One Motor and One Sensory Test for Each Hand Nerve
A nerve injury missed at the first exam can leave a patient with a numb fingertip or a hand that cannot pinch, and the chance to repair it early is lost.
Key terminology
- Autonomous zone
- This is a patch of skin supplied by only one nerve, with no overlap from its neighbors. Numbness there pins the injury to that one nerve.
- Anterior interosseous nerve (AIN)
- This purely motor branch leaves the median nerve just below the elbow. It powers FPL, index FDP and pronator quadratus and supplies no skin.
- Froment sign
- This sign is a bent thumb tip while pinching paper between the thumb and index finger. The median FPL is substituting for a weak ulnar adductor pollicis.
- Posterior interosseous nerve (PIN)
- This deep motor branch of the radial nerve passes through the supinator into the back of the forearm. It extends the fingers and thumb and supplies no skin.
- Two-point discrimination
- This exam asks if the patient feels 1 or 2 blunt points touching the fingertip. The smallest gap felt as 2 points is the bedside measure of fingertip sensation.
- Neurapraxia
- This injury is the mildest kind, a conduction block with the nerve still in continuity. It typically recovers on its own over weeks to months.
Takeaways
- Test each nerve's sensation in its autonomous zone, the skin no other nerve reaches. The median nerve owns the volar pad of the index finger and the ulnar nerve owns the volar pad of the small finger. The superficial radial nerve owns the dorsal first web space. Hand surgeons test the index and small fingers to reflect median and ulnar function (Lundborg et al, Scand J Plast Reconstr Surg Hand Surg 1992;26:275-279, PMID 1335164).
- AIN palsy weakens FPL, index FDP and pronator quadratus with no sensory loss. The patient cannot bend the thumb tip or index fingertip, so the OK sign collapses into a flat pinch. Sensation is normal because the AIN supplies no skin. Pronator quadratus weakness is hard to see at the bedside (Keiner et al, Acta Neurochir 2011;153:2225-2229, PMID 21786008; Ikumi et al, J Med Case Rep 2023;17:253, PMID 37301873).
- The median motor test in the hand is palmar abduction of the thumb. The thenar branch powers abductor pollicis brevis, which lifts the thumb straight up off the palm. Thenar wasting is a late sign of median nerve compression at the wrist. After a thenar branch transfer, visible thumb abduction was the sign of return (Lee et al, J Neurosurg Case Lessons 2026;11:CASE25744, PMID 41569653; Banach et al, Przegl Lek 2004;61:120-125, PMID 15230155).
- The ulnar motor test is finger abduction and key pinch. Ulnar nerve injury weakens pinch through paralysis of the first dorsal interosseous and adductor pollicis. A positive Froment sign is the bent thumb tip that results. In 1 nerve transfer series, 10 of 12 patients kept a positive Froment sign even after pinch strength returned (Rieker et al, J Hand Surg Am 2026;51:974-981, PMID 42189016).
- PIN palsy drops the fingers and thumb but spares wrist extension and sensation. The patient cannot extend the fingers at the MCP joints, and the intrinsics still extend the IP joints. ECRL and brachioradialis branch off above the PIN, so the wrist still extends (often toward the radial side). There is no sensory loss (Fansa et al, Handchir Mikrochir Plast Chir 1997;29:335-338, PMID 9483433).
- Two-point discrimination is the bedside test of fingertip sensation after a hand injury. In 624 healthy adults the mean static value at the fingertips was 2.78-3.5 mm, and the mid-palm was coarser at 5.39-7.13 mm. Children discriminated even finer points. Test each side of the pad, since each digital nerve supplies 1 side of the finger (K et al, Indian J Plast Surg 2024;57:263-269, PMID 39345675).
- A humeral shaft fracture puts the radial nerve at risk, and most palsies recover without surgery. Radial nerve palsy followed 11.8% of 4,517 shaft fractures, mostly in the middle and middle-distal thirds and in transverse or spiral patterns. Overall recovery was 88.1%, and 70.7% recovered spontaneously with nonoperative care (Shao et al, J Bone Joint Surg Br 2005;87:1647-1652, PMID 16326879).
- In an extension-type supracondylar fracture, the AIN is the nerve most often hurt. Neurapraxia occurred in 11.3% of 5,148 children, and the AIN made up 34.1% of nerve injuries in extension-type fractures. Ulnar nerve injury dominated flexion-type fractures. A medial pin raised the risk of iatrogenic ulnar nerve injury (1 extra injury per 28 crossed-pin cases) (Babal et al, J Pediatr Orthop 2010;30:253-263, PMID 20357592; Slobogean et al, J Pediatr Orthop 2010;30:430-436, PMID 20574258).