Part 1. Injuries to the brachial plexus: mechanisms of injury and identification of risk factors.

Upper-arm weakness (paresis) or paralysis indicates peripheral-nerve damage to the brachial plexus, a network of lower cervical and upper thoracic spinal nerves supplying the arm, forearm, and hand. Physical findings reflect muscle paralysis from spinal nerve roots. The mechanism of injury includes maternal, obstetric, and infant factors that apply traction on or compression to the anatomically vulnerable brachial plexus. Nerve regeneration can occur if nerve tissue components are preserved. Recovery is affected by multiple factors, including the type and site of injury, intervention timing, and developmental factors. The majority of injuries recover in days or months; however, residual deficits can persist. Part 1 of 2 of this article provides an overview of the neurophysiology of peripheral-nerve damage and nerve regeneration. The multifactorial etiology of brachial plexus injuries will be reviewed. Photographs and on-line video clips will enhance the description of the brachial plexus injury classifications and illustrate mechanisms of shoulder dystocia and obstetric relief maneuvers. A systematic approach to the physical examination will be explored in Part 2. Serial evaluation of motor function recovery is essential and is accomplished by appropriate referrals and follow-up. Part 2 will also describe treatment options and discuss anticipatory parent guidance.