Robert M. Levy MD, PhD Anesthesia Pain Care, Tamarac, FL, USA The dorsal root ganglion (DRG) is a potentially attractive therapeutic target. DRG stimulation is an alternative to spinal cord stimulation (SCS) and has several advantages. The area of stimulation is smaller, more specific, minimizing the paresthesia outside the painful area [1]. DRG stimulation may cover areas difficult to capture with SCS [2–3]. DRG stimulation is less affected by postural changes, and there may be less loss of efficacy noted over time with traditional stimulation, and may not require parasthesia to induce pain relief [1, 4]. The DRG contains the cell bodies of the peripheral primary sensory neurons (PSN), which then project to the dorsolateral sulcus of the spinal cord. Located within and just outside the neural foramen, the DRG is located between the medial and lateral aspects of the pedicles and just below the superior pedicle on anteroposterior radiographs [1, 5]. Over the past few decades, the critical role of the PSNs and the DRG in the development of chronic neuropathic pain have been well elucidated [1, 6, 7]. DRG cells have a somewhat unique morphology; these “pseudounipolar” neurons appear to travel from their peripheral endings to the spinal cord. At the level of the DRG, there is a T-junction connecting the DRG cell body to the traversing axon. Contrary to classical thinking, the DRG cell bodies do more than simply provide energy and nutrition to the primary axons. The activity of the DRG cell bodies work through the T-junction to modulate sensory signals, of particular relevance to pain, as they travel to the spinal cord [4, 8]. Over the past few decades, the critical role of the PSNs and the DRG in the development of chronic neuropathic pain have been well elucidated [1, 6, 7]. DRG stimulation is a relatively new technique with an associated learning curve. It is more technically demanding than SCS. Current FDA labeling in the United States is for the device to be placed most rostrally at or below the T10 spinal level, with the proviso that “The safety and efficacy of implantation of leads implanted above the T10 vertebral level have not been evaluated”. The implanting physician should review neuroimaging to determine whether a patient’s spinal canal, lateral recess, foramen, and epidural space are adequate for the needle, introducer and lead placement at each target level. If not skilled at radiologic interpretation, the implanting physician should consult with a radiology specialist for a report focusing on these specific issues. In patients where MRI is contraindicated, CT with or without myelography is a valid alternative. Neuromonitoring or conversation with the awake patient have both been suggested to improve safety. Many DRG implants are performed below the level of the conus medullaris, thus eliminating the risk of spinal cord injury and making the risk of serious injury with DRG stimulation less common at L2 or lower. The only FDA and CE approved system for DRG stimulation was specifically designed, tested and validated for the clinical use and safety of epidural access and placement of a lead within the neuroforamen to lay over the DRG. While other approaches have been utilized for DRG lead placement, these other systems have either been standard SCS systems or systems not designed or intended to be anatomically placed near the DRG [9–11]. The use of these approaches has resulted in a relative lack of long-term efficacy as well as higher safety event incidence [9, 10]. DRG leads are sited percutaneously from inside to outside the canal. An introducer sheath containing the lead is passed via a Tuohy needle into the epidural space and fluoroscopically guided into a posterosuperior position in the intervertebral foramen (Figure 52.1a,b). A trial stimulator may then be attached and the lead tested, with immediate patient feedback. A successful trial may be indicated by relief of pain, reduction of allodynia, and/or adequate coverage with paraesthesia. Once the lead tip is in place in the intervertebral foramen. a strain relief loop is deployed in the epidural space to reduce the risk of migration, and the introducer and needle are withdrawn. This is critical to the success of the implant. An implantable pulse generator (IPG) may then be placed subcutaneously and the lead(s) connected to it to complete the system, or the leads may be externalized if an extended ambulatory trial period is desired. Present IPGs can accommodate up to four quadripolar DRGS leads, thus allowing stimulation of up to four DRGs simultaneously, although most commonly only one or two are used. Post-operatively, stimulation is titrated to achieve optimal results and the patient is provided with a remote controller which can be used to modulate the stimulation amplitude [12]. Figures 52.1 AP (a) and lateral fluoroscopic (b) images showing DRG stimulator wires at L4 and L5. The wires are posterosuperiorly placed within the foramen and strain relief loops have been placed in the epidural space to minimize risk of displacement. (Source: Used with permission.) Due to the unique placement techniques for the epidural needle and the placement of the DRG leads into the neural foramen, there are some unique risks and contraindications for DRG stimulation compared to SCS devices. First, the L5–S1 interspace is commonly accessed for DRG stimulation, as this is often required for coverage of neuropathic foot pain. The L5–S1 level often has a thinner ligamentum flavum and less capacious epidural space than other lumber levels; thus extra care must be taken to avoid dural puncture here. Second, a contralateral epidural access technique to L5–S1 is advised for DRG lead placement. Third, the DRG introducer and/or lead is placed into the intervertebral foramen. Accordingly, the target foramen must not be critically stenotic. One important mitigation strategy arises from our knowledge of the output of the DRG fibers. There is significant divergence and convergence of the DRG fibers as they enter the DRG itself and exit to travel to the spinal cord. As a result, and in light of the significant hypersensitivity to stimulation of DRG cell bodies upstream from injured peripheral fibers, stimulation at the level above or below the target level may well provide stimulation induced paresthesias and pain relief within the target area. Facing a severely stenotic target neuroforamen, implanters should first consider trial stimulation at the level above and/or below. Complications might be related to the operator, the hardware or the patient. These included temporary motor stimulation, cerebrospinal fluid (CSF) leaks, infection, lead migration, and lead fracture. The Neuromodulation Appropriateness Consensus Committee (NACC) of the International Neuromodulation Society (INS) has identified these complications and suggested techniques for mitigation of these events (Table 52.1). Table 52.1 Complications from DRG Therapy and Mitigation Techniques.
52
Complications of Dorsal Root Ganglion Stimulation for the Treatment of Chronic Neuropathic Pain
Introduction
Anatomy
Indications
Contraindications
Technique
Complications
Complication
Mechanism of Complication
Mitigation Technique
Nerve injury
Needle puncture
Appropriate angle, landmarks and pre-procedure imaging
Nerve injury
Lead or sheath trauma
Gentle technique, pre- procedure imaging, patient conversation or neuromonitoring
Dural puncture or CSF leak
Needle puncture
Shallow angle, appropriate pathway to space. Loss of resistance with lateral view
Lead migration
Lack of proper strain relief, not piercing the ligaments
S-loop strain relief, ensure sheath is through ligaments
Infection
Surgically acquired
Follow NACC guidance
Bleeding
Perioperatively
Follow NACC guidance
Lead fracture
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