USMLE Step 1 Question 374

Question:

A 2 day old newborn is found to have persistent adduction and internal rotation of his left arm with extension at the elbow, pronation of the forearm, and flexion at the wrist. The delivery of the baby was complicated by a vaginal delivery in which the baby’s shoulder was caught on the mother’s sacrum. The obstetrician was required to pull hard on the baby’s head during delivery. Which nerve roots of the brachial plexus are most likely to be damaged in this situation?

Correct Answer: C.  C5, C6

The brachial plexus is composed of nerve roots C5-T1 which form trunks composed of roots C5-C6, C7 and C8-T1. In this situation, the baby has a upper brachial plexus injury. It is caused when there is significant traction put on the upper trunk of the brachial plexus. The nerve roots affected are most commonly C5 and C6, and much less frequently, C7. There is an increased risk of developing Erb-Duchenne palsy when a baby is delivered in the breech position, yanking on the shoulder (either in during delivery or in a situation when a falling person grabs a branch/bar to stop their fall) or when there is a traumatic fall on the side of the neck.

The description of the baby’s arm in the question stem results from a paralysis of the abductors of the arm, lateral rotators of the shoulder, and the biceps.

Another kind of traumatic palsy is Klumpke’s palsy (thoracic outlet syndrome) is caused by an embryologic defect compressing the inferior trunk of the brachial plexus. The effected nerve roots are C8 and T1 (choice E) and can present with atrophy of the thenar and hypothenar eminences, sensory deficits on the medial side of the forearm and hand, and disappearance of the radial pulse upon moving the head toward the opposite side.

Nerve roots C3-C5 (choices A and B) are involved in the innervation of the of the diaphragm (“3,4,5 keeps the diaphragm alive”), but C3 and C4 are not nerves innervating muscles of the arm.