Tenosynovitis of the first extensor compartment is characteristic of De Quervain tenosynovitis and it is usually seen at the level of radial styloid. The finding is probably attributed to repetitive gliding of the tendons against the underlying bone.  Some people may have complete septum between the two tendons and may require two separate steroid injections to treat the inflammation. Note that the appearance of several fascicles within the abductor pollicis tendon is considered a normal variant which can be mistaken for tendon tear.