Also known as De Quervain's tenosynovitis, an inflammation of the sheaths of the tendons that move the thumb up and out (away from the hand). It is technically referred to as a stenosing tenosynovitis because both the tendons and their surrounding tissue sheaths are involved where the tendon has to go through a small fibro-osseous tunnel at the wrist. The inflamed tenosynovium becomes painful at the tunnel area.