Neuromodulation for Pain
Tory McJunkin, MD, DABA
Founder, Arizona Pain Specialists
Clinical Instructor, Mayo Clinic Rochester
Nicole E. Berardoni, MD
Research Director, Arizona Pain Specialists
Radiology Resident, Maricopa Integrated Health Systems
Dr. McJunkin is a consultant for St. Jude Medical and Boston Scientific. Dr. Berardoni has no relevant financial conflicts to disclose.
Chronic pain is a pandemic that affects approximately 11% of people in the United States, and is estimated to cost the nation $100 billion per year in medical expenses, lost income, and productivity.1,2 Chronic pain interferes with all aspects of life, including personal enjoyment, work, and relationships. Although pain is universal, the etiology of many painful conditions is not well understood, making treatment difficult.
Forms of neuromodulation were used long before the discovery of electricity. The Egyptians first attempted electric stimulation to treat pain as early as 2,500 BC as seen on stone carvings depicting electric catfish being placed on people.4 The torpedo fish, which also generates electric currents, was used to treat pain and common maladies like headaches and arthritis during the times of Socrates.5 Electric shocks from these animals can reach 220 volts, enough to kill a human adult. In ancient Greece and Rome, the shocks from the species Torpedo nobiliana were used as a treatment for gout, headache, and other sources of pain.4
Grounded in the theory that non-noxious stimuli can reduce noxious pain perception, modern neuromodulation works by introducing an electric current near the source of chronic pain impulses or along pain pathways. In 1967, Shealy introduced neuromodulation into contemporary medicine; however, the approach did not gain popularity for the treatment of pain until the mid-1980s.6,7
Stimulation is becoming increasingly popular for the treatment of refractory pain conditions. It is reversible, safe, cost-effective, and nonpharmacologic. The most common forms of stimulation offered to treat pain are spinal cord stimulation (SCS), peripheral nerve stimulation (PNS), motor cortex stimulation, and deep brain stimulation. This review focuses on SCS and PNS.
Most implantable pulse generators (IPGs) are rechargeable, programmable, and can connect to one or two leads. The IPG produces a low-voltage current, which creates a sensation that blocks the ability of the brain to sense previously perceived noxious stimuli. The intensity of the stimulator can be changed, the pattern of the electrical field can be varied, and the system can be turned on and off as necessary to provide optimal pain relief as experienced by the patient.
A stimulator trial is performed prior to permanently implanting the IPG and leads. During a stimulator trial, one or two leads containing varying amounts of electrodes are implanted percutaneously into the epidural space (SCS) or close to pain-generating nerves (PNS) with local anesthetic and IV drug sedation. If pain and function improve by a margin of 50% or more (assessed on a visual analog scale
Spinal Cord Stimulation
Spinal cord stimulation has proven to be an effective treatment modality in improving daily function and quality of life for many patients suffering from severe neuropathic pain of various causes, most commonly chronic back and limb pain (11). However, treatment of chronic neuropathic pain in the region of the face, neck, head, and low back are challenging for pain specialists. The pain is typically refractory to many of the conventional treatment options, including medical therapies (non-steroidal anti-inflammatory drugs [NSAIDs], anticonvulsants, local anesthetics, antidepressants, and opiates), physical therapy, and nerve blocks or ablations.8
Peripheral Nerve Stimulation
Peripheral nerve stimulation (PNS) is a type of neuromodulation that is becoming increasingly popular for the treatment of many causes of chronic pain and peripheral neuralgias. This innovative treatment works by delivering low-level electric impulses close to pain-generating nerves that interfere with the perception of nociceptive pain.3
Peripheral nerve stimulation can be further divided into two entities. The first is PNS of an identifiable pain-generating nerve (such as the sciatic nerve in a patient with foot pain). The second type of PNS, peripheral nerve field stimulation (PNfS), involves the subcutaneous placement of electrical leads over the generalized area of pain (eg, occipital stimulation for occipital neuralgia). An advantage of PNS over SCS is the ability to place the lead directly adjacent to the affected nerve, which provides a greater surface area of pain relief.
Head, Neck, and Facial Pain
The first through third cervical nerve roots are commonly involved when a patient presents with head, neck, and facial pain. Spinal nerve roots are not always the cause of these cephalic neuralgias and muscle contractures. Patients who have failed other treatments for their pain have responded positively to PNfS. In 2007, Melvin et al conducted a study of patients with occipital headaches. After four weeks of receiving PNfS for their headaches, 46% of the patients rated the relief as excellent, and at 12 weeks no patients enrolled in the study reported poor pain relief.9 Percutaneous implantation of peripheral stimulator leads has been shown to provide good to excellent results in treating intractable occipital neuralgia, which has been a suggested trigger for migraines.8 In a clinical investigation of patients receiving PNS for craniofacial pain, 73% of the participants experienced significant improvement in pain intensity in the first three months.9
Chronic Back Pain
The evaluation and treatment of back pain, either acute or chronic, poses economic hardship on society, as well as an emotional and physical destitution to the patient.10 Back pain can arise from a number of structures causing irritation to central and/or peripheral nerves. Localizing a specific offending structure (nerve root, facet joint, disc, ligament, muscle) as the cause of the pain can be difficult. In PNS, the electrode is placed adjacent to the nerve causing the pain, improving the likelihood of sufficient nociceptive blockade.
A study of PNfS for chronic axial low back pain without a radicular component after failed back surgery syndrome found a significant reduction in mean VAS scores of 4.18+/-1.42 in patients who reported a successful outcome (one of 13 patients reported a poor outcome).3 Leads were placed subcutaneously over the area of worst pain.
Abdominal, Pelvic, and Ilioinguinal Pain
Abdominal, pelvic, and ilioinguinal pain are common, and research on the efficacy of pain relief from PNS in these conditions is insufficient. However, a peripheral nerve likely is implicated and, if identified or if the patient has a discreet and focal area of pain, PNS may prove beneficial.
Lead migration has been a constant complication for some patients following implantation in SCS. When leads migrate in the epidural space from SCS, the stimulation coverage changes and may no longer provide adequate pain relief. Although peripheral leads are also prone to migration, they are more accessible and easier to manipulate in order to receive reestablish sufficient stimulation coverage. However, the most evident benefit of PNS over SCS is that in the former technique the leads are placed outside of the epidural space, thus reducing the risk for epidural neurologic insult and epidural hemorrhage.1,4
The number of centers using SCS and PNS for a multitude of chronic pain syndromes is dramatically increasing as the research on these treatment modalities exponentially rises. Even more, the use of PNS for craniofacial pain and other peripheral nerve syndromes is increasing. In the future, a wider acceptance of this treatment is expected because it is minimally-invasive, can be trialed, is reversible, and has adjustable settings. These unique qualities may eventually make PNS the preferred modality for otherwise intractable conditions.3
Spinal cord stimulation has been proven to reduce pain by approximately 50% in the near term; however, controversy exists as to whether it provides adequate long-term pain relief.3 PNS is a relatively new treatment. As a result, little evidence-based medicine supports its long-term efficacy. Nevertheless, it has relatively few contraindications, and the majority of patients who undergo PNS report a significant decrease in pain and the use of pain medication 12 months after implantation.3 Therefore, PNS may be considered an effective means of relieving pain and decreasing the use of pain medication, particularly opiates.
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8. Franzini A, Leone M, Messina G, et al. Neuromodulation in treatment of refractory headaches. Neurol Sci. 2008;29(suppl 1): S65-S68.
9. Melvin EA, Jordan FR, Weiner RL, Primm D. Using peripheral stimulation to reduce the pain of C2-mediated occipital headaches: a preliminary report. Pain Physician. 2007;10(3):453-460.
10. Rubin DI. Epidemiology and risk factors for spine pain. Neurol Clin. 2007;25(2):353-371.
11. Sundaraj SR et al. Spinal Cord Stimulation: A seven year audit. Journal of Clinical Neuroscience 2005 12(3), 264-270
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