Anterior Interosseous Nerve Innervation
The anterior interosseous nerve (AIN) innervates three muscles in the forearm: the flexor pollicis longus (FPL), the flexor digitorum profundus (FDP) to the index and middle fingers, and the pronator quadratus. 1, 2, 3
Specific Muscle Innervation
The AIN is a pure motor branch of the median nerve that supplies the following deep forearm flexors:
Flexor Pollicis Longus (FPL): Responsible for flexion of the interphalangeal joint of the thumb 1, 2
Flexor Digitorum Profundus (FDP) to Index and Middle Fingers: Controls flexion of the distal interphalangeal joints of the index (FDP1) and middle (FDP2) fingers 1, 3
Pronator Quadratus: The deepest muscle in the anterior forearm, responsible for pronation of the forearm 1, 4, 3
Clinical Significance
Isolated paralysis of these three muscles without sensory deficits defines anterior interosseous nerve syndrome (AIN syndrome or Kiloh-Nevin syndrome). 4, 5
Common Presentations
Incomplete palsy patterns are frequent, with isolated FPL weakness being most common, followed by combined FPL and FDP1 involvement 1
Clinical testing reveals inability to flex the interphalangeal joint of the thumb and the distal interphalangeal joint of the index finger, creating an abnormal "OK sign" or pinch pattern 2
Pain in the forearm accompanies the motor deficit in approximately two-thirds of cases 1
Diagnostic Pitfall
The AIN is purely motor with no sensory component, so the presence of any sensory deficits should prompt consideration of alternative diagnoses such as median nerve compression at a more proximal level. 3 This distinguishes AIN syndrome from carpal tunnel syndrome or proximal median neuropathy, which would include sensory changes in the median nerve distribution.