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 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 and peripheral nerve 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 spinal cord and peripheral nerve 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.
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