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Brachial Plexus Injury in the Newborn Infant

Brachial plexus palsy occurs at a frequency of approximately four cases in 1,000 births. The most obvious signs of trauma to the brachial plexus in the newborn are limp hypotonic arm, paucity or lack of movements, and abnormal posture of the affected limb.
 ETIOLOGY
 In most cases, there has been a complicated delivery with prolonged, difficult labor. The infant is often large. The mechanism of injury is traction upon the brachial plexus. Excessive stretching may occur in either breech on vertex presentation, as the head and neck are forced into lateral flexion to deliver the shoulders. The slightly higher incidence on the right side is attributed to the greater frequency of left occiput anterior presentation.
 Experimental studies have shown that resistance to structural disruption by tensile forces differs in various segments of the brachial plexus. Neuronal elements surrounded by soft tissues are less vulnerable to injury than nerve roots; the latter are adjacent to bone as they exit through the foramina and course along the grooves of the transverse processes.
 ANATOMY
 Clinical signs relate to the site of the lesion. The distribution of weakness reflects the anatomy of the brachial plexus (Figure), whereas the abnormal postures of the affected extremity are caused by the imbalance and unopposed action of muscles that remained innervated.