Biology
Brachial Plexus Injury Patterns in Shoulder Dystocia
Quick fact
Erb's palsy, affecting the upper brachial plexus (C5-C6), occurs in about 1-2 per 1000 live births and is more common than Klumpke's palsy, which affects the lower roots (C8-T1) and is far rarer but often more severe.
Why this is interesting
During a difficult delivery, a baby's shoulder gets stuck—and the next few minutes can determine whether a nerve injury is temporary or permanent. Why do some babies lift their arm again, while others never regain movement?
Read the full explanation
Understanding Brachial Plexus Injury Patterns in Shoulder Dystocia
To understand brachial plexus injuries in shoulder dystocia, think of the brachial plexus as a bundle of nerve cables that run from the spinal cord through the neck and into the arm. When the fetal shoulder becomes impacted behind the maternal pubic bone, the head and neck are pulled to one side to free it. This traction stretches the brachial plexus, which is relatively fixed at both ends—the spinal cord and the arm. Depending on the angle and force of the pull, different parts of the plexus are overstretched. The upper roots (C5-C6) are most vulnerable, leading to Erb's palsy, where the baby holds the arm adducted and internally rotated with the elbow extended and wrist flexed—the classic 'waiter's tip' position. In contrast, lower root injury (C8-T1) results in Klumpke's palsy, producing a flail hand with Horner's syndrome (ptosis, miosis, anhidrosis) if T1 is involved. The severity ranges from simple stretching (neurapraxia), which resolves in weeks, to complete tearing (avulsion) from the spinal cord, which is permanent.
A deeper explanation
The mechanism of injury is mechanical traction that exceeds the elastic limits of the nerve fibers. The plexus is composed of axons surrounded by connective tissue layers (epineurium, perineurium, endoneurium). In mild cases, the stretch disrupts the myelin sheath but leaves the axon intact—this is neurapraxia, where conduction is blocked but regeneration is possible. More severe stretch can rupture axons inside intact connective tissue (axonotmesis), requiring slow axonal regeneration. In the most severe cases, the root tears from the spinal cord (avulsion), which is a rootlet avulsion from the CNS and cannot regenerate. The patterns correlate with the type of shoulder dystocia maneuver: lateral traction on the head increases the angle of the neck, stretching the upper roots; while downward traction on the shoulder can affect the lower roots. Understanding these patterns helps clinicians predict recovery and plan interventions like physical therapy or nerve surgery.