DeQuervain's tenosynovitis is inflammation of the tendons of the thumb. It causes pain and swelling near the base of the thumb, which is worsened by movements involving the thumb and wrist. While the cause is often unknown, overuse can make the symptoms worse. Diagnosis involves a physical test called Finkelstein's test. Treatment may include rest, anti-inflammatory drugs, steroid injections, physiotherapy including exercises, and potentially surgery if other treatments are not effective. The document provides details on exercises that can help relieve symptoms.