Anterior interosseous nerve syndrome, often referred to as AIN palsy, is a rare but important cause of weakness in the forearm and hand. It usually affects athletes, manual workers, and individuals performing repetitive gripping or rotational activities. Unlike other median nerve problems, AIN syndrome causes muscle weakness without sensory symptoms such as tingling or numbness in the hand or fingers.

Anatomy and Function

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

  • Flexor pollicis longus (FPL) – flexes the thumb tip

  • Flexor digitorum profundus (FDP) – flexes the tips of the index and middle fingers

  • Pronator quadratus (PQ) – helps rotate the forearm inward (pronation)

Because the AIN carries only motor fibres, injury leads to weakness but no sensory loss.

Symptoms of Anterior Interosseous Syndrome 

Patients with anterior interosseous nerve syndrome often notice sudden or gradual weakness in their thumb and index finger rather than pain. The hallmark sign is difficulty forming the “OK” gesture — the tips of the thumb and index finger cannot touch because of weakness in the FPL and FDP.

Other common symptoms include:

  • Weakness when pinching or gripping small objects

  • Fatigue during writing or fine motor activities

  • Deep, aching forearm discomfort in some cases

  • Weak pronation (turning the palm down), especially with a flexed elbow

Since the AIN is purely motor, there is no numbness or tingling, distinguishing AIN palsy from carpal tunnel syndrome or lacertus syndrome. 

Clinical Presentation of Anterior Interosseous Syndrome

Patients with AIN palsy show weakness in flexion of the thumb’s interphalangeal joint and the index and middle fingers’ distal joints. When asked to make an “OK” sign, the thumb and index finger form a flattened shape rather than a circle.

There is usually normal sensation throughout the hand and fingers, which helps confirm the diagnosis. Pronation may be weak, and in chronic cases, mild forearm muscle wasting can develop.

The condition can occur due to several mechanisms, including compression by fibrous bands, inflammation (neuritis), or trauma such as forearm fractures or heavy lifting. Brachial neuritis is also a recognised cause. Over 50% of cases of AIN swelling are secondary to acute brachial neuritis. 

Investigations

Diagnosis is mainly clinical, but investigations help confirm the diagnosis of anterior interosseous nerve syndrome and exclude other causes.

  • Electromyography (EMG) and nerve conduction studies are the most valuable tests. They confirm denervation of the FPL, FDP, and PQ muscles.

  • MRI of the forearm may identify compression sites, inflammatory changes, or muscle oedema consistent with denervation.

  • Ultrasound can visualise nerve swelling or fibrosis along its course and rule out space-occupying lesions.

  • Laboratory tests are used to rule out systemic causes such as diabetes or inflammatory neuropathy.

Treatment

Most cases of AIN palsy recover with conservative management. The condition often improves spontaneously within 3–6 months, particularly when caused by neuritis rather than structural compression.

Non-surgical treatment includes rest, avoiding aggravating activities, and physiotherapy. Therapy focuses on maintaining range of motion, preventing stiffness, and encouraging gradual strengthening as recovery begins. Pain can be managed with anti-inflammatory medications, and in some cases, we use ultrasound-guided nerve hydrodissection to free the nerve from surrounding tissue adhesions.

If there is clear evidence of compression or no improvement after 3–6 months, we consider surgical decompression. This involves releasing potential compression points near the pronator teres and the deep forearm fascia.

Prognosis

The prognosis for anterior interosseous nerve syndrome is generally good, though recovery can take several months. Partial nerve injuries recover faster than complete palsy. Early diagnosis, activity modification, and targeted physiotherapy improve outcomes and help athletes return to sport safely.

Final Word from Sportdoctorlondon about Anterior Interosseous Syndrome

AIN palsy is a rare but important cause of forearm weakness. Because it produces weakness without sensory symptoms, it may be mistaken for tendon injury or fatigue. Awareness of anterior interosseous nerve syndrome, careful clinical assessment, and appropriate investigations are essential for accurate diagnosis and effective management.

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