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.And speaking of compression, the radial nerve can also be affected by compression injuries.

This happens when the radial nerve is compressed against the hard surface of the humerus, often occurring when we are sitting down and have our arm spread out over a chair next to us.

This is commonly referred to as ‘Saturday Night Palsy’, because it might happen on a Saturday night if you fall asleep with your arm hanging over the chair, but it can also happen from any direct pressure on the radial nerve, for example when using crutches.Spending an extensive amount of time in that position can cause weakness in elbow extension, and wrist extension, classically known as wrist drop, in addition to loss of sensation over the dorsal hand and thumb, as well as the posterior forearm or arm depending on how high up the nerve damage is.Finally, the axilla is also home to the axillary lymph nodes, which are often dissected and sent for pathologic analysis in the evaluation of breast cancer.

Axillary nerve dissection, however, comes with a risk of long thoracic nerve injury, which clinically translates as a winged scapula.A winged scapula is more apparent when a person tries to push against a wall: the medial border and inferior angle of the scapula pull away from the posterior thoracic wall, making the scapula look like a wing.

Besides protracting the scapula against the posterior thoracic wall, the serratus anterior also helps abduct the arm above the horizontal plane, so patients may have trouble doing overhead movements like, for example, brushing their hair.Allright, as a quick recap… Superior brachial plexus injuries cause Erb palsy, and the classic finding is a “waiter’s tip position”, which reflects arm adduction and medial rotation, as well as forearm extension and pronation.Inferior brachial plexus injuries cause Klumpke palsy, and the classic finding is a claw hand, due to flexion of the interphalangeal joints, and extension of the metacarpophalangeal joints.Compression of structures at the superior thoracic aperture can result in superior thoracic outlet syndrome, which often affects the lower trunks of the brachial plexus or subclavian vessels.Compression of the axilla can result in radial nerve palsy, resulting in wrist drop.

Finally, the axilla is also home to the axillary lymph nodes, and during axillary lymph node dissection, damage to the long thoracic nerve may result in a