The concept of nerve stimulation/peripheral nerve stimulation (PNS) is not new and has been in practice since the early 19 century. However, clinical studies were done much later in this field. In 1976, Campbell et al. published the reports of the first set of clinical studies done in the field of pain management using PNS. Since then, PNS has been actively explored as a treatment of modality for managing chronic pain conditions. Chronic pain conditions, including low back pain, neck pain, and neuropathic pain states like chronic regional pain syndrome, causalgia, and diabetic neuropathy, afflict a large group of population. They pose a substantial economic burden on society. These pain conditions are the leading cause of disability in the world. Currently, there is an active opioid crisis in the United States, with over 15,000 deaths involving prescription opioids reported in 2015. Also, managing these chronic pain conditions with conventional medical management can lead to multiple drug-drug interactions resulting in intolerable side effects. PNS offers a relatively safe and effective treatment modality in the treatment of such chronic pain conditions.
The determination of the nerve to be stimulated corresponds with the area where the pain relief is required. We can manage any focal area of pain in the body by stimulation of the peripheral nerve supplying that particular dermatome. Nerve stimulation is broadly divided into electrical nerve stimulation and magnetic nerve stimulation.
Electrical nerve stimulation
The mechanism of electrical nerve stimulation is not fully understood, but the idea originates from the "gate control theory."  The most simplistic explanation of this theory is that electrical stimulus is applied to the A-beta fibers (that carry non-painful stimuli) in the peripheral nerve, which blocks the impulses carried in C fibers (carry painful stimuli). Stimulation of the A-beta fibers in the vicinity of C-fibers can result in the closure of the "gate" that exists in the dorsal horn of the spinal cord, and thus result in blockade of painful stimuli. Another mechanism that has been proposed includes inhibition of wide dynamic range hyperexcitability through GABA release. While some evidence points to the pain relief secondary to the stimulation-induced release of serotonin, adenosine, and noradrenaline.
Magnetic nerve stimulation
A magnetic stimulator is not an implantable device and works by a different mechanism as compared to an electrical nerve stimulator. The transcranial magnetic stimulator is a commonly used application of this technology. It consists of a capacitor that may be able to discharge current through a hand-held wire coil, which then produces a magnetic field lasting 100-200 microseconds. When held against the skull, this magnetic field can produce depolarization of cortical neurons. The following mechanisms have been proposed for the therapeutic effects of transcranial magnetic stimulation.
1) Changes in brain monoamines
2) Changes in cerebral blood flow
3) Reduction of oxidative stress to neurons
For the purpose of this article, we will be describing implantable electric peripheral nerve stimulators in detail in the upcoming sections.
PNS limits the amount of energy by using focalized current and has been used to treat a variety of chronic pain disorders. Prior studies have shown that there are good outcomes from PNS on the median, ulnar, sciatic, ilioinguinal, and genito-femoral nerves. Weiner et al. placed the first percutaneous PNS lead to manage of occipital neuralgia. Schoenen et al. published the results of a multicenter study evaluating the efficacy of sphenopalatine ganglion (SPG) stimulation in the treatment of cluster headaches. The authors reported that 67% of subjects achieved pain relief, and 36% of patients achieved a reduction in attack frequency. Saper et al. published the results of the first randomized study on the use of occipital nerve stimulation (ONS) for the treatment of migraine. 39% of patients in the study showed a 50% or greater reduction in the number of headache days per month. It may also be effective in other headache disorders. PNS is also efficacious in treating sacroiliac joint pain. Improvements were noted during the 12-month follow up in the study, in International Patient Satisfaction Index (IPSI), Visual analog scale (VAS), and Oswestry Disability Index 2.0 (ODI). PNS is also efficacious in the treatment of shoulder pain secondary to subacromial impingement syndrome. The study reported improvements in pain and functionality.
Recently results of a study evaluating the use of PNS in the treatment of low back pain were reported. PNS was used to stimulate the medial branches of dorsal ramus for 30 days, and after that, the leads were explanted. Subjects reported statistically significant reductions in the average pain intensity (ANOVA, p<0.005) and worst pain intensity (ANOVA, p<0.001) scores at the end of the trial (EOT). They continued to experience benefit at the 4-month follow up period, and the mean reduction in average pain intensity and worst pain intensity was 84% and 78%, respectively. At 12-month follow-up, responders showed a 63% reduction in pain intensity and a 32-point decrease in disability scores.
There have also been reports of the effectiveness of Gasserian ganglion stimulation in the treatment of post-stroke facial pain.
One of the main contraindications to performing this procedure is the presence of local infection at the site of stimulator lead placement. Performing this procedure in patients with active bleeding disorder requires extra caution as it can lead to hematoma formation.
PNS requires the availability of a nerve stimulation kit that usually includes a transducer needle and a stimulation electrode. After the patient is prepped under sterile conditions using a chlorhexidine/iodine solution, a sterile drape is used to cover the area of surgery. We identify the desired location of needle entry and administer local anesthesia. The target nerve is identified with the help of ultrasound guidance. A transducer needle is used to access the target nerve. The ideal target of stimulation is approximately 1 cm away from the target nerve. A PNS electrode is introduced through the transducer needle and is ideally set parallel to the nerve. The battery source is connected to the proximal end of the electrode. Once a connection is established, the electrode is tested intraoperatively to assess the distribution of paresthesia. If the paresthesia corresponds to the area of pain, the electrode is tunneled under the skin and secured in place using a suture.
This procedure is also performed by the direct exploration of the nerve where a surgeon (usually a neurosurgeon) dissects the area surrounding the nerve and places the electrode on top of the nerve.
Recently, surgeons have directed focus on using wireless nerve stimulation devices that preclude the use of an implantable pulse generator. These wireless devices use an implantable electrode that has inbuilt microelectrodes, which are stimulated by an external source generator, usually in the form of a patch or wearable device. In the United States, the Food and Drug Administration has approved mainly 4 devices for wireless peripheral nerve stimulation.
Typically neurosurgeons, anesthesiologists trained in interventional pain medicine, and interventional radiologists perform peripheral nerve stimulator implantation. It is advised that physicians must have performed this procedure an adequate number of times under supervision before performing it independently.
Like any other invasive procedure, common complications of this procedure include infection, bleeding/hematoma formation, surgical site pain, and nerve injury.
Device related complications include migration of the stimulator lead, lead fracture, and device malfunction. Even though these complications are minor, but require a minor revision of the procedure for the re-implantation of the stimulator lead.
Chronic pain is one of the most prevalent health problems and poses a considerable burden on the economy and society. Amid the current opioid crisis, there is a need for exploration of alternative pain therapies. Peripheral nerve stimulation offers a relatively safe option for the management of such painful pain conditions. The use of peripheral nerve stimulation can provide regional anesthesia without exposing the patient to the systemic side effects of opioid medications. Since these procedures are minimally invasive, they offer an alternative for patients who are not candidates for major joint replacement surgeries secondary to contraindication to general anesthesia.
For the success of this therapy, it is critical that the physician performing PNS is adequately trained in the procedure. A prior discussion is vital with the patient explaining all the risks and benefits of the procedure. A realistic expectation needs to be set with the patient about the outcomes of the therapy regarding his/her pain relief.
There is a need for more randomized controlled trials to completely assess the full potential of this novel therapy.
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