What is it
De Quervain’s tenosynovitis is a painful inflammation of the tenosynovium in the wrist, which will affect the two tendons that move the thumb. These two major tendons, the extensor pollicis brevis (EPB) and the abductor pollicis longus (APL), connect the thumb to the forearm. On their way to the thumb, the APL and EPB tendons go side by side along the inside edge of the wrist. They pass through a tunnel near the end of the radius bone of the forearm (the tunnel helps hold the tendons in place), which is covered by the tenosynovium, a slippery coat that allows the two tendons to glide easily back and forth. The inflammation of this slippery coat constricts the movement of the tendons within the tunnel. 
The only sign, initially, could be soreness on the thumb side of the arm, near the wrist. If the problem isn’t treated, pain may spread up the forearm or further down into the wrist and thumb.
As the friction increases, the two tendons could begin to squeak as they move through the constricted tunnel. This noise is called crepitus. There could be also swelling along the tunnel near the edge of the wrist. 
Overuse of the wrist and hand, like extending the thumb, or repeatedly pinch or twist something with the thumb while turning the wrist, may lead to inflammation of the tenosynovium and tendons, thus limiting the motion of the tendons. 
Left untreated, the inflammation and progressive narrowing (stenosis) can cause scarring that further limits thumb movement. De Quervain’s tenosynovitis is sometimes called “baby wrist” or “mother’s thumb” because it often develops in new mothers, possibly because of the repetitive movements needed to care for an infant. It can also be caused by an injury to the thumb or by an inflammatory condition such as rheumatoid arthritis. 
De Quervain’s tenosynovitis occurs most often in individuals between 30 and 50 years old, being 10 times more common in women. 
Mild to moderate cases may experience pain with, or be unable to perform, writing, infant and child care, office tasks (e.g. filing or keyboarding), lifting items like books or milk cartons, performing ball or racquet sports, or other tasks requiring sustained or repetitive use of the thumb, or grasping/lifting involving the thumb.
Several complications can happen if you don’t seek immediate treatment for de Quervain’s Tenosynovitis. Physical therapy is the initial treatment for this syndrome, and a very important one. Recurrence of the symptoms is very rare.
Delaying treatment may lead to chronic inflammation, thickening of the tissues, and occasionally even locking of the tendon, preventing full thumb motion. Pain in the base of the thumb may also lead to unnatural ways of moving that may cause stress on other areas, such as the elbow or shoulder. 
However, if physical therapy doesn’t work as an initial treatment, surgery is needed. But, although surgery is undertaken, physical therapy is also needed after surgery to restore thumb and wrist motion, and to treat some of the complication that can happen due to the surgery Some of that complications are irritation of the small nerves which give feeling to the area of the skin on the wrist and back of the hand, causing numbness or a burning sensation in that area and subluxation (abnormal movement) of the released tendons. 
 A Patient’s Guide to: de Quervain’s Tenosynovitis. Chiropractic Performance & Sports Therapy Center.
 Tendon trouble in the hands: de Quervain’s tenosynovitis and trigger finger. (2010). Harvard’s Women’s Health Watch: April; 4-6
 Fact Sheet: de Quervain’s Tenosynovitis. Wesley Hand Centre.
 de Quervain’s Tenosynovitis (2002). American Society of Hand Therapists.