Anterior Interosseous Nerve Syndrome

Описание: The anterior interosseous nerve (AIN) arises from the median nerve approximately 4-6 cm distal to the elbow, about one-third of the way down the forearm. It branches from the anterolateral aspect of the median nerve and courses between the radius and ulna along the interosseous membrane, running deep to the flexor digitorum profundus (FDP) and flexor pollicis longus (FPL) muscles. The AIN is purely motor and innervates two and a half muscles in the deep compartment of the ventral forearm:

Flexor pollicis longus (FPL)
Flexor digitorum profundus (FDP) (to the index and middle fingers)
Pronator quadratus (PQ)
The AIN travels dorsal to the pronator quadratus muscle alongside the anterior interosseous artery and also provides innervation to the volar wrist capsule. Its terminal branches contribute to innervation of the carpal joint capsule.

Etiology of AIN Injury
Traumatic causes:

Penetrating injuries (e.g., stab wounds)
Forearm fractures (e.g., Monteggia fractures)
Venipuncture
Prolonged cast fixation
Complication of open reduction and internal fixation (ORIF) of fractures
Non-traumatic causes:

Compression neuropathy (e.g., fibrous bands, aberrant muscles, mass lesions)
Brachial plexus neuritis (Parsonage-Turner Syndrome, PTS)
Idiopathic causes (spontaneous AIN palsy)
Clinical Presentation
Deep, aching pain in the volar forearm
Motor deficits without sensory loss
Weakness in flexion of the thumb and index finger at the distal interphalangeal (DIP) joints
No sensory deficits, differentiating AINS from median nerve injuries
Clinical Examination
1. O.K. Sign / Circle Sign Test
The patient is asked to form an "O.K." sign by touching the thumb and index finger tips together.
Positive test: The distal phalanx of the thumb and index finger cannot flex properly, resulting in a flattened or incomplete O.K. sign due to weakness of the FPL and FDP to the index finger.
2. Benedictine Sign
The patient is asked to make a fist.
Positive test: The first two digits (index and middle fingers) fail to flex, showing partial weakness of the third digit, while the fourth and fifth digits flex normally.
Differentiation: Unlike ulnar claw hand (which occurs with a distal ulnar nerve lesion and is evident at rest), the Benedictine sign occurs when the patient attempts to make a fist.
3. Differential Diagnosis
High median nerve injury (e.g., Pronator Teres Syndrome - PTS)
Sensory deficits present (unlike in AINS)
Martin-Gruber anastomosis
Weakness in intrinsic hand muscles (misleading for AINS)
Parsonage-Turner Syndrome (PTS)
Painful onset with patchy weakness in the brachial plexus distribution
Diagnosis
Imaging & Electrodiagnostic Studies
MRI
Identifies soft tissue compression, mass lesions, or muscle denervation changes
Electromyography (EMG) & Nerve Conduction Studies (NCS)
NCS: Often normal (since AIN is motor only)
EMG: Abnormal findings in FPL, FDP (index & middle), and PQ
Management
Non-Surgical Treatment (First Line)
Observation (most cases resolve spontaneously)
NSAIDs for pain control
Splinting (some patients report improvement)
Surgical Intervention (If No Improvement in 4-6 Months)
Surgical exploration and nerve decompression
If traumatic: Nerve repair or grafting may be required
Quiz Questions
1. The anterior interosseous nerve arises from which nerve?
A. Median nerve
B. Ulnar nerve
C. Radial nerve
D. Musculocutaneous nerve

Explanation: The anterior interosseous nerve is a branch of the median nerve and supplies motor function to deep forearm muscles.

2. What is the primary function of the anterior interosseous nerve?
A. Motor only
B. Sensory only
C. Both motor and sensory
D. Autonomic

Explanation: The AIN is a purely motor nerve with no sensory function.

3. Which muscle is NOT innervated by the AIN?
A. Flexor pollicis longus
B. Pronator quadratus
C. Flexor digitorum profundus (index/middle)
D. Flexor carpi radialis

Explanation: The flexor carpi radialis is innervated by the median nerve, not the AIN.

4. The “O.K. sign” test evaluates which nerve?
A. Anterior interosseous nerve
B. Radial nerve
C. Ulnar nerve
D. Axillary nerve

Explanation: The O.K. sign tests the AIN by assessing the flexor pollicis longus and flexor digitorum profundus.
5. Which condition can mimic AINS but includes sensory deficits?
A. Monteggia fracture
B. Martin-Gruber anastomosis
C. Pronator teres syndrome
D. Carpal tunnel syndrome
Explanation: PTS affects the median nerve proximally and includes sensory loss, unlike AINS.
6. Which of the following fractures is most commonly associated with AIN palsy?
A. Supracondylar humerus fracture in children
B. Scaphoid fracture
C. Distal radius fracture
D. Clavicle fracture
Explanation: AIN palsy is commonly seen in supracondylar humerus fractures due to traction or direct nerve injury.
7. What is the main distinguishing feature of AINS compared to a high median nerve injury?
A. Presence of motor weakness
B. Wrist flexion weakness
C. Thumb opposition weakness
D. Absence of sensory loss