A 16-year-old boy is brought to the physician for an annual health maintenance exam. His mother has observed an unusual prominence on the right side of the upper back and requests the physician to have a look at it. Past medical history is unremarkable. On physical examination of the back with the arms hanging by the side, the medial border of the right scapula is noted to be prominent compared to the left, and the entire right scapula is displaced laterally. There is no exaggerated prominence of the medial border of the right scapula when the patient is asked to do a wall push-up, and the level of both shoulders is observed to be the same.
Based on these findings, which of the following nerves is most likely to be injured?
B) Dorsal scapular nerve
The rhomboid major arises from the spinous processes of the thoracic vertebrae T2 to T5 as well as the supraspinous ligament, and inserts into the medial border of the scapula. The rhomboids help to hold the scapula (and thus the upper limb) onto the ribcage. Other muscles that perform this function include the serratus anterior and pectoralis minor. The rhomboids are supplied by the dorsal scapular nerve (C5). When they are non-functional (paralyzed), the medial border which was previously held onto the ribcage is now let free and appears prominent even when the arms are hanging by the side of the patient. Since the rhomboids pull the entire scapula towards the vertebral column, this function is lost when paralyzed, and the entire scapula is displaced laterally due to imbalance of forces.
Option A: Axillary nerve, is incorrect. The axillary nerve innervates the deltoid, teres minor, and the long head of triceps brachii muscles, none of which are associated with winging of the scapula.
Option C: Long thoracic nerve, is incorrect. The long thoracic nerve innervates the serratus anterior muscle, and injury of this nerve is the most common cause of scapular winging. The wall push-up test, in which the patient is asked to place the palms on the wall and simulate a push-up. A positive test would show prominence of the the medial border of the scapula compared to when the arms were by the side of the patient. The question stem reports there is no exaggerated prominence when the wall push-up test was performed, so this can be ruled out.
Option D: Spinal branch of accessory nerve, is incorrect. Although the trapezius may cause winging to some extent, there is also associated shoulder droop. The question stem mentions the shoulder level being the same on inspection, so the trapezius can be ruled out.
Option E: Subscapular nerve, is incorrect. The subscapular nerve innervates the subscapularis muscle which is not related to winging but causes the medial rotation and adduction of the arm.
Key Learning Point
The rhomboids muscles are supplied by the dorsal scapular nerve. Winging of the scapula as a result of damage to the dorsal scapular nerve can be differentiated from other causes as follows: 1) there is no associated shoulder droop as would be seen if the trapezius were involved; 2) the scapula is prominent even when the arms are hanging by the side of the patient, and the prominence is not exaggerated when the wall push-up test is performed (in contrast to when the serratus anterior is affected.)
