Anatomy clinical correlates: Axilla

The axilla, also known as the armpit, is first and foremost, incredibly ticklish. But from an anatomical standpoint, it’s a key location that contains many important structures that may be damaged, causing significant functional deficits. The axilla is like a train station, where a number of vascular, nervous and lymphatic structures pass between the trunk and the upper limb.

One very important structure is the brachial plexus, which can be divided into five roots, three trunks, six divisions, three anterior and three posterior cords, and five terminal branches. The order can be remembered using the mnemonic “Remember To Drink Cold Beer.” But you may want to wait until the end of the video before you act on that!

Now, an upper brachial plexus injury affects the superior roots, namely spinal nerves C5 and C6, and a classic example of an upper brachial plexus injury is Erb palsy, which can happen in adults as a shoulder trauma that results in an increase in the angle between the neck and the shoulder, or in newborns, when excessive stretching of the neck occurs during childbirth.

The clinical consequences reflect the affected nerves, which are the ones that are derived solely from C5 and C6 roots, namely, the musculocutaneous, axillary, and suprascapular nerves. This causes paralysis of muscles like the biceps brachii, which normally allows forearm flexion and supination, and the infraspinatus and teres minor, so lateral rotation of the arm is affected, as well as the deltoid and supraspinatus muscles, which would usually cause arm abduction but would also be affected. So with superior brachial plexus injuries, the classic finding is a “waiter’s tip position”, which reflects arm adduction and medial rotation, and forearm extension and pronation.