Which median nerve syndrome is typically motor-only with preserved sensation?

• A. Anterior interosseous syndrome
• B. Pronator syndrome
• C. Carpal tunnel syndrome
• D. Median nerve palsy at elbow

Answer: (B)

Understanding that some median-nerve injuries spare sensation because they affect only a motor branch helps here. Anterior interosseous syndrome involves the anterior interosseous nerve, a motor branch of the median nerve that innervates the deep forearm flexors — flexor pollicis longus and the flexor digitorum profundus to the index and middle fingers. When this branch is affected, you lose finger- and thumb-flexion at the interphalangeal joints, leading to an inability to form the precise pinch, often described as trouble making the “OK” sign. Sensation remains intact because the sensory fibers of the median nerve are not involved.

By contrast, pronator syndrome compresses the median nerve at the elbow and can cause both motor and sensory symptoms in the hand, carpal tunnel syndrome produces primarily sensory symptoms in the hand with possible later motor issues, and a median nerve palsy at the elbow typically causes broader motor and sensory deficits distal to the elbow. The motor-only with preserved sensation pattern best fits anterior interosseous syndrome.