These have several causes. Continued or repeated severe pressure may damage a nerve seriously, as in the case of a crutch pressing into the armpit and causing drop-wrist. Bruising due to a blow which drives a superficially placed nerve against a bone may damage, say, the radial nerve behind the upper arm. A sharp wound, for example from a knife, may sever nerves, along with other structures; this accident is specially liable to occur to the ulnar nerve in front of the wrist, for example when a person accidentally puts a hand through a pane of glass.
When a sensory nerve is injured or diseased, sensation is immediately impaired in the part of the body supplied by the nerve. Ulceration or death of the tissue supplied by the defective nerve may then occur. For motor nerves, the muscles governed through them are instantly paralysed. In the latter case, the portion of nerve beyond the injury degenerates and the muscles gradually waste, losing their power of contraction in response to electrical stimuli. Eventually, deformities result and the joints become fixed. This is particularly noticeable when the ulnar nerve is injured, the hand and fingers taking up a claw-like position. The skin may also be affected.
Damaged or severed (peripheral) nerve fibres should be sewn together, using microsurgery. Careful realignment of the nerve endings gives the fibres an excellent chance of regenerating along the right channels. Full recovery is rare but, with regular physiotherapy to keep paralysed muscles in good shape and to prevent their shortening, the patient can expect to obtain a reasonable return of function after a few weeks, with improvement continuing over several months.