A 28-year-old man is brought to the emergency department after falling asleep with his right arm draped over the back of a chair after drinking alcohol. When he woke, he was unable to extend his right wrist. He has no neck pain. Past medical history is negative. Physical examination shows weakness of wrist extension, finger extension, and thumb extension. Sensation is decreased over the dorsal first web space. Triceps strength and reflex are normal. Sensation over the middle finger is intact.
Which of the following structures is most likely injured?
The correct answer is:
D) Radial nerve
This patient has compressive radial neuropathy, often called “Saturday night palsy,” due to prolonged compression of the radial nerve in the spiral groove. The radial nerve innervates the wrist extensors, finger extensors, and thumb extensors; injury can cause wrist drop. Sensory loss over the dorsal first web space is a classic finding because this region is supplied by the superficial radial nerve. Normal triceps strength and reflex suggest that the lesion is distal to the radial nerve branches to the triceps, which typically arise proximal to or within the axilla.
Answer choice A: C7 nerve root, is incorrect. C7 radiculopathy can cause weakness of elbow extension, wrist flexion, finger extension, or finger flexion, and sensory symptoms often involve the middle finger. This patient has preserved triceps strength and reflex and intact sensation over the middle finger, making C7 radiculopathy less likely than a radial nerve lesion.
Answer choice B: Median nerve, is incorrect. Median nerve injury can impair thumb opposition, wrist flexion, forearm pronation, and sensation over the palmar lateral 3.5 digits. It does not cause isolated wrist drop or sensory loss limited to the dorsal first web space.
Answer choice C: Musculocutaneous nerve, is incorrect. The musculocutaneous nerve innervates the biceps brachii, brachialis, and coracobrachialis and provides sensation to the lateral forearm through the lateral antebrachial cutaneous nerve. Injury would cause weakness of elbow flexion and forearm supination, not wrist and finger extension weakness.
Answer choice E: Ulnar nerve, is incorrect. Ulnar nerve injury causes weakness of interossei, medial lumbricals, adductor pollicis, and hypothenar muscles, with sensory loss over the medial hand and medial 1.5 digits. It does not explain wrist drop or decreased sensation over the dorsal first web space.
Key Learning Point
Radial nerve compression in the spiral groove causes wrist and finger extension weakness with sensory loss over the dorsal first web space, while triceps function is typically preserved.
