A Prospective Analysis of Neuromonitoring for Confirmation of Lead Placement in Dorsal Root Ganglion Stimulation

A Prospective Analysis of Neuromonitoring for Confirmation of Lead Placement in Dorsal Root... Abstract BACKGROUND Dorsal root ganglion stimulation is a neuromodulation therapy used for chronic neuropathic pain. Typically, patients are awakened intraoperatively to confirm adequate placement. OBJECTIVE To determine whether neuromonitoring can confirm placement in an asleep patient. METHODS This is a prospective analysis of 12 leads placed in 6 patients. Lead confirmation was confirmed by awake intraoperative testing, as well as asleep testing utilizing neuromonitoring. Patients were used as their own control. Sensory and motor thresholds for each patient with awake and asleep neuromonitoring testing were recorded. Intraoperative impedance and postoperative programming were also recorded. RESULTS In each patient, paresthesias were generated prior to motor contractions in the awake patient. For each patient, somatosensory evoked potential responses were present after lowering below the dropout threshold of electromyogram responses with neuromonitoring. There were varying degrees of separation in the thresholds that did not appear to be consistent across level or diagnosis. Smaller degrees of separation between thresholds during awake testing also held true in the asleep patient. This was further confirmed with postoperative programming. Impedances did not alter the separation in thresholds or amount of stimulation required for responses. One patient was combative during awake testing, and therefore motor thresholds were not obtained. This same patient was determined to have a ventral placement, confirmed with awake and asleep neuromonitoring testing. CONCLUSION This series demonstrates that the proposed neuromonitoring protocol can be used in an asleep patient to assure proper positioning of the dorsal root ganglion electrode in the dorsal foramen by generating somatosensory evoked potential responses in the absence of electromyogram responses. Dorsal root ganglion stimulation, Neuromodulation, Spinal cord stimulation, Chronic pain, CRPS, RSD, Complex regional pain syndrome, Reflex Sympathetic dystrophy ABBREVIATIONS ABBREVIATIONS CRPS complex regional pain syndrome DRG dorsal root ganglion EMG electromyogram SSEP somatosensory evoked potential The dorsal root ganglion (DRG) is a spinal structure that includes bundles of sensory cell bodies and may play a role in chronic neuropathic pain. Each spinal level has a corresponding DRG from a specific dermatome of the body. The anatomy and positioning of the DRG is consistent and reproducible. The size of the DRG increases in size from superior to inferior in the lumbar spine. In addition, the DRG moves more lateral in the foramen as it moves lower down the spine. It is the most lateral at L5.1 The dorsal aspect of the DRG is involved in sensory perception, while the ventral aspect of the DRG includes motor activity. DRG stimulation is presently a neuromodulation therapy used in the treatment of chronic neuropathic pain. It involves stimulation of the DRG at its junction between the peripheral nervous system and the central nervous system. It is presently approved by the Food and Drug Administration in the United States for the treatment of intractable pain in the lower limbs of patients with the diagnosis of complex regional pain syndrome (CRPS) type I and II, but has also been used for the treatment of failed back surgery syndrome and postsurgical pain.2,3 TABLE 1. Patient Demographics, Diagnosis, and Lead Placement Patient  Age (years old)  Sex  Diagnosis  Lead placement  SG  51  M  Complex regional pain syndrome  Left L5  TD  21  M  Complex regional pain syndrome  Left L3 and L4  MC  43  M  Failed back surgery syndrome, complex regional pain syndrome  Right L1 and L2  GN  70  M  Peripheral neuropathy  Bilateral L4 and L5  DM  57  F  Complex regional pain syndrome  Left L4 and L5  RD  55  M  Peripheral neuropathy  Bilateral L5  Patient  Age (years old)  Sex  Diagnosis  Lead placement  SG  51  M  Complex regional pain syndrome  Left L5  TD  21  M  Complex regional pain syndrome  Left L3 and L4  MC  43  M  Failed back surgery syndrome, complex regional pain syndrome  Right L1 and L2  GN  70  M  Peripheral neuropathy  Bilateral L4 and L5  DM  57  F  Complex regional pain syndrome  Left L4 and L5  RD  55  M  Peripheral neuropathy  Bilateral L5  View Large TABLE 1. Patient Demographics, Diagnosis, and Lead Placement Patient  Age (years old)  Sex  Diagnosis  Lead placement  SG  51  M  Complex regional pain syndrome  Left L5  TD  21  M  Complex regional pain syndrome  Left L3 and L4  MC  43  M  Failed back surgery syndrome, complex regional pain syndrome  Right L1 and L2  GN  70  M  Peripheral neuropathy  Bilateral L4 and L5  DM  57  F  Complex regional pain syndrome  Left L4 and L5  RD  55  M  Peripheral neuropathy  Bilateral L5  Patient  Age (years old)  Sex  Diagnosis  Lead placement  SG  51  M  Complex regional pain syndrome  Left L5  TD  21  M  Complex regional pain syndrome  Left L3 and L4  MC  43  M  Failed back surgery syndrome, complex regional pain syndrome  Right L1 and L2  GN  70  M  Peripheral neuropathy  Bilateral L4 and L5  DM  57  F  Complex regional pain syndrome  Left L4 and L5  RD  55  M  Peripheral neuropathy  Bilateral L5  View Large The placement of the leads involves a standard percutaneous technique for introduction of the lead into the epidural space. A 4-contact electrode is placed inferior to the pedicle of the desired level on the dorsal aspect of the DRG in the foramen. At present, targeted nerve root levels include the lower thoracic and lumbar-sacral levels. Typically, once the electrode is placed under fluoroscope imaging, the patient is awakened to confirm placement in the dorsal foramen with a sensory perception threshold. This case series looks at a novel protocol for the use of neuromonitoring to confirm placement in an asleep patient. METHODS This study is a prospective analysis of 6 patients who underwent DRG stimulation surgery in a single institution performed by a single surgeon. St. Luke's University Health Network IRB approval process utilized. The patient was made aware during the procedure consent process of the utilization of 2 testing methods. Diagnoses of these patients included CRPS, peripheral neuropathy, and failed back surgery syndrome. The case series included a total of 12 leads placed in 6 patients, involving L1-L5 (Table 1). Lead placement ranged from 1 to 4 leads per patient. Lead confirmation was confirmed by awake intraoperative testing as well as asleep testing utilizing neuromonitoring. Patients were used as their own control. Each patient underwent trial placement of leads, which were connected to tunneled extensions for the 1-wk trial period. The leads placed during the trial were therefore placed as a possible permanent implant. None of the patients needed to have the leads revised when presenting after the trial. All patients had successful trials and were subsequently converted directly to permanent implant, with removal of the extensions and connection to the generator. System implant included the St. Jude Medical Axium Neurostimulator System (St. Jude Medical Inc., St. Paul, Minnesota). Placement of the neuromonitoring electrodes was performed under mild sedation while in the supine position prior to placing into the prone position for the procedure. Placement of the trial leads was performed while under light sedation with propofol. Fluoroscopic imaging was utilized for placement and confirmation within the foramen. Each patient was then awakened, and test stimulation was given to determine sensory thresholds. Motor thresholds were also determined if possible. At this point, the patient was placed back under sedation, and neuromonitoring was then utilized for threshold determinations utilizing SSEP (somatosensory evoked potential) and EMG (electromyogram) testing. Further analysis included a lead in which awake intraoperative testing returned low motor thresholds from ventral placement. This lead was also tested with neuromonitoring prior to being repositioned into the dorsal foramen. Intraoperatively, the stimulation parameters remained consistent across awake and asleep testing. Pulse width ranged from 200 to 300, while rate was maintained at 20 Hz. Contact configuration for testing was based on the position of the contacts within the foramen, in which the 2 contacts below the pedicle were used in a bipole configuration. This was most commonly 2– (cathode) and 3+ (anode). However, contact configuration could be adjusted based on responses generated. Postoperative programming was also kept consistent for both pulse width and rate, but contact configuration was adjusted based on patient feedback. Protocol Performing the procedure using neuromonitoring differs from the traditional awake-patient technique in that the patient remains anesthetized throughout the procedure, and the accuracy of the placement and expected sensory and motor thresholds for stimulation are determined objectively using a combination of SSEP and free-run EMG testing. The electrode is placed utilizing fluoroscopic imaging, and the device is then connected to the neuromonitoring system via a custom adapter, currently under patent (Figure 1), that allows the electrode contacts to be supplied with electrical stimulation from the constant-current stimulator, which also runs the traditional SEP modes. FIGURE 1. View largeDownload slide Custom adapter allowing connection of the electrode to the constant-current stimulator. FIGURE 1. View largeDownload slide Custom adapter allowing connection of the electrode to the constant-current stimulator. Monitoring for the surgery consists of bilateral ulnar and posterior tibial nerve SSEPs and free-run EMG for muscles representing the appropriate nerve level(s) for the stimulator placements. It is suggested that recording electrodes be placed in multiple muscles primarily supplied by the targeted nerve root level as well as in an additional muscle primarily supplied by the nerve root above and one supplied by the level below, to allow for variations in an individual's nerve root level to supplied muscle. Table 2 lists the muscles that are most commonly monitored; however, additional muscles may be used for any of these levels. TABLE 2. Typical Muscle Relationships to Nerve Root Levels Nerve root level  Representative muscles*  L1  Iliopsoas  L2  Adductor brevis and longus  L3  Adductor magnus, quadriceps  L4  Quadriceps  L5  Tibialis anterior, adductor hallucis  S1  Gastrocnemius  Nerve root level  Representative muscles*  L1  Iliopsoas  L2  Adductor brevis and longus  L3  Adductor magnus, quadriceps  L4  Quadriceps  L5  Tibialis anterior, adductor hallucis  S1  Gastrocnemius  View Large TABLE 2. Typical Muscle Relationships to Nerve Root Levels Nerve root level  Representative muscles*  L1  Iliopsoas  L2  Adductor brevis and longus  L3  Adductor magnus, quadriceps  L4  Quadriceps  L5  Tibialis anterior, adductor hallucis  S1  Gastrocnemius  Nerve root level  Representative muscles*  L1  Iliopsoas  L2  Adductor brevis and longus  L3  Adductor magnus, quadriceps  L4  Quadriceps  L5  Tibialis anterior, adductor hallucis  S1  Gastrocnemius  View Large SSEPs and EMG are monitored as for any typical spine surgery. At the time of testing the stimulator electrode placement, regular SSEP testing is temporarily suspended, but free-run EMG continues. The same-side posterior tibial nerve SSEP stimulator channel is switched to a low-current mode for better resolution in determining thresholds. The SSEP trial is started and the stimulation current level is slowly raised from 0 mA until compound motor-unit action potential responses in the muscle(s) corresponding to the stimulated nerve root level are visualized; this is the motor threshold for the targeted DRG. If the stimulation level significantly exceeds the typical threshold level, different combinations of contacts and/or polarity conventions can be tried until the motor threshold is at an acceptable level. Once desired motor threshold is determined, the SSEP trial is stopped, cleared, restarted, allowed to run to completion, and stored. For that trial, a time marker is placed at the cortical trough or peak. Then the SSEP is run at progressively lower levels than the motor threshold until the SSEP cortical peak disappears. The last stimulation level at which the cortical peak remains present is considered the sensory threshold (Figure 2). A sensory threshold that is equal to or even greater than the motor threshold indicates a potential ventral placement of the electrode and should prompt repositioning. FIGURE 2. View largeDownload slide SSEP and EMG responses with intraoperative testing. A, EMG response threshold in the adductor hallucis muscle for L5 dermatome at 3.3 mA. B, Presence of an SSEP response below EMG threshold at 1.5 mA. FIGURE 2. View largeDownload slide SSEP and EMG responses with intraoperative testing. A, EMG response threshold in the adductor hallucis muscle for L5 dermatome at 3.3 mA. B, Presence of an SSEP response below EMG threshold at 1.5 mA. Interpretation After proper placement of the DRG electrode under fluoroscopic imaging, further confirmation is generally performed by awake-patient testing. The intraoperative fluoroscopic imaging is valuable in determining proper placement, but a form of patient testing is used and recommended. During awake testing of the patient, paresthesias are generated by stimulation of the DRG. Proper lead placement in the dorsal foramen is confirmed with the onset of paresthesias prior to motor contractions. The procedure performed with the patient asleep, without awakening, would preclude this awake paresthesia testing. In this case series, neuromonitoring was utilized by interpreting the presence of SSEP and EMG. SSEP was used as a marker of generated paresthesia, while EMG would indicate motor contraction. Proper lead placement in the dorsal foramen was confirmed by generating SSEP responses in the absence of EMG responses. To determine proper placement, the stimulation was performed until an EMG response was activated. The stimulation was then lowered until the EMG response completely resolved and the presence of SSEP was confirmed. This was then lowered to find its threshold. The presence of an SSEP below the EMG threshold confirmed dorsal placement. The absolute thresholds and levels of stimulation for both SSEP and EMG were considered to be of less significance. If the SSEP was not present at the EMG threshold, it would prompt a repositioning of the lead secondary to a ventral placement. RESULTS Participants The demographic information for the patients is shown in Table 1. There were 12 lead placements in 6 patients in this study. Patients ranged from 21 to 70 yr of age, with 5 males and 1 female. Diagnoses of these patients included CRPS, peripheral neuropathy, and failed back surgery syndrome. Lead placement ranged from 1 to 4 leads per patient. Data The data collected from each patient are included in Table 3. Data include sensory and motor thresholds for each patient with asleep neuromonitoring testing, sensory and motor thresholds (when able) for awake testing, and postoperative programming parameters. One patient was combative during the awake testing period, and therefore motor thresholds were not obtained. The intraoperative fluoroscopic imaging appeared to have a dorsal placement. However, this same patient was determined to have a ventral placement on the first attempt at a left L4 placement during the testing. He returned with motor contractions at 0.5 mA without generation of paresthesia. This lead was subsequently tested under neuromonitoring prior to repositioning. EMG thresholds returned at 0.4 mA without the presence of SSEP responses below this threshold. TABLE 3. Response Thresholds With Stimulation Patient  Lead location  Awake  Neuromonitoring  Postoperative programming  Impedance intra-op  1  Left L5—sensory threshold  1.2  1.2  1.23  3137      Left L5—motor threshold  3.3  3.3  1.8      2  Left L3—sensory threshold  1.18  1.3  0.4  1665      Left L3—motor threshold  3.5  1.7  0.8        Left L4—sensory threshold  2.5  0.5  0.25  1690      Left L4—motor threshold  5.5  0.8  0.55      3  Right L1—sensory threshold  0.4  1.6  2.5  977      Right L1—motor threshold  1.35  2.2  3.2        Right L2—sensory threshold  0.25  0.5  0.5  1351      Right L2—motor threshold  0.45  1.15  0.85      4  Left L4—first attempt- sensory threshold  Not obtained  Not present at or below 0.4      Note: This lead was considered ventral and repositioned    Left L4—first attempt- motor threshold  0.5  0.4          Left L4—sensory threshold  0.875  0.5  1.68  1855      Left L4—motor threshold  Not obtained  0.6      Note: Patient did not tolerate awake testing. Combative upon awakening and was difficult for assessment. Awake motor thresholds not determined.    Right L4—sensory threshold  0.85  1.4  1.65  1118      Right L4—motor threshold  Not obtained  1.7          Left L5—sensory threshold  1.5  1.3  0.45  2209      Left L5—motor threshold  Not obtained  2.3          Right L5—sensory threshold  0.55  0.4  0.55  1491      Right L5—motor threshold  Not obtained  0.9        5  Left L4—sensory threshold  0.5  0.55  0.7  2214      Left L4—motor threshold  0.75  0.7  1        Left L5—sensory threshold  0.5  1.3  0.7  2894      Left L5—motor threshold  0.75  1.75  1      6  Left L5—sensory threshold  0.4  0.6  0.4  1986      Left L5—motor threshold  0.8  1.36  0.8        Right L5—sensory threshold  0.5  0.25  0.25  1567      Right L5—motor threshold  0.9  0.31  0.7      Patient  Lead location  Awake  Neuromonitoring  Postoperative programming  Impedance intra-op  1  Left L5—sensory threshold  1.2  1.2  1.23  3137      Left L5—motor threshold  3.3  3.3  1.8      2  Left L3—sensory threshold  1.18  1.3  0.4  1665      Left L3—motor threshold  3.5  1.7  0.8        Left L4—sensory threshold  2.5  0.5  0.25  1690      Left L4—motor threshold  5.5  0.8  0.55      3  Right L1—sensory threshold  0.4  1.6  2.5  977      Right L1—motor threshold  1.35  2.2  3.2        Right L2—sensory threshold  0.25  0.5  0.5  1351      Right L2—motor threshold  0.45  1.15  0.85      4  Left L4—first attempt- sensory threshold  Not obtained  Not present at or below 0.4      Note: This lead was considered ventral and repositioned    Left L4—first attempt- motor threshold  0.5  0.4          Left L4—sensory threshold  0.875  0.5  1.68  1855      Left L4—motor threshold  Not obtained  0.6      Note: Patient did not tolerate awake testing. Combative upon awakening and was difficult for assessment. Awake motor thresholds not determined.    Right L4—sensory threshold  0.85  1.4  1.65  1118      Right L4—motor threshold  Not obtained  1.7          Left L5—sensory threshold  1.5  1.3  0.45  2209      Left L5—motor threshold  Not obtained  2.3          Right L5—sensory threshold  0.55  0.4  0.55  1491      Right L5—motor threshold  Not obtained  0.9        5  Left L4—sensory threshold  0.5  0.55  0.7  2214      Left L4—motor threshold  0.75  0.7  1        Left L5—sensory threshold  0.5  1.3  0.7  2894      Left L5—motor threshold  0.75  1.75  1      6  Left L5—sensory threshold  0.4  0.6  0.4  1986      Left L5—motor threshold  0.8  1.36  0.8        Right L5—sensory threshold  0.5  0.25  0.25  1567      Right L5—motor threshold  0.9  0.31  0.7      View Large TABLE 3. Response Thresholds With Stimulation Patient  Lead location  Awake  Neuromonitoring  Postoperative programming  Impedance intra-op  1  Left L5—sensory threshold  1.2  1.2  1.23  3137      Left L5—motor threshold  3.3  3.3  1.8      2  Left L3—sensory threshold  1.18  1.3  0.4  1665      Left L3—motor threshold  3.5  1.7  0.8        Left L4—sensory threshold  2.5  0.5  0.25  1690      Left L4—motor threshold  5.5  0.8  0.55      3  Right L1—sensory threshold  0.4  1.6  2.5  977      Right L1—motor threshold  1.35  2.2  3.2        Right L2—sensory threshold  0.25  0.5  0.5  1351      Right L2—motor threshold  0.45  1.15  0.85      4  Left L4—first attempt- sensory threshold  Not obtained  Not present at or below 0.4      Note: This lead was considered ventral and repositioned    Left L4—first attempt- motor threshold  0.5  0.4          Left L4—sensory threshold  0.875  0.5  1.68  1855      Left L4—motor threshold  Not obtained  0.6      Note: Patient did not tolerate awake testing. Combative upon awakening and was difficult for assessment. Awake motor thresholds not determined.    Right L4—sensory threshold  0.85  1.4  1.65  1118      Right L4—motor threshold  Not obtained  1.7          Left L5—sensory threshold  1.5  1.3  0.45  2209      Left L5—motor threshold  Not obtained  2.3          Right L5—sensory threshold  0.55  0.4  0.55  1491      Right L5—motor threshold  Not obtained  0.9        5  Left L4—sensory threshold  0.5  0.55  0.7  2214      Left L4—motor threshold  0.75  0.7  1        Left L5—sensory threshold  0.5  1.3  0.7  2894      Left L5—motor threshold  0.75  1.75  1      6  Left L5—sensory threshold  0.4  0.6  0.4  1986      Left L5—motor threshold  0.8  1.36  0.8        Right L5—sensory threshold  0.5  0.25  0.25  1567      Right L5—motor threshold  0.9  0.31  0.7      Patient  Lead location  Awake  Neuromonitoring  Postoperative programming  Impedance intra-op  1  Left L5—sensory threshold  1.2  1.2  1.23  3137      Left L5—motor threshold  3.3  3.3  1.8      2  Left L3—sensory threshold  1.18  1.3  0.4  1665      Left L3—motor threshold  3.5  1.7  0.8        Left L4—sensory threshold  2.5  0.5  0.25  1690      Left L4—motor threshold  5.5  0.8  0.55      3  Right L1—sensory threshold  0.4  1.6  2.5  977      Right L1—motor threshold  1.35  2.2  3.2        Right L2—sensory threshold  0.25  0.5  0.5  1351      Right L2—motor threshold  0.45  1.15  0.85      4  Left L4—first attempt- sensory threshold  Not obtained  Not present at or below 0.4      Note: This lead was considered ventral and repositioned    Left L4—first attempt- motor threshold  0.5  0.4          Left L4—sensory threshold  0.875  0.5  1.68  1855      Left L4—motor threshold  Not obtained  0.6      Note: Patient did not tolerate awake testing. Combative upon awakening and was difficult for assessment. Awake motor thresholds not determined.    Right L4—sensory threshold  0.85  1.4  1.65  1118      Right L4—motor threshold  Not obtained  1.7          Left L5—sensory threshold  1.5  1.3  0.45  2209      Left L5—motor threshold  Not obtained  2.3          Right L5—sensory threshold  0.55  0.4  0.55  1491      Right L5—motor threshold  Not obtained  0.9        5  Left L4—sensory threshold  0.5  0.55  0.7  2214      Left L4—motor threshold  0.75  0.7  1        Left L5—sensory threshold  0.5  1.3  0.7  2894      Left L5—motor threshold  0.75  1.75  1      6  Left L5—sensory threshold  0.4  0.6  0.4  1986      Left L5—motor threshold  0.8  1.36  0.8        Right L5—sensory threshold  0.5  0.25  0.25  1567      Right L5—motor threshold  0.9  0.31  0.7      View Large Main Results In each patient, paresthesias were generated prior to motor contractions in the awake patient. There were varying degrees of separation in the thresholds, and they did not appear to be consistent across level or diagnosis. For each patient, SSEP responses were present after lowering below the dropout threshold of EMG responses with neuromonitoring. There were varying degrees of separation in the thresholds, and they did not appear to be consistent across level or diagnosis. A smaller degree of separation between sensory and motor thresholds during awake testing appeared to also hold true in the asleep patient for SSEP and EMG thresholds. This was further confirmed with postoperative programming. Impedances did not appear to alter the separation in thresholds or amount of stimulation required for responses. DISCUSSION DRG stimulation has been shown to be an effective therapy in the treatment of chronic neuropathic pain and is presently used to treat regions of focal pain2. Its most common use in the United States is for the diagnosis of CRPS type I and II, but it is also used for the treatment of failed back surgery syndrome, postsurgical pain such as that following an inguinal hernia repair, and peripheral neuropathy.2,3 The placement of the lead on the DRG requires confirmation of its position in the dorsal aspect of the foramen to ensure it is stimulating the sensory fibers. Ventral positioning would lead to uncomfortable motor contractions without the desired effect of pain control. Fluoroscopic imaging, in both an anterior-posterior and lateral projection, is generally utilized as a first step in confirmation, but in certain placements can be equivocal. A second step in confirmation has been generally accepted with awake intraoperative paresthesia testing. This second step of confirmation will not only confirm a dorsal placement but helps to identify ideal placement of the electrodes within the dorsal foramen. This case series looks at a novel protocol for the use of neuromonitoring to confirm placement in an asleep patient. Neuromonitoring has been demonstrated in the placement of spinal cord stimulators in the asleep patient under general anesthesia.4,5 These studies have also demonstrated the potential benefits of placing neuromodulatory devices in the asleep patient such as accuracy of placement, degree of comfort for the patient, and possible lower risk of adverse events. The protocols published have demonstrated utilizing SSEP and EMG for confirmation of lead placement with a spinal cord stimulator.6 To the author's knowledge, there are no studies at present looking at the placement of a DRG electrode in an asleep patient using neuromonitoring to guide and confirm placement. The results of this study confirm that the proposed protocol can be used to assure proper positioning of the DRG electrode in the dorsal foramen. Proper lead placement in the dorsal foramen was confirmed by generating SSEP responses in the absence of EMG responses. Potential benefits of using this technique in an asleep patient are improved comfort of the procedure for the patient and physician, as well as a method of verification to confirm proper placement without awake intraoperative testing. Specifically, the degree of comfort to the patient may be of additional interest in the placement of DRG electrodes while asleep, as the brushing of the electrode across the DRG can be a painful experience in the awake patient. Awake testing in one of the patients of this series led to incomplete testing given the patient was combative and difficult to assess, which also demonstrates the benefit of asleep testing. This study utilized the patients as their own controls, directly comparing awake and asleep testing with lead placement. The absolute thresholds and levels of stimulation for both SSEP and EMG were considered to be of less significance. However, a smaller degree of separation between sensory and motor thresholds during awake testing appeared to also hold true in the asleep patient for SSEP and EMG thresholds. This was further confirmed with postoperative programming demonstrating the accuracy of neuromonitoring for placement. Furthermore, the accuracy of placement was confirmed by none of the patients needing their leads revised after the 1-wk buried trial with extension and therefore proceeding directly to permanent implant with the generator and removal of extensions. If the SSEP was not present at the EMG threshold, it would prompt a repositioning of the lead secondary to a ventral placement. This was seen on the initial placement of a left L4 lead in one of the patients, which demonstrated motor contractions without paresthesia during awake testing, and was further confirmed by having an EMG threshold with neuromonitoring below that of the SSEP threshold. Contact configuration could also be adjusted based on responses for further clarification and testing. Potentially, neuromonitoring can be used to determine the best contacts to use and to guide the depth of placement in the foramen to generate the best coverage with the middle contacts. We were able to generate reliable and interpretable SSEPs for L1 and below. It is expected that this will also be the case with cervical placements, which may be approved in the future. This is true secondary to the robust cortical representation of the upper limbs. The utility and importance of asleep cervical placements will be far greater given the higher risk of cord injury in this location with patient movement. However, we have not looked into thoracic placements, where cortical representation of thoracic nerve root SSEPs is quite low. This may require a revised protocol, which has not been formulated but would likely utilize recorded cervical spinal SSEP signals from the stimulation of the thoracic nerve roots, as the signal should be stronger prior to projecting to the cortex. The neuromonitoring needles were placed with mild sedation with the patient in the supine position, but the patient was eventually placed into the prone position for the procedure. The increased risk of prone sedated patients should be minimal, as the procedure is relatively short and would be expected to carry the same risk involved with a spinal cord stimulator placement. There is the option of performing the procedure under general anesthesia, but this was not possible for this study secondary to using the patients as their own controls. It is important to consider that placement of the lead in the foramen as it passes the DRG may be painful. While some have advocated for mild sedation at that time in the awake patient, this would potentially compromise an awake patient's ability to monitor themselves for nerve injury at the crucial point it is needed. It is therefore advised that the leads be placed with the patients completely awake for the entirety of the procedure or that a form of neuromonitoring be used to inform the physician of potential injury, such as sustained EMG activity with placement. The authors have transitioned to performing these procedures under general anesthesia with neuromonitoring. Lastly, a multicenter prospective trial was performed comparing the placement of spinal cord stimulators in either awake patients or those asleep with neuromonitoring that demonstrated a significantly lower adverse event profile with an asleep patient.7 Limitations Limitations of this study include the small sample size, as well as potential changes across the various levels in different patients. Possible confounding factors include decreased cortical representation of upper lumbar levels compared to lower, as evidenced by the typical depiction of the sensory homunculus. Also, the presence of scar tissue or thickened dura may inhibit a clear separation of motor and sensory thresholds or cause excessive threshold levels compared to expectations. Further studies with a larger patient population would be required to delineate the possible differences across levels and diagnoses. CONCLUSION This series demonstrates that neuromonitoring can be used in an asleep patient to assure proper positioning of the DRG electrode in the dorsal foramen by generating SSEP responses in the absence of EMG responses. Disclosure The authors have no personal, financial, or institutional interest in any of the drugs, materials, or devices described in this article. REFERENCES 1. Silverstein MP, Romrell LJ, Bensel EC, Thompson N, Griffith S, Lieberman IH. Lumbar dorsal root ganglia location: an anatomic and MRI assessment. Int J Spine Surg.  2015; 9. doi:10.14444/2003. 2. Liem L, Russo M, Huygen FJ et al.   One-year outcomes of spinal cord stimulation of the dorsal root ganglion in the treatment of chronic neuropathic pain. Neuromodulation.  2015; 18( 1): 41- 49. Google Scholar CrossRef Search ADS PubMed  3. Liem L, Russo M, Huygen FJ et al.   A multicenter, prospective trial to assess the safety and performance of the spinal modulation dorsal root ganglion neurostimulator system in the treatment of chronic pain. Neuromodulation  2013; 16( 5): 471- 482; discussion 482. Google Scholar CrossRef Search ADS PubMed  4. Falowski SM, Celii A, Sestokas AK, Schwartz DM, Matsumoto C, Sharan A. Awake vs. asleep placement of spinal cord stimulators: a cohort analysis of complications associated with placement. Neuromodulation.  2011; 14( 2): 130- 134; discussion 134-135. Google Scholar CrossRef Search ADS PubMed  5. Shils JL, Arle JE. Intraoperative neurophysiologic methods for spinal cord stimulator placement under general anesthesia. Neuromodulation.  2012; 15( 6): 560- 571; discussion 571-562. Google Scholar CrossRef Search ADS PubMed  6. Falowski S, Dianna A. Neuromonitoring protocol for spinal cord stimulator cases with case descriptions. Int J Acad Med.  2016; 2( 2): 132- 144. 7. Falowski S, Sharan A, McInerney J, Jacobs D, Venkatesan L, Agnesi F. NAPS—Non-awake versus awake placement of spinal cord stimulators: comparing safety and efficacy. Presented at: North American Neuromodulation Society Annual Meeting , January 19-22, 2017. COMMENTS The authors have attempted to provide a method for determining whether a DRG lead has been placed into its position appropriately, relative to distinguishing between sensory and motor components of the nerve root, as well as determining whether or not the appropriate dermatomal region is being covered. The latter is obtained by having the patient awake and asking them where they feel the stimulation, in the traditional way. The somewhat innovative aspect of the study is to use IOM to provide a ratio of sensory and motor thresholds whereby if the sensory (SSEP) thresholds are notably lower than the motor, then the lead is considered to be closer to the sensory components of the nerve and not too close to motor. With a few caveats, this is an interesting application of IOM for a neuromodulation procedure and I applaud the authors for developing interest and data in this area. While this idea is worth considering, perhaps in a larger study, as the authors point out, I did have a few thoughts to consider in moving forward. Only L1-5 regions were examined. DRG, however, is likely useful for very localized areas, such as a single or few thoracic dermatomes, and these areas are virtually impossible to obtain an SSEP from. Even proximal lumbar roots are not straightforward for obtaining reliable SSEPs because of the small region of cortex activated, compared with L5 or S1 (or median and ulnar in UE) carried signals. This method therefore seems to require reliance on a fairly specific region of potential DRG placement. Perhaps it will be helpful in cervical areas as DRG is developed more there in the future. In addition, the ability to place IOM needles while the patient is awake, and then using the patient to provide feedback during the procedure likely requires some sedation, and this may compromise safety or feedback, as the authors appreciate. The authors themselves note they have taken to doing DRG procedures under general anesthesia to avoid these issues. However, this eliminates one of the potential advantages of DRG as more of an office-based procedure. Finally, I am not yet convinced the ratios of motor and sensory thresholds, within patients, and across patients, will be reliable enough to use over simply using fluoroscopy, especially once scar or variations in knowing what is optimal in any given patient comes more to light with DRG. Time will tell. Jeffrey Arle Boston, Massachusetts Chronic stimulation of the dorsal root ganglion is a recent introduction to the neuromodulation armamentarium. Awake intraoperative testing has been recommended by the device manufacturer to assure appropriate dorsal placement of the lead. Most practitioners with experience implanting DRG stimulators are aware that placement of the sheath and electrode into the neural foramen in an awake patient generates significant discomfort, and thus the ability to correctly place leads with the patient under anesthesia would be beneficial. The authors report the use of intraoperative neurophysiology to guide appropriate “asleep” placement, although it should be noted that the patients are not truly “asleep”, but under heavy sedation (unlike the use of general anesthesia and neuromonitoring for spinal cord stimulation placement). Comparison of sensory and motor thresholds, ie a sensory threshold equal to or greater than the motor threshold, indicates inappropriate ventral placement. The next logical step for the authors would be to perform this procedure under true general anesthesia, as has been analogously reported for traditional spinal cord stimulation with identical (if not better) results as compared with awake testing. However, while the authors state that X-ray fluoroscopy is “equivocal at times” in determining dorsal vs ventral placement, I have found lateral fluoro to be highly reliable, and I have moved to implanting all DRG electrodes under general anesthesia with X-ray guidance only, and without the need for intraoperative monitoring. If that technique were to fail me, I will consider adopting the authors’ recommendations. Alon Y. Mogilner New York, New York Dorsal root ganglion stimulation has expanded the neuromodulation toolbox and afforded us the opportunity to treat pain that has not been adequately treated in many cases by spinal cord stimulation, such as post-herniorrhaphy, knee, and foot pain. Though DRG stimulation has come into favor in the last several years, the idea of targeting the DRG to treat pain has been realized for decades. Animal studies suggest that the DRG has somatotopy similar to regions in the central nervous system. This idea, which remains to be explored in humans, and the difficulties of DRG stimulation in the awake patients suggest that intraoperative DRG monitoring may have numerous benefits, both scientifically and practically. In addition, this monitoring allows for safety during placement of epidural devices near the conus and potentially could help further identify specific foramina in the sacrum which often overlap on intraoperative fluoroscopy. Julie Pilitsis Albany, New York Copyright © 2017 by the Congress of Neurological Surgeons This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Operative Neurosurgery Oxford University Press

A Prospective Analysis of Neuromonitoring for Confirmation of Lead Placement in Dorsal Root Ganglion Stimulation

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Congress of Neurological Surgeons
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Copyright © 2017 by the Congress of Neurological Surgeons
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2332-4252
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2332-4260
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10.1093/ons/opx172
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Abstract

Abstract BACKGROUND Dorsal root ganglion stimulation is a neuromodulation therapy used for chronic neuropathic pain. Typically, patients are awakened intraoperatively to confirm adequate placement. OBJECTIVE To determine whether neuromonitoring can confirm placement in an asleep patient. METHODS This is a prospective analysis of 12 leads placed in 6 patients. Lead confirmation was confirmed by awake intraoperative testing, as well as asleep testing utilizing neuromonitoring. Patients were used as their own control. Sensory and motor thresholds for each patient with awake and asleep neuromonitoring testing were recorded. Intraoperative impedance and postoperative programming were also recorded. RESULTS In each patient, paresthesias were generated prior to motor contractions in the awake patient. For each patient, somatosensory evoked potential responses were present after lowering below the dropout threshold of electromyogram responses with neuromonitoring. There were varying degrees of separation in the thresholds that did not appear to be consistent across level or diagnosis. Smaller degrees of separation between thresholds during awake testing also held true in the asleep patient. This was further confirmed with postoperative programming. Impedances did not alter the separation in thresholds or amount of stimulation required for responses. One patient was combative during awake testing, and therefore motor thresholds were not obtained. This same patient was determined to have a ventral placement, confirmed with awake and asleep neuromonitoring testing. CONCLUSION This series demonstrates that the proposed neuromonitoring protocol can be used in an asleep patient to assure proper positioning of the dorsal root ganglion electrode in the dorsal foramen by generating somatosensory evoked potential responses in the absence of electromyogram responses. Dorsal root ganglion stimulation, Neuromodulation, Spinal cord stimulation, Chronic pain, CRPS, RSD, Complex regional pain syndrome, Reflex Sympathetic dystrophy ABBREVIATIONS ABBREVIATIONS CRPS complex regional pain syndrome DRG dorsal root ganglion EMG electromyogram SSEP somatosensory evoked potential The dorsal root ganglion (DRG) is a spinal structure that includes bundles of sensory cell bodies and may play a role in chronic neuropathic pain. Each spinal level has a corresponding DRG from a specific dermatome of the body. The anatomy and positioning of the DRG is consistent and reproducible. The size of the DRG increases in size from superior to inferior in the lumbar spine. In addition, the DRG moves more lateral in the foramen as it moves lower down the spine. It is the most lateral at L5.1 The dorsal aspect of the DRG is involved in sensory perception, while the ventral aspect of the DRG includes motor activity. DRG stimulation is presently a neuromodulation therapy used in the treatment of chronic neuropathic pain. It involves stimulation of the DRG at its junction between the peripheral nervous system and the central nervous system. It is presently approved by the Food and Drug Administration in the United States for the treatment of intractable pain in the lower limbs of patients with the diagnosis of complex regional pain syndrome (CRPS) type I and II, but has also been used for the treatment of failed back surgery syndrome and postsurgical pain.2,3 TABLE 1. Patient Demographics, Diagnosis, and Lead Placement Patient  Age (years old)  Sex  Diagnosis  Lead placement  SG  51  M  Complex regional pain syndrome  Left L5  TD  21  M  Complex regional pain syndrome  Left L3 and L4  MC  43  M  Failed back surgery syndrome, complex regional pain syndrome  Right L1 and L2  GN  70  M  Peripheral neuropathy  Bilateral L4 and L5  DM  57  F  Complex regional pain syndrome  Left L4 and L5  RD  55  M  Peripheral neuropathy  Bilateral L5  Patient  Age (years old)  Sex  Diagnosis  Lead placement  SG  51  M  Complex regional pain syndrome  Left L5  TD  21  M  Complex regional pain syndrome  Left L3 and L4  MC  43  M  Failed back surgery syndrome, complex regional pain syndrome  Right L1 and L2  GN  70  M  Peripheral neuropathy  Bilateral L4 and L5  DM  57  F  Complex regional pain syndrome  Left L4 and L5  RD  55  M  Peripheral neuropathy  Bilateral L5  View Large TABLE 1. Patient Demographics, Diagnosis, and Lead Placement Patient  Age (years old)  Sex  Diagnosis  Lead placement  SG  51  M  Complex regional pain syndrome  Left L5  TD  21  M  Complex regional pain syndrome  Left L3 and L4  MC  43  M  Failed back surgery syndrome, complex regional pain syndrome  Right L1 and L2  GN  70  M  Peripheral neuropathy  Bilateral L4 and L5  DM  57  F  Complex regional pain syndrome  Left L4 and L5  RD  55  M  Peripheral neuropathy  Bilateral L5  Patient  Age (years old)  Sex  Diagnosis  Lead placement  SG  51  M  Complex regional pain syndrome  Left L5  TD  21  M  Complex regional pain syndrome  Left L3 and L4  MC  43  M  Failed back surgery syndrome, complex regional pain syndrome  Right L1 and L2  GN  70  M  Peripheral neuropathy  Bilateral L4 and L5  DM  57  F  Complex regional pain syndrome  Left L4 and L5  RD  55  M  Peripheral neuropathy  Bilateral L5  View Large The placement of the leads involves a standard percutaneous technique for introduction of the lead into the epidural space. A 4-contact electrode is placed inferior to the pedicle of the desired level on the dorsal aspect of the DRG in the foramen. At present, targeted nerve root levels include the lower thoracic and lumbar-sacral levels. Typically, once the electrode is placed under fluoroscope imaging, the patient is awakened to confirm placement in the dorsal foramen with a sensory perception threshold. This case series looks at a novel protocol for the use of neuromonitoring to confirm placement in an asleep patient. METHODS This study is a prospective analysis of 6 patients who underwent DRG stimulation surgery in a single institution performed by a single surgeon. St. Luke's University Health Network IRB approval process utilized. The patient was made aware during the procedure consent process of the utilization of 2 testing methods. Diagnoses of these patients included CRPS, peripheral neuropathy, and failed back surgery syndrome. The case series included a total of 12 leads placed in 6 patients, involving L1-L5 (Table 1). Lead placement ranged from 1 to 4 leads per patient. Lead confirmation was confirmed by awake intraoperative testing as well as asleep testing utilizing neuromonitoring. Patients were used as their own control. Each patient underwent trial placement of leads, which were connected to tunneled extensions for the 1-wk trial period. The leads placed during the trial were therefore placed as a possible permanent implant. None of the patients needed to have the leads revised when presenting after the trial. All patients had successful trials and were subsequently converted directly to permanent implant, with removal of the extensions and connection to the generator. System implant included the St. Jude Medical Axium Neurostimulator System (St. Jude Medical Inc., St. Paul, Minnesota). Placement of the neuromonitoring electrodes was performed under mild sedation while in the supine position prior to placing into the prone position for the procedure. Placement of the trial leads was performed while under light sedation with propofol. Fluoroscopic imaging was utilized for placement and confirmation within the foramen. Each patient was then awakened, and test stimulation was given to determine sensory thresholds. Motor thresholds were also determined if possible. At this point, the patient was placed back under sedation, and neuromonitoring was then utilized for threshold determinations utilizing SSEP (somatosensory evoked potential) and EMG (electromyogram) testing. Further analysis included a lead in which awake intraoperative testing returned low motor thresholds from ventral placement. This lead was also tested with neuromonitoring prior to being repositioned into the dorsal foramen. Intraoperatively, the stimulation parameters remained consistent across awake and asleep testing. Pulse width ranged from 200 to 300, while rate was maintained at 20 Hz. Contact configuration for testing was based on the position of the contacts within the foramen, in which the 2 contacts below the pedicle were used in a bipole configuration. This was most commonly 2– (cathode) and 3+ (anode). However, contact configuration could be adjusted based on responses generated. Postoperative programming was also kept consistent for both pulse width and rate, but contact configuration was adjusted based on patient feedback. Protocol Performing the procedure using neuromonitoring differs from the traditional awake-patient technique in that the patient remains anesthetized throughout the procedure, and the accuracy of the placement and expected sensory and motor thresholds for stimulation are determined objectively using a combination of SSEP and free-run EMG testing. The electrode is placed utilizing fluoroscopic imaging, and the device is then connected to the neuromonitoring system via a custom adapter, currently under patent (Figure 1), that allows the electrode contacts to be supplied with electrical stimulation from the constant-current stimulator, which also runs the traditional SEP modes. FIGURE 1. View largeDownload slide Custom adapter allowing connection of the electrode to the constant-current stimulator. FIGURE 1. View largeDownload slide Custom adapter allowing connection of the electrode to the constant-current stimulator. Monitoring for the surgery consists of bilateral ulnar and posterior tibial nerve SSEPs and free-run EMG for muscles representing the appropriate nerve level(s) for the stimulator placements. It is suggested that recording electrodes be placed in multiple muscles primarily supplied by the targeted nerve root level as well as in an additional muscle primarily supplied by the nerve root above and one supplied by the level below, to allow for variations in an individual's nerve root level to supplied muscle. Table 2 lists the muscles that are most commonly monitored; however, additional muscles may be used for any of these levels. TABLE 2. Typical Muscle Relationships to Nerve Root Levels Nerve root level  Representative muscles*  L1  Iliopsoas  L2  Adductor brevis and longus  L3  Adductor magnus, quadriceps  L4  Quadriceps  L5  Tibialis anterior, adductor hallucis  S1  Gastrocnemius  Nerve root level  Representative muscles*  L1  Iliopsoas  L2  Adductor brevis and longus  L3  Adductor magnus, quadriceps  L4  Quadriceps  L5  Tibialis anterior, adductor hallucis  S1  Gastrocnemius  View Large TABLE 2. Typical Muscle Relationships to Nerve Root Levels Nerve root level  Representative muscles*  L1  Iliopsoas  L2  Adductor brevis and longus  L3  Adductor magnus, quadriceps  L4  Quadriceps  L5  Tibialis anterior, adductor hallucis  S1  Gastrocnemius  Nerve root level  Representative muscles*  L1  Iliopsoas  L2  Adductor brevis and longus  L3  Adductor magnus, quadriceps  L4  Quadriceps  L5  Tibialis anterior, adductor hallucis  S1  Gastrocnemius  View Large SSEPs and EMG are monitored as for any typical spine surgery. At the time of testing the stimulator electrode placement, regular SSEP testing is temporarily suspended, but free-run EMG continues. The same-side posterior tibial nerve SSEP stimulator channel is switched to a low-current mode for better resolution in determining thresholds. The SSEP trial is started and the stimulation current level is slowly raised from 0 mA until compound motor-unit action potential responses in the muscle(s) corresponding to the stimulated nerve root level are visualized; this is the motor threshold for the targeted DRG. If the stimulation level significantly exceeds the typical threshold level, different combinations of contacts and/or polarity conventions can be tried until the motor threshold is at an acceptable level. Once desired motor threshold is determined, the SSEP trial is stopped, cleared, restarted, allowed to run to completion, and stored. For that trial, a time marker is placed at the cortical trough or peak. Then the SSEP is run at progressively lower levels than the motor threshold until the SSEP cortical peak disappears. The last stimulation level at which the cortical peak remains present is considered the sensory threshold (Figure 2). A sensory threshold that is equal to or even greater than the motor threshold indicates a potential ventral placement of the electrode and should prompt repositioning. FIGURE 2. View largeDownload slide SSEP and EMG responses with intraoperative testing. A, EMG response threshold in the adductor hallucis muscle for L5 dermatome at 3.3 mA. B, Presence of an SSEP response below EMG threshold at 1.5 mA. FIGURE 2. View largeDownload slide SSEP and EMG responses with intraoperative testing. A, EMG response threshold in the adductor hallucis muscle for L5 dermatome at 3.3 mA. B, Presence of an SSEP response below EMG threshold at 1.5 mA. Interpretation After proper placement of the DRG electrode under fluoroscopic imaging, further confirmation is generally performed by awake-patient testing. The intraoperative fluoroscopic imaging is valuable in determining proper placement, but a form of patient testing is used and recommended. During awake testing of the patient, paresthesias are generated by stimulation of the DRG. Proper lead placement in the dorsal foramen is confirmed with the onset of paresthesias prior to motor contractions. The procedure performed with the patient asleep, without awakening, would preclude this awake paresthesia testing. In this case series, neuromonitoring was utilized by interpreting the presence of SSEP and EMG. SSEP was used as a marker of generated paresthesia, while EMG would indicate motor contraction. Proper lead placement in the dorsal foramen was confirmed by generating SSEP responses in the absence of EMG responses. To determine proper placement, the stimulation was performed until an EMG response was activated. The stimulation was then lowered until the EMG response completely resolved and the presence of SSEP was confirmed. This was then lowered to find its threshold. The presence of an SSEP below the EMG threshold confirmed dorsal placement. The absolute thresholds and levels of stimulation for both SSEP and EMG were considered to be of less significance. If the SSEP was not present at the EMG threshold, it would prompt a repositioning of the lead secondary to a ventral placement. RESULTS Participants The demographic information for the patients is shown in Table 1. There were 12 lead placements in 6 patients in this study. Patients ranged from 21 to 70 yr of age, with 5 males and 1 female. Diagnoses of these patients included CRPS, peripheral neuropathy, and failed back surgery syndrome. Lead placement ranged from 1 to 4 leads per patient. Data The data collected from each patient are included in Table 3. Data include sensory and motor thresholds for each patient with asleep neuromonitoring testing, sensory and motor thresholds (when able) for awake testing, and postoperative programming parameters. One patient was combative during the awake testing period, and therefore motor thresholds were not obtained. The intraoperative fluoroscopic imaging appeared to have a dorsal placement. However, this same patient was determined to have a ventral placement on the first attempt at a left L4 placement during the testing. He returned with motor contractions at 0.5 mA without generation of paresthesia. This lead was subsequently tested under neuromonitoring prior to repositioning. EMG thresholds returned at 0.4 mA without the presence of SSEP responses below this threshold. TABLE 3. Response Thresholds With Stimulation Patient  Lead location  Awake  Neuromonitoring  Postoperative programming  Impedance intra-op  1  Left L5—sensory threshold  1.2  1.2  1.23  3137      Left L5—motor threshold  3.3  3.3  1.8      2  Left L3—sensory threshold  1.18  1.3  0.4  1665      Left L3—motor threshold  3.5  1.7  0.8        Left L4—sensory threshold  2.5  0.5  0.25  1690      Left L4—motor threshold  5.5  0.8  0.55      3  Right L1—sensory threshold  0.4  1.6  2.5  977      Right L1—motor threshold  1.35  2.2  3.2        Right L2—sensory threshold  0.25  0.5  0.5  1351      Right L2—motor threshold  0.45  1.15  0.85      4  Left L4—first attempt- sensory threshold  Not obtained  Not present at or below 0.4      Note: This lead was considered ventral and repositioned    Left L4—first attempt- motor threshold  0.5  0.4          Left L4—sensory threshold  0.875  0.5  1.68  1855      Left L4—motor threshold  Not obtained  0.6      Note: Patient did not tolerate awake testing. Combative upon awakening and was difficult for assessment. Awake motor thresholds not determined.    Right L4—sensory threshold  0.85  1.4  1.65  1118      Right L4—motor threshold  Not obtained  1.7          Left L5—sensory threshold  1.5  1.3  0.45  2209      Left L5—motor threshold  Not obtained  2.3          Right L5—sensory threshold  0.55  0.4  0.55  1491      Right L5—motor threshold  Not obtained  0.9        5  Left L4—sensory threshold  0.5  0.55  0.7  2214      Left L4—motor threshold  0.75  0.7  1        Left L5—sensory threshold  0.5  1.3  0.7  2894      Left L5—motor threshold  0.75  1.75  1      6  Left L5—sensory threshold  0.4  0.6  0.4  1986      Left L5—motor threshold  0.8  1.36  0.8        Right L5—sensory threshold  0.5  0.25  0.25  1567      Right L5—motor threshold  0.9  0.31  0.7      Patient  Lead location  Awake  Neuromonitoring  Postoperative programming  Impedance intra-op  1  Left L5—sensory threshold  1.2  1.2  1.23  3137      Left L5—motor threshold  3.3  3.3  1.8      2  Left L3—sensory threshold  1.18  1.3  0.4  1665      Left L3—motor threshold  3.5  1.7  0.8        Left L4—sensory threshold  2.5  0.5  0.25  1690      Left L4—motor threshold  5.5  0.8  0.55      3  Right L1—sensory threshold  0.4  1.6  2.5  977      Right L1—motor threshold  1.35  2.2  3.2        Right L2—sensory threshold  0.25  0.5  0.5  1351      Right L2—motor threshold  0.45  1.15  0.85      4  Left L4—first attempt- sensory threshold  Not obtained  Not present at or below 0.4      Note: This lead was considered ventral and repositioned    Left L4—first attempt- motor threshold  0.5  0.4          Left L4—sensory threshold  0.875  0.5  1.68  1855      Left L4—motor threshold  Not obtained  0.6      Note: Patient did not tolerate awake testing. Combative upon awakening and was difficult for assessment. Awake motor thresholds not determined.    Right L4—sensory threshold  0.85  1.4  1.65  1118      Right L4—motor threshold  Not obtained  1.7          Left L5—sensory threshold  1.5  1.3  0.45  2209      Left L5—motor threshold  Not obtained  2.3          Right L5—sensory threshold  0.55  0.4  0.55  1491      Right L5—motor threshold  Not obtained  0.9        5  Left L4—sensory threshold  0.5  0.55  0.7  2214      Left L4—motor threshold  0.75  0.7  1        Left L5—sensory threshold  0.5  1.3  0.7  2894      Left L5—motor threshold  0.75  1.75  1      6  Left L5—sensory threshold  0.4  0.6  0.4  1986      Left L5—motor threshold  0.8  1.36  0.8        Right L5—sensory threshold  0.5  0.25  0.25  1567      Right L5—motor threshold  0.9  0.31  0.7      View Large TABLE 3. Response Thresholds With Stimulation Patient  Lead location  Awake  Neuromonitoring  Postoperative programming  Impedance intra-op  1  Left L5—sensory threshold  1.2  1.2  1.23  3137      Left L5—motor threshold  3.3  3.3  1.8      2  Left L3—sensory threshold  1.18  1.3  0.4  1665      Left L3—motor threshold  3.5  1.7  0.8        Left L4—sensory threshold  2.5  0.5  0.25  1690      Left L4—motor threshold  5.5  0.8  0.55      3  Right L1—sensory threshold  0.4  1.6  2.5  977      Right L1—motor threshold  1.35  2.2  3.2        Right L2—sensory threshold  0.25  0.5  0.5  1351      Right L2—motor threshold  0.45  1.15  0.85      4  Left L4—first attempt- sensory threshold  Not obtained  Not present at or below 0.4      Note: This lead was considered ventral and repositioned    Left L4—first attempt- motor threshold  0.5  0.4          Left L4—sensory threshold  0.875  0.5  1.68  1855      Left L4—motor threshold  Not obtained  0.6      Note: Patient did not tolerate awake testing. Combative upon awakening and was difficult for assessment. Awake motor thresholds not determined.    Right L4—sensory threshold  0.85  1.4  1.65  1118      Right L4—motor threshold  Not obtained  1.7          Left L5—sensory threshold  1.5  1.3  0.45  2209      Left L5—motor threshold  Not obtained  2.3          Right L5—sensory threshold  0.55  0.4  0.55  1491      Right L5—motor threshold  Not obtained  0.9        5  Left L4—sensory threshold  0.5  0.55  0.7  2214      Left L4—motor threshold  0.75  0.7  1        Left L5—sensory threshold  0.5  1.3  0.7  2894      Left L5—motor threshold  0.75  1.75  1      6  Left L5—sensory threshold  0.4  0.6  0.4  1986      Left L5—motor threshold  0.8  1.36  0.8        Right L5—sensory threshold  0.5  0.25  0.25  1567      Right L5—motor threshold  0.9  0.31  0.7      Patient  Lead location  Awake  Neuromonitoring  Postoperative programming  Impedance intra-op  1  Left L5—sensory threshold  1.2  1.2  1.23  3137      Left L5—motor threshold  3.3  3.3  1.8      2  Left L3—sensory threshold  1.18  1.3  0.4  1665      Left L3—motor threshold  3.5  1.7  0.8        Left L4—sensory threshold  2.5  0.5  0.25  1690      Left L4—motor threshold  5.5  0.8  0.55      3  Right L1—sensory threshold  0.4  1.6  2.5  977      Right L1—motor threshold  1.35  2.2  3.2        Right L2—sensory threshold  0.25  0.5  0.5  1351      Right L2—motor threshold  0.45  1.15  0.85      4  Left L4—first attempt- sensory threshold  Not obtained  Not present at or below 0.4      Note: This lead was considered ventral and repositioned    Left L4—first attempt- motor threshold  0.5  0.4          Left L4—sensory threshold  0.875  0.5  1.68  1855      Left L4—motor threshold  Not obtained  0.6      Note: Patient did not tolerate awake testing. Combative upon awakening and was difficult for assessment. Awake motor thresholds not determined.    Right L4—sensory threshold  0.85  1.4  1.65  1118      Right L4—motor threshold  Not obtained  1.7          Left L5—sensory threshold  1.5  1.3  0.45  2209      Left L5—motor threshold  Not obtained  2.3          Right L5—sensory threshold  0.55  0.4  0.55  1491      Right L5—motor threshold  Not obtained  0.9        5  Left L4—sensory threshold  0.5  0.55  0.7  2214      Left L4—motor threshold  0.75  0.7  1        Left L5—sensory threshold  0.5  1.3  0.7  2894      Left L5—motor threshold  0.75  1.75  1      6  Left L5—sensory threshold  0.4  0.6  0.4  1986      Left L5—motor threshold  0.8  1.36  0.8        Right L5—sensory threshold  0.5  0.25  0.25  1567      Right L5—motor threshold  0.9  0.31  0.7      View Large Main Results In each patient, paresthesias were generated prior to motor contractions in the awake patient. There were varying degrees of separation in the thresholds, and they did not appear to be consistent across level or diagnosis. For each patient, SSEP responses were present after lowering below the dropout threshold of EMG responses with neuromonitoring. There were varying degrees of separation in the thresholds, and they did not appear to be consistent across level or diagnosis. A smaller degree of separation between sensory and motor thresholds during awake testing appeared to also hold true in the asleep patient for SSEP and EMG thresholds. This was further confirmed with postoperative programming. Impedances did not appear to alter the separation in thresholds or amount of stimulation required for responses. DISCUSSION DRG stimulation has been shown to be an effective therapy in the treatment of chronic neuropathic pain and is presently used to treat regions of focal pain2. Its most common use in the United States is for the diagnosis of CRPS type I and II, but it is also used for the treatment of failed back surgery syndrome, postsurgical pain such as that following an inguinal hernia repair, and peripheral neuropathy.2,3 The placement of the lead on the DRG requires confirmation of its position in the dorsal aspect of the foramen to ensure it is stimulating the sensory fibers. Ventral positioning would lead to uncomfortable motor contractions without the desired effect of pain control. Fluoroscopic imaging, in both an anterior-posterior and lateral projection, is generally utilized as a first step in confirmation, but in certain placements can be equivocal. A second step in confirmation has been generally accepted with awake intraoperative paresthesia testing. This second step of confirmation will not only confirm a dorsal placement but helps to identify ideal placement of the electrodes within the dorsal foramen. This case series looks at a novel protocol for the use of neuromonitoring to confirm placement in an asleep patient. Neuromonitoring has been demonstrated in the placement of spinal cord stimulators in the asleep patient under general anesthesia.4,5 These studies have also demonstrated the potential benefits of placing neuromodulatory devices in the asleep patient such as accuracy of placement, degree of comfort for the patient, and possible lower risk of adverse events. The protocols published have demonstrated utilizing SSEP and EMG for confirmation of lead placement with a spinal cord stimulator.6 To the author's knowledge, there are no studies at present looking at the placement of a DRG electrode in an asleep patient using neuromonitoring to guide and confirm placement. The results of this study confirm that the proposed protocol can be used to assure proper positioning of the DRG electrode in the dorsal foramen. Proper lead placement in the dorsal foramen was confirmed by generating SSEP responses in the absence of EMG responses. Potential benefits of using this technique in an asleep patient are improved comfort of the procedure for the patient and physician, as well as a method of verification to confirm proper placement without awake intraoperative testing. Specifically, the degree of comfort to the patient may be of additional interest in the placement of DRG electrodes while asleep, as the brushing of the electrode across the DRG can be a painful experience in the awake patient. Awake testing in one of the patients of this series led to incomplete testing given the patient was combative and difficult to assess, which also demonstrates the benefit of asleep testing. This study utilized the patients as their own controls, directly comparing awake and asleep testing with lead placement. The absolute thresholds and levels of stimulation for both SSEP and EMG were considered to be of less significance. However, a smaller degree of separation between sensory and motor thresholds during awake testing appeared to also hold true in the asleep patient for SSEP and EMG thresholds. This was further confirmed with postoperative programming demonstrating the accuracy of neuromonitoring for placement. Furthermore, the accuracy of placement was confirmed by none of the patients needing their leads revised after the 1-wk buried trial with extension and therefore proceeding directly to permanent implant with the generator and removal of extensions. If the SSEP was not present at the EMG threshold, it would prompt a repositioning of the lead secondary to a ventral placement. This was seen on the initial placement of a left L4 lead in one of the patients, which demonstrated motor contractions without paresthesia during awake testing, and was further confirmed by having an EMG threshold with neuromonitoring below that of the SSEP threshold. Contact configuration could also be adjusted based on responses for further clarification and testing. Potentially, neuromonitoring can be used to determine the best contacts to use and to guide the depth of placement in the foramen to generate the best coverage with the middle contacts. We were able to generate reliable and interpretable SSEPs for L1 and below. It is expected that this will also be the case with cervical placements, which may be approved in the future. This is true secondary to the robust cortical representation of the upper limbs. The utility and importance of asleep cervical placements will be far greater given the higher risk of cord injury in this location with patient movement. However, we have not looked into thoracic placements, where cortical representation of thoracic nerve root SSEPs is quite low. This may require a revised protocol, which has not been formulated but would likely utilize recorded cervical spinal SSEP signals from the stimulation of the thoracic nerve roots, as the signal should be stronger prior to projecting to the cortex. The neuromonitoring needles were placed with mild sedation with the patient in the supine position, but the patient was eventually placed into the prone position for the procedure. The increased risk of prone sedated patients should be minimal, as the procedure is relatively short and would be expected to carry the same risk involved with a spinal cord stimulator placement. There is the option of performing the procedure under general anesthesia, but this was not possible for this study secondary to using the patients as their own controls. It is important to consider that placement of the lead in the foramen as it passes the DRG may be painful. While some have advocated for mild sedation at that time in the awake patient, this would potentially compromise an awake patient's ability to monitor themselves for nerve injury at the crucial point it is needed. It is therefore advised that the leads be placed with the patients completely awake for the entirety of the procedure or that a form of neuromonitoring be used to inform the physician of potential injury, such as sustained EMG activity with placement. The authors have transitioned to performing these procedures under general anesthesia with neuromonitoring. Lastly, a multicenter prospective trial was performed comparing the placement of spinal cord stimulators in either awake patients or those asleep with neuromonitoring that demonstrated a significantly lower adverse event profile with an asleep patient.7 Limitations Limitations of this study include the small sample size, as well as potential changes across the various levels in different patients. Possible confounding factors include decreased cortical representation of upper lumbar levels compared to lower, as evidenced by the typical depiction of the sensory homunculus. Also, the presence of scar tissue or thickened dura may inhibit a clear separation of motor and sensory thresholds or cause excessive threshold levels compared to expectations. Further studies with a larger patient population would be required to delineate the possible differences across levels and diagnoses. CONCLUSION This series demonstrates that neuromonitoring can be used in an asleep patient to assure proper positioning of the DRG electrode in the dorsal foramen by generating SSEP responses in the absence of EMG responses. Disclosure The authors have no personal, financial, or institutional interest in any of the drugs, materials, or devices described in this article. REFERENCES 1. Silverstein MP, Romrell LJ, Bensel EC, Thompson N, Griffith S, Lieberman IH. Lumbar dorsal root ganglia location: an anatomic and MRI assessment. Int J Spine Surg.  2015; 9. doi:10.14444/2003. 2. Liem L, Russo M, Huygen FJ et al.   One-year outcomes of spinal cord stimulation of the dorsal root ganglion in the treatment of chronic neuropathic pain. Neuromodulation.  2015; 18( 1): 41- 49. Google Scholar CrossRef Search ADS PubMed  3. Liem L, Russo M, Huygen FJ et al.   A multicenter, prospective trial to assess the safety and performance of the spinal modulation dorsal root ganglion neurostimulator system in the treatment of chronic pain. Neuromodulation  2013; 16( 5): 471- 482; discussion 482. Google Scholar CrossRef Search ADS PubMed  4. Falowski SM, Celii A, Sestokas AK, Schwartz DM, Matsumoto C, Sharan A. Awake vs. asleep placement of spinal cord stimulators: a cohort analysis of complications associated with placement. Neuromodulation.  2011; 14( 2): 130- 134; discussion 134-135. Google Scholar CrossRef Search ADS PubMed  5. Shils JL, Arle JE. Intraoperative neurophysiologic methods for spinal cord stimulator placement under general anesthesia. Neuromodulation.  2012; 15( 6): 560- 571; discussion 571-562. Google Scholar CrossRef Search ADS PubMed  6. Falowski S, Dianna A. Neuromonitoring protocol for spinal cord stimulator cases with case descriptions. Int J Acad Med.  2016; 2( 2): 132- 144. 7. Falowski S, Sharan A, McInerney J, Jacobs D, Venkatesan L, Agnesi F. NAPS—Non-awake versus awake placement of spinal cord stimulators: comparing safety and efficacy. Presented at: North American Neuromodulation Society Annual Meeting , January 19-22, 2017. COMMENTS The authors have attempted to provide a method for determining whether a DRG lead has been placed into its position appropriately, relative to distinguishing between sensory and motor components of the nerve root, as well as determining whether or not the appropriate dermatomal region is being covered. The latter is obtained by having the patient awake and asking them where they feel the stimulation, in the traditional way. The somewhat innovative aspect of the study is to use IOM to provide a ratio of sensory and motor thresholds whereby if the sensory (SSEP) thresholds are notably lower than the motor, then the lead is considered to be closer to the sensory components of the nerve and not too close to motor. With a few caveats, this is an interesting application of IOM for a neuromodulation procedure and I applaud the authors for developing interest and data in this area. While this idea is worth considering, perhaps in a larger study, as the authors point out, I did have a few thoughts to consider in moving forward. Only L1-5 regions were examined. DRG, however, is likely useful for very localized areas, such as a single or few thoracic dermatomes, and these areas are virtually impossible to obtain an SSEP from. Even proximal lumbar roots are not straightforward for obtaining reliable SSEPs because of the small region of cortex activated, compared with L5 or S1 (or median and ulnar in UE) carried signals. This method therefore seems to require reliance on a fairly specific region of potential DRG placement. Perhaps it will be helpful in cervical areas as DRG is developed more there in the future. In addition, the ability to place IOM needles while the patient is awake, and then using the patient to provide feedback during the procedure likely requires some sedation, and this may compromise safety or feedback, as the authors appreciate. The authors themselves note they have taken to doing DRG procedures under general anesthesia to avoid these issues. However, this eliminates one of the potential advantages of DRG as more of an office-based procedure. Finally, I am not yet convinced the ratios of motor and sensory thresholds, within patients, and across patients, will be reliable enough to use over simply using fluoroscopy, especially once scar or variations in knowing what is optimal in any given patient comes more to light with DRG. Time will tell. Jeffrey Arle Boston, Massachusetts Chronic stimulation of the dorsal root ganglion is a recent introduction to the neuromodulation armamentarium. Awake intraoperative testing has been recommended by the device manufacturer to assure appropriate dorsal placement of the lead. Most practitioners with experience implanting DRG stimulators are aware that placement of the sheath and electrode into the neural foramen in an awake patient generates significant discomfort, and thus the ability to correctly place leads with the patient under anesthesia would be beneficial. The authors report the use of intraoperative neurophysiology to guide appropriate “asleep” placement, although it should be noted that the patients are not truly “asleep”, but under heavy sedation (unlike the use of general anesthesia and neuromonitoring for spinal cord stimulation placement). Comparison of sensory and motor thresholds, ie a sensory threshold equal to or greater than the motor threshold, indicates inappropriate ventral placement. The next logical step for the authors would be to perform this procedure under true general anesthesia, as has been analogously reported for traditional spinal cord stimulation with identical (if not better) results as compared with awake testing. However, while the authors state that X-ray fluoroscopy is “equivocal at times” in determining dorsal vs ventral placement, I have found lateral fluoro to be highly reliable, and I have moved to implanting all DRG electrodes under general anesthesia with X-ray guidance only, and without the need for intraoperative monitoring. If that technique were to fail me, I will consider adopting the authors’ recommendations. Alon Y. Mogilner New York, New York Dorsal root ganglion stimulation has expanded the neuromodulation toolbox and afforded us the opportunity to treat pain that has not been adequately treated in many cases by spinal cord stimulation, such as post-herniorrhaphy, knee, and foot pain. Though DRG stimulation has come into favor in the last several years, the idea of targeting the DRG to treat pain has been realized for decades. Animal studies suggest that the DRG has somatotopy similar to regions in the central nervous system. This idea, which remains to be explored in humans, and the difficulties of DRG stimulation in the awake patients suggest that intraoperative DRG monitoring may have numerous benefits, both scientifically and practically. In addition, this monitoring allows for safety during placement of epidural devices near the conus and potentially could help further identify specific foramina in the sacrum which often overlap on intraoperative fluoroscopy. Julie Pilitsis Albany, New York Copyright © 2017 by the Congress of Neurological Surgeons This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)

Journal

Operative NeurosurgeryOxford University Press

Published: Aug 2, 2017

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