Wrist drop and spared triceps in high radial nerve injury
A high radial nerve injury after a fracture of the shaft of the humerus (the upper arm bone) causes wrist drop (wrist falling into flexion) but usually spares elbow extension.
Remember
When the radial nerve is injured at the humeral shaft, the muscles that lift the wrist and straighten the fingers fail and the wrist drops into flexion, but elbow extension is usually preserved because the branches to the long and lateral heads of the triceps leave the nerve above the radial groove (a shallow spiral channel on the back of the humerus).
The radial nerve supplies the extensor muscles, the muscles on the back of the arm and forearm that straighten joints. It winds around the back of the humerus (the upper arm bone) in the radial groove, a shallow spiral channel on the back of the shaft, where it is tightly bound to the bone. A fracture of the humeral shaft can therefore damage it. This is called a high injury because the nerve is damaged in the arm, before it reaches the forearm.
Damage in the groove cuts off everything the nerve supplies further down. The extensor muscles of the wrist and fingers lose their nerve supply, so the patient cannot lift the hand. The flexor muscles on the front of the forearm have other nerve supplies and now act without opposition, so the wrist falls into flexion. This posture is wrist drop. The fingers cannot be straightened at the metacarpophalangeal joints (the knuckles), so they hang flexed.
The triceps (the muscle that straightens the elbow) is the classic exam trap. Its branches to the long and lateral heads leave the nerve before the groove, so elbow extension is usually spared, although the muscle may be weakened because the branch to its medial head leaves in the groove. An injury higher up, in the armpit, can affect the triceps.
Sensation over the back of the thumb and of the index and middle fingers can also be altered, excluding the fingertips. Treatment typically involves a splint that restricts movement, giving the nerve time to heal, or surgery may be required.
Key facts
- Cause
- a fracture of the humeral shaft can damage the radial nerve where it lies in the radial groove.
- Wrist
- the wrist extensors lose their nerve supply, so the wrist falls into flexion (wrist drop).
- Fingers
- they cannot be straightened at the metacarpophalangeal joints (the knuckles), so they hang flexed.
- Triceps
- elbow extension is usually preserved, because the branches to its long and lateral heads leave the nerve above the groove.
- Sensation
- altered over the back of the thumb and of the index and middle fingers, excluding the fingertips.
- Higher lesion
- an injury in the armpit can affect the triceps.
Exam traps
- Loss of elbow extension is not the usual finding when the radial nerve is injured at the humeral shaft, because the branches to the triceps usually leave the nerve above the injury, so elbow extension is usually preserved.
- Loss of sensation over the palm side of the little finger does not fit a radial nerve injury, because the ulnar nerve supplies that skin, while a radial injury alters sensation over the back of the hand and the back of the thumb and of the index and middle fingers.
- Clawing of the ring and little fingers is a classic sign, but it comes from an ulnar nerve injury, whereas a radial injury leaves the wrist unable to extend and the fingers unable to straighten at the knuckles.
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