Chapter Overview
Chapter Synopsis: Peripheral subcutaneous stimulation (PSS) provides a minimally invasive form of neurostimulation for intractable pain. This technique, also called peripheral nerve field stimulation , targets the small, arborized fibers in subcutaneous tissue. Originally developed to treat areas difficult to reach with stimulation of the nerve trunk or spinal cord, PSS shows promise in other areas, rendering more central implantation unnecessary. Technical details of device selection should always be considered in advance of implantation. PSS may be optimized with cylindrical rather than paddle leads used in some other applications. Further consideration should be given to the size and site of the painful area to be treated. Large areas of the body can be treated with multiple, widely spaced leads. This chapter provides several technical considerations for optimal implantation. The primary indications for PSS are back pain and headaches that may be neuropathic or nociceptive in origin. The most promising patient candidates can pinpoint areas of their worst pain and the area from which pain originates. Because stimulation is achieved directly at the pain site, precise placement is critical to success. Transcutaneous external nerve stimulation can be considered a less invasive option to PSS and, if effective, should be used in place of PSS. Although PSS is minimally invasive and avoids many risks associated with spinal stimulation, some complications are common to it, including infection and lead migration.
Important Points:
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Map the pain areas very carefully. Highlight the areas of “worse pain.”
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Confirm with the patient the mapped pain areas.
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Avoid allodynic areas.
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If the areas of worse pain are very extensive (several square inches), the patient might not be a candidate for the procedure.
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Combine it with intraspinal stimulation if indicated.
Clinical Pearls:
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A 100% pain relief at trial is very worrisome and means strong placebo effect.
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If possible, have the patient or a family member map the pain areas before showing up for surgery.
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Always document the lead position with an intraoperative/postoperative radiograph.
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Always cover with the lead placement either the areas of “worse pain” or the areas where the pain starts (or both, if possible).
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During the first 24 to 48 hours following lead placement, the electrical parameters might not reflect the true characteristics of the stimulation.
Clinical Pitfalls:
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Make sure that the patient is fully awake before starting intraoperative sensory testing.
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Carefully secure the leads, particularly in the cervical area.
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Tip of the lead skin erosion is a common issue in the scalp area.
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When tunneling, avoid hypersensitive areas.
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If the patient is thin, place anchors, connectors, and extensions under the fascia if possible to avoid a later revision.
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 . 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.
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.
Technique
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 , ( Figs. 20-1 to 20-4 ).
