The successful “intradural” placement of stimulating electrodes represented an important key technical advance to demonstrate that spinal electrodes could be safely placed in stable positions and did not have to reside in the subarachnoid space to be effective. It then became apparent, from clinical experience, that the most effective pain relief from spinal neurostimulators was primarily that involving the lower extremities and, to a lesser degree, low back and this remained true until the recent advent of tripole electrode systems.
Once epidural electrode placement was established as a generally safe location multi-contact catheter and plate electrodes were designed to be placed in this location. Various designs for epidural application are shown here
The recent development of tripole multicontact electrodes have now allowed for greater control of stimulation patterns, most importantly in providing better control of low back pain.
Medtronic, Inc. has been the primary pioneer neurostimulator company. Unfortunately, because they primarily a cardiac pacemaker company they made a corporate decision a number of decades ago to turn their back on the more reliableradiofrequency coupled implanted systems and focus their marketing on internal pulse generators (IPGs). By this act Medtronic allowed other companies the opportunity entering into the neurostimulation business.
Shown here is what has typically been the bulky Medtronic neurostimulator IPG which is similar to those used in cardiac pacemakers. IPGs are powered by internal batteries which have required surgery replacement. Typical clinical life in the past was 2-3 years.
The present generation of IPGs represent a real advance in that they are quite compact and rechargeable from an external source. Shown here is the St. Jude “Mini Eon” which is estimated to provide a 8-10 year life expectancy.
The new generation of mini-IPGs are now about the same size of a compact Radio Frequency (RF) receiver (shown here) which have been the most reliable systems over the past 30 years. Because they are activated by an external source the implanted components are inert.
This illustration shows a radio-frequency (RF) coupled system where the batteries are in an external pulse generator. The RF receiver, which is about the size of a silver dollar, is placed under the skin and is activated by a small coil placed on the outside of the skin. The external transmitting coil is usually held in place with an adhesive pad although other options exist.
In the illustration to the left a catheter electrode has been inserted through a percutaneously placed needle into the epidural space. This technique, which is performed under local anesthesia with sedation, is typically used for screening purposes because it is relatively simple to perform. This ease of implantation has unfortunately made PENS electrodes the definitive implant system for many pain management specialists (who are not surgeons).
Shown here is an example of test stimulation screening where a catheter electrode has been advanced into the thoracic area. The system is externalized and the patient then has the opportunity of using this, on a ambulatory trial basis. At this point in time the potential neurostimulator candidate has already undergone drug and psychological screening.
In the example shown above the catheter electrodes have been placed as a definitive implant by a non-surgeon. Over a two month period electrode migration has occurred causing failure of neurostimulator efficacy requiring revision. This is not an uncommon circumstance when dependence is placed on non-surgically implanted systems.
Shown here is a surgically implanted plate electrode being inserted into the epidural space. This procedure is typically performed under general anesthesia and requires a small amount of bone removal. Epidural plate electrodes have a high degree of stability and much less failure. Care must be exercised in their placement in order to determine that the epidural space is patent
One of the more exciting observations, evident over the years of implanting neurostimulators, has been that some patients who have previously been disabled by pain have been able, by markedly increasing their exercise and activity in conjunction with decreased drug use to be able to reach a point where their implanted neurostimulator is no longer required for use.
It is presently being predicted that in the near future implanted neurostimulators will be “MRI safe”. This will represent a meaningful patient benefit when it occurs.
The Editor’s experience (direct and indirect), based on over 2,500 spinal cord neurostimulators implanted for the relief of intractable pain over a 35 year period, has indicated that when patients are carefully screened and the procedure is performed by appropriately trained and experienced specialists that there can, and should be, be a long-term success record. As with all medical devices and as with all surgeries there are potential serious complications and informed consent is important in regard to this. Fortunately, however, with neurostimulators the most likely complication is that of failure of the device itself, something which is typically repairable.