Pronator Teres Syndrome vs Anterior Interosseous Syndrome: Proximal Median Nerve Lesions
Nerve Injuries · 8 min read · 2026-03-28
Introduction
Most students learn carpal tunnel syndrome cold but freeze when a median nerve problem presents above the wrist. Two proximal median nerve lesions dominate the differential: pronator teres syndrome and anterior interosseous nerve (AIN) syndrome. Each has a signature exam finding that, once memorised, makes the diagnosis nearly automatic.
Anatomy You Need
The median nerve descends the arm without branching, then enters the forearm by passing between the two heads of pronator teres. Just distal to this, it gives off the anterior interosseous nerve (AIN) — a purely motor branch that supplies:
- Flexor pollicis longus (FPL)
- Flexor digitorum profundus to the index and middle fingers (FDP I–II)
- Pronator quadratus
The main median nerve continues distally, providing sensation to the lateral palm via the palmar cutaneous branch (which arises proximal to the carpal tunnel) and to the radial three-and-a-half digits.
Pronator Teres Syndrome
Compression of the median nerve at the two heads of pronator teres — typically from repetitive forearm pronation, a fibrous band, or muscular hypertrophy.
- Sensory loss: lateral palm AND radial three-and-a-half digits (because the palmar cutaneous branch is also affected — the key distinction from carpal tunnel)
- Motor: weakness of all median-innervated forearm and hand muscles, including FPL, FDP I–II, FDS, and the thenar group
- Provocation: resisted pronation with the elbow extended reproduces symptoms
- No nocturnal symptoms (unlike carpal tunnel)
- Negative Phalen's and Tinel's at the wrist
Anterior Interosseous Nerve Syndrome (Kiloh-Nevin)
An isolated lesion of the AIN — often spontaneous, viral, or from forearm trauma.
- No sensory loss whatsoever — AIN is purely motor
- Cannot make the "OK sign": loss of FPL and FDP to the index produces a flat pinch instead of a rounded "O" between thumb and index
- Weakness of pronator quadratus: tested by resisted pronation with the elbow fully flexed (eliminates the contribution of pronator teres)
- No wrist or thenar weakness
The Differentiating Algorithm
The OK Sign Test
Ask the patient to make a circle with the thumb and index finger. Watch carefully:
- Normal: a round "O" with flexion at both the IP joint of the thumb and the DIP joint of the index
- AIN lesion: a flattened pinch — the patient pinches pulp-to-pulp because FPL and FDP I cannot flex the distal joints
This single observation is pathognomonic for AIN syndrome and takes three seconds to perform.
Common Pitfall
Students often miss pronator teres syndrome because they only test for Phalen's and Tinel's at the wrist, then conclude "not carpal tunnel" and stop. The clue is sensory loss extending into the lateral palm — the carpal tunnel cannot affect this region because the palmar cutaneous branch passes superficial to the flexor retinaculum.
Clinical Pearl
AIN syndrome is sometimes called "spontaneous neuralgic amyotrophy" of the median nerve and may follow a viral illness or vaccination. Many cases recover spontaneously over months to a year — premature surgery is a known overtreatment.
Summary
Pronator teres syndrome = median nerve sensory loss including the lateral palm, no Phalen's. AIN syndrome = pure motor loss with a flat OK sign. Carpal tunnel = digital sensory loss with palm spared and a positive Phalen's. Three sites, three patterns, three diagnoses.