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Spinal Cord StimulationIn 1644, French philosopher René Descartes described the Specificity Theory of Pain. He proposed that pain was the result of an impulse produced by an injury, which traveled along a particular nerve to a center in the brain. According to this philosophy, the amount of pain experienced should be directly proportional to the extent of the injury. Therefore, it made sense to treat pain by removing the source of injury or blocking the pain pathway. For over 300 years, scientists focused their efforts on trying to discover pharmacological agents to block pain. As neurology became more sophisticated, nerve destruction procedures were introduced. When these procedures failed to accomplish the desired effect, the brain was targeted through lobotomy.
A major milestone in the understanding of pain occurred in 1965 when Canadian psychologist Ronald Melzack and British anatomist Patrick Wall proposed the Gate Control Theory of Pain. (1) They suggested that a mechanism in the spinal cord behaved like a gate, which could facilitate or inhibit the transmission of pain from a source of injury. Descending information from the brain could “close” the gate and block the perception of pain.
The Gate Control Theory of Pain was bolstered by the discovery of different types of nerve fibers in the spinal cord. The A-delta fibers are small myelinated fibers that transmit sharp pain while the C-fibers are small unmyelinated nerve fibers that transmit dull or aching pain. The A-beta fibers are the largest myelinated fibers, and they transmit touch, mild pressure, joint position sense, and vibration. When pain occurs, the gate is “opened” by the signals traveling up the A-delta or C-fibers, and “closed” by the A-beta fibers, thus blocking pain signals.
Neurosurgeon C. Norman Shealy was the first to put this theory into clinical practice. He used low voltage electrical stimulation of the dorsal column of the spinal cord to “close” the gate and block the perception of pain.(2)
Advances in technology and patient selection have made spinal cord stimulation (SCS) a significant tool in the arsenal of the pain specialist. Neuropathic pain is sometimes opioid-resistant, and many patients require the addition of adjuvant agents like antidepressants and anticonvulsants for pain relief. Patients who do not respond to standard pain management and nerve blocks may be candidates for spinal cord stimulation. This treatment is most often used in patients with post-laminectomy syndrome, epidural scarring, arachnoiditis, complex regional pain syndrome, peripheral neuropathy, and ischemic pain.
Patients who are considered for spinal cord stimulation must demonstrate at least 50 percent pain reduction during an initial trial via a partly external system before undergoing permanent implantation. The permanent device consists of a battery which is usually implanted under the skin in the upper buttock, and a wire or paddle lead electrode passed into the epidural space. Studies have shown that 57 to 84% percent of carefully chosen patients have pain relief with this device. Whiles outcomes are good with SCS, there is a 25 to 30 percent complication rate, usually due to migration or breakage of the epidural lead.(3)
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