A 29-year-old man comes in with a stab wound, cannot raise his arm above horizontal, and exhibits a condition known as “winged scapula.” Which of the following structures of the brachial plexus would most likely be damaged?
**Core Concept**
The brachial plexus is a complex network of nerves that arise from the spinal cord and supply the upper limb. Damage to these nerves can result in weakness, paralysis, or altered sensation in the affected limb. In this case, the patient's inability to raise his arm above horizontal and the presence of a "winged scapula" (a condition where the scapula protrudes due to paralysis of the serratus anterior muscle) suggest damage to the long thoracic nerve, which is a branch of the brachial plexus.
**Why the Correct Answer is Right**
The long thoracic nerve arises from the spinal cord at C5, C6, and C7 and courses along the lateral aspect of the thorax to innervate the serratus anterior muscle. Damage to this nerve results in paralysis of the serratus anterior, leading to the characteristic "winged scapula" deformity. This occurs because the serratus anterior muscle is responsible for rotating the scapula upward and downward, and its paralysis allows the scapula to protrude. The brachial plexus is formed by the ventral rami of C5 to T1, and its roots, trunks, divisions, and cords can be affected by trauma, leading to a range of clinical presentations.
**Why Each Wrong Option is Incorrect**
**Option A:** The axillary nerve, which arises from the posterior cord of the brachial plexus, primarily supplies the deltoid and teres minor muscles, and its damage would result in weakness of shoulder abduction, not a winged scapula.
**Option B:** The radial nerve, which arises from the posterior cord of the brachial plexus, primarily supplies the extensor muscles of the forearm and wrist, and its damage would result in weakness of wrist extension, not a winged scapula.
**Option C:** The ulnar nerve, which arises from the medial cord of the brachial plexus, primarily supplies the intrinsic muscles of the hand, and its damage would result in weakness of hand intrinsic muscles, not a winged scapula.
**Clinical Pearl / High-Yield Fact**
The long thoracic nerve is particularly susceptible to damage in the setting of a traumatic brachial plexus injury, often referred to as a "stretch injury" or "traction injury." This occurs when the brachial plexus is stretched or torn due to a sudden, forceful movement of the arm, such as in a motor vehicle accident or a fall.
**Correct Answer:** C. The long thoracic nerve.