I'm always interested in causes behind symptoms that often get lumped in under "repetitive stress injuries" (RSI). We blame typing and poor ergonomics for a lot of our woes, but when I was having a lot of trouble with hand pain, it was a matter of changing my diet, not my desk configuration. Of course, it took a couple doctors before someone thought to have me tested for food allergies (first I had to suffer through the here-take-some-drugs-and-wear-wrist-splints phase followed by the let's-sign-you-up-for-expensive-physical-therapy-you-can't-afford phase). That's when I found out I was intolerant to gluten and dairy and that eliminating them helped tremendously.
If you're in the same boat where you've tried a lot of the common fixes for hand/wrist/joint pain etc. and haven't found them all that helpful, it's definitely worth trying to get to the bottom--figuring out what's causing the symptoms rather than just trying to treat them.
I was recently surfing around the web and found a list of the common symptoms of vitamin B12 deficiency. The fact that numbness/tingling in the hands was on the list, of course, caught my eye. Here's a list of some of the typical symptoms:
1. Itchy or tingling tongue. The tongue suddenly itches from time to time without warning. This occurs on the edge of the tongue, along one side or the other or at the tip. Some individuals experience stinging, pain, or tingling instead of itching.
2. White spots in the skin, resulting from melanin becoming absent in the area. These often occur on the outside of the forearm, but may occur in other places. The longer these spots are there, the whiter they get. As time goes by, the spots become very dry and flaky to the extent that small raw spots of skin may be exposed.
3. Sharp stabbing, tingling pain in the palm of one or both hands. This occurs suddenly and for no apparent reason in a spot directly below the ring finger, approximately where the first palm crease is. If B12 deficiency is not treated, a tingling pain may begin to occur along the outside edge of the hand, starting from the wrist. This pain occurs when the wrist is extended.
4. Sores at the corners of the mouth, sometimes extending along the edge of the lip. These are raw spots, not blisters, and they tend to come and go.
5. Nerve shock in the side of the body. It can be felt coming on a few seconds before it hits, and then it hits almost like a mild but deep electric shock and quickly subsides. It can occur at the side of either hip or on either side of the upper body, along the ribs. It can occur consecutively in at least two or three locations, one right after the other.
6. Shortness of breath, but without chest pain. This can occur when walking just a few yards.
7. Eye twitch, usually in one eye or the other. It can occur on the eyelid or just below the eye.
8. Facial pain, usually on only one side of the face at a time. This pain varies so much that it would be difficult to describe all the possibilities. This can present as a dull pain in the cheek bone right underneath an eye, or a sharp shooting pain across the forehead, sometimes coming downward from the scalp to the edge of the nose by the eye.
9. Tingling along the back of one or both thighs, starting at the hips and shooting downward.
10. Memory loss and/or disorientation.
11. Migraine headaches. These may be preceded by a temporary blind spot in the center of the field of vision, usually lasting about ten minutes, and sometimes followed by facial pain under the eyes. After the blind spot vanishes, there may be zigzag streaks through the vision that may last up to hours. Even in the same person, there may be extreme variations in the headaches themselves. They may be quite severe with nausea or they may be virtually nonexistent.
This list is from the wikipedia article on Vitamin B12 Deficiency. I assume not everyone is going to have all of the symptoms (I know I had a couple of the less common gluten intolerance symptoms and few of the common ones when I was tested for that, but eliminating it from my diet was key in getting over my RSI). Another article on B12 anemia (from WebMD) mentions simply "Numbness or tingling in your fingers and toes."
If any of this stuff sounds familiar, it's definitely worth getting a blood test.
There are several causes of B12 deficiency including simply not eating enough meat (vegans and vegetarians may have the problem and can take a vitamin to overcome it) or not being able to digest it sufficiently (such as may happen with celiac disease or bacteria overgrowth in the intestines).
Taking vitamin pills may not be that effective if you're having trouble digesting B12. Alternatives are B12 shots (which you can give yourself after a doctor shows you how and writes a prescription) or B12 patches which you can buy over the counter (or even over the web--I saw some at Amazon).