The anterior interosseous nerve is a pure motor branch of the median nerve supplying the FPL, index-middle FDP, and pronator quadratus. AIN palsy — either from nerve compression (Kiloh-Nevin syndrome) or fascicular compression within the median nerve — produces inability to make the OK sign (the distal phalanges of the thumb and index hyperextend because FPL and FDP-index are paralysed). It has no cutaneous territory, distinguishing it from carpal tunnel syndrome.
| Origin | Median nerve in the proximal forearm (below the elbow, after passing through the pronator teres) |
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AIN syndrome presents with inability to flex the thumb IP joint (FPL palsy) and the index DIP joint (FDP-index palsy) — the patient cannot make a pinch circle (OK sign). No sensory deficit. Spontaneous recovery within 6-12 months occurs in most cases (the syndrome often represents a neuralgic amyotrophy fascicular variant). Surgery is reserved for complete non-recovery at 12 months.
AIN paralysis producing inability to make OK sign managed with observation for 12 months — surgery for complete non-recovery.
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