Anterior Interosseous Nerve Syndrome (AINS, Kiloh–Nevin Syndrome). A rare nerve condition that causes weakness of the thumb, index finger, and hand

Łukasz Birycki
5 days ago
3 min read

What Is the Anterior Interosseous Nerve?

The median nerve is one of the main nerves of the arm. It starts in the neck, travels through the shoulder and upper arm, passes the elbow, and continues into the hand.

The anterior interosseous nerve (AIN) is a motor (movement) branch of the median nerve. It separates near the elbow and controls muscles responsible for:

Bending the thumb
Bending the index and middle fingers
Rotating the forearm inward (pronation)

Unlike many other nerve problems, AINS does not cause numbness or tingling, because this nerve controls movement only.

What Causes Anterior Interosseous Nerve Syndrome?

Compression or irritation of the anterior interosseous nerve may be caused by:

Swelling of the biceps tendon bursa
Bone changes after fractures of the arm
Anatomical variations (extra muscle or small bone growth)
Direct trauma
Shoulder or elbow dislocation

In some people, symptoms may appear without any clear injury, especially if an anatomical variation is present.

Symptoms – How It Usually Feels

Symptoms may appear:

Immediately after an injury
Or gradually after a long symptom-free period

Common symptoms include:

Weakness in the forearm and hand
Difficulty performing precise hand movements
Fast fatigue of the hand
Inability to pinch or grip small objects

A classic sign is the “OK sign” problem:

When trying to make an “OK” sign, the thumb and index finger stay straight instead of forming a circle
This creates the so-called “blessing hand” appearance

Symptoms may:

Appear suddenly
Or slowly worsen over time, depending on the cause

Diagnosis

Diagnosis starts with a detailed medical history, which may reveal past injuries or risk factors.

Clinical examination includes:

Testing thumb and index finger strength
Asking the patient to hold a coin or a piece of paper against resistance
Difficulty picking up a coin from a flat surface is a common finding

The most reliable test is:

Electromyography (EMG), which objectively measures nerve function

Differential Diagnosis – What Needs to Be Ruled Out

It is important to make sure symptoms are not coming from:

The neck (cervical spine)
The shoulder or thoracic outlet

AINS can resemble:

Pronator teres syndrome
Carpal tunnel syndrome

Specific clinical tests (resisted forearm rotation, stretching, Phalen’s test) help distinguish between these conditions.

Treatment

Conservative (Non-Surgical) Treatment

If EMG shows partial nerve function is preserved, conservative treatment is recommended.

Physiotherapy focuses on:

Releasing nerve compression
Improving nerve mobility (nerve gliding)
Restoring movement control

Additional treatments may include:

Electrical stimulation to activate the nerve
Physical therapy modalities
Anti-inflammatory medication to reduce swelling around the nerve

Surgical Treatment

If EMG shows severe nerve damage or if symptoms persist longer than 6 months, treatment options include:

Steroid injections at the compression site
Endoscopic nerve release surgery

Physiotherapy After Surgery

Surgery does not replace physiotherapy.

After the procedure:

Scar therapy is essential
Movement and nerve mobility must be restored
Strength and coordination need to be rebuilt gradually

When to See a Physiotherapist

You should seek professional help if you notice:

Sudden or progressive hand weakness
Difficulty gripping small objects
Fatigue of the hand without numbness
Problems forming the “OK” sign

Early diagnosis greatly improves recovery outcomes.

References

Rodner C. M. et al. Pronator Syndrome and Anterior Interosseous Nerve Syndrome. JAAOS, 2013.
Chi Y., Harness N. G. Anterior Interosseous Nerve Syndrome. Journal of Hand Surgery, 2010.
Keiner D. et al. Anterior interosseous nerve compression syndrome. Acta Neurochirurgica, 2011.