Trauma is the most common cause of brachial plexus injury. Trauma may occur during pregnancy prior to the onset of labor (intrauterine trauma), during labor, or at the time of delivery. The term obstetrical brachial plexus injury is used for brachial plexus palsy that occur during labor or at the time of delivery. Obstetrical brachial plexus injury is more frequent than brachial plexus palsy due to intrauterine trauma.

Intrauterine trauma prior to the onset of labor
Intrauterine brachial plexus palsy injuries during pregnancy (before the onset of labor) are due to compression of the brachial plexus by the uterine wall (uterine malformation, uterine fibromas, uterine maladaptation). Neonates with brachial plexus palsy due to intrauterine compression usually have a fixed anatomical deformity (Figure 260.1) in the position usually seeing when neonates with obstetric brachial plexus palsy try to move the affected arm. Bone deformities may be present. The arm may be thinner. Fibrillation potentials are found in the affected muscles at birth. These four findings (fixed deformity in the position usually seeing when neonates with obstetric brachial plexus palsy, bone abnormality, thinner arm, and early fibrillation potential) help to distinguish intrauterine onset of brachial plexus palsy from obstetrical brachial plexus injury.

Figure 260.1.— Fixed right arm position with decreased movements.

Trauma during labor and delivery (obstetrical brachial plexus palsy)
Trauma during labor and delivery is the most common cause of brachial plexus palsy in neonates. Risk factors for obstetrical brachial plexus palsy include diabetes mellitus, fetal macrosomia, instrumented midpelvic delivery, prolonged second stage of labor, postdates gestation, multiparity, shoulder dystocia, and an intrauterine weak arm (Figure 260.2).

Figure 260.2.— Fetal ultrasound 18-20 weeks gestation demonstrating a thin fixed right arm with decreased movements.

Decreased fetal arm movements during pregnancy leading to weakening of the forces that protect the brachial plexus may contribute to obstetrical brachial plexus palsy. (Click on clip)

Neonates born with intrauterine evidence of decreased arm movements such as anatomical deformities, thinner arm (Figure 260.3), or evidence of intrauterine arm injury that in addition have a brachial plexus injury during delivery show no fibrillation potentials during the the first week of life and do so at a later day.

Figure 260.3.— Brachial plexus palsy due to intrauterine factors predisposing for obstetrical brachial plexus palsy. Left arm is thinner.

Obstetrical brachial plexus injury occurs because the brachial plexus is pulled in a direction that causes the angle formed by the spinal cord and the plexus to increase beyond the stretching capacity of the nerve fibers (Figure 260.4).

Figure 260.4.— Brachial plexus palsy due to intrauterine compression. [A] Fixed anatomical deformity; [B] left arm is thinner.

Obstetrical brachial plexus injury is due to acute trauma as witnessed by the presence of swelling of the brachial plexus on MRI for several days after the injury (Figure 260.5). Evidence of trauma in other areas may be present. Frequent structures involved are the scalp (caput cephalohematoma), facial and hypoglossal nerves, phrenic nerve (diaphragmatic paralysis), oculosympathetic nerve fibers (Horner syndrome), clavicle (fracture), shoulder (posterior dislocation of the head of the humerus), and humerus (fracture). Specific investigations may be required to diagnose these injuries.

Figure 260.5.— MRI of the brachial plexus showing edema in the area of the left brachial plexus.

The cornerstone of the treatment of obstetrical brachial plexus injury is to avoid: (1) muscular atrophy in the affected muscles, and (2) muscular contractures of the healthy muscles. Atrophy of the affected muscles renders them unsuitable for reinnervation. Contracture of the healthy muscles produces muscle group imbalance and joint deformities. Muscular atrophy is prevented with physical therapy. Physical therapy involving the shoulder is recommended after 7 to 10 days of age. The waiting period is recommended to avoid stretching the plexus during a time when the plexus is vulnerable to further injury. Physical therapy should include active and passive exercises. Splinting is generally contraindicated unless low-weight splints are used, and only to arrest or reverse deformities. Some authors recommend electrical stimulation.