Peripheral Nerve Stimulation

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Chapter 16 Peripheral Nerve Stimulation

Ashwin Viswanathan, Diaa Bahgat, Jonathan Miller, Kim J. Burchiel

Chapter Overview

Chapter Synopsis: Electrical stimulation of a peripheral nerve (PNS) can be used to treat neuropathic pain, ideally that arises from a single nerve and often follows peripheral nerve damage. This chapter considers the issues relevant to successful PNS. The conditions most amenable to PNS include occipital neuralgia, peripheral neuropathic pain, and trigeminal branch pain. With a more regional subcutaneous stimulation, PNS can affect lower back pain and migraine headache. Similar to spinal cord stimulation, our understanding of the mechanism behind PNS is rooted in Melzack and Wall’s gate control theory of pain: stimulation of large-diameter fibers decreases the pain signaling from small-fiber neurons. Other hypotheses suggest that PNS may result in peripheral nerve block or other peripheral changes in action potential kinetics to suppress pain. PNS was traditionally delivered with a cuff electrode wrapped around the target nerve, but this can lead to perineural fibrosis. Today, plate or wire electrodes are more commonly implanted, either surgically or percutaneously. An implantable pulse generator is commonly implanted in the abdominal wall or gluteal region. With surgical implantation, a piece of fascia may be implanted between the electrode and the nerve itself, fulfilling a role analogous to the dura mater in implantation of a spinal cord stimulator. Percutaneous implantation carries its own set of considerations. More recently, the leadless bionic neuron (BION) system has been developed with an eye toward PNS.

Important Points:

PNS is most effective for neuropathic pain attributable to a single peripheral nerve.

Some sensory preservation should be present in the region of pain.

Implantation of PNS should be preceded by a successful trial of stimulation.

Outcome evidence for subcutaneous and regional field stimulation is in progress.

Clinical Pearls:

Lack of improvement with TENS is not a contraindication to a trial of PNS.

Fascial cuff can be placed between peripheral nerve and plate electrode during surgical implantation (serves an analogous to role to the dura in spinal cord stimulation).

Percutaneous trial of stimulation for occipital neuralgia and trigeminal branch neuralgia can be successfully performed under general anesthesia.

Clinical Pitfalls:

Risk of infection is significant; use of local antibiotics may reduce risk.

Incidence of lead migration can be minimized through careful anchoring and tunneling of leads.

Cuff electrodes should not be placed around peripheral nerves because of risk of perineural fibrosis.

Introduction

Peripheral nerve stimulation (PNS) has been recognized as a treatment modality for peripheral neuropathic pain beginning with Wall and Sweet’s original description in 1967.¹ By inserting percutaneous needle electrodes into their own infraorbital regions, these authors were able to test the effects of stimulation on peripheral nerves. Through stimulation with square-wave pulses at 100 Hz and with a pulse width of 100 msec, they were able to induce diminished sensation to pin prick in the area of the stimulation. This led the way to investigations of PNS as a modality for treating neuropathic pain.

The last two decades have seen an increased interest in the use of PNS with application to occipital neuralgia, facial pain, and complex regional pain syndrome.² In addition to targeting defined peripheral nerves, clinicians are also applying the techniques of PNS to subcutaneous and regional stimulation.^3,⁴ This has led to a number of trials assessing the efficacy of PNS for a wide array of conditions, including lower back pain, postherniorrhaphy inguinal pain, sacral pain, and migraine headaches.⁵

Basic Science

Our understanding of the mechanism underlying PNS is still being developed. One hypothesis for achieving pain control with PNS involves the gate control theory of pain management proposed by Melzack and Wall in 1965.⁶ In their initial description of the gate theory, the authors postulated that large- and small-diameter fibers both send input to inhibitory neurons within the substantia gelatinosa. They theorized that small-diameter fibers provided inhibitory input to the substantia gelatinosa and the large-diameter fibers provided excitatory input. The summation of these inputs modulated the overall inhibitory connections from the substantia gelatinosa neurons to the dorsal horn transmission (T) cells, the projections of which formed the anterolateral system. In accordance with this theory, an increase in large-diameter afferent input would lead to increased inhibitory output from the substantia gelatinosa and consequently decreased transmission of nociceptive input to suprasegmental centers. Thus neurostimulation of peripheral nerves through its effect on the lowest threshold large-diameter fibers would act to “close the gate,” effectively inhibiting the transmission of small-diameter pain fibers. Although the gate control theory provides a framework for understanding the mechanism of PNS, the specifics have been challenged.⁷

Other investigators have shown additional mechanisms that may contribute to the potential efficacy of PNS. Campbell and Taub ⁸ explored the mechanism of PNS through transcutaneous nerve stimulation of the median nerve. The authors stimulated the median nerve proximally with a 100-Hz, 1-msec stimulus. They found that the response elicited depended on the stimulus amplitude. At 10 V to 12 V touch threshold was elevated. With increased voltage pain thresholds were also elevated, and at intensities of 50 V analgesia was elicited. In addition, the development of analgesia was correlated with the loss of the Aδ portion of the compound action potential, suggesting that peripheral nerve transmission block may underlie the suppression of pain by PNS.

Ignelzi and Nyquist ⁹ further explored the mechanism of PNS by placing cuff PNS electrodes around the sural and superficial radial nerves of cats. They found that, with stimulation, all of the components of the compound action potential were affected, although the Aδ fiber peak was more affected following neurostimulation than were the Aα or Aβ peaks. The changes were represented by either a reduction in amplitude or an increase in the latency of these waves. These findings also support the involvement of a more peripheral mechanism underlying the analgesic effect of PNS.

Indications/Contraindications

Indications

PNS is usually indicated for patients with peripheral neuropathic pain that can be attributed to a single peripheral nerve.¹⁰ Candidates for PNS should have undergone multimodal therapy, including medical management, anesthetic blockade, and physical therapy. As with patients who are being evaluated for spinal cord stimulation, neuropsychological testing can be valuable. In addition, before permanent implantation of the internal pulse generator, patients should have undergone a successful trial of stimulation with a predetermined therapeutic benefit.

More recent studies of subcutaneous target stimulation (STS) and regional stimulation have broadened the traditional inclusion criteria of neuropathic pain attributable to a single peripheral nerve.¹¹ Retrospective case series of subcutaneous stimulation applied to painful areas has demonstrated therapeutic benefit for lower back pain, neck pain, inguinal pain, and others. However, the longer-term efficacy for this application is unknown.

Contraindications

PNS is a well-tolerated procedure. Consequently the contraindications to implantation are generally those related to the patient’s ability to undergo a surgical procedure. Specifically the patient should not be coagulopathic and should be able to tolerate the stress of general anesthesia. Since the procedure involves the implantation of a medical device, the patient should not have an active infection, and the surgical area should be free of infection.

Equipment

The system for PNS includes the electrode through which stimulation is applied to the target nerve and the implantable pulse generator (IPG). Initial electrode designs were cuff electrodes, which were wrapped around the target nerves. This electrode design was found to lead to an increase in perineural fibrosis with some association of peripheral nerve injury. Newer electrode designs are either plate electrodes, which are surgically implanted, or wire electrodes, which may be implanted percutaneously.¹²

Both types of electrodes may be used for a trial of PNS. If the plate electrodes are used during the trial, an extension cable is externalized during the trial and is removed should the trial prove successful. If a percutaneous trial is conducted, the trial wire is removed before the implantation of the permanent system.

The IPG may be implanted in various sites, depending on the site of the stimulation electrode. Locations for the IPG include infraclavicular region, abdominal wall, gluteal region, or lateral thigh. The life span of the IPG varies, depending on the level of stimulation required for clinical efficacy, but 3 to 5 years is a reasonable estimate with current generators.

There has been significant recent research on BION (bionic neuron) technology, which is being applied to the field of PNS.¹³ BION is an implantable stimulator that can be used to target nerves and muscles. The design is quite unique from currently used technologies since it is a leadless system, in which the generator and electrode are incorporated into a single miniature apparatus. Technology is being developed to percutaneously implant these devices to be used as peripheral nerve stimulators.¹⁴ Reports have been published with this technique targeting the occipital and pudendal nerves, among others.

Technique

Peripheral nerve stimulators can be implanted through either an open surgical or a percutaneous approach. Box 16-1 lists nerves that are commonly targeted for PNS. If using a surgical approach, the first step is the exposure of the target nerve. An approximate 4-cm distance of the nerve is dissected free from the surrounding tissues. Care is taken not to excessively disrupt the vascularity of the nerve. Once the nerve has been dissected free, the next step is placing the electrode under the nerve so the electrode contacts lie in proximity to the nerve. The electrode can then be sutured to the surrounding tissues to prevent migration. Some authors have advocated placing a piece of fascia between the stimulating electrodes and the nerve.¹² This is thought to be analogous to the dura in spinal cord stimulation, which separates the spinal cord–stimulating electrode from the underlying spinal cord. Fig. 16-1 illustrates the surgical technique for placement of an ulnar nerve stimulator.

Fig. 16-1 Illustration of the surgical technique used during implantation of a paddle electrode for ulnar nerve stimulation. The electrode may be implanted either above or below the target nerve. A fascial cuff can be secured around the paddle electrode and to the underlying tissues to prevent migration. The lead can then be tunneled to the infraclavicular region for generator implantation.

Created by Andy Rekito, MS, and published with permission from Oregon Health & Science University.

The techniques for percutaneous implantation of peripheral nerve stimulators to specific target nerves are discussed in the following paragraphs. A recent report by Huntoon and Burgher ¹⁵ describes an ultrasound-guided technique for percutaneously implanting PNS in close proximity to the target peripheral nerve. They have used this technique to implant PNS around the median, radial, ulnar, peroneal, and posterior tibial nerves with efficacy similar to surgically implanted leads.

Occipital Nerve Stimulation

A trial of occipital nerve stimulation may be performed under local or general anesthesia. The senior author (KJB) has found radiographic targeting effective and thus performs the trial under general anesthesia to aid in patient comfort. For a unilateral trial the patient may be positioned supine on the operating table with head turned contralateral to the side of implantation. If a bilateral trial is to be performed, the patient may be positioned either in the lateral position or prone on the operating room table. Anteroposterior (AP) fluoroscopy is used to guide the positioning of the electrode. A Tuohy needle can be gently curved to approximate the curvature of the suboccipital region, and an entry point is chosen slightly medial to the mastoid tip. The needle can then be advanced subcutaneously with the beveled edge down radiographically angled toward the tip of the odontoid as a guide to the superior/inferior plane (Fig. 16-2). The electrode is then passed to the midline through the Tuohy needle. Alternatively some authors^12,¹⁶ prefer to aim the electrode along the posterior arch of C1. Care is taken so the Tuohy needle does not penetrate the scalp as it passes medially.

Fig. 16-2 Anteroposterior radiograph of the skull demonstrating a four-contact occipital nerve stimulator. From an entry point medial to the mastoid tip, the electrode is aimed toward the tip of the odontoid process.

Following a successful trial, permanent implantation can be performed under general anesthesia with the IPG placed in the infraclavicular region. During the permanent implantation a small 2-cm vertical incision is made in the retromastoid incision to allow anchoring of the lead to the underlying fascia (Fig. 16-3).

Fig. 16-3 Technique for occipital nerve stimulator placement. Through a 2-cm retromastoid incision, the Tuohy needle and electrode can be passed toward the midline aimed toward the tip of the odontoid. The distal electrode can then be tunneled to the infraclavicular region where the generator is implanted.

Created by Andy Rekito, MS, and published with permission from Oregon Health & Science University.

Supraorbital and Infraorbital Stimulation

Implantation of supraorbital and infraorbital stimulator electrodes may be performed with the patient awake or under general anesthesia. Similarly to occipital nerve stimulation, we prefer to implant the trial electrode using radiographic guidance under general anesthesia for patient comfort. For supraorbital stimulation, a small stab incision is made in the anterior portion of the temple above the orbital ridge. A gently curved Tuohy needle is advanced subcutaneously toward the midline, and the electrode is passed through the needle with the goal of spanning the supraorbital foramen. For permanent implantation the trial electrode is removed, and a new percutaneous electrode is inserted as described. This electrode is then tunneled to the ipsilateral retromastoid region, where it can be anchored to the underlying fascia through a small 2-cm incision. The electrode and extension cable can then be tunneled and connected to the IPG through an infraclavicular incision (Fig. 16-4).

Fig. 16-4 Technique for infraorbital and supraorbital electrode placement. Through a small stab incision either superolateral to orbital roof (supraorbital stimulation) or inferolateral to orbital floor (infraorbital stimulation), the percutaneous electrode can be advanced to span the affected territory. The electrode is then anchored in the retromastoid region and tunneled to the infraclavicular region where the generator is implanted.

Created by Andy Rekito, MS, and published with permission from Oregon Health & Science University.

For infraorbital stimulation, an entry point is chosen lateral and inferior to the orbital floor, and the electrode is advanced so it spans the infraorbital foramen (Fig. 16-5). During permanent lead implantation, a 2-cm incision is made approximately 1 cm in front of the tragus, and the electrode is tunneled to this incision where it is anchored to the fascia. The electrode and extension cable can then be tunneled to an infraclavicular incision where the IPG will be implanted.

Fig. 16-5 Anteroposterior radiograph of the skull with an infraorbital stimulator aimed to span the infraorbital foramen.

Subcutaneous Stimulation

Trial and implantation of electrodes for subcutaneous stimulation is a newer technique. Technically, however, the implantation of the electrode and pulse generator is identical as for other percutaneously placed peripheral nerve stimulators. For subcutaneous stimulation, leads are implanted with the goal of placing the active contacts in the center of the painful area.¹¹

Some authors have advocated peripheral nerve field stimulation techniques by subcutaneously placing two parallel electrodes on either side of the painful region (Fig. 16-6). In applying regional stimulation, one lead can serve as the anode, and the other parallel lead can serve as the cathode.³

Fig. 16-6 Anteroposterior radiograph of the lumbar spine after implantation of two paramedian subcutaneous electrodes for field stimulation in the treatment of lower back pain.

Outcomes Evidence

Peripheral Nerve Stimulation

In 1976 Campbell and Long ¹⁷ reported their series of 23 patients who had been treated with PNS for a variety of conditions, including peripheral nerve injuries, sciatica, and lower back pain. The authors used a bipolar electrode that was wrapped around the affected peripheral nerve. At an average follow-up of 12 months, 14 of the 23 patients were found to have failed treatment. Four patients in the series had excellent outcomes, and five patients had intermediate results. In reviewing the patient characteristics and clinical outcomes, the authors found that the patient’s diagnosis was the factor most correlated with their outcome. None of the patients who were implanted with a sciatic nerve stimulator for lower back pain had good outcomes. Conversely, patients implanted with PNS for chronic peripheral nerve injury had the best outcomes and the highest chance for success.

Reports of PNS applied to patients with peripheral nerve injury have been very favorable. Law, Swett, and Kirsch ¹⁸ reported their results with 22 patients who had been treated with PNS for chronic posttraumatic neuropathy. At a mean follow-up of 25 months, 13 (59%) patients reported benefit from their stimulators and continued to use them.¹⁸ Similarly Waisbrod and associates¹⁹ reported their experience using a cuff electrode from Avery in the treatment of 19 patients with traumatic peripheral nerve injuries. Their series incorporated a range of targets, including the sciatic, femoral, median, posterior tibial, and other nerves. At a mean follow-up of 11.5 months, the authors report complete pain relief in 11 patients (58%) and significant pain relief in four other patients (21%). Poor pain relief was seen in the remaining four (21%) patients.

Van Calenbergh and associates ²⁰ reported long-term outcomes of a small group of patients who had been implanted with PNS in the treatment of neuropathic pain after nerve injury. The authors used a circumferential Avery electrode that was surgically implanted around the injured nerve proximally. Of the 10 patients available for follow-up, four were not using the device because of hardware complications or infection. The remaining five patients were found to have excellent pain relief from the PNS at a mean follow-up of 22 months.

Occipital Nerve Stimulation

Slavin, Nersesyan, and Wess ¹⁶ and Slavin and associates²¹ reported their experience with unilateral and bilateral occipital nerve stimulation for occipital neuralgia. In their series five patients underwent a trial of unilateral stimulation, and nine patients had a bilateral electrode trial. A successful trial was determined by a 50% reduction in pain by the visual analog pain scale. On the basis of this criterion, 10 patients (71%) underwent permanent implantation. In a mean follow-up of 22 months, 70% of patients who were implanted were found to have beneficial effects of stimulation.

In a series of patients who underwent implantation of an occipital nerve stimulator following a successful trial, Weiner ¹² reported excellent pain relief in 55%, good relief in 27%, and fair relief in 15%.

Trigeminal Branch Stimulation

Amin and associates ²² reported their experience with 16 patients who underwent a trial of PNS in the treatment of supraorbital neuralgia. Ten patients experienced a greater than 50% reduction in the visual analog score and consequently were implanted with the IPG. In the 30-week follow-up after implantation, patients were found to have a significantly decreased pain score and a significantly reduced requirement for morphine equivalents.

In a longer-term follow-up study, Johnson and Burchiel ²³ reported a series of 10 patients who had undergone implantation of supraorbital or infraorbital nerve stimulators for trigeminal neuropathic pain following facial trauma or herpes zoster infection. At a mean follow-up of 26 months, 70% of patients were found to have continued benefit from the stimulation and experienced greater than 50% reduction in their pain compared with preimplantation. All of the patients with posttraumatic neuropathic pain had greater than 50% pain reduction and reduced their intake of medications.

Slavin and associates ²¹ shared their follow-up of 12 patients who had undergone a trial of PNS to the trigeminal branches for facial pain, reporting that 9 patients had a successful trial warranting implantation. Over a mean follow-up of 44 months, five of these nine patients maintained greater than 50% pain relief. One patient underwent explantation because of infection, another was explanted as his pain improved, and the remaining two patients did not maintain benefit.

Subcutaneous Target Stimulation

Subcutaneous stimulation of painful areas of the body without targeting a specific peripheral nerve is a newer technique that has been applied since the late 1990s. In 2006 Goroszeniuk, Kothari, and Mamann ⁴ reported their technique and outcomes with a series of three patients who suffered from chronic pain that had been unresponsive to multimodal therapy. During a subcutaneous trial of stimulation using slow frequency stimulation at 2 Hz for 2 minutes, the authors were able to achieve several hours of pain control in each of the patients. The patients went on to permanent implantation with positive results. Of note, the authors found that each of these patients did not receive significant benefit with a transcutaneous electrical nerve stimulation (TENS) unit.

In 2010 Sator-Katzenschlager and associates ¹¹ reported a multicenter retrospective series of 119 patients who underwent a trial of STS between 1999 and 2007. The indications for STS included low back pain, failed back surgery syndrome, neck pain, and postherpetic neuralgia. Of the 119 patients, 111 reported a more than 50% reduction in the numerical rating scale (NRS) for pain with the trial of stimulation and underwent permanent implantation. In short-term follow-up of 3 months, the average reduction in the NRS was from 8.2 before surgery to 4.0 after surgery. As the authors conclude, prospective long-term studies will be needed to fully assess the efficacy of this treatment modality.

Risk and Complication Avoidance

Risks associated with PNS are generally related to those of any implantable device. The most notable risks include infection, lead breakage, migration of the stimulating electrode, and nerve injury. The rates of complications range from 5% to 43% in published series, with infection remaining the major risk.¹⁰ As a means for limiting the risk of device explantation, the present authors have found local injection of antibiotics before skin closure to be effective in reducing the risk of surgical site infections. Miller, Acar, and Burchiel²⁴ found a 5.7% risk of infection associated with implantation of neurostimulation and intrathecal devices with prophylaxis by intravenous antibiotics before surgery. When a local injection of neomycin/polymyxin before skin closure was used in addition to intravenous antibiotics, the infection rate dropped to 1.2%.