Introduction

Peripheral subcutaneous stimulation (PSS) is a new and exciting area of neurostimulation. It belongs to the general category of stimulation of the peripheral nervous system. However, instead of stimulating a well-defined nerve trunk, the stimulation is applied to the small terminal branches of one or more peripheral nerves. The target area for the stimulation is the subcutaneous tissue, where the small nervous endings of the nerves arborize in a widespread network.

The technique is known with several different names, such as subcutaneous stimulation, peripheral nerve field stimulation, regional stimulation, and peripheral nerve stimulation.^1–6 All of these definitions point to the fact that the target is the small peripheral nervous system fibers in the subcutaneous tissue. This is a paradigm switch from previous neurostimulation modalities, in which the stimulation is applied to a well-defined large neural structure (i.e., a large peripheral nerve, the nerve roots, or the spinal cord).

Although the mechanisms of action are unknown, they are most likely similar to the ones described for peripheral nerve stimulation.⁷

This technique has determined a revolutionary change in the paradigm of classical neurostimulation as it has been performed for several decades. In the classical neurostimulation paradigm, the goal is to stimulate some major nervous sensory structures upstream from the painful area to generate paresthesias in that area. With PSS the lead is actually placed within or near the area of the pain (or the area of the projection of the pain). This technique was developed with the goal of stimulating areas that are notoriously difficult or impossible to reach from the spinal cord or major nerve trunk level, including the posterior axial surface of the body from the neck to the lumbar spine. Although originally developed for these difficult situations, PSS is sometimes being used as a first, minimally invasive, neurostimulation procedure if the pain is limited to a relatively small and well-defined area.

Equipment

In the author’s experience, PSS is best accomplished with percutaneously placed cylindrical leads and not with paddle leads. The reason lies in the target of the stimulation. When performing dorsal column, nerve root, or large peripheral nerve stimulation, the target is a well-defined neural structure, and the lead(s) is (are) placed on its surface. Therefore the current is unidirectional and best delivered by a paddle lead. With PSS, instead the lead is placed within the target, which is made of all the small sensory nerve endings within the subcutaneous tissues. Therefore the best electrical field is one that is circumferential, like the one delivered by a percutaneously placed cylindrical lead. The type and contact-spacing of the lead depends on the size of the pain area and the number of leads being used. For a very small pain area, a quadripolar lead with 5- to 7-mm intercontact spacing is most likely sufficient. For a large pain area, one or more leads with 9 mm or more intercontact spacing is more appropriate.

Currently there is no implantable equipment developed specifically for this modality. The leads used are the same ones used for intraspinal stimulation. The pulse generators are also borrowed from the spinal cord stimulation (SCS) line of products.

Technique¹

The principle of PSS is that the lead should be placed within or as near as possible to the painful area. Each electrical contact spreads a circumferential electrical field, which is about 2.5 cm in diameter. Therefore the number, spacing, and distribution of the electrical contacts should be carefully planned according to the size and shape of the painful area. The current can also be driven across leads placed at a distance, thereby increasing the size of the affected electrical field. Larger areas might require several leads placed strategically. I have placed up to four widely spaced percutaneous leads in an effort to cover larger areas of the body.

Almost any area of the body can be reached with this technique. The most commonly addressed areas include lumbar, posterior thoracic, scapular, inguinal, and various regions of the head and face^{1–6, 8–17} (Figs. 20-1 to 20-4).

Fig. 20-1 Severe bilateral lumbar pain following several lumbar spine surgical procedures. Test trial patient in right lateral position. A, Pain areas. B, Tuohy needles placed in pain areas. C, Leads placed in pain areas. D, Leads attached to extensions and taped with transparent adhesive dressing on left flank.

Fig. 20-2 Permanent surgical implant. A, Preoperative marking showing pain areas, planned incisions, and planned lead trajectories (asterisks). B, Three leads placed through midline lumbar incision; one lead placed through IPG incision; three leads tunneled to IPG incision; all leads connected to two bifurcated extensions; two extensions plugged into IPG. Leads being tested intra-operatively (asterisks). C, Midline lumbar incision. Each of the three leads is anchored with two anchors and a small strain relief loop between the two anchors. IPG, Implantable pulse generator.

Fig. 20-3 Temporary trial. A, B, Posterior cervical and medial scapular border pain. C, Four leads: one in the posterior cervical area, one in the upper medial scapular border, and two in the lower thoracic area. D, Two Tuohy needles inserted in a caudal direction in the lower pain areas.

Fig. 20-4 A, C, Cervical and thoracic leads. Percutaneous trial with temporary leads. Leads externalized. B, D, AP x-rays of the cervical and thoracic spine with the leads in place.

A bendable introducer with a blunt obturator is very useful in areas of great curvature such as the forehead, the cervical region, and the knee. A long introducer (6 to 8 inches) can also be useful to avoid placing an incision near painful areas.

The depth of the needle (and therefore the lead) is crucial to the success of the procedure. The lead should be placed in the superficial layer of the subcutaneous tissues. If it is placed in the deep layers of the subcutaneous tissues, no paresthesias will be perceived. If the lead is under the fascia or very close to the muscles, motor contractions will be elicited. If the lead is placed within the dermis, the stimulation will be perceived as extremely painful.

The depth of the needle should satisfy three criteria:

There should be no indentations in the skin. Indentations mean that the needle is actually in the dermis. If the needle is in the dermis, advancing it is very difficult, and one meets a lot of resistance. If the patient is semiawake, he or she demonstrates a lot of resistance since advancing a needle in the dermis is extremely painful.

One should be able to visualize a slight “tenting up” caused by the needle in the subcutaneous tissue, and certainly one should be able to readily feel the needle with finger palpation. If the needle cannot be felt, its location is too deep.

During a percutaneous trial, entry in the subcutaneous tissue is usually characterized by a “pop” feeling caused by the loss of resistance once the needle has penetrated through the epidermis/dermis.

The trial is performed without an incision or with a small nick by inserting the leads in the subcutaneous tissue through the needle provided in the kit or some other inserting device. Even though the procedure is simple, it has to be done with heavy (albeit short) intravenous sedation.

Heavy sedation is highly recommended since inserting a large-bore needle in the subcutaneous tissue parallel to the skin can be extremely painful, particularly since the needle tract cannot be infiltrated with local anesthetic. On insertion of the leads, the trial is undertaken just like a regular SCS trial. One caveat is that sometimes the trauma of the needle insertion causes the tissues to temporarily react abnormally to the stimulation. Impedances can be high, and stimulation might not be perceived fully. A reprogramming session 24 to 48 hours following the lead insertion usually yields much more reliable results.

If the trial is successful in reducing the pain, the whole system is implanted at a second sitting. It is possible to perform a trial with permanent electrodes, but this entails an incision and the use of one or more extension cables.

Implantation of the whole system requires excellent surgical technique and experience. The areas of pain to be addressed must be marked. Possible areas of hypersensitivity or hyperalgesia that must be avoided are then mapped out. The implantable pulse generator (IPG) location must be decided with the above mapping in mind. Sometimes tunneling several electrodes subcutaneously in the posterior thoracic area is not desirable. The same applies to the strategic placement of anchors and connections with extension cables. If not properly placed, these elements alone could lead to later removal of the device. Migration of the leads is the most common complication, and one should make every effort to prevent that occurrence. Anchoring the lead is crucial. Several methods of anchoring are available. One could use the commercially available anchors that come with the lead kit. If more than one anchor per lead is placed, the anchors can be trimmed so as to not be too bulky. Alternatively one could place two or three “drain stitches,” with the knots tied semiloosely around the lead so as to not damage it. This is a very effective way of “trapping” the lead without the bulk of an anchor. I always place a loop, which is then secured with a second anchor or another suture. The loop is probably the most significant protection against migration. In some areas such as the posterior cervical region, migrations are much more common.

The implantation of the whole system can be a substantial surgical procedure that cannot be underestimated. Often it requires several incisions and multiple tunneling efforts. A complex PSS procedure could be much more demanding than a simple SCS implant.

Indications

Both nociceptive and neuropathic pain can respond positively to this modality.

Criteria for the trial procedure include: (1) severe pain of at least a 6-month duration, (2) failure of conservative treatment, and (3) psychological stability. Both axial and limb painful areas can be targeted (Figs. 20-5 to 20-7). In the author’s practice the most common indications include intractable axial lumbar pain (whether the patient has received surgical intervention or not), intractable posterior thoracic pain, intractable scapular pain, inguinal pain, thoracic wall pain, shoulder pain, headaches, and atypical facial pain. I have not placed subcutaneous leads in the perineum. I have implanted many patients with severe intractable low back pain whose other alternative was a multilevel spine fusion. When dealing with exclusively axial pain and in the presence of multilevel lumbar spine degenerative disease, a trial with PSS seems to be a reasonable option.

Fig. 20-5 Intractable knee pain. Status post–knee replacement. A, Pain areas and planned IPG placement. B, Medial lead placement and IPG incision. C, Permanent implant. D, Lateral lead placement and tunneling tool to IPG (white arrow). E, Incisions (black arrows). IPG, Implantable pulse generator.

Fig. 20-6 Permanent implant. A, Planned incisions (asterisks). B, IPG shown in incision. IPG, Implantable pulse generator.

Fig. 20-7 Intractable neuropathic pain following wrist replacement. Temporary test trial. A, Planned leads placement. B, Tuohy needles for leads placement (arrow). C, Leads in place for temporary trial. D, AP and lateral views of lead in proper position.

The most important part of the procedure is careful planning. Unlike SCS, in which the lead insertion incision is almost invariably either in the upper lumbar or upper thoracic midline, with PSS the incisions can be numerous and must be strategically placed according to the pain areas to be covered and the position of the implantable pulse generator (IPG). As a first step, the patient and the implanting physician must agree on which pain areas are to be addressed. Many times, when presented with the question, “where is your pain?” the patient points to very large body areas, often larger than what it is reasonable to try to cover with the stimulation. I then usually ask two key questions: (1) Where is the area of your worst pain? (2) Where is the area from which your pain originates? (if there is one). If the patient points to a vague large area and cannot pinpoint the answer to these questions, he or she is not deemed a good candidate for the procedure. Next the areas of pain to be addressed must be outlined in detail. Areas that qualify for criteria 1 and/or 2 are the ones that carry priority in lead placement.

The success of the procedure is directly related to the ability of the patient to guide the implanter to the most crucial pain areas. Experienced implanters often ask the patient to mark the areas of pain before coming to the hospital. Some implanters go as far as asking the patient to assign numerical values to the various pain areas as a reflection of the pain severity.

Strategic placement of the leads can be accomplished either by placing the lead(s) in the center of the painful area or by “bracketing” the area with leads flanking the area. The latter is definitely the preferred approach in the case of allodynic areas. Leads placed directly in the allodynic area might be physically bothersome, and the stimulation might be perceived as painful. It is crucial to carefully map the transition zone between the allodynic and nonallodynic area and place the lead exactly in that zone. A centimeter difference could substantially impact the success or failure of the modality.

PSS can also be used in conjunction with intraspinal stimulation and/or large peripheral nerve stimulation.⁸ I have several patients in whom both modalities were successfully implemented jointly, either as separate procedures or as part of a single implant procedure. It is not unusual to be able to adequately cover pain in the lower extremities with intraspinal leads, but to rely on leads placed subcutaneously to address pain in the axial low lumbar area. Another common situation for a “mixed” lead placement is in patients who have upper-extremity and scapular pain. In this instance one lead can be placed intraspinally in the cervical area, and another in the scapular area subcutaneously. Both the intraspinal and the subcutaneous leads can be plugged in the same IPG (provided that the total number of contacts does not exceed the capacity of the system). The current can be programmed to flow between the intraspinal and the subcutaneous leads; this might lead to an even broader field of stimulation.

Peripheral Subcutaneous Stimulation and Transcutaneous External Nerve Stimulation

In some respects PSS could be considered the next step in the neurostimulation continuum, after transcutaneous external nerve stimulation (TENS). In fact, many of the indications for PSS are also indications for TENS. The TENS modality(ies) should always be tried before PSS, unless clear contraindications exist: allodynia, pain location not suitable (head, hairy area, perineum). If the TENS treatment is satisfactory, PSS should not be considered.

There are four reasons why TENS might not work:

The stimulation itself is not effective in reducing the pain.

The patient might have skin hypersensitivity or allodynia in the pain areas and might not tolerate the electrodes on the skin.

Some body conditions might not make it possible to use the TENS unit regularly: (a) excessive sweat, (b) hairy areas, and (c) patient might not be able to reach the pain areas and place the electrode pads.

Even though the TENS helps, the patient might find it awkward to implement it on a daily basis.

If the TENS unit did not work because of the first reason, PSS might still have superior efficacy in relieving pain. If the TENS modality did not work because of reasons two through four, a trial with PSS is definitely indicated; it might be a much more suitable long-term treatment option.

Risk and Complication Avoidance

Because the whole procedure is performed at the level of the subcutaneous tissues, the risk of neurological damage is remote and certainly not in the same category as intraspinal stimulation. The three most common complications are lead migration, skin erosion, and infection. Lead migration can easily occur if the leads are not properly secured as outlined previously. Unlike SCS, in which a small lead migration can be corrected by programming, in PSS, if the lead moves out of the painful area, no reprogramming will be effective in recapturing the pain relief. Skin erosion is possible if the anchors are placed too superficially in the subcutaneous tissue. This is a more frequent issue with leads placed in the scalp area. Skin erosion can also occur at the tip of the lead; this is also a more frequent problem with scalp stimulation. The risk of infection is not different from other neurostimulation procedures.

Results

Only case reports are available in the literature at the time of writing of this chapter. There are no large series reports or prospective studies. Positive results have been reported in patients with intractable lumbar axial pain, inguinal neuralgia, neuropathic pain, abdominal pain, postherpetic neuralgia, and cervicogenic headaches, to name a few. ^{3–6, 8–19}

The most comprehensive retrospective study is the one by Verrills and associates.³ The authors collected data on 13 consecutive patients who had successful trials and were subsequently implanted with Octrode percutaneous leads placed subcutaneously within the major area of pain. Eleven patients met diagnostic criteria for failed back surgery syndrome. A questionnaire assessed outcomes, including pain, analgesic use, and patient satisfaction. The response rate was 93% (13/14; average follow-up time was 7 months). There was a significant decrease in pain levels: an average reduction of 3.77 visual analog scale (VAS) points. Eleven patients (85%) reported successful outcomes and an average pain reduction of 4.18 points, but two reported a poor response. Pain relief was highly correlated with reduced analgesia and patient satisfaction. No complications were reported. Before peripheral nerve stimulation (PNS), the mean VAS was 7.42 ± 1.16 (range 5 to 10). Following PNS, the mean VAS was 3.92 ± 1.72 (range 1 to 6), representing a mean improvement in VAS of 3.77 ± 1.65 (range 1 to 6.5). As a response to PNS, patients were asked to rank their satisfaction with the procedure. One patient considered the outcome to be completely satisfactory. Two patients were very satisfied, and seven were satisfied with the procedure. Two patients were not completely satisfied, and one was not satisfied with PNS.

The results are promising, but prospective randomized studies are necessary before reaching definitive conclusions for the role of this modality in the management of chronic pain conditions.

Conclusions

Stimulation of the small peripheral nerve fibers in the subcutaneous tissue is promising to become a very important modality in the management of severe pain conditions that have failed more conservative measures. The attractiveness of the modality lies both in its (relative) simplicity and in the fact that one can cover with neurostimulation areas previously deemed almost impossible. A trial is a simple procedure; however, the actual surgical implant is far more complex and tedious and requires substantial intraoperative planning.

The exact role of this modality in the management of refractory pain conditions will have to be defined by large prospective clinical studies.