Causes / typical injury mechanism: The brachial plexus is a network of nerves composed of the C5-T1 spinal roots. It is located by the axilla that extends distally to the arm, providing motor and sensory innervation to the different parts of the arm, forearm, and hand. The roots then converge into 3 trunks, then divisions, then cords, until they become terminal branches. These 5 terminal branches of the brachial plexus are known as the musculocutaneous, median, ulnar, axillary, and radial nerves. Brachial plexus injuries result from damage to any component of the plexus, from the root to the terminal nerve branches. Most brachial plexus injuries are caused by forceful longitudinal pulling or stretching of the nerves. Other traumatic events that can result in brachial plexus injuries include shoulder dislocations, motor vehicle collisions, gunshot wounds, grasping injuries during falls, and motorcycle accidents.

Additionally, neonates can experience brachial plexus injuries during childbirth. There are a handful of brachial plexus injuries that are more strictly defined, each resulting from rather specific causes (see below).

Compression neuropraxia, loss of peripheral nerve function without compromise of the axonal structure, is a common manifestation of brachial plexus injuries in older individuals.

Classic history and presentation: The presentation of brachial plexus injuries is dependent on the location of the lesion, which specific nerves are affected, and how proximal or distal that injury is located. Some common lesions of the brachial plexus have been given colloquial names based on the unique hand / forearm deformity that ensues:

• Erb palsy, or waiter's tip, causes the inability to abduct, laterally rotate, flex, and supinate the arm due to deltoid, supraspinatus, infraspinatus, and biceps brachii deficits. It is a lesion of C5 and C6 nerve roots, located rather proximally along the brachial plexus, causing both large sensory and motor deficits.
• Klumpke palsy results from a traction or tear from the C8-T1 nerve roots. This is classified as a "lower" brachial plexus palsy affecting intrinsic hand muscles including the lumbricals, interossei, thenar, and hypothenar muscles, resulting in a "claw hand" appearance. This occurs from the inability to flex the metacarpophalangeal (MCP) joints and to extend the interphalangeal (IP) joints.
• Wrist drop is a phenomenon resulting from a lesion at the posterior cord that later diverges into the axillary and radial nerves. A similar presentation can occur, resulting in "Saturday night palsy" (radial nerve injury) or deltoid paralysis (axillary nerve injury). Saturday night palsy specifically occurs in the triceps brachii, brachialis, brachioradialis, and extensor carpi radialis longus muscles. Deltoid paralysis occurs from axillary nerve injury. The remaining 3 of the 5 terminal branches of the brachial plexus – the musculocutaneous, median, and ulnar nerves – are all susceptible to injury. A musculocutaneous nerve-specific lesion causes loss of function to the coracobrachialis, biceps, brachii, and brachialis, causing an inability to flex the elbow, and is usually associated with variable sensory loss.
• Median nerve and ulnar nerve injuries can present similarly, but to deduce the cause depends on the location of the lesion. "Pope's blessing" is a proximal median nerve injury that causes the inability to flex all IP joints in the lateral digits. This presents similarly to a distal ulnar nerve injury, "ulnar claw," that causes the inability to extend the digits. Alternatively, "median claw," a distal median nerve lesion, occurs from the paralysis of lateral digit extension, while proximal ulnar nerve injury causes the "OK gesture," named from the inability to flex the medial digits.

Prevalence: Severe trauma that results in a lesion in any part of the brachial plexus is most often seen in young males. Brachial plexus injuries can also result from childbirth trauma.

Risk factors: Risk factors for brachial plexus injuries in a neonate include:

• Shoulder dystocia – The shoulder of the neonate is caught on the pubic symphysis of the mother, causing traction on the ipsilateral brachial plexus.
• Large gestational size.
• Prolonged labor.
• Breech presentation at birth.
• Instrumented deliveries.

Specifically for injury resulting from childbirth: shoulder dystocia (large shoulders), large gestational size, prolonged labor, and breech presentation at birth.

Pathophysiology:
Longitudinal traction upon the brachial plexus:

• Childbirth
• Forces required to reduce shoulder dislocations
• Fall from height causing forced arm abduction as patient tries to grab onto object to prevent fall
• Posterior shoulder dislocations