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.

Lower brachial plexus injuries are much more uncommon, and they affect the inferior roots of the brachial plexus, namely C8 and T1. This can happen because of excessive abduction of the arm, aka an increased angle between the trunk and the upper limb. This could happen to a person falling from a tree and grabbing onto a branch, or during delivery, if the newborn is pulled out by the arm.

The result is what’s clinically known as Klumpke palsy, which has serious effects on nerves derived from the C8 and T1 roots, like the median and ulnar nerves, and causes loss of sensation along the medial side of the arm, and paralysis of the intrinsic hand muscles. The classic finding is a claw hand, which is due to flexion of the interphalangeal joints, and extension of the metacarpophalangeal, or MCP, joints.

This is mainly due to paralysis of the lumbricals, which normally act to flex the MCP joints and extend the interphalangeal joints, as if you were to be waving “bye-bye”, resulting in unopposed flexion of the interphalangeal joints, and unopposed extension of the MCP joints.

Lower brachial plexus injuries can also occur due to compression of the lower trunk, for example, by an extra (or cervical) rib, or by an apical lung tumor, called a Pancoast tumor. This is an example of thoracic outlet syndrome, and results in the same deficits as seen in Klumpke palsy.

However, thoracic outlet syndrome can also result from damage to any structures that enter and exit the lower neck and upper limbs via the superior thoracic aperture, which is the region above the first rib between the clavicle, sternum, and cervical vertebrae. These structures include the subclavian vessels, which are usually compressed in the scalene triangle, formed by the anterior and middle scalene muscles. The brachial plexus trunks along with the subclavian artery pass through this scalene triangle, where the subclavian vein travels anterior to it. Compression of the subclavian vessels results in upper extremity swelling and exertional arm pain.