The thenar region of the palm is the group of muscles in the thick pad just underneath the thumb. Thenar atrophy is an erosion of muscle tissue, which can impair control over the thumb and leave the hand disfigured. The problem can be several different conditions and disorders, including carpal tunnel syndrome, acromegaly, or direct trauma to the wrist or thumb. Additionally, thenar atrophy can occur if the wrist and hand are immobilized in a cast for several weeks or months. Advanced carpal tunnel syndrome is the most common cause of thenar atrophy. Three muscles, several tendons, and a small area of protective cartilage tissue are found in the thenar region. The muscle group that controls the movement of the thumb is stimulated by a long, vital structure called the median nerve. Most cases of thenar atrophy have to due with problems related to the median nerve. When it is compressed or severed, the resulting lapse of electrical activity in the hand means that the muscles are never exercised; unstimulated muscle tissue wastes away over time.