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.