Anterior interosseous syndrome

Overview

Anterior Interosseous Syndrome (AIS) is a rare neurological condition that results from compression or injury of the anterior interosseous nerve (AIN), a motor branch of the median nerve. The AIN innervates specific muscles in the forearm responsible for fine motor movements such as pinching and gripping. Unlike typical median nerve injuries, AIS does not affect sensation, since the AIN carries only motor fibers.

This condition is sometimes referred to as anterior interosseous nerve palsy and may present spontaneously or following trauma, repetitive motion, or infection.

Causes

Anterior Interosseous Syndrome can be caused by several factors that result in compression, inflammation, or injury to the anterior interosseous nerve. These include:

• Trauma: Fractures, crush injuries, or surgical complications near the elbow or forearm

• Repetitive strain: Prolonged use of tools or repetitive forearm movements (e.g., in athletes, typists, or factory workers)

• Neuritis: Idiopathic inflammation of the nerve (Parsonage-Turner syndrome or brachial neuritis)

• Compression: By abnormal fibrous bands, tendinous arches (especially of the flexor digitorum superficialis), or cysts

• Iatrogenic injury: Following orthopedic or neurosurgical procedures in the arm or elbow

Symptoms

The hallmark of AIS is weakness or paralysis of the muscles supplied by the anterior interosseous nerve without any associated sensory loss. Key symptoms include:

• Inability to make an “OK” sign: Due to weakness of the flexor pollicis longus and the flexor digitorum profundus to the index finger

• Weak pinch grip: Difficulty holding small objects between the thumb and index finger

• Forearm pain: Dull aching pain near the proximal forearm may precede motor weakness

• No numbness or tingling: Distinguishing feature from other median nerve lesions

In severe cases, complete paralysis of the affected muscles may occur, leading to significant hand dysfunction.

Diagnosis

Diagnosis of Anterior Interosseous Syndrome involves clinical evaluation supported by diagnostic tests:

• Physical examination: Includes the “OK sign test” and strength testing of affected muscles

• Electromyography (EMG) and nerve conduction studies: Confirm the diagnosis and localize the nerve dysfunction

• Imaging: MRI or ultrasound may be used to identify compressive lesions, inflammation, or muscle denervation

• Rule out differential diagnoses: Including cervical radiculopathy, median nerve entrapment at the elbow, and tendon rupture

Treatment

Treatment for AIS depends on the underlying cause and severity. Initial management is usually conservative:

Non-Surgical Treatment:

• Rest and activity modification: Avoid activities that aggravate symptoms

• Non-steroidal anti-inflammatory drugs (NSAIDs): To reduce pain and inflammation

• Physical therapy: To maintain range of motion and strengthen surrounding muscles

• Splinting: In some cases to support the affected hand during healing

Surgical Treatment:

• Indicated if symptoms persist for more than 3–6 months despite conservative management or if a compressive lesion is identified

• Decompression surgery: Involves releasing the anterior interosseous nerve from entrapment

Prognosis

The prognosis for Anterior Interosseous Syndrome varies based on the cause and timing of intervention. Many patients recover fully within several months, especially those with neuritis or mild compression. Recovery can be slower or incomplete if diagnosis is delayed or if there is significant nerve damage.

Early diagnosis and appropriate therapy, whether conservative or surgical, improve the chances of a favorable outcome.