Causes of Erb’s Palsy
Erb’s palsy is most commonly caused due to excessive lateral traction or stretching of the fetal head and neck in opposite directions during delivery, usually in association with shoulder dystocia. This may happen following the delivery of the head when the head is deviated away from the axial plane of the fetal body. The brachial plexus can be compressed causing it to stretch and tear. Sometimes pulling on the infant’s shoulders during the delivery or excessive pressure on the baby’s raised arms during a breech (feet first) delivery can cause brachial plexus injury. During labor, two potentially harmful forces act on the brachial plexus, i.e. the natural expulsive force of the uterus and the traction force applied by the obstetrician. Clinicians can underestimate the force they are applying during downward traction as significant force.
Risk Factors of Erbs Palsy
Shoulder dystocia increases the risk for obstetric brachial plexus palsy nearly 100-fold. The reported incidence of brachial palsy after shoulder dystocia can vary from 4% to 40%. Other risk factors include large infants, maternal diabetes mellitus, vacuum extraction, small mothers, low or mid-forceps delivery, obesity or excessive weight gain, prolonged pregnancy, prolonged second stage of labor of over 60 minutes, persistent fetal mal-position, operative delivery, breech extraction of a small baby, and previous delivery of an infant with an obstetric brachial plexus injury. Babies with higher birth weight are at increased risk of brachial plexus injury as the shoulder can be caught behind a part of the mother’s pelvic bone. Erb's Palsy after caesarean section accounts for one to four percent of cases.
Large babies and babies with breech presentation are at a greater risk of brachial plexus injury. Gestational diabetes mellitus is associated with higher risk of brachial plexus injury. The strongest predictor of brachial plexus injury is found to be a fetal birth weight of greater than 4,000 g. The most commonly seen case of brachial plexus injury is a large baby with vertex delivery who suffers shoulder dystocia.
In rare instances, brachial plexus injury can occur without relation to traction and sometimes in the absence of any identifiable risk factors suggesting a possible intrauterine origin of injury. In this case, the injury may occur by another mechanism. Uterine abnormalities can cause abnormal intrauterine pressure and injury to the brachial plexus in rare cases subsequently causing Erbs Palsy.