Musculoskeletal System
Median nerve cut at the wrist: thenar wasting, with palm sensation that can be spared
A median nerve cut at the wrist paralyses the thenar muscles (the bulk at the base of the thumb), yet lateral palm sensation can be preserved.
Remember
A wrist-level median nerve injury paralyses the thenar muscles, yet the palm can keep its sensation, because the palmar cutaneous branch (the branch that supplies the skin of the lateral palm) leaves the nerve in the forearm.
A low median nerve injury means damage to the median nerve at the wrist rather than at the elbow.
The thenar muscles are supplied through the recurrent branch of the median nerve, and this branch lies beyond a wrist cut, so the muscles are paralysed. Once the muscles are paralysed they eventually waste (they have lost their nerve supply), so the thenar eminence, the fleshy pad at the base of the thumb, flattens. The thumb's opposition, which is swinging it across to meet the fingertips, is also weakened. The forearm flexors (the forearm muscles that flex the wrist and fingers) are supplied higher up, so they keep working, and that separates a low injury from a high one at the elbow.
The palmar cutaneous branch supplies the skin of the lateral palm. It leaves the median nerve in the forearm, before the carpal tunnel (the tunnel whose roof is a tough band called the flexor retinaculum), and passes over that band. A cut at the wrist lies beyond this point, so the skin of the lateral palm can keep its sensation. The digital branches (the branches to the fingers) do lie beyond the cut, so the palmar skin of the thumb, index and middle fingers and the lateral half of the ring finger goes numb.
Palm sensation should be tested and not assumed. A wound at the wrist may injure the palmar cutaneous branch separately, so sensation over the palm needs testing. Prompt hand-surgery assessment is needed when a penetrating wound shows signs of median nerve injury, and the nerve needs repair if it has been cut.
Key facts
- Motor loss
- the recurrent branch supplies the thenar muscles, so a cut at the wrist causes wasting and weak thumb opposition.
- Lumbricals
- the lateral two lumbricals (small hand muscles that help bend the index and middle fingers at the knuckles) may also be affected, and weakness of these muscles is distinct from the thenar changes.
- Numb skin
- the palmar skin of the thumb, index and middle fingers and the lateral half of the ring finger.
- Palm skin
- lateral palm sensation can be preserved, through the palmar cutaneous branch, which leaves the nerve in the forearm.
- Forearm
- the forearm flexors still work, unlike an injury higher up at the elbow.
- Caution
- test palm sensation and do not assume it, because a wound at the wrist may injure the palmar cutaneous branch separately.
Exam traps
- The index fingertip does not keep its feeling after a wrist-level cut, because the digital branches to the index finger lie beyond the cut, so its palmar skin is numb.
- A median cut at the wrist does not numb the whole palm: the medial palm is supplied by the ulnar nerve, and the palmar cutaneous branch leaves the median nerve above the wrist, so the lateral palm can keep its sensation.
- The recurrent branch is not the spared nerve: it is the motor branch to the thenar muscles and carries no skin sensation, and the flattened thenar eminence shows it is affected.
- The ulnar palmar cutaneous branch also arises in the forearm, but its skin territory is the medial half of the palm, not the lateral palm.
- Wrist flexion is not weakened by a wrist-level median cut, because the injury lies below the forearm muscles, and the back of the hand is supplied mainly by the radial nerve, not the median nerve.
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