11681
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 collision and has had difficulty raising 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?
A.Axillary
B.Dorsal scapular
C.Long thoracic
D.Median
E.Musculocutaneous
F.Radial
G.Thoracodorsal
H.Ulnar
THE CORRECT ANSWER IS : A. Axillary (78%)
EXPLANATION
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 such as anterior shoulder dislocation or fracture of the surgical neck of the humerus. Patients often have sensory loss over the upper lateral arm and weakness on shoulder abduction due to denervation of the deltoid muscle.
(Choice B) The dorsal scapular nerve provides motor innervation to the rhomboids (which retract the scapula) and levator scapulae muscles (which elevate the scapula).
(Choices C and G) Injury to the long thoracic nerve or thoracodorsal nerve can occur during axillary lymph node dissection. Long thoracic nerve injury results in paralysis of the serratus anterior muscle, leading to winging of the scapula. Thoracodorsal nerve injury results in paralysis of the latissimus dorsi muscle, leading to weakness of arm adduction, extension, and internal rotation at the shoulder.
Subject: anatomy
System: Nervous System
Topic: JB
