Introduction

Peripheral nerve stimulation (PNS) is a technique of direct electrical stimulation of peripheral nerve(s) to relieve pain or to assist with the functional restoration of patients with severe neurological injuries, including those to the spinal cord. The term PNS has also been used to describe the superficial placement of electrical leads in areas of the body such as the back, thorax, or abdomen where no discrete named nerves exist but small nerve fibers near the dermis can be targeted to produce local analgesia. This type of stimulation is mechanistically poorly understood and will be referred to as peripheral field stimulation (not the subject of this chapter). PNS became conceptually possible after the development of the gate control theory. Wall and Sweet’s experiments with PNS tested the concept of electrical stimulation–produced analgesia, initially of the authors’ infraorbital nerves and subsequently in eight patients. Soon many neurological and orthopedic surgeons became interested in the technique, and several case series of these peripheral nerve surgeries were described The early history of the development of these techniques is chronicled by Michael Stanton-Hicks and colleagues, who describe the indications, outcomes, and complications of a series of 91 patients at the Cleveland Clinic. Despite early investigator interest and the development of novel lead and battery technology, innovations in programmability, and other technical improvements for stimulation of the peripheral nerves, the application and study of PNS has been less vigorous in scope than that for spinal cord stimulation. The absence of a comparable minimally invasive trial system that became available with the introduction of percutaneous spinal cord stimulation electrodes in the 1980s may be one reason that peripheral nerve surgeries have not advanced as rapidly. Other potential reasons for decreased interest in PNS may be frequent complications such as lead migration (requiring revision surgery), neurological damage to the axons from mechanical or stimulation-induced nerve injury, and poor patient selection leading to poor outcomes. PNS has always been an open technique since its inception more than 40 years ago, but recently minimally invasive techniques were described. Possible advantages to peripheral nerve stimulator placement via a minimally invasive technique might include: (1) minimal nerve trunk dissection and manipulation (potential trauma); (2) the omission of the need for a fascial graft or fat pad interposed between the electrode and the nerve (the fat around the epineurium in situ is minimally perturbed by the percutaneous electrode); (3) a true percutaneous trial avoiding dissection and open surgery; and (4) the ability to perform intraoperative electrical stimulation testing in an awake patient with minimal sedation compared to a patient under general anesthesia. Cadaver feasibility studies were performed in 2008, suggesting that ultrasound (US) image guidance might allow safe placement of peripheral nerve stimulating electrodes near target peripheral nerves, similar to nerve catheter placement. Following these feasibility studies, a case series of nine patients who received PNS placements using US was published, based on the principles described in the cadaver studies. In two of the nine cases, patients with good paresthesia coverage did not achieve analgesia and were not implanted, thus avoiding an unnecessary surgical dissection. Most cases produced durable analgesia beyond 1 year. The programming of multiple different stimulation parameters intraoperatively allowed the additional placement of second electrodes parallel to the first or on either side of a nerve trunk to more optimally stimulate the desired fascicles in some cases ( Fig. 18-1 ). Others have described a stimulus router concept that eliminates the need for an implanted generator. A surgeon places a passive implant delivery terminal near the target nerve. Current is passed between a surface cathodal electrode and a pick-up terminal, a small fraction of which (10% to15%) is passed to the delivery terminal. This results in PNS at threshold. It is unclear at this time if this type of implant would be less prone to migration without additional study ( Fig. 18-2 ).

An upper-extremity radiograph after placement of dual four-contact electrodes parallel to the radial nerve. The electrodes can be seen in horizontal orientation to the humerus. In addition, tension loops at the site of anchoring and in the flank are noted.

Stimulus router configuration is depicted from reference 10. Both the passive delivery terminal and nerve implant are shown.

Currently the only commercial U.S. Food and Drug Administration (FDA)–approved electrodes for PNS are four-contact flat paddle arrays, which must be placed via an open surgical dissection, often with an interposed fascial graft, which is thought to protect the nerve. Neurological surgeons usually place the electrodes longitudinally to maximize the number of contacts in direct apposition to the nerve. Since ideal electrode availability and long-term safety testing is minimal, neurosurgical open procedures with the paddle electrodes are likely to remain the preferred method of placement of these devices in the near term. Whether the use of percutaneous techniques might allow future patients to undergo trial placements without significant investment in an open procedure or whether percutaneous permanent placements will become a reality will depend on the pace of new electrode design, future studies of PNS outcomes, and FDA approval of novel leads or other devices.

Current Evidence

At this juncture no prospective randomized and blinded studies have been performed. A recent paper by Bittar and Teddy reviewed the current evidence for PNS. Unfortunately the greatest need for evidence at this time is from prospective, randomized controlled, and blinded studies. Until now these studies have been difficult if not impossible to perform because of patient perception of a physical sensation (paresthesia), which hampers blinding, and the lack of minimally invasive, ethical controls. In an editorial on the subject of peripheral neuromodulation, Davis discussed some of the factors that may make interpretation of currently available studies difficult. Questions regarding the role of external neurolysis (the removal of external scarring around the nerve) on the analgesia seen after PNS, unclear placebo effects, the unequal application of physical therapy in some patient groups, lack of standardization of potentially analgesic drugs (including neuromodulators and opioids), or the increased attention to the patient needs during study protocols are all possible confounding factors. Long-term studies are difficult, but some studies seem to suggest an extremely long-standing effect of PNS, with the possibility of nerve healing over time. Van Calenbergh and colleagues looked back at a group of 11 patients who had been chronically implanted with radiofrequency-coupled peripheral nerve stimulators over an average period of 22 years! Of these 11, four had been explanted, one had died of unrelated causes to his or her stimulator, and one could not participate because of distance from the study center. The remaining five patients had long-term circumferential electrode arrays. These patients demonstrated a durable improvement in their pain, level of function, and use of analgesics when comparing stimulation off to stimulation on. They had sustained minimal complications. Remarkably, benefits persisted despite the passage of over two decades on average. One patient had even demonstrated gradual resolution of his neuropathic pain in the face of neural stimulation. The largest clinical series in print to date are those from Eisenberg, Waisbrod, and Gerbershagen and the Cleveland Clinic. In Eisenberg, Waisbrod, and Gerbershagen’s series, 46 patients with isolated peripheral nerve syndromes were treated. Positive results were noted in 78% of patients, whereas 22% had poor results. Decreases in visual analog pain scores from 69 ± 12 before surgery to 24 ± 28 after surgery were seen. The major pathologies treated in the PNS protocols were for the following: (1) nerve lesions incurred from surgeries around the hip or knee, (2) nerve entrapments, (3) persistent pain after nerve graft surgeries, or (4) postaccidental nerve injection injuries. In the previously unpublished Cleveland Clinic series, results on average were positive, with a frequency of revision surgery of 1.6 per patient, not including battery replacements.