MD; 2Plastic Surgery, Johns Hopkins University, Baltimore, MD; 3Plastic Surgery, Dellon Institute for Peripheral Nerve Surgery, Towson, MD

Background: Neurologic thoracic outlet syndrome (TOS) remains a controversial diagnosis as brachial plexus compression at the thoracic inlet is not well-documented. Though the brachial plexus in live patients cannot be excised, a subclinical investigation of known plexus compression sites in cadavers may allow us to document evidence of chronic compression. We therefore undertook a study with the specific aim of providing histologic analysis of known brachial plexus compression sites in cadavers. Additionally, as we have noted clinical evidence of wide variation in lower trunk formation from C8 and T1 nerve roots, this was investigated in this same group of cadavers.
Materials and Methods: Brachial plexuses from twenty-five cadavers were dissected and excised. Histologic analysis was directed at the junctures of 1) the upper trunk and anterior scalene muscle, and 2) C8 and T1 nerve roots (lower trunk) with the posterior border of the first rib. Plexus specimens were formalin-fixed, stained with toluidine blue, and examined under light microscopy. Specimens were analyzed for histopathologic sequela from chronic compression including epineurial and perineurial thickening/fibrosis, myelin thinning, and presence of Renaut bodies (whorled area of fibrosis associated with persistent mechanical stress). Anatomical variations in formation of the lower trunk were documented and analyzed with descriptive statistics.
Results: Histologic analysis demonstrated epineurial and perineurial fibrosis, myelin thinning, and Renaut bodies at junctions of the upper trunk with the anterior scalene muscle and the lower trunk with the first rib. These histologic findings were absent in the C7 root (plexus middle trunk). Lower trunk formation occurred on or lateral to, not medial to (as is normally depicted), the posterior border of the first rib (66%). A muscle of Albinus was present in 36% of cadavers, positioned to compress the lower trunk. A large dorsal scapular artery coursed through 36% of plexuses and was related to a high, arched subclavian artery, which also can compress the lower trunk. Bilateral plexus asymmetry was documented in 32% of cadavers.
Conclusions: We report histologic changes consistent with chronic compression of the upper and lower trunk of the brachial plexus in the thoracic inlet that may correlate with the clinical neck/shoulder (upper trunk) and "ulnar nerve¬like" (C8-T1/lower trunk) symptoms of neurologic TOS. Intraoperatively, the “lower trunk” observed over the first rib is likely to be the C8 nerve root.


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