 It damages the tendons which results in pain and tenderness on the outer part of the elbow.De Quervain's syndrome is a medical condition when the synovial sheath surrounding tendons in the first extensor tendon compartment becomes inflamed, so called tenosynovitis.The tendons of the abductor pollicis longus and the extensor pollicis brevis run narrower due to the thickening of the synovial sheath, which causes pain when extending and moving the thumb outward.