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|Year : 2020 | Volume
| Issue : 8 | Page : 224-230
Peripheral Neuromodulation for Chronic Pain
Department of Neurosurgery, West Virginia University Health Sciences Center, Morgantown, WV, USA
|Date of Web Publication||5-Dec-2020|
Dr. Milind Deogaonkar
Department of Neurosurgery, Rockefeller Neuroscience Institute, West Virginia University Health Sciences Center, 33 Medical center drive, Morgantown, WV, 26505
Source of Support: None, Conflict of Interest: None
Background: Chronic, focal, neuropathic pain is difficult to treat. Local nerve blocks are either ineffective or do not last. Regular neuromodulation modalities like spinal cord stimulation (SCS) or pain pump are invasive and affect a larger area.
Objectives: To discuss the indications, technique, nuances, programming, and outcomes of peripheral neuromodulation
Methods: The article reviews published literature and the author's own experience of over 500 cases of peripheral neuromodulation.
Results and Conclusion: Peripheral neuromodulation using peripheral nerve field stimulation (PNFS) is an effective, minimally invasive, targeted method of treatment. It is a relatively new modality in the field of neuromodulation but is used more often.
Keywords: Chronic pain, face pain, field stimulation, focal pain, headache, migraine, nerve stimulation, neuropathic pain, peripheral neuromodulation, post-herpatic neuralgia
Key Message: Peripheral neuromodulation using peripheral nerve field stimulation (PNFS) is an effective, minimally invasive, targeted method of treatment.
|How to cite this article:|
Deogaonkar M. Peripheral Neuromodulation for Chronic Pain. Neurol India 2020;68, Suppl S2:224-30
In a pain practice, chronic focal pain is commonly encountered. These types of pain patients pose a challenge for treating physicians. Various medications, nerve blocks, nerve ablations, neurectomies, and invasive neuromodulation of the spinal cord and brain are used for treating this pain. This article discusses a relatively new method of neuromodulation that focuses on the nerve and field stimulation, peripheral field-nerve stimulation (PFNS).
Nerve blocks have temporary benefits, neurectomies can lead to deafferentation and recurrent pain. Spinal cord stimulation (SCS) can cause uncomfortable spillover of stimulation. Deep brain stimulation (DBS) for pain is still experimental and invasive.
In comparison, PFNS, delivers highly focused coverage to the affected area, as the leads are placed right below the painful area or over the nerve supplying the region. It is essentially a subcutaneous procedure with no major systemic complications.
PFNS can be used in any chronic, focal pain condition. The smarter, newer systems have left no area in the body inaccessible. In our experience, we have used PFNS in neuropathic pain conditions such as post-herpetic neuralgia, neuropathic face pain, chronic daily headaches, inguinal and genital pain, and many such complex pain indications.
This review discusses the nuances in patient selection, technique, and regional peculiarities with each of these indications.
| » Mechanism of Action|| |
The mechanism of action of PFNS is at the best speculative. There are various theories to explain the pain relief provided by PFNS [Figure 1]. These can be broadly divided into peripheral and central mechanisms.
|Figure 1: Mechanisms of action of PFNS. a. conduction block, b. competitive block, c. central activation|
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- Peripheral Mechanisms
- Conduction block of the nerve: This was the earliest theory [Figure 1]. The amount of energy needed to block a myelinated nerve and keep it in a steady-state of depolarization is significantly high. The kilohertz frequencies and sinusoidal waveforms are not seen in the commercially available devices. Besides, it does not explain the selective blockage of pain stimuli without affecting motor or sensory fibers in a mixed peripheral nerve.
- Competitive block: This represents the gate control theory of pain [Figure 1]. Peripheral stimulation is shown to recruit the A-delta and A-beta fibers and suppress the C-fibers in experimental animals. In practice too, if there are no preserved tactile sensations in the area of pain, PFNS fails to provide long-term benefits.
- Central analgesic mechanisms of PFNS: Peripheral stimulation of any kind is known to activate central circuits in various parts of the brain [Figure 1]. Central neuromodulation with occipital nerve stimulation (ONS) has been documented. The central effects may also work due to the effects of medial pain pathways or by modulating central nociceptive processing of pain.
| » Indications and Patient Selection|| |
Peripheral neuromodulation in the form of PFNS can be used in any type of chronic, intractable, focal pain. Following factors should be considered while selecting a candidate for peripheral neuromodulation:
- Chronic and intractable pain: Any chronic pain that has not responded to medications (nonsteroidal anti-inflammatory, analgesic, opioid analgesics, neuropathic pain medications), physical therapy, and more than two interventions (nerve blocks, direct surgery, trigger point injections, botulinum toxins, radio-frequency ablation, transcutaneous electrical nerve stimulation [TENS], acupuncture, behavioral therapy, chronic pain rehabilitation).
- Focal nature: Large areas of pain are not amenable to this form of neuromodulation. The pain or the source of pain must be in an area that is not bigger than 8 inches across or the width of two palms of a hand. Pain involving larger areas needs modulation efforts targeted a node higher to the spinal cord or in even larger areas a node even higher to the brain.
- Effect on quality of life: The pain should affect the activities of daily living (ADL). Unless the pain has an impact on the work or home activities, an implant is unwarranted.
- Psychological comorbidities: All patients need to undergo pain psychology evaluation and need to be cleared by a psychologist.,, Any red flags in psychology testing or any untreated depression or anxiety, is not compatible with good outcomes in any form of pain neuromodulation. It is uncommon to find a patient with chronic, long-standing pain without coexisting psychological comorbidities. If they are under treatment and controlled, they still can be candidates. Subtle red flags are patients who are consumed by their pain, their life revolves around it, who is on disability due to pain, who tell you that you are their last hope, who want 100% pain relief. Any patients with on-going litigations about the causative event of pain are to be included with caution.
- Type of pain: Neuropathic and musculoskeletal pain both respond well to PFNS. Patients with severe allodynia sometimes present a challenge. In these patients, the electrodes should be placed bracketing the pain area rather than beneath the pain area.
- Associated sensory loss in the area of pain: Complete sensory loss in the pain area does not give good results. The absence of enough recruitable A-delta and A-beta fibers in the area is a probable reason. More than 50% of sensory loss should be offered trial stimulation with caution and with the explicit understanding that it may not help in reducing pain.
- Mobile areas: Pain in mobile parts of the body needs special attention to lead anchoring. There is an increased chance of lead migration in these areas. In face pain, the mandibular lead or the leads on extremities come under this category. When placing the implantable pulse generator (IPG) in extremity leads, an effort should be made not to cross more than one joint if possible, to be recruited by stimulation.
- Location of pain in the body: Most of the locations for PFNS are standard locations such as ONS for migraines and supraorbital nerve stimulation (SONS) for chronic daily headaches. Some of the locations though are uncommon, the lead placements need to be carefully planned. We will discuss these issues in the forthcoming sections.
- Influence of nerve blocks and TENS in patient selection: Nerve blocks work by a completely different mechanism. So, they do not have any predictive value in deciding a candidate for PFNS. TENS uses a completely different plane of stimulation also does not offer any predictive value in patient selection.
- Lessons from trials: Every patient must undergo an externalized trial. Trial benefits dictate the permanent implant as in SCS., More than 50%, sustained reduction in pain during the trial is necessary for an implant. Apart from this core information, there are subtle indicators that can help refine the patient selection. They are:
- Chasing the pain: Though the extent of coverage during trial helps decide on the final position of leads, there will be patients who will focus on the pain outside the coverage area. These patients may need repeated programming and may have low satisfaction with the therapy.
- Immediate disappearance of pain: Within a few hours of trial placement if the pain goes away completely, this indicates a strong placebo effect. It again indicates a possible early failure of therapy.
- Need for magnetic resonance imaging (MRI): Since PFNS is not compatible with MRI, before proceeding, it is important to make sure that the patient does not need MRI for unrelated conditions regularly.
Though PFNS can be used in any chronic, focal intractable pain, there are some common pain disorders that we will discuss in detail. From personal experience of more than 500 PFNS cases [Table 1], each of these indications has certain nuances, which will be discussed in the forthcoming sections. These common pain disorders are as follows
|Table 1: Table showing the spread of indications for PFNS in the author's practice|
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- ONS for migraines
- Supraorbital nerve stimulation for chronic daily headaches
- Neuropathic face pain
- Inguinal neuralgia
- Post-herpatic neuralgia (PHN)
- Axial back and neck pain
- Chronic joint pain
- Abdominal pain
- Meralgia paresthetica
- Other pain conditions.
The surgical technique for PFNS is straightforward. There are two procedures involved. First is the trial placement and the second is permanent placement with IPG placement. Both these procedures should be done in the operating room under general anesthesia. Local anesthesia is not enough to alleviate procedural discomfort as it involves tunneling in and outside of the pain area. Though we have used open dissection of nerve and placement of paddle leads on it, PFNS essentially involves the placement of cylindrical leads, either percutaneously or through small incisions.
- Planning of procedure: Before surgery, trial and implant, it is essential to measure the area of pain, correlate with lead contact location and length, plan for placement of leads, plan on the entry point, decide on the length of leads, length of tunneler, location of additional incisions for tunneling, decide on the location of strain relief loops and proposed location of the battery. It is important to do it before anesthesia and make sure you have leads and tunnelers of the right length.
- Trial placement of leads: The principle behind lead placement remains the same in the trial as well as a permanent implant. Trial leads are placed percutaneously, while permanent leads are placed via a small incision. Externalized part of trial leads is anchored to the skin and is kept covered by a tape, close to the skin so that they do not dislodge during the trial week. The trial is for 7 days and the patient is advised to keep the area dry. The lead placement is done by a thin, flexible, blunt tunneler, or needle. We prefer a tunneler, as it is blunt and flexible and can be molded to fit the contours of the area to be stimulated. The most important principle, while placing the lead is to stay in the subcutaneous plane [Figure 2]. When making the percutaneous entry, it helps to pinch the skin and by molding a curve on the tunneler, the tip can be seen lifting the whole skin [Figure 2]. If the tip comes too superficial, dimpling of the skin can be seen, if the tip goes too deep, it will be less visible.
- Permanent placement of leads: Same principles of planning and lead placement applies to the permanent placement of electrodes, except it is done through a small incision. If the electrode is too deep it may cause muscle stimulation and may increase the perception and therapeutic thresholds. If it is too superficial there is a risk of erosion and can make stimulation intolerable at low voltages. Some surgeons use ultrasound guidance for lead placement.,, Leads could be either under the painful area or close to the nerve (in case of nerve injury) or bracketing the area of pain (in case of severe allodynia or deep ear pain) and always 1 to 2 cm away from a scar (as the innervation around the scar tissue is aberrant).
- Anchors: Any commercial anchors can be used for the implanted leads. I did not use any anchor and kept the lead in place with two silk sutures through a loop of the lead. The anchors themselves become trigger points and a source of pain over time, hence I tried to avoid using them.
- IPG placement: Subclavicular IPG placement for craniofacial PFNS is preferred. For proximal extremity cases, a truncal IPG and distal extremity cases a proximal extremity IPG is most efficient. Avoiding crossing too many joints, avoiding areas that can cause discomfort (pant belt line, lower hip region), and avoiding too superficial placement of IPGs can prevent complications like erosion and lead migration. The new IPGs are smaller and hence easier to place.
|Figure 2: Technique of subcutaneous placement of the lead. Top: while placing the lead, pinch the skin to gain subcutaneous access, Bottom: while tunneling make sure the tip if the tunneler is visible and tents the skin|
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Let us look at the additional nuances in the 10 common pain syndromes discussed before in the article.
ONS for migraines
Out of all the PFNS, this is the commonest and most written about procedure.,,,,,, Various electrodes and methods have been used for ONS. The common denominator is the subcutaneous placement of leads that cross the greater and lesser occipital nerves [Figure 3]a,[Figure 3]b,[Figure 3]c,[Figure 3]d. In my practice, I placed the leads from midline directed towards the mastoid process in patients with migraine, occipital neuralgia, and chronic daily headaches [Figure 3]a. If there is a large component of cervicogenic pain then, I placed leads horizontally from one side to another [Figure 3]b. In hemicranial headaches, I placed one lead, more on the affected side but still crossing the midline [Figure 3]c. I did not use X-rays or ultrasound guidance, but if it is used, the lead should be placed above the C2 level and not below it [Figure 3]d. Multiple loops should be kept in the incision to avoid migration. If anchors are used, make sure the most proximal contact does not come below the anchor. The contacts close to midline are most useful. In ONS, field stimulation is as important as nerve stimulation. Over years, I have believed that field stimulation has more analgesic effects than nerve stimulation. Subclavicular IPG placement is better than placement on the back as it prevents lead migration due to the breaking of the arc during flexion. Due to that reason, it is better to position the patient laterality.
|Figure 3: Occipital nerve stimulator placement. (a) Midline access with leads directed towards the mastoid area, (b) lateral access with horizontal leads, (c) lateral access with one lead placed for unilateral pain, (d) Postoperative radiograph showing ONS leads placed from the midline. Note the leads are above the C2 level|
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SONS for chronic daily headaches
SONS can be done alone or in combination with ONS [Figure 4]a and [Figure 4]b. When doing only SONS, the entry point is in the supra-auricular region, behind the hairline. Both leads are placed on the same side. It is prudent to pick the side of more pain to place the leads. A soft, flexible tunneler can be contoured to the curve of the forehead and placed in the plane close to the bone. The tip of the distal lead should not be too superficial. The leads should not be placed too close to the eyebrows as the skin gets thin near the eyebrows. The leads should be at least 1.5 cm above the eyebrows. In the case of unilateral SONS, the lead should cross the midline. If ONS and SONS are being placed simultaneously, all four leads can be placed through one supra-auricular incision.
|Figure 4: Supraorbital stimulation. (a) Supraorbital stimulation in combination with ONS, (b) SONS alone, with leads placed from supra-auricular access|
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Neuropathic face pain
Complex craniofacial pain has limited treatment options. Peripheral neuromodulation is one of the best options to treat these pain syndromes. These pain syndromes are variously called atypical face pain, neuropathic face pain, deafferentation pain, and posttreatment trigeminal neuropathic pain. The common feature is constant burning pain. The prerequisite for PFNS is preserved sensation. If there is complete anesthesia dolorosa, PFNS is not useful and then we must think about motor cortex stimulation (MCS). The leads are placed from a supra-auricular entry point. The leads can be placed in all three subdivisions of the trigeminal nerve [Figure 5]a, or the ones affected by pain [Figure 5]b or in all three subdivisions [Figure 5]b and also in the occipital area in cases with microvascular decompression-induced occipital neuralgia [Figure 5]b. Supraorbital lead is described earlier. The infraorbital lead should be placed with a lazy curve on the tunneler, to keep the lead away from the lateral canthus. If the lead is too close to the eye, it will cause blepharospasm with stimulation. Another precaution while placing this lead is to make sure it does not go too low and too close to the upper buccal mucosa of the upper jaw. The mandibular lead should be tunneled close to the tragus while going down, to avoid any contact with the superficial branches of the facial nerve. All three leads should be close to midline (infraorbital lead close to the nasolabial fold).
|Figure 5: PFNS for face pain. (a) Schematic diagram showing the placement of leads for face pain, (b) Various postoperative radiographs showing 2, 3, or 4 leads placed for face pain depending on the location of pain|
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Inguinal neuralgias are commonly seen after surgery for inguinal hernia or pelvic surgeries. PFNS helps immensely with pain control in these patients. The entry point of the lead is in the lower lateral abdominal wall and the leads are placed parallel to the inguinal ligament [Figure 6]a and [Figure 6]b. The leads should be at least a centimeter away from any previous scars due to lack of recruitment in scar tissue and the possibility of adhered abdominal viscera. Leads placed closer to the midline in the inguinal region will also cause scrotal stimulation. The incisions and IPG should not be placed on the beltline as far as possible.
|Figure 6: Inguinal PFNS placement. (a) Schematic, (b) Postoperative radiograph of inguinal leads and IPG placement|
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PHN causes debilitating pain. PFNS works very well in these patients. The leads should be placed along the line of pain with one lead each, on either end of the pain area [Figure 7]. In cases with severe allodynia, the leads should bracket the area and should not be placed under the area. A midaxillary entry point with leads going in either direction works well for thoracic PHN.
|Figure 7: Postoperative radiograph of thechest showing PFNS placement for post-herpatic neuralgia|
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Axial back and neck pain
Chronic, intractable, axial back, and neck pain responds well to PFNS. Patients must have localized pain and they must have undergone physical therapy and pain interventions in the form of trigger point injections and facet joint blocks. The lead placement here is planned to cover the whole area. Various ways of placing the leads are shown in [Figure 8]. The leads should be away from previous scars. They should not be placed too deep. Deeper leads need a lot more energy to provide pain relief. In the neck, a central entry point can help place the leads in all four directions [Figure 8].
|Figure 8: Various ways of placing PFNS leads to axial back and neck pain|
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Chronic joint pain
Chronic knee, ankle, or shoulder pain where no other surgical treatment is available, can be treated with PFNS. The leads should overlay the most painful spot on the joint [Figure 9] or should bracket the joint from all four directions.
Chronic abdominal pain
In patients with chronic pancreatitis, chronic abdominal pain is a major problem. This is different from the pain of acute flare-up. It is a constant ache in the epigastrium. PFNS leads placed over the region of pain help control it [Figure 10]. These patients must try celiac plexus blocks first and should not have any surgically treatable lesions inside. Patients with nonspecific abdominal pain, however, do not respond to stimulation as well. Four leads can be placed through two small incisions and routed to the IPG placed in the lower abdominal wall.
|Figure 10: A radiograph of the abdomen showing anterior abdominal wall PFNS placed for chronic abdominal pain|
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Meralgia parasthetica (MP)
MP has excellent surgical solutions. Still, the patients who fail this can be candidates for PFNS if they do not have anesthesia in the area. The leads can be placed in the pain area [Figure 11]. If the nerve is intact, one of the leads can be placed just below the anterior superior iliac spine, from where the nerve runs. IPG can go on the thigh or the anterior abdominal wall.
|Figure 11: Thigh PFNS placement for meralgia parasthetica. Note the IPG on the medial aspect of the thigh in the second X-ray|
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Nerve injuries causing causalgias, extremity pain [Figure 12], craniotomy scar pain, multiple shunt revision site pain [Figure 13], coccydynia, rectal pain, and pudendal pain can be treated by PFNS. In the genital, pudendal, and perirectal region, use a long tunneler to keep the incisions and points of entry away. In extremities, keep generous loops, so the leads do not migrate.
|Figure 12: PFNS placements in forearm and hand for forearm pain and neuropathic pain due to medial nerve injury|
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|Figure 13: PFNS leads to placement for post-craniotomy scar pain and pain due to multiple shunt revisions|
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Programming of PFNS
PFNS programming is quite simple. Key points are to use wide bipolar combinations, keep pulse width low, and keep rate low. Do not use too many cathodes. Do not use guarded cathodes as far as possible. A rate of 30 to 50 Hz is well tolerated. Pulse width depends on the depth of the lead, but in general, a pulse width of 100 to 200 ms is well tolerated. Remind patients that changing positions may change stimulation.
Outcomes and complications
In well-selected patients, with well-planned lead placements and appropriate programming, a sustained improvement of 50% is seen in the majority of patients. There are some treatment failures when the follow-up goes up to 15 years. There are various reasons for the system explant. In a small group of patients, the pain goes away over time and they do not need the stimulation. In some, the pressing need for MRI for an unrelated or related condition necessitates the explant. In another small group of patients, over time, the efficacy starts to go down. It sometimes results in the explant.
There are no major, systemic complications seen with PFNS. The complications include infection, erosion with exposed hardware, and migration of leads. The rate of complication is extremely low in well-selected patients and well-placed leads and IPGs.
| » Conclusion|| |
Peripheral neuromodulation using PFNS is a safe and effective therapy for chronic, focal, intractable pain. It can be used in anatomically complex areas. The benefits are equal to any central neuromodulation intervention and the risks are much less. Large, randomized, double-blind, prospective trials are needed for understanding the exact efficacy of PFNS. With new wireless implants, it has become more elegant, efficient, and simple.
This work was not funded by any grant or support.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12], [Figure 13]