Obstetric brachial plexus palsy (OBPP) was first described by the Scottish obstetrician William Smellie in 1779 who described, in a newborn, a case of unilateral arm paralysis which rapidly recovered. He attributed this palsy to compression of the axillary nerve against the humerus, probably wrongly in light of subsequent knowledge. The main risk factor for OBPP is high birth weight (>4000 g) and intrapartum shoulder dystocia causing traction on the brachial plexus. It has been demonstrated in cadaveric models that traction on the arm with the neck laterally flexed in the opposite direction causes rupture or avulsion of the brachial plexus. Upper roots are more likely to rupture due to strong ligamentous attachment to the spine whereas the lower roots are more likely to be avulsed. For unknown reasons, however, in the rare circumstance of small babies born breech, upper root avulsion is common. Other risk factors for OBPP include a previous child affected, prolonged labour, instrumented delivery, and multiparity, although each of these may be a surrogate for large birth weight.
Time-specific microRNA changes during spinal motoneuron degeneration in adult rats following unilateral brachial plexus root avulsion: ipsilateral vs. contralateral changes
