Radial Nerve Palsy vs Posterior Interosseous Syndrome: How to Tell the Difference
Nerve Injuries · 8 min read · 2026-03-21
Introduction
Wrist drop is one of the most recognizable clinical signs in upper limb neurology, yet students frequently struggle to distinguish between a full radial nerve palsy and an isolated posterior interosseous nerve (PIN) syndrome. The distinction matters enormously — one threatens grip strength and sensation, while the other spares the wrist extensors entirely.
Anatomy Refresher
The radial nerve (C5–T1) descends the spiral groove of the humerus before dividing at the level of the lateral epicondyle into two terminal branches:
- Superficial branch — purely sensory, supplying the dorsal hand and first three-and-a-half digits
- Deep branch (PIN) — purely motor, diving through the supinator muscle via the arcade of Frohse to supply the finger and thumb extensors
This anatomical divergence is the entire basis for differentiating the two conditions.
High Radial Nerve Palsy
A lesion proximal to the bifurcation — classically in the spiral groove ("Saturday night palsy") — produces:
- Wrist drop: loss of extensor carpi radialis longus (ECRL) and brevis
- Finger drop: inability to extend at the MCPs
- Thumb drop: loss of extensor pollicis longus and brevis
- Sensory loss: over the dorsal first web space and radial dorsum of the hand
- Weakened supination: brachioradialis and supinator are affected
- Triceps: may or may not be involved depending on the exact level
The key here is the combination of motor and sensory loss.
Posterior Interosseous Nerve Syndrome
A PIN lesion — often compressed at the arcade of Frohse — produces:
- Finger drop: inability to extend digits at the MCPs
- Thumb extension loss: extensor pollicis longus affected
- NO wrist drop: the ECRL is innervated proximal to the PIN takeoff, so wrist extension is preserved (though the wrist may deviate radially because extensor carpi ulnaris is lost)
- NO sensory loss: the superficial branch is entirely separate
- No brachioradialis weakness: innervated proximal to the split
The Clinical Decision Points
Common Pitfall
Students often see finger drop and immediately call it "radial nerve palsy." The critical question is: can the patient extend their wrist? If yes, it's a PIN lesion. If no, the lesion is proximal.
The second clue is sensation — any sensory deficit on the dorsum of the hand rules out an isolated PIN syndrome.
Clinical Pearl
Radial deviation of the wrist during attempted extension is a subtle sign of PIN syndrome. The ECRL still works (pulling radially), but the extensor carpi ulnaris (ulnar-side wrist extensor) is denervated. This asymmetric pull causes the wrist to deviate toward the radial side.
Summary
The distinction between radial nerve palsy and PIN syndrome comes down to three questions: Is wrist extension preserved? Is there sensory loss? Is brachioradialis intact? Master these three checks and you'll never confuse the two again.