Anterior interosseous/median nerve latency ratio.

This report describes a new diagnostic technique for evaluating anterior interosseous nerve injuries (Kiloh Nevin syndrome). The resultant "AIM" (anterior interosseous/median nerve) score establishes a normal ratio of the pronator quadratus latency to the abductor pollicis brevis latency after one stimulus to the proximal median nerve and simultaneous recordings of the evoked response at the respective muscles. One hundred normal nerves were tested in 61 patients. The result was an AIM score of 0.60 (SD = 0.06). Five patients with Kiloh Nevin syndrome were evaluated. The average AIM score was 0.76 (SD = 0.04). Five patients with severe carpal tunnel syndrome were evaluated. The average AIM score was 0.38 (SD = 0.06). Ten patients with borderline carpal tunnel syndrome were evaluated. The average AIM score was 0.48 (SD = 0.06). Anterior interosseous nerve entrapment or compression injury remains a difficult clinical diagnosis because it is mainly a motor nerve and the syndrome is often mistaken for finger ligamentous injury. We describe an easily performed electrodiagnostic latency ratio technique to diagnose this injury. This technique may also be helpful as a screen for carpal tunnel syndrome when it is difficult to control for systemic illness.