Anomalous Course of the Extensor Pollicis Longus With Multiple Absences of Thumb Muscles

We report a rare case of a 10-year-old girl with anomalous course of extensor pollicis longus (EPL) muscle, which exists with absence of thenar muscles and muscles in the 1st extensor compartment. Her chief complaint was severe atrophy on the right thenar eminence. On physical examination, there was no obvious functional abnormality on her right thumb. On magnetic resonance imaging, we found that the abductor pollicis brevis, opponens pollicis, abductor pollicis longus, and extensor pollicis brevis muscles were absent. The tendon of the EPL muscle was found, but it had abnormal insertion on the radial side of the proximal phalanx, not on the distal phalanx. This variation was thought to have played a major role in compensating for impaired abduction of the thumb, which is usually accompanied by agenesis of major abductors of the thumb.