There are a number of conditions and diseases that can cause a tingling sensation in the body. This sensation is known as paresthesia, and is quite common in the hands and feet, but not in a location like the back of the knees. This is usually the result of restricted blood flow to the limb, which in turn may be caused by your posture, or perhaps pressure from your own body or some external object. Due to the decreased blood flow, nerve cells temporarily stop functioning normally, which gives you a sensation of numbness and tingling. If this is the cause, then paresthesia is of course harmless - this is a regular occurrence known as the leg (or arm, and so on) 'falling asleep', and results in no permanent damage. However, paresthesia can also be a sign of actual nerve damage; this is of course dangerous and requires proper diagnosis and treatment.
Your first course of action should be to rule out the possibility of this being a simple case of impaired circulation. For example, your chair may be such that it cuts off circulation to a small portion of your legs, without affecting the whole of each limb. If this is ruled out, then you need to consult a doctor - nerve problems cannot be diagnosed or treated without a thorough physical examination, and may often require scans and tests as well.
A simple nutritional deficiency can sometimes affect blood supply and nerve functions, and such problems can usually be treated quite easily. However, there are a number of dangerous conditions that can also cause paresthesia. Injuries can sometimes cause such symptoms, and so can many other diseases and conditions. Although this may seem like a mild symptom, it may only be the beginning. Diabetes is one common cause of nerve damage and paresthesia, and if you already have diabetes, you should inform your doctor about these new symptoms. Hypothyroidism also sometimes causes paresthesia, and once again, if you are already being treated for the disorder, it is important that you keep your doctor updated. Artherosclerosis is known to cause paresthesia as a result of the narrowing and sometimes complete blockage of blood vessels. Multiple sclerosis, on the other hand, involves direct damage to the central nervous system, as a result of a malfunctioning immune system. Although pain is a more common feature of shingles (medically known as herpes zoster), the nerve damage can sometimes result in tingling or numbness instead.
answered by M W