Fundamentals and Mechanisms of Dorsal Root Ganglion Stimulation

Introduction

Electrical neuromodulation for the relief of chronic pain has a long and colorful history, beginning with the often-repeated anecdotes involving electric fish ( ), spanning galvanic applications of somewhat dubious merit ( ), and encompassing a number of reports of do-it-yourself approaches in the present day ( ). However, on the peer-reviewed publication of prospective investigations ( ) and the contemporaneous development of an explanatory mechanistic model based on a modern understanding of neurophysiology ( ), neuromodulation has entered the contemporary era.

For more than 50 years, the neural–technology interface has been refined. Today, dozens of hardware designs exist for pain management via neuromodulation, and thousands of patients have experienced meaningful improvements of their symptoms, function, and quality of life (QOL). Spinal cord stimulation (SCS) is the most widely used of the implanted neuromodulation options and is typically used for neuropathic pain of the trunk and/or limbs. Traditionally, for SCS, electrical contacts are placed epidurally above the dorsal columns (DCs) of the spinal cord. Two styles of leads exist: (1) cylindrical linear leads that are maneuvered to their epidural sites through an epidural needle/catheter in a percutaneous procedure under fluoroscopic guidance and (2) unidirectional paddle-shaped leads that are placed via open laminotomy or laminectomy. The former is minimally invasive and can be performed by anesthesiologists or other nonsurgical specialists, while the latter requires neurosurgical or orthopedic involvement.

A recent systematic review and meta-analysis calculated that the average pain relief with SCS was 58% ( ), and measures of QOL, function, and sleep likewise improve ( ). In accordance with the International Classification of Functioning, Disability, and Health (ICF model), this makes it an extremely valuable intervention for otherwise-intractable cases. Currently, more than 30,000 Americans receive SCS systems each year ( ). Despite its undisputed benefit, SCS does not meet the needs of all people with chronic neuropathic pain for various reasons:

• 1.

  Pain relief may be insufficient. Adequate response to SCS is generally defined as relief of 50% or more of baseline pain as measured on a standard visual analog scale (VAS) or numeric rating scale (NRS), although this is only a rule of thumb; the minimal clinically important difference for back pain is 2–3 cm on a 10-cm VAS ( ). About 20% of patients do not respond to initial treatment during the temporary trial, and about 25% of those implanted do not respond long term ( ). Thus, a substantial proportion of patients, having already exhausted conventional pain management interventions, are not served by SCS either.

• 2.

  Pain in certain regions may not be amenable to treatment. Pain–paresthesia concordance, known to be a necessary condition for achieving pain relief of the painful trunk and legs, but not angina pain, with traditional tonic SCS ( ), is readily achieved for the limbs but is more difficult for trunk sites. This is likely due to an inability to recruit the necessary DC fibers as a result of spinal anatomy and electrode location ( ). This may result in SCS patients having residual pain (e.g., patients may report that SCS relieves their leg pain but not their back pain) or patients being deemed unsuitable for SCS treatment in the first place due to the site of their pain.

• 3.

  Treatment is highly dependent on lead location. Because there is a strong relationship between dermatomal coverage and spinal level, there is a narrow rostrocaudal range within which lead placement is appropriate and efficacious ( ). Midline versus lateral lead placement is also relevant ( ). Patients with epidural adhesions or other spinal abnormalities may not be good SCS candidates due to potential complications during implantation.

• 4.

  Side effects may be intolerable. Some SCS patients report extraneous paresthesias in nonpainful regions, abrupt changes in the perceived intensity of stimulation with changes in position ( ), and/or uncomfortable or “segmental” stimulation, relating to the preferential recruitment of collaterals ( ). These side effects may limit the effective usage range of SCS.

• 5.

  Lead migrations are a common complication (although, certainly, not the only one; ). Migration rates have been reported at 8.5% ( ), 13.2% ( ), and 15.5% ( ) in comprehensive reviews of the literature. Migrations are most often detected due to a complaint of loss of therapy or movement of paresthesia to unwanted areas and generally require surgical resolution.

• 6.

  Some SCS programs can have high power consumption. In primary cell devices (nonrechargeable battery), surgical procedures for battery replacement are needed every 3–4 years ( ). In rechargeable devices, frequent recharging is necessary ( ).

DRG stimulation was developed in response to these unmet needs and challenges related to conventional SCS. Some preliminary work investigated the DRG as a target for neuromodulation ( ) but did not progress due to the technical difficulty of introducing standard percutaneous leads into the vertebral foramen. The development of specialized technology has now made DRG stimulation feasible. The sections below describe the current knowledge base for this intervention.