There was much debate at the turn of the century as to whether these children's difficulties were due to a compression (by the collarbone, delivery forceps or the obstetrician’s fingers, or traction on the nerves). Studies, first by Clark and then Sever, were able to generate frank rupture of the nerve roots only by applying traction to the plexus (the traction was applied by pulling on the upper arm or depressing the shoulder). As the force of traction increased, injury to the upper roots became more severe and lower roots became involved in the injury. Thus a picture emerged for injury to the infant born head-first. Once the head of the baby is delivered, shoulders, overly large for the birth canal, hang up. The force then needed to deliver the shoulders results in the shoulder being depressed away from the neck while the neck is laterally flexed away from the shoulder. This applies a traction to the upper roots of the plexus.
The C5 and C6 roots are particularly prone to injury in such a state and, for each of these roots, the part of the nerve dealing with motor (muscle movement) function is much more vulnerable than the sensory (bringing sensation from the arm to the spinal cord) part. The C7 root is next in vulnerability, and its injury signifies a significant increase in injury force being applied, with permanent injury being more common. When C8 and T1 roots are involved in the injury, even more force has been applied and frank root avulsion (nerve roots pulled out of the spinal cord) is not uncommon. Isolated injury to the C8 and T1 nerves is only seen in association with breech (feet-first) or face-first deliveries and is very unusual. Several risk factors have been identified as being associated with injury to the plexus during birth. Head-first delivery with shoulder dystocia (too large to fit through the birth canal easily), increased birth weight, prolonged delivery and multiparous mothers (several prior births) are common factors. Babies born breech with their arms extended over their heads can sustain injury to their lower roots, and this is frequently a bilateral injury. Interestingly, there is a 175-fold increase in risk of injury to the plexus with breech deliveries. There is also a 14-fold increase in risk of injury to succeeding infants born to mothers of children with obstetrical brachial plexus injuries.