Sciatica is a word we use to describe a pain running down the back of the leg. It is usually thought to come from the sciatic nerve, which develops from several little nerve roots branching off the spine. When one or more of these nerve roots are "pinched" by a herniated disc, Sciatica develops, or at least so we have been lead to believe.

But is this the whole story? Not always, because Sciatica is not always what it seems to be. In fact, often no nerve is pinched at all, but rather pain is referred down the leg from painful structures in the back.

Referred pain is different from the pain of a pinched nerve. Referred pain is harder to understand because it does not follow the pathways of any nerve and is often felt in locations quite removed from its origin. An example that many people know about is referred pain from the heart --pain which is often felt in the left arm. No nerve runs from the heart to the elbow, but somehow the brain perceives pain coming from the elbow as well as (or instead of) the heart. This may be a mistake on the brain's part, but it probably doesn't matter much, because the purpose of the pain is really to make the patient lie down and rest for a while until the heart heals.

Sciatica due to referred pain is similar to pain from a pinched nerve, but at the same time is different in some respects. This distinction is important because treatment of the two conditions may be quite different.

Sciatica from a pinched nerve tends to go down the back of the leg to the foot. It may be felt as a band right down the back of the leg, or it may be felt in only part of the back of the leg -- the calf, for example. Often pain or numbness is felt in part of the foot or the toes as well. Coughing will often provoke a shot of pain into the back of the leg, and sometimes there is weakness in the leg or ankle.

Sciatica from referred pain may go down the back, front or side of the leg. When it goes down the back, it will skip the backof the knee. This is one of the most important ways to tell the difference between Sciatica caused by a pinched nerve and Sciatica caused by referred pain. Sometimes there is pain or numbness in the foot, but it tends to be more vague than that froma pinched nerve.

X-rays are not much help in separating these two types of Sciatica. CT scans and MRIs may be useful, but are far from fool-proof. They sometimes show us more than we really want to know. In fact, 30 percent of pain- free people will have bulging or herniated discs if we look for them, yet are experiencing no pain problems whatsoever. So for someone with Sciatica, a bulging disc on the CT scan is no guarantee that the disc is the cause of the pain.

What it boils down to is that there is no replacement for a careful examination to separate these two types of Sciatica.