Anterior interosseous nerve syndrome — often called AIN palsy — is a rare but important cause of forearm and hand weakness. It typically affects athletes, manual workers, and anyone doing repetitive gripping or rotation. Unlike other median nerve problems, AIN syndrome causes muscle weakness without numbness or tingling. So how do we spot AIN palsy, and how is it treated?

What is the anterior interosseous nerve?

The anterior interosseous nerve is a branch of the median nerve that arises just below the elbow, near the pronator teres muscle. It runs deep in the forearm and supplies three muscles:

  • Flexor pollicis longus (FPL) — bends the tip of the thumb
  • Flexor digitorum profundus (FDP) — bends the tips of the index and middle fingers
  • Pronator quadratus (PQ) — helps rotate the forearm inward (pronation)

Crucially, the AIN carries only motor fibres — it supplies no skin. So when it’s affected, there’s weakness but no numbness or tingling. This is the single most useful fact about the condition.

What causes anterior interosseous nerve syndrome?

The cause isn’t fully settled. For years it was thought to be a compression (entrapment) of the nerve in the forearm. Still, current evidence suggests many cases are actually a form of neuralgic amyotrophy — the same inflammatory nerve condition as Parsonage-Turner syndrome — with tiny constrictions within the nerve fascicles near the elbow. It often comes on spontaneously, sometimes after an inciting event such as a viral illness, trauma, unusual exertion, or pregnancy. True external compression (by fibrous bands, a mass, or a forearm fracture) is a less common cause.

Symptoms of AIN palsy

People with AIN palsy often notice weakness rather than pain — a sudden or gradual weakness of the thumb and index finger. There may be a forearm ache first, which then gives way to the weakness.

The hallmark sign is difficulty making the “OK” gesture: because the FPL and FDP are weak, the tip of the thumb and index finger can’t pinch into a proper circle, and instead form a flattened, triangular pinch. This is sometimes called the Kiloh-Nevin sign.

Because the AIN is purely motor, sensation is completely normal — there’s no numbness or tingling. That’s what distinguishes AIN palsy from carpal tunnel syndrome and lacertus syndrome, which both cause sensory symptoms.

How is anterior interosseous nerve syndrome diagnosed?

The diagnosis is mainly clinical — the pattern of weakness with normal sensation, and the abnormal “OK” sign. Investigations confirm it and exclude other causes:

  • Nerve conduction studies and EMG — the preferred test, showing abnormal activity in the FPL, FDP, and pronator quadratus
  • Ultrasound or MRI — to look for nerve constrictions, a compressing mass, or muscle changes, and to help distinguish the inflammatory from the compressive type

How is AIN palsy treated?

Most cases are managed conservatively, and many recover on their own — often over a period of six weeks to twelve months.

  • Observation and time — since spontaneous recovery is common, especially in the inflammatory (neuralgic amyotrophy) type
  • Physiotherapy — maintaining movement, nerve gliding, and strengthening as the nerve recovers, with splinting where helpful
  • Nerve-pain medication — if there’s a painful phase early on
  • Surgery — reserved for cases with a clear compressing lesion, or where there’s no recovery after a prolonged period (usually beyond a year). The role of surgery is debated, and for many it isn’t needed

Because recovery can take many months and isn’t guaranteed, ongoing review matters — a small proportion of people are left with lasting weakness.

Frequently asked questions about anterior interosseous nerve syndrome

What is the classic sign of AIN palsy?

The inability to make a normal “OK” sign. Weakness of the muscles that bend the tips of the thumb and index finger means the pinch collapses into a flattened, triangular shape rather than a circle. This is the Kiloh-Nevin sign, and it’s the clinical hallmark.

How is AIN syndrome different from carpal tunnel syndrome?

AIN palsy causes motor weakness without sensory loss because the nerve innervates muscles but not skin. Carpal tunnel syndrome, by contrast, causes numbness and tingling in the hand. So the presence or absence of numbness is a key indicator for telling them apart. Also, in carpal tunnel syndrome, patients are able to amke the OK sign. 

Will AIN palsy recover on its own?

Often, yes — especially the inflammatory (brachial neuritis) type, which tends to recover over six weeks to twelve months. Recovery isn’t guaranteed, though, and a minority are left with weakness, so the condition is monitored. Surgery is reserved for a clear compressing cause or a lack of recovery over time.

Is AIN palsy the same as Parsonage-Turner syndrome?

They’re closely related. Current evidence suggests many cases of AIN palsy are actually a localised form of neuritis— the same inflammatory nerve condition as Parsonage-Turner syndrome— rather than a simple compression in the forearm. This has shifted treatment towards patience and rehabilitation, rather than early surgery, in most cases.

Final word from Sport Doctor London about anterior interosseous nerve syndrome

Anterior interosseous nerve syndrome is a rare cause of forearm and hand weakness, with a distinctive hallmark: weakness of thumb and index-finger pinch — an abnormal “OK” sign — with completely normal sensation. That pattern separates it from carpal tunnel and lacertus syndrome. Most cases recover with time and physiotherapy, and surgery is reserved for the few with a clear compressing cause or no recovery.

If you have unexplained hand or forearm weakness, Dr Masci can assess you in London, including ultrasound in clinic. Contact the team here or call +44 (0) 203 488 0350.