Q: I have suffered from chronic leg pain ever since I had a disc problem and surgery to remove the disc. I now have a spinal cord stimulation (SCS) device. It doesn't get rid of all the pain but about 60 per cent is gone. If it can reduce the pain, why can't it eliminate it altogether?

A: That is a very good question. Spinal cord stimulation (SCS) is used for 1000s of people with chronic, debilitating pain. The device delivers a low level electrical current through wires. The wires are placed in the area near the spinal cord. The device is similar in size to a pacemaker. How it works exactly is still a mystery.

Patients are usually told upfront before ever receiving the unit that it does not provide a cure for the pain. A 50 percent or greater decrease in pain can be expected, however. Some patients get as much as 90 per cent relief of painful symptoms.

Even a 50 per cent reduction in pain should allow recipients to be more active. Also, the need for less pain medication is considered a successful result. A trial with an external device for about a week is done, before having the device implanted.

The original theory behind how these devices work is called the gate control theory of pain. According to this theory, electrical stimulation generated by the stimulator replaces the pain impulses.

The gate theory suggests that when the electrical stimulation reaches the spinal cord first, the "gate" closes and blocks the pain impulses. As pain lessens, the muscles around the sore area relax, further lessening pain.

But studies over the years have shown that this theory only explains some of the pain relief obtained. It probably isn't the only way the stimulator works. There is evidence now that spinal cord stimulation actually affects not one, but several different neural pathways.

If you have more than one pathway involved in the formation of your symptoms, then the device may not inhibit or turn off all pathways involved. But the fact that you get a significant portion of your pain eliminated suggests that the primary problem has been taken care of.

As more and more is understood about pain pathways, the use of spinal cord stimulation may expand. Eventually, it may include patients with all types of nerve pain as well as motor disturbances.

Reference: Daniel Ciampi de Andrade, et al. Neurophysiological Assessment of Spinal Cord Stimulation in Failed Back Surgery Syndrome. In PAIN. September 2010. Vol. 150. No. 3. Pp. 485-491.