Signals generated through the spinal cord and peripheral nerves in the form of nerve impulses are the information pathways that the brain learns about the environment and reacts to the stimulation (using muscle contraction such as the withdrawal reflex). This pathway includes skin information (touch, temperature and pressure), position sense called proprioception (you know where your arms and legs are in space without looking) and pain (nociception) sensation.
Pain is an obviously important sensation for self-preservation. If you are burning your hand, it behooves you to remove your hand from the fire. If you are wearing through a substantial layer of your skin (a blister), pain is the indication that you need to protect this area of skin from further injury.
There are times that these nerve signal pathways themselves can become damaged. In cases like this, the signals delivered to the brain by the damaged nerve or nerves are false or incorrect. The skin may feel like it is on fire but there is no injury at the site of the pain. Sensation can even be altered at the level of the lower brain. Light touch can be interpreted as severe pain (allodynia). When the nerves themselves are damaged, burning, aching or stabbing pain are the typical descriptions of this type of pain. This is called neuropathic pain. If the pain does not subside after some length of time, this pain is called chronic neuropathic pain.
Chronic neuropathic pain can result from nerve compression (disc herniation, bone spur), from injury to the peripheral nerve (stingers or brachial plexopathy), from infection (shingles, Lyme disease), from autoimmune disorders (Lupus), from metabolic disorders (diabetes) and from trauma.
In cases of chronic neuropathic pain, even though you feel like you want to have these nerves “cut”, cutting these nerves will do nothing to stop the pain. Neuromas (tangles of the ends of these cut nerves) would result, which would create more pain and dysfunction than if the nerves were left alone. The injured nerves also have some intact functions, which need to continue to work. Any severing of this nerve would eliminate these useful functions.
Initial treatment to deal with chronic radiculopathy or spinal cord injury induced pain is the use of medications and time. There are occasions that this pain can reduce or extinguish itself over some period of time (see the section on nerve damage and healing to understand nerve recovery). If however this pain does not extinguish itself and becomes intolerable, there is a technique that can help reduce this perceived pain. It is the use of a spinal cord or peripheral nerve stimulator.
How Stimulation Works
The stimulator really trades one sensation for another. Think of the “white noise” produced when a TV is turned on but the cable is disconnected. This “noise” is similar to the sensation that one trades for pain reduction. This trading has to do with how the nervous system deals with stimuli of different intensities.
Do you remember the days you were in elementary school on the playground and bruised your forearm? If you rubbed your arm vigorously enough, you would gain temporary relief from that deep pain. This “fast rubbing” of your arm produced a flood of input from the touch receptors. This flooding of the sensory system overwhelmed the pain pathway and the pain “magically” disappeared.
This essentially is the same technique utilized by the spinal cord stimulator. The name for this phenomenon is the Gate Theory of pain. Two scientists, Melzak and Wall were on the short list for the Nobel Prize for this discovery.
The thalamus, located in the lower brain is the switchbox that routes pain signals from the body to the upper portions of the brain. Think of the nerve pathways from the body to the thalamus as pipes carrying messages. The pipes that deliver sensory messages are much larger than the pipes carrying pain messages. If the “pain pipe” delivers its messages unimpeded, the thalamus will “notice pain” and relay it to the upper brain, especially if the sensory pipe is not “flowing with much information”.
The sensory pipe has the capacity for a much larger volume of information than can be delivered by the pain pipe. If this flow of sensory information is large enough, the pressure of this flow will “close a valve”, which cuts off the flow of the pain pipe. The thalamus will then “sense” only sensory information and pain will be eliminated.
The stimulator, through electrical discharges from the implantable leads, creates this sensory overload. The leads of the stimulator however, have to be placed at the correct location for this pain valve to be shut off. This is the precise job of the implant specialist. If the leads are placed correctly, the “pain valve” can be shut off and only “white noise” will be noticed.
Appropriate Application of Stimulation
One of the problems with spinal cord stimulation is that this technique has to be used in the appropriate situation. If there is basic surgical pathology causing pain, this disorder should be surgically corrected before consideration of spinal cord stimulation.
Some of the individuals who specialize in implantation of these stimulation devices are not spine surgeons or neurosurgeons. They might not understand the disorder well enough to know that there are surgical solutions to these painful spinal disorders.
I have seen patients with typical pain generators (degenerative spondylolisthesis, spinal stenosis, foraminal stenosis- see website) undergo stimulator placement when the disorder should have been surgically repaired instead. I recommend that if the implanting specialist is an interventional radiologist, anesthesiologist or a physical medicine and rehabilitation doctor (PM&R), that the patient have a consultation with a spine surgeon or neurosurgeon prior to the implantation of the stimulator to make sure the correct decision is being made.
One of the beauties of stimulation is that a trial of stimulation can be performed on an outpatient basis. Leads can be placed percutaneously (through needles) and the wires connected to the electrodes can be left out of the body (temporarily of course). The wires are connected to a small generator that can be adjusted by the patient. If the trial is effective in blocking pain, the permanent implant should be equally as effective.
Implant Failure Over Time
The typical stimulators can be placed along the spinal cord in the epidural space or along a nerve or plexus (junction of a number of nerves). Over time, the stimulators need to be adjusted as in some cases, a fibrous membrane develops which surrounds the leads and “insulates” them. This requires increased signal to overcome the insulator effect of the additional fibrous tissue. Batteries rarely fail but can fail. Infection can occur in these implants requiring removal.
The diagnosis of chronic neuropathy or spinal cord injury that is not surgically fixable is the main indication for spinal cord or peripheral nerve stimulation. Make sure that there are no disorders that can be surgically repaired.