Guyon canal syndrome
Guyon's tunnel is a space in the wrist which contains the ulnar artery and vein as well as the ulnar nerve, which is one of the major nerves in the arm. Compression of the nerve at this point can lead to, among other symptoms, loss of sensation and loss of ability to stretch and flex the fourth and fifth fingers, and general weakness in the hand.
This kind of compression can be induced by activities such as using the palm of the hand as a hammer (also see hypothenar hammer syndrome), using a screwdriver or pliers, or long-distance cycling.
Treatment options can include avoiding activities which aggravate the condition, splinting, anti-inflammatory medication, steroid injections, and physiotherapy. If these are unsuccessful, surgery to decompress the nerve may be needed.