Answer to Woodstock: Hitchhiker’s Nightmare

Correct answer is A, the inferior part of the brachial plexus. The rationale for this answer is based on the scenario presented. The hitchhiker is unable to extend his thumb, an action of the extensor pollicis longus muscle that is innervated by the radial nerve. Hence, stem of the question leads one to look for a radial nerve in the images, but none are labeled. #2 indicates the musculocutaneous nerve emerging from the brachial plexus. #3 labels the medial root from the medial cord. #4 is the median nerve. #5 indicates the cutaneous component of the musculocutaneous nerve. Only #1 presents as a plausible answer, in which the lower trunk of the brachial plexus, consisting of the C8 and T1 spinal roots is labeled. Indeed, such an injury to the lower trunk is not uncommon in difficult deliveries if the child’s arm exits the birth canal and the child is yanked prior to full dilation of the canal. The yanking may stretch the nerves to the point of injury (neuropraxia) as a function of compression and traction.

This type of injury is demonstrated in lecture 4 of the upper limb series.