Patients with neuropathic pain who do not respond or do not have a sufficient response to conventional treatment may benefit from neuromodulation techniques. The gate control theory, introduced by Melzack and Wall in 1965, provided the theoretical foundation for the use of implanted electrical stimulation (Melzack and Wall 1965). Although the specific mechanism of action of SCS remains elusive, neural and neurochemical changes, perhaps resulting from stimulation in the dorsal roots, dorsal root entry zone, or dorsal columns, have been implicated.
The systems in current use employ a totally implantable impulse generator system that utilizes an implantable receiver and external transmitter. The system is flexible to accept multiple leads, depending upon the need for bilateral extremity stimulation or wider unilateral or axial coverage. Similarly, various electrode configurations exist, varying in the number and spacing of electrodes. It is also possible to take advantage of complex programming options to fine-tune or change stimulation patterns.
Prior to permanent placement, a test stimulation is performed. Generally, temporary percutaneous stimulator leads are used for this purpose. Once an appropriate stimulation pattern is obtained, most protocols require at least a 50% reduction in pain scores. The length of the trial period varies but usually ranges from 3 to 7 days in most centers.
Lead migration and breakage are common problems with long-term stimulation. Lead migration may produce unwanted paresthesias or diminish benefit in the original area of stimulation. This can sometime be overcome by reprogramming of the electrode but may require lead replacement. Serious infection, bleeding, and nerve injury are uncommon complications. Impulse generator failure is unlikely; however, fully implantable generators will require replacement, depending upon use and battery life. Other contraindications to implantation such as the presence of localized infection, systemic sepsis, severe immune suppression, and coagulopathies are similar to other implantable devices.
The most critical issues in patient selection consist of identifying a well-founded diagnosis and the presence of specific neuropathic or ischemic pain states. A multidisciplinary approach including a psychological evaluation is often recommended.
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