A 25-year-old man comes to the office due to right arm weakness. The patient injured his right shoulder several months ago in a motor vehicle accident and has had difficulty elevating his arm since. On examination, there is diminished pinprick and temperature sensation over a small area in the right upper lateral arm and prominent atrophy of the right deltoid with 3/5 strength on right shoulder abduction. Reflexes and handgrip strength are normal in the upper extremities bilaterally. This patient most likely injured which of the following nerves?

Question

A 25-year-old man comes to the office due to right arm weakness.  The patient injured his right shoulder several months ago in a motor vehicle accident and has had difficulty elevating his arm since.  On examination, there is diminished pinprick and temperature sensation over a small area in the right upper lateral arm and prominent atrophy of the right deltoid with 3/5 strength on right shoulder abduction.  Reflexes and handgrip strength are normal in the upper extremities bilaterally.  This patient most likely injured which of the following nerves?

Quizplus · Accepted Answer

11ec160b_3597_6bb5_9249_117a98bfa5c1_MD0001_00 The axillary nerve originates from the posterior cord of the brachial plexus and carries fibers from C5 and C6. It courses deep in the axilla below the shoulder joint, then runs through the quadrangular space (bounded superiorly by the teres minor, laterally by the humerus, medially by the long head of the triceps, and inferiorly by the teres major) and along the surgical neck of the posterior humerus. At this point, it gives rise to motor branches that supply the deltoid and teres minor muscles, and a sensory branch that innervates the skin over the lateral shoulder. Axillary nerve injury most commonly occurs in the setting of shoulder trauma (eg, anterior dislocation, proximal humeral fracture). Patients often have sensory loss over the upper lateral arm and weakness on shoulder abduction due to denervation (and possibly atrophy) of the deltoid muscle. (Choice B) The dorsal scapular nerve provides motor innervation to the rhomboids (retract the scapula) and levator scapulae muscles (elevate the scapula). (Choice C) Long thoracic nerve injury classically occurs during axillary lymph node dissection and results in paralysis of the serratus anterior muscle, leading to winging of the scapula. (Choice D) Proximal injury to the median nerve (eg, due to supracondylar humerus fracture) may result in palmar sensory loss over the first 3 digits and impairment of thumb flexion/opposition, flexion of the second/third digits, and wrist flexion/abduction. (Choice E) Injury to the musculocutaneous nerve can cause sensory loss over the lateral forearm and weakened elbow flexion due to denervation of the biceps brachii and brachialis muscles. (Choice F) Injury to the radial nerve at the axilla (eg, "crutch palsy") typically causes weakness of the forearm, hand, and finger extensor muscles (eg, wrist drop, absent triceps reflex) with sensory loss over the posterior arm, forearm, and dorsolateral hand. (Choice G) The thoracodorsal nerve provides motor innervation to the latissimus dorsi muscle, which is responsible for shoulder extension, adduction, and internal rotation. (Choice H) Ulnar nerve injury most commonly occurs at the elbow, resulting in sensory loss over the medial 1½ digits of the hand and weakness on wrist flexion/adduction, finger abduction/adduction, and flexion of the fourth/fifth digits. Educational objective: Injury to the axillary nerve most commonly occurs in the setting of shoulder trauma (eg, anterior dislocation, humeral fracture) and presents with sensory loss over the lateral shoulder and weakness on shoulder abduction (due to denervation of the deltoid muscle). __________ References: Peripheral nerve entrapment and injury in the upper extremity. (http://www.ncbi.nlm.nih.gov/pubmed/20082510) Axillary nerve injury. (http://www.ncbi.nlm.nih.gov/pubmed/10613150)

A 25-Year-Old Man Comes to the Office Due to Right