Carpal Tunnel Syndrome is one of many RSI or Repetitive Strain Injury. RSI occur when a particular movement is performed over and over again and places undue strain on a particular structure. The body is a mechanical structure. Just like a machine, if the muscles or tendons are under constant strain, eventually something will give. If you have a pulley system and the rope is constantly being run through the pulley, eventually that spot on the rope will begin to fray.
Inflammation of the area creates compression on the median nerve. Inflammation can be caused by constant strain and pressure on the tendons. This strain can happen from doing the same motion over and over again. Using a mouse, typing on a computer, knitting, or performing sign language are just a few examples. Tendons are covered in a protective sheath. When there is constant strain this sheath can be worn away. It is similar to the covering of electrical wires. When the live wires come into contact with each other you get sparks. When the sheaths wear away the tendon becomes inflamed. Inflammation is part of the bodies healing process.
The problem is that there is very little space in the tunnel. When the area becomes inflamed there is nowhere for anything to go, and you get pressure of the median nerve.
Compression of this nerve can cause pain, tingling, numbness in the thumb, first two fingers, and up the arm.
One further complexity is that certain trigger points present in the shoulder and upper arm can mimic carpal tunnel symptoms. These trigger points can present as pain down the arm, numbness in the hand and fingers, or tingling in the arm and hand.