A 35-year-old, right-handed mechanic comes to the clinic due to progressive right hand weakness. The patient frequently uses a screwdriver at work. Neurologic examination shows preserved sensation in the upper limbs. There is weakness on extension of the fingers and thumb in the right hand. Strength is otherwise intact. Triceps reflexes are 2+ and bilaterally symmetric. The nerve affected in this patient was most likely injured at which of the following locations?
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The radial nerve enters the forearm anterior to the lateral epicondyle and divides into superficial and deep branches. The superficial branch provides purely somatic sensory innervation to the radial half of the dorsal hand, and the deep branch innervates the extensor compartment muscles in the forearm. After passing between the superficial and deep parts of the supinator muscle, the deep branch continues as the posterior interosseous nerve, which innervates muscles involved in finger and thumb extension.
Injury to the deep branch of the radial nerve at the supinator muscle may occur due to repetitive pronation/supination of the forearm (eg, frequent screwdriver use), direct trauma, or dislocation of the head of the radius. Patients typically have weakness on finger and thumb extension (ie, finger drop). The triceps brachii (responsible for elbow extension and triceps reflex) and extensor carpi radialis longus (responsible for wrist extension) are typically unaffected because the branches supplying these muscles diverge proximal to the supinator muscle. Cutaneous sensory branches are similarly preserved.
(Choices A and E) 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) with absent triceps reflex and sensory loss over the posterior arm, forearm, and dorsolateral hand. Injury to the nerve at the humeral midshaft along the spiral groove usually causes similar forearm/hand deficits with sparing of the triceps muscle.
(Choice B) Carpal tunnel syndrome can result from any condition that reduces the size of the carpal tunnel and compresses the median nerve (eg, pregnancy, hypothyroidism). Patients typically have pain/paresthesia affecting the first 3 and a half digits along with thenar atrophy and thumb abduction/opposition weakness as the disease progresses.
(Choice C) The coracobrachialis muscle lies deep to the biceps brachii and is innervated by the musculocutaneous nerve. Injury to the musculocutaneous nerve may result in decreased strength on forearm flexion and sensory loss over the lateral forearm.
(Choice D) In the wrist, the ulnar nerve passes between the hook of hamate and the pisiform bone in the Guyon canal. Ulnar nerve injury at this site can cause weakness on finger abduction/adduction and clawing of the fourth and fifth digits.
Educational objective:
Injury to the radial nerve at the supinator muscle may occur due to repetitive pronation/supination of the forearm, direct trauma, or subluxation of the radius. Patients typically have weakness during finger and thumb extension (ie, finger drop) without wrist drop or sensory deficits.