Anterior Interosseous Syndrome
Jarrod Yamanaka, MD
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The anterior interosseous nerve (AIN) is a motor branch of the median nerve that innervates the pronator quadratus, flexor pollicis longus, and flexor digitorum profundus serving the index finger.
Anterior interosseous syndrome (AIS) is a “catch-all” term for neuropathies that result in paralysis of these muscles secondary to several causes.
Overall AIS is a rare condition with a paucity of research (1).
ETIOLOGY AND PATHOPHYSIOLOGY
The most frequent causes of AIS are direct traumatic nerve damage and external compression. Insidious onset of the condition thought to be a transient neuritis (1):
Chronic vibration (cyclists)
Bulky, tendinous origin of the deep head of the pronator teres
Soft tissue mass such as lipoma, ganglion, or tumor
Fibrous band originating from the superficial flexor
Pain in the forearm
Sensory loss not noted
Weakness noted as difficulty with writing or with fine-pinch activities
The signature finding is weakness of the flexor pollicis longus, flexor digitorum profundus indicis, and partial innervation of the 3rd digit as well as pronator quadratus.
Weakness of the flexor pollicis longus and flexor digitorum profundus to the index finger is indicated by an inability to make the “OK sign.” Rather, the distal interphalangeal (DIP) joint of the index finger and interphalangeal (IP) joint of the thumb are hyperextended during attempted tip-to-tip pinch.
The pronator quadratus is difficult to isolate clinically. Weakness may be detected by asking the patient to forcibly pronate the forearm against resistance (resist supination) with the elbow flexed at 90 degrees (1)[C].
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