Introduction

Spinal cord stimulation (SCS) has been accepted as a therapeutic modality for chronic complex pain conditions of the lumbar and cervical spine ( ). Although conventional SCS has been successful for the treatment of radicular pain, either upper extremity or lower extremity, it has been less effective for the relief of axial pain such as chronic low back pain (CLBP). The main reason for this lack of efficacy may be related to the difficulty for SCS to cover the painful lower lumbar area with epidural stimulation (paresthesia). This has led to the development of new approaches to more selectively stimulate areas of pain and provide more efficient paresthesia coverage, such as the introduction of peripheral nerve field stimulation (PNFS).

PNFS is described as electrical stimulation of a painful area by the percutaneous deployment of electrode(s) under the skin to electrically stimulate the end “arbors” of a peripheral nerve ( ). When electrodes are subcutaneously placed in the pain area, they stimulate the regional field of pain, including the local affected nerves and cutaneous afferents in the dermatomal distribution of the nerves, which then converge back into the spinal cord ( ). Successful use of PNFS has been reported for a variety of conditions, such as pain in the chest and abdominal wall, the pelvis or even shoulder girdle ( ), and axial CLBP and neck pain ( ) with good outcomes. Moreover, PNFS has been used as an add-on or combination therapy when SCS alone has not been sufficient in treating low back pain ( ). Recently, PNFS has become more popular and widespread because of its effective, reversible, and minimally invasive nature associated with low complication rates. It appears that the use of PNFS has opened new vistas for the management of a variety of painful conditions. In this chapter, we review procedural details of PNFS and results of the published studies to provide an overview and update on PNFS.

Patient Selection

Appropriate patient selection is a key factor for efficacy and good outcomes of PNFS. The inclusion criteria ( ) are the following:

• 1.
  

  A clearly defined, discrete focal area of pain with a neuropathic or combined somatic/neuropathic pain component

  
  
• 2.
  

  Failure to respond to medications, psychological therapies, physical therapy, pain management programs, and other surgical intervention, such as SCS

  
  
• 3.
  

  Psychological clearance

  
  
• 4.
  

  No coexisting chronic pain problems or neurologic diseases, and

  
  
• 5.
  

  No coexisting conditions that would increase procedural risks (e.g., sepsis, coagulopathy)

Surgical Implantation

Prior to the surgery itself, it is important to plan for the number of electrodes that will be required to successfully treat the pain in a person and in what position the electrodes should be placed to cover the painful area in its entirety ( ). Usually two to four electrodes are needed for most pain conditions, given that current implantable pulse generators (IPGs) offer multiple channels of stimulation. For this purpose, the anatomic atlas produced by Barolat is an excellent planning tool that can be used for proper choice of location and insertion approaches for subcutaneous electrodes ( ). Depending on the shape and size of the pain area, the electrodes could be placed in either a vertical, horizontal, or diagonal direction. For example, if the CLBP is located on only one side of the midline, one electrode is used and placed subcutaneously in the area of the worst back pain, in a direction perpendicular to the spinal axis. If the low back pain is on both sides of the midline, two electrodes can be placed, one on each side of the spinal column.

Efficacy of PNFS appears to be directly related to several key factors that include proper electrode placement and eliciting the appropriate sensation within the region of pain. The optimal depth of electrode placement is 10–12 mm beneath the skin surface ( ). More superficial placements of electrodes in the dermal layer may cause painful stimulation; on the other hand, implantation within deeper tissues may result in muscle recruitment and discomfort ( ).

The choice of the lead type is at the discretion of the physician. The goals of lead selection and placement are to cover the area of pain as completely as possible with paresthesia and to enable the greatest flexibility in subsequent programming. Either eight or four contact electrodes are used, in various combinations, to total 16, 24, or 32 programmable contacts, depending on the system/manufacturer used. Over time, as the overlap between maximum paresthesia coverage and maximum pain may vary or change, electrodes with more contacts provide greater programming options and versatility after surgery and therefore are selected in most cases ( ). The use of multicontact electrodes increases the options for programming parameters to stimulate precise areas of pain and may allow patients to independently modify the location and degree of stimulation in response to changes in activity or positioning, or fluctuations in pain severity ( ).

Electrode Implantation

Electrode implantation is the first stage of the PNFS procedure, which is usually performed under monitored anesthetic care (MAC). Use of pure local anesthesia is usually avoided because it makes the stimulation area numb and hinders intraoperative testing. Similarly, implanting electrodes under general anesthesia or deep sedation prevents intraoperative confirmation of concordance between the pain region and the location of paresthesias; however, general anesthesia may be considered an option in patients who are unable to tolerate positioning or other surgical steps that are needed for electrode insertion. Based on the patient’s report, the pain area is clearly outlined and marked on the skin surface. The locations for placement of each subcutaneous electrode are marked in either the center of the painful area with the patient in an upright position or on the outer edges of the painful region, if the plan is to flank the pain with multiple electrode leads. Once the patient is taken to the operating room and properly positioned (usually prone for lumbar pain interventions), the surgical area is prepped and draped in standard sterile fashion. The skin is infiltrated with 1% lidocaine at the insertion point only. A slightly bent or curved 14-gauge Tuohy needle is inserted subcutaneously. After the Tuohy needle is correctly positioned, the stylet is removed and then each electrode lead is placed subcutaneously in the desired location under C-arm fluoroscopic guidance.

One way to perform this is with pure temporary electrodes that will be removed at the end of the trial period. In this way, the procedure can be performed in the office or a procedure room and no incisions are needed. A disadvantage of this procedure is that if the trial is successful, a new set of electrode leads has to be used and there is no guarantee that the new leads will be in exactly same location or depth as the trial ones – and therefore, the results of the trial, in terms of paresthesia coverage and the pain relief, may not be reproduced after the permanent implantation.

Another option for trialing is to place the electrode leads as if they were permanent, anchor them in place (subcutaneously and via surgery), and tunnel temporary extensions outside of the skin. The advantage of this technique is that the location of the leads remains unchanged after the trial is completed and the same electrode leads may be then connected to an IPG. A disadvantage of this “tunneled” approach is that (1) it requires an incision and deep dissection for the anchoring and (2) the patient has to have this incision reopened if the trial fails in order to have the electrode removed.

After the electrode leads are positioned subcutaneously in the painful area, the intraoperative stimulation trial is initiated. The goals of the intraoperative trial are to test the location of paresthesias and to adjust the electrode position to obtain optimal paresthesia coverage. The electrode is connected to the screening pulse-generating device, either directly or via a temporary extension cable, and a variety of different configurations and stimulation parameters are tested by asking the patient about his or her sensations until good paresthesia coverage of the painful area(s) is reached. The use of multiple contacts and electrode leads allows one to move the active (negative) contact along the electrode array without actually repositioning the electrodes.

Once the intraoperative trial is completed, the electrodes are anchored to the fascia. Following this, temporary extension cables are tunneled subcutaneously and brought to the skin surface 10 cm or farther from the original midline incision. The site of the temporary extension’s exit through the skin is infiltrated with local anesthetic. For tunneling during the first stage of surgery, we do not recommend using the “passers” provided by the manufacturer with the electrodes or extensions (they are too large and are not needed). Instead, we use the same Touhy needles that were used for the lead insertion and advance them over the stylets in a retrograde fashion, as has been described in the past ( ). Considering the final position of the PNFS system, the temporary extension cables are usually tunneled to the side opposite of the eventual IPG location to avoid crossing the path of the permanent extension cables with the temporary one. Each extension is connected to the electrode lead, and the connection is secured with set screws. The incision is then closed. These cables are connected to the screening cable, and the impedance of all contacts is checked to confirm its value to be within normal range. The patient’s trial usually lasts 5–7 days.

Electrode Internalization and Implantation of Spinal Cord Stimulation Generator

This second stage of PNFS surgery is usually performed under general anesthesia to avoid a painful electrode-tunneling process for the patient. If the stimulation trial is satisfactory, achieving at least 50% pain relief without intolerable side effects, the patient is prepared for electrode internalization. If the patient drives an automobile, the generator is usually implanted in the left side of abdominal area to avoid the seatbelt line. Alternative locations (paraspinal, buttock, flank, etc.) may be used, based on the patient’s and physician’s preference.

After anesthesia is initiated, the patient is placed for proper positioning to allow appropriate access to the targeted painful area and planned IPG location. For lateral positioning, the patient’s body is secured appropriately with a beanbag or appropriate padded taping, and then both the pain area and the IPG location are prepped and draped in a standard sterile fashion. The tunneled externalized electrode extensions should be free and away from the sterile field and placed in a position so that some person, usually the anesthesia staff, can pull on it to remove. After application of local anesthetic, first the suture line (for original electrode placement) in the pain region is reopened carefully to avoid cutting the electrode lying above the fascia. The soft tissues are dissected, and the connectors between the electrodes and extension cables are identified. The extension cables are cut in two, and their distal cut part is gently pulled and removed from outside of the sterile field by the anesthesia staff. The presumed sterile connectors are then disconnected from the electrodes and discarded. The electrodes are wiped clean and inspected for integrity. The electrodes are checked for length in order to determine the location of the generator placement and if the extension cables are needed.

After the administration of local anesthetic, a second incision is made for the IPG placement, usually on the left side of the abdomen or buttock, taking care to ensure that the final generator site does not overlay bony structures, such as ribs and iliac crest. The tissues are dissected in the subcutaneous plane above the fascia to form the pocket for generator, but not too far under the skin, because a thick layer of tissue above the generator may interfere with programming and/or charging process. A special passer is used to connect the two incisions; the electrode leads are passed through the passer, and connected to the IPG, according to the manufacturer’s instructions. A bifurcated extension cable or a stretch coil extension cable can also be used to connect electrode leads to IPG depending on the choice of hardware and physician’s preference. The connection is secured with set screws. The generator is then placed into the pocket and attached to the underlying fascia with nonabsorbable sutures. Care must be taken to place any redundant wires behind the IPG, within the pocket. The device is interrogated to ensure that the impedance of all contacts is within normal limits. Both incisions are then closed in layers after the hemostasis is completed with bipolar coagulation (monopolar coagulation should be avoided due to risk of inadvertent thermal damage next to the exposed metal surfaces of the electrodes and generator), and the surgical areas are irrigated with antibiotic solution.