Growth of the brachial nerve plexus with reference to topographical relation of the medianus nerve ansa with the thoracic wall and shoulder: a histologic study using human embryos and fetuses

There is currently no information on positional changes in the brachial nerve plexus during prenatal growth. The subclavian–axillary artery passing through the medianus nerve ansa is considered a good landmark for evaluating the height of the plexus. We used histologic sections from 9 embryos and 17 fetuses (approximately 6–15 weeks of gestational age) to identify the height of the ansa by referring to the level of the rib and the glenohumeral joint. The nerve ansa was usually (23 plexuses) observed at the level of the first and/or second ribs. However, it was sometimes observed above the first rib, at a distance equal to or more than an intercostal width (7 plexuses). In the latter group, the ansa was usually located below the glenohumeral joint. Thus, the joint was located higher than the first rib, although the upper extremities were in the anatomic position for all specimens. The left–right difference in the height of the plexus corresponded to or was less than the width of the first intercostal space. Despite the synchronized growth between the thorax and shoulder girdle, the brachial plexus showed a considerable variation in comparative height; the range corresponded to twice of an intercostal width. Whether the nerve plexus is located high or low is determined at an early developmental stage and is maintained during the later growth stages. The high-positioned plexus might cause nerve injury at delivery, followed by a glenohumeral joint deformity because of the fragility without fixation in the thorax.