Review Article
The Dorsal Root Ganglion in Chronic Pain and as a Target for Neuromodulation: A Review

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Background

In the not-too-distant past, the dorsal root ganglion (DRG) was portrayed as a passive neural structure without involvement in the development or maintenance of chronic neuropathic pain (NP). The DRG was thought of as a structure that merely “supported” physiologic communication between the peripheral nervous system (PNS) and the central nervous system (CNS). Newer scientific information regarding the anatomic and physiologic changes that occur within the DRG as a result of environmental pressures has dispelled this concept and suggests that the DRG is an active participant in the development of NP. This new information, along with new clinical data showing that stimulation of the DRG reduces intensity of pain, suggests that the DRG can be a robust target for neuromodulation therapies.

Methods

A review of the anatomical and physiological literature regarding the role of the DRG in the development of NP was performed utilizing SciBase, PubMed, and Google Scholar. The information gathered was used to lay an anatomic and physiologic foundation for establishing the DRG as a relevant target for neuromodulation therapies and to formulate a hypothesis as to how electrical stimulation of the DRG might reverse the process and perception of NP.

Conclusions

The DRG is an active participant in the development of NP. DRG stimulation has multiple effects on the abnormal changes that occur within the DRG as a result of peripheral afferent fiber injury. The sum total of these stimulation effects is to stabilize and decrease hyperexcitability of DRG neurons and thereby decrease NP.

Section snippets

INTRODUCTION

There are numerous targets for neuromodulation therapies used today, including the brain, the spinal cord, the cardiovascular system, the peripheral neuromuscular system, the peripheral nervous system (PNS), the gastroesophageal system, and the sacral nerves. Various intraspinal structures are often targeted in both specific and non-specific ways by the application of electrical fields to modulate their function as a greater part of what is referred to as spinal cord stimulation. This review

Structure and Function

The DRG contains the cell bodies of the primary sensory neurons responsible for transducing and modulating sensory information and transmitting it to the spinal cord. There are several types of DRG neurons, classified by the size of the cell bodies and their function. Type A DRG neurons are large and are responsible for touch, vibration, and proprioception; type B neurons are small in size and are responsible for nociception. Histological studies estimate that the number of small neurons (type

Development of Neuropathic Pain

Injury to a PAF results in hyperexcitability only in axotomized DRG neurons, sparing nonaxotomized neurons (34). Injured DRG neurons become more excitable; their SGC sheaths increase their number of cells (35,36), and they exhibit ectopic firing (37,38). Wall and Devor showed that electrical impulses in PAF injury may originate not only from the damaged PAF, but from within the DRG itself (38), and that systemic application of lidocaine suppressed ectopic impulse discharges generated both at

ELECTRICAL STIMULATION OF THE DRG

Recent studies indicate that low-intensity electrical stimulation is functionally equivalent to the administration of various growth factors, enhancing and guiding growth of spinal neurons (80,81), fostering regeneration in bone (82,83) and muscle mass (84), and promoting wound healing (85,86). Because activity of growth factors and of the growth-associated protein GAP-43 within the DRG does play a role in the development of NP, and as it is known that electrical stimulation stimulates the

CONCLUSION

Electrical stimulation around the DRG may result in the relief of chronic aberrant pain, and direct electrical stimulation of the DRG results in decreased hyperexcitability of the DRG neurons. The DRG is a vibrant and active organ participating in the origination and modulation of electrical activity in response to environmental pressures. In response to injury or inflammation, a whole cascade of events occurs within and around DRG cell bodies, including changes in cytokine and chemokine

Authorship Statement

Dr. Krames is the sole author of this manuscript.

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