There is a high signal within the extensor pollicis longus (EPL) tendon just distal to Lister's tubercle as it crosses over the second extensor compartment tendons (ECRL and ECRB) and is flattened by the overlying extensor retinaculum.
There is localized tenosynovitis around this area and a small amount of increased fluid within the EPL and ECRL tendon sheaths.