Spinal cord stimulation and intrathecal therapy are advanced therapeutic modalities used for treating patients with chronic intractable pain. They are essentially reserved for patients in whom continuing pain is not the symptom, but rather the disease. Together, these modalities consist of technology that is considered “neuromodulatory.”
Neuromodulation is electric or chemical alteration of the central nervous system to significantly reduce chronic pain or improve neurologic function by precise delivery of small doses of electricity or drugs directly to targeted nerve sites.
Electricity to treat pain dates back hundreds of years and was usually met with considerable skepticism. However, in 1967, Shealy and associates
32 reintroduced the use of electricity in treatment of patients in pain based on the Melzack and Walls publication of the gate control theory.
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In the early 1970s, spinal cord stimulation technology lost enthusiasm as technical failures and poor patient selection resulted in limited success in treating patients with chronic pain. During the past 30 years, however, many well-controlled studies have provided a substantial level of clinical experience. Subsequently, more specific patient selection criteria (
Figure 7) and technologic changes have resulted in successful utilization of electricity in management of chronic pain. Many common chronic pain conditions such as chronic radiculopathy, neuralgia, peripheral ischemia pain, and phantom limb pain respond to electrical neuromodulation (
Figure 8).
Currently, the drugs approved by the US Food and Drug Administration for intrathecal use include morphine sulfate, baclofen, and, most recently, ziconotide. Other drugs commonly administered by physicians experienced with this technology include other opioids such as hydromorphone, as well as agents such as bupivacaine and clonidine hydrochloride. Intrathecal therapy is used for malignant and nonmalignant pain as well as for spasticity not relieved with oral agents.
Systemic analgesics administered either orally or transdermally, as well as other conservative modes of therapy, are usually effective in reducing symptoms in most patients with malignant and in those with nonmalignant pain. However, for patients with chronic pain not responding to more conventional treatment modalities, intrathecal therapy may be an option. It is reserved for those who failed all of the more conservative approaches, including systemic delivery of analgesic medications such as the many sustained-release opioids that are now available. Intrathecal therapy is considered a last-resort therapy. When delivered intrathecally, opioids exert a potent analgesic effect via spinal and supraspinal receptors, without significantly affecting motor, sensory, and sympathetic reflexes.
The most recent advance in intrathecal therapy is the now marketed ziconotide, a synthetic equivalent of a conotoxin derived from a marine snail. Ziconotide selectively and reversibly blocks N-type voltage-sensitive calcium channels, thereby inhibiting the release of neurotransmitters from primary afferent nociceptors located in the dorsal horn of the spinal cord. Although slow titration is required to minimize the occurrence of adverse effects, tolerance to ziconotide does not appear to develop. Early clinical experience suggests that individual response can differ markedly.
34 Ziconotide is currently not being used as a first-line intrathecal medication.