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Original research
Pocket pain, does location matter: a single-centre retrospective study of patients implanted with a spinal cord stimulator
  1. Ganesan Baranidharan1,2,
  2. Beatrice Bretherton2,3,
  3. Gemma Richert4,
  4. Thomas Kay2,
  5. Nathan Marsh2,
  6. Bethan Roberts2 and
  7. Charlotte Romanis2
  1. 1 School of Medicine, University of Leeds Faculty of Medicine and Health, Leeds, UK
  2. 2 Pain Management, Leeds Teaching Hospitals NHS Trust, Leeds, UK
  3. 3 School of Biomedical Sciences, University of Leeds Faculty of Biological Sciences, Leeds, UK
  4. 4 Hull York Medical School, University of Hull, Hull, Kingston upon Hull, UK
  1. Correspondence to Dr Ganesan Baranidharan, Pain Management, Leeds Teaching Hospitals NHS Trust, Leeds, UK; g.baranidharan{at}nhs.net

Abstract

Objectives Spinal cord stimulation (SCS) is an effective therapy for alleviating pain but reported complication rates vary between healthcare centers. This study explored the prevalence of pain associated with Implantable Pulse Generators (IPGs), the component that powers the SCS system.

Methods This was a retrospective, single site study analyzing data from 764 patients who had a fully implanted SCS between September 2013 and March 2020. Demographic data were collected together with IPG site and type, patient reported presence of IPG site pain, revisions, explants and baseline scores for neuropathic pain (using the Self-Administered Leeds Assessment of Neuropathic Symptoms and Signs questionnaire). Data were statistically analyzed by one-way analysis of variance, independent sample t-tests, X2 tests of independence and logistic regression modeling.

Results IPG site pain occurred in 127 (17%) of 764 patients. These patients had higher baseline neuropathic pain scores than those who reported no IPG site pain. This complication was more common in females than males. The lowest rates of IPG site pain occurred after posterior chest wall placement and the highest rates occurred after abdominal implants. 7% of patients had revision surgery for IPG site pain (n=55) and 10 of 95 explanted patients stated that IPG site pain was a secondary influencing factor.

Conclusions These findings suggest that IPG site pain is a common complication, contributing to SCS revisions and explantation. This study shows that anatomical factors and baseline characteristics of individual patients may contribute to IPG site pain and indicates that exploration of potential factors leading to IPG revision is required.

  • spinal cord stimulation
  • postoperative complications
  • pain
  • postoperative

https://doi.org/10.1136/rapm-2020-101752

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    Introduction

    Recent innovations in neuromodulation therapies have focused on delivering alternative methods of current delivery,1 2 targeting novel anatomical structures (dorsal root ganglion, peripheral nerve and vagus nerve stimulation)3–6 and implementing wireless communications and MRI compatibility. These improvements are steered not only to enhance therapeutic efficacy, but also to lower common complications associated with these therapies. These advancements have been beneficial in spinal cord stimulation (SCS) where novel stimulation paradigms have been shown to provide superior pain relief over traditional tonic SCS.7–10 The physical properties of the component that powers the neuromodulation system, that is, the implantable pulse generator (IPG) have decreased in size over 10 years. This is similar to other implantable devices, such as pacemakers, which have become smaller over recent years. However, despite these developments, the anatomical placements of IPGs have remained relatively unchanged.

    Currently, there are two types of SCS IPG available: rechargeable and non-rechargeable that deliver either constant-current or constant-voltage stimulation, depending on device manufacturers. Both types of IPG deliver power and can be externally programmed for different stimulation types, controlled by an external programmer. The IPG is usually placed in a subcutaneous pocket. However, it is sometimes positioned in the submuscular plane to try to reduce irritation of cutaneous nerves. Typical surgical IPG placements include in the buttock given its straightforward surgical access and the large amount of subcutaneous fat and in the abdomen. In our unit, we have adopted a flank position for some patients (away from ribs) and/or the thoracic cage over the T10/11 ribs (see figure 1 for an illustration of the IPG positions).

    Figure 1
    Figure 1

    IPG positions. (A) Typical placement of IPG in the abdomen: between costal margin and pelvic inlet. (B) Placement of IPG in buttock: lateral superior quadrant of buttock, caudal to posterior superior iliac crest and lateral to sacroiliac joint prominence. (C) Placement of IPG in posterior chest wall: over rib cage, between inferior border of scapula and T11 ribs. (D) Placement of IPG in flank: between T12 rib and posterior superior iliac crest. IPG, implantable pulse generator.

    The chest wall IPG technique involves placement directly over the pectoralis major (cardiac implants) or between the posterior mid clavicular line and posterior axillary line above the latissimus dorsi muscle plane (spinal implants). It is important to avoid the T12 rib, below the brassiere underband in females and choose the side opposite to the patient’s preferential sleeping position (see figure 2).

    Figure 2
    Figure 2

    Typical placement for posterior chest wall IPG. Patient prone and IPG pocket created over right T10/11 rib cage. Right paramedian anchor site parallel to IPG site, reducing stretch on flexion and extension of spine and avoiding brassiere line. IPG, implantable pulse generator.

    Recent advances in IPG technology have largely been restricted to wireless communications between the programmer and IPG and compatibility issues. However, there have been few changes in IPG physical dimensions, anatomical depth and placement choices to improve efficacy by reducing complications. Indeed, the Neuromodulation Appropriateness Consensus Committee recommended that further research and development is needed to limit IPG size to improve efficacy by reducing complications.11

    There is increasing recognition of IPG site pain as a complication associated with SCS. Rates of IPG site pain, described as needing revision surgery, have been reported in 0.9%–12% of cases.12–21 Furthermore, the severity of IPG site pain varies between patients. A recent survey of 153 patients with SCS rated pain using a numerical rating scale (NRS) of 0–11; 63% experienced IPG site pain≥1, 30% had NRS≥4 and 8% reported severe IPG site pain 7–10.22 There is preliminary evidence suggesting that the location of an IPG can influence the rate of surgical revisions.23 It is therefore possible that the incidence of IPG site pain may be due to differences in IPG placement and other physical characteristics of patients. This retrospective single site study aimed to examine the influence of anatomical position, physical characteristics and rates of surgical revision and explant in patients who received a fully implanted SCS and experienced IPG site pain.

    Methods

    This was a retrospective single-center study that evaluated the prevalence of pain associated with IPGs. The study received local audit board registration.

    Participants and methods

    We set out to collect data on all patients who had received a fully implanted SCS system in the Leeds Teaching Hospitals National Health Service (NHS) Trust between September 2013 and March 2020. Seven hundred and seventy-five patients were identified. The following data were collected from hospital paper and electronic records: age, gender, pain diagnosis, type of IPG (classed as large rechargeable (>30 cm3), small rechargeable (<30 cm3) or non-rechargeable (>30 cm3)), site of IPG implant (categorized as posterior chest wall, buttock, flank or abdomen), implant date, IPG site pain (quantified as present or absent) and IPG pain-associated revisions and explants. Baseline scores for neuropathic pain (using the Self-Administered Leeds Assessment of Neuropathic Symptoms and Signs (SLANSS) questionnaire) were recorded.

    Statistical analysis

    All statistical analyzes were conducted in SPSS (V.25) and normality was ascertained via the Shapiro-Wilk test. The alpha level was set to 0.05 and all statistical tests were two tailed. To examine differences in baseline characteristics (age and baseline neuropathic pain scores) between IPG sites (posterior chest wall, buttock, flank, abdomen), one-way analysis of variance (or Kruskall-Wallis tests for non-normally distributed data) were performed. To correct for multiple comparisons following statistically significant effects, the Bonferroni correction was applied. Independent sample t-tests (or Mann-Whitney U tests) ascertained differences in baseline characteristics between those who did and did not experience IPG site pain. X2 tests of independence examined associations between gender, IPG site, IPG type and IPG site pain. To examine the extent to which baseline characteristics predicted the presence of IPG site pain, logistic regression modeling was undertaken.

    Results

    Data were collected from 775 patients (361 males and 414 females) who had received a fully implanted SCS system between September 2013 and March 2020 in the Leeds Teaching Hospitals NHS Trust (see table 1 for a summary). IPGs were most frequently implanted in the posterior chest wall (n=376), buttock (n=292), flank (n=50) and abdomen (n=46). As there were missing data on IPG location in 11 patients (with none reporting IPG site pain), these patients were excluded from all subsequent analyzes. This resulted in a final sample of 764 patients (359 males and 405 females, see table 1 for a summary). This comprised 396 patients who had a large rechargeable IPG (>30 cm3), 250 who had a small rechargeable IPG (<30 cm3) and 118 who had a non-rechargeable IPG (>30 cm3). There were 433 patients with failed back surgery syndrome (FBSS) and/or postsurgical persistent pain at cervical spine, 87 with complex regional pain syndrome (CRPS), 69 with visceral pain, 51 with peripheral mononeuropathy, 22 with pelvic/perineal pain and 13 with postamputation pain. Implanting physicians included: four anesthetists/pain physicians (majority of cases) and one neurosurgeon (10 paddle leads per year).

    View this table:
    Table 1

    Summary of characteristics for the initial dataset (n=775) and the final sample (n=764, excluding those for whom IPG location were missing n=11)

    IPG site was associated with age and gender

    Age differed significantly between the IPG sites (χ2(3)=31.22, p<0.001, see table 2). The group of patients who had IPGs located in the posterior chest wall were significantly younger (95% CI 51 to 53 years) compared with those who had IPGs implanted in the buttock (95% CI: 53 to 56 years; p=0.007), flank (95% CI 54 to 62 years; p=0.020) and abdomen (95% CI 57 to 64 years; p<0.001). In addition, patients with IPGs located in the buttock were significantly younger than those who had IPGs implanted in the abdomen (p=0.015). A X2 test of independence also revealed that gender was associated significantly with IPG site (χ2(3)=11.14, p=0.011, see table 2): a larger percentage of males than females had IPGs implanted in the posterior chest wall, and a larger percentage of females than males had IPGs implanted in the buttock. Additionally, IPG type was associated significantly with IPG site (χ2(6)=34.60, p<0.011, see (table 2): most large rechargeable IPGs were implanted in the posterior chest wall (58%). Baseline neuropathic pain (SLANSS) did not differ significantly between the IPG sites (p>0.05).

    View this table:
    Table 2

    Summary of characteristics for each IPG site

    View this table:
    Table 3

    Prevalence of IPG site pain for the final sample and IPGs located in the posterior chest wall, buttock, flank and abdomen

    Prevalence of IPG site pain was lowest for posterior chest wall and non-rechargeable IPGs

    IPG site pain was reported in 127 patients (47 males and 80 females), amounting to a prevalence rate of 17% (see table 3). IPG site pain was associated significantly with location of implant (χ2(3)=8.88, p=0.031): prevalence of IPG site pain was lowest for IPGs implanted in the posterior chest wall (13%) and highest for IPGs located in the abdomen (28%) followed by the buttock (19%).

    IPG type was associated significantly with the occurrence of IPG site pain in the final sample (χ2(2)=6.06, p=0.048, see table 4): prevalence of IPG site pain was lowest for non-rechargeable IPGs (4%) and highest for large rechargeable IPGs (9%). Although the following patterns did not reach statistical significance (p>0.05), non-rechargeable IPGs had the lowest rates of IPG site pain for posterior chest wall and abdominal implants (see table 4). For buttock placement, small rechargeable IPGs had the lowest rates of IPG site pain, and for flank positioned IPGs, small rechargeable and non-rechargeable IPGs had equally low rates of IPG site pain.

    View this table:
    Table 4

    Prevalence of implantable pulse generator (IPG) site pain for large rechargeable (LR), small rechargeable (SR) and non-rechargeable (NR) IPGs for the final sample and IPGs located in the posterior chest wall, buttock, flank and abdomen.

    IPG site pain was associated with high baseline neuropathic pain scores

    In the final sample, patients who experienced IPG site pain had significantly higher neuropathic pain scores at baseline (95% CI 16.2 to 19.5) compared with those who had no IPG site pain (95% CI 14.7 to 16.2; U=9515.50, p=0.008, see table 5). In addition, gender was associated significantly with the occurrence of IPG site pain (gender: χ2(1)=6.09, p=0.014): 20% of females reported IPG site pain whereas only 13% of males experienced IPG site pain. Logistic regression modeling was performed to ascertain the effects of baseline neuropathic pain scores, gender and IPG type (see table 4) on the likelihood that patients would encounter pain at IPG sites. Baseline neuropathic pain score, gender and IPG type were entered into the model. The Hosmer and Lemeshow Test did not reach statistical significance (χ2(8)=6.78, p>0.05) and the model explained 6% (Nagelkerke R2) of the variance in IPG site pain and correctly classified 85% of cases.

    View this table:
    Table 5

    Summary of differences between patients who did and did not experience implantable pulse generator (IPG) site pain

    In the group who had IPGs implanted in the posterior chest wall, gender was associated significantly with the occurrence of IPG site pain (χ2(1)=4.27, p=0.039): a greater percentage of females (62%) than males (38%) experienced pain at the IPG site (see table 5). Age also differed significantly between those who did and did not experience IPG site pain (U=5993.00, p=0.003). Indeed, those who had pain at the IPG site were younger (95% CI 43 to 50 years) compared with those who reported no pain at the IPG site (95% CI 51 to 54 years). Baseline neuropathic pain scores tended to be higher for those who experienced IPG site pain (95% CI 15.7 to 20.0) compared with those who did not (95% CI 14.9 to 16.7, see table 5), although this failed to reach statistical significance (U = 3774.00, p=0.062). To explore whether gender and age predicted whether patients would encounter IPG site pain, logistic regression modeling was undertaken. The Hosmer and Lemeshow test was not statistically significant (χ2(8)=3.24, p>0.05) and the model explained 7% (Nagelkerke R2) of the variance in IPG site pain and correctly classified 87% of cases. Inclusion of baseline neuropathic pain scores had little effect on the logistic regression outcome (Hosmer and Lemeshow test: χ2(8)=7.92, p>0.05; Nagelkerke R2: 9%; percentage of correctly classified cases: 85%).

    There were no significant differences in baseline characteristics between the IPG site pain and no IPG site pain groups for IPGs implanted in the buttock (p>0.05, see table 5). Although it was not possible to undertake statistical analyzes due to the small numbers of patients with IPG site pain for the flank and abdomen, descriptive statistics are provided in table 5.

    Revisions, explants and IPG site pain

    In total, 207 patients (27%) underwent a revision (96 males and 111 females). IPG site pain was provided as a reason in 55 cases (21 males and 34 females, see table 6), giving a rate of 7% for surgical revision due to IPG site pain. Rates of surgical revision due to IPG site pain were lowest for IPGs implanted in the posterior chest wall (5%), followed by the buttock (7%), and highest for IPGs positioned in the abdomen (26%). Large rechargeable IPGs had the highest rates of surgical revision due to IPG site pain.

    View this table:
    Table 6

    Summary of characteristics of patients who underwent surgical revision due to implantable pulse generator (IPG) site pain

    In total, 95 individuals (44 males and 51 females) underwent a full system explantation (12%). IPG site pain was stated as a secondary reason for explantation in 10 cases. This included three which were implanted in the posterior chest wall, six in the buttock and one in the flank. The primary reasons for these explants included insufficient pain relief (n=8) and infection (n=2).

    Discussion

    In this retrospective study, IPG site pain occurred in 17% of patients who received a fully implanted SCS with 7% needing pocket revision surgery. Patients who self-reported IPG site pain had significantly higher neuropathic pain scores prior to implant compared with those who reported no IPG site pain. A higher percentage of females than males reported IPG site pain. The prevalence of IPG site pain was lowest for IPGs implanted in the posterior chest wall and highest for IPGs implanted in the abdomen. Rates of IPG site pain were lowest for non-rechargeable IPGs and highest for large rechargeable IPGs. IPG site pain was a contributing factor to revision in 55 of 207 patients who underwent a revision. Although IPG site pain was not the primary contributing factor for explantations, it was cited as a secondary reason in 10 of 95 individuals who underwent a full system explantation.

    Anatomy influences IPG site pain

    Although SCS is associated with improvements in self-reported pain,7–9 pain associated with IPG sites is common.22 In this large retrospective cohort of patients, we found that IPG site pain occurred in 17% with a fully implanted SCS. This complication may compromise the overall outcome of an otherwise successful SCS. This was demonstrated by the 7% of revisions that were prompted by pain at the IPG site which is consistent with prior work.24 Importantly, rates of IPG site pain differed between the IPG locations. The posterior chest wall site was associated with the lowest rates of IPG pain and surgical revisions prompted by IPG site pain. In contrast, the abdomen was associated with the highest rates of IPG pain, although the validity of this result may have been biased by the small number of patients who had abdominal placements (n=46). Overall, our findings suggest that the anatomical positioning of IPGs may contribute to the occurrence of IPG site pain.

    Chest wall

    IPG placement over the posterior (rather than anterior) chest wall was adopted as standard practice in our unit. The placement is normally at the same level as the anchor site minimizing the stretch between the anchor and the IPG site. The cardiac implant literature demonstrated that low IPG site pain and having a bony substrate reduces movement related pain. In our study, the most frequent site of IPG implantation was the posterior chest wall, accounting for 49% of SCS implants. More importantly, posterior chest wall IPGs were associated with the lowest rates of IPG site pain (13%). These are novel findings, suggesting that with further refinement and clinical uptake, pain after IPGs are implanted in the posterior chest wall may become less prevalent.

    Although this location can be associated with less discomfort during sleeping or sitting in comparison to IPGs placed in the buttock, it does carry the risk of causing damage to the supraclavicular nerves (anterior pectoral placement) and the anterior cutaneous divisions of the upper thoracic nerves (posterior chest wall). Irritation of the cutaneous branches of the supraclavicular, upper thoracic and thoracoabdominal nerves has the potential to cause long-term disabling effects. This is most evident in patients implanted with cardiac pacemakers. For example, in five case studies, two patients who had either a pacemaker or defibrillator developed chest pain.25 The implantation of a cardiac pacemaker has also been associated with the development of type I CRPS in a single case study.26 Therefore, due to the complex distribution of cutaneous nerves and the variation in subcutaneous fat in the chest, IPG placement in this area should be carefully considered. However, the bony ribcage makes movement-related pain less, perhaps providing an advantage over other potential IPG sites, such as the buttock and abdomen.

    Buttock

    Due to the higher density of subcutaneous fat in the buttock, it is thought to be an area well-suited for IPG implantation.12 Although the buttock was the second most popular site for IPGs (accounting for 38% of cases), it was associated with the second highest rate of IPG site pain (19%) and the greatest need for revisions due to IPG site pain (n=20). Perhaps this is due to the innervation of the site. The superior cluneal nerve (SCN), arising from lumbar spinal nerves 1, 2 and 3, supplies the uppermost skin of the buttock and the medial cluneal nerve (MCN), arising from sacral spinal nerves 1, 2 and 3, innervates the skin of the buttock closest to the midline. Compression of the cluneal nerves from an IPG may cause pain in the areas supplied by the SCN and MCN, resulting in pain or numbness radiating into the lumbar spine or down the leg and being exacerbated by lumbar movement.27 Landmark bony projections such as the posterior superior iliac spine are also at risk of direct irritation of the periosteum due to their close proximity to the IPG pocket particularly in patients with a low body mass index (BMI).

    Abdomen

    The density of subcutaneous fat makes the abdomen a convenient site for a subcutaneous pocket. However, the abdominal wall has complex neurovasculature, perhaps explaining why we found the prevalence of IPG site pain to be highest for this site. The skin of the abdomen is supplied superiorly to inferiorly by the medial and lateral cutaneous branches of the subcostal, thoracoabdominal and iliohypogastric nerves. Irritation or compression of these nerves can cause intrusive pain (with allodynia and hyperalgesia) and sensory disturbance over the implant site. Pain may be exacerbated by bending forward, during movement of the abdominal muscles (eg, when coughing or straining) and when the IPG is placed close to lines of clothing. However, optimum placement should be tailored to individual patients. For instance, IPGs located in the abdomen for sacral nerve stimulation were found to be a good alternative to the buttock in wheelchair dependant patients or those lacking gluteal fat and was not found to be associated with significant postoperative pain in these patients.28 In contrast, the abdomen should be avoided in patients with scars from prior surgery, a stoma, underlying pathology that may require abdominal surgery and females of childbearing age. This site could also be problematic in obese patients as there is a risk of IPG migration, especially with weight loss after implant. Anatomical considerations highlight the importance of carefully choosing IPG sites and developing procedures to further reduce the prevalence of IPG site pain.

    The influence of baseline parameters and IPG type on IPG site pain

    Patients reporting IPG site pain had significantly higher neuropathic pain (SLANSS) scores prior to SCS compared with those who reported no IPG site pain. In addition, gender was associated with the occurrence of IPG site pain, where higher rates of self-reported IPG site pain were seen in females compared with males. Similar patterns emerged for gender following analysis of the group of patients who had IPGs located in the posterior chest wall. Intriguingly, for the posterior chest wall group, age also differed significantly between those who did and did not experience IPG site pain; the former was younger than the latter. This, therefore, suggests that simple demographic details gathered at the outset may be useful in identifying individuals who are more likely to self-report IPG site pain.

    Type of IPG, categorized as either a large rechargeable, small rechargeable or non-rechargeable, also influenced rate of IPG site pain. Non-rechargeable IPGs were associated with lower rates of IPG site pain compared with small and large rechargeable IPGs. When recharging, induction heat is generated, in turn increasing the sensitivity of skin. Regular interaction with the IPG is needed for rechargeable systems, especially for patients who are required to recharge weekly or even daily. As greater pain severity is correlated with lower heat perception thresholds,29 the additional heat generated from rechargeable systems may result in a predisposition of higher rates of IPG site pain for rechargeable IPGs. This, therefore, suggests that the functional and physical characteristics of the IPG may have an influential role in IPG site pain. Although differing patterns between IPG sites did not reach statistical significance, it is unclear whether the association between IPG type and IPG site pain interacts with IPG position, as well as other parameters (eg, age and gender). This, therefore, warrants further exploration in future research. It may be possible to implement procedures that subsequently reduce the likelihood of IPG site pain occurring as a complication with fully implanted SCS systems after further investigating the contribution of anatomy and baseline characteristics to identify patients who are likely to report IPG site pain.

    Limitations and future directions

    It is important to acknowledge that this study was limited to exploring the influence of five variables that may potentially contribute to IPG site pain that is, IPG site, IPG type, age, gender and baseline neuropathic pain scores. Other variables may be important for example, IPG site pain severity, body composition metrics (eg, BMI and body fat distribution), physical activity, chronic pain diagnoses, comorbidities, efficacy of SCS therapy, quality of life and challenges during surgery. Although protocols are in place to guide the pain team during patient visits, the reporting of the presence and severity of IPG site pain was not formalized. This, therefore, renders it challenging to retrospectively generate an accurate account of which variables are associated with a greater propensity to experience IPG site pain and undergo surgical revisions. To overcome these limitations, current practice and prospective clinical trials should systematically explore potential contributing factors.

    It would be worthwhile to conduct a health economics assessment of the costs associated with IPG site pain. This should generate a more comprehensive picture of this SCS complication, detailing the wider effects of IPG site pain on the working life of patients, direct and indirect healthcare costs and societal impacts. The rate of IPG site pain seems to vary between research studies,24 it is, therefore, important that this and other SCS complications are accurately reported in future work. Despite the limitations associated with retrospective studies, the findings reported here have the potential to generate an initial evidence base that can inform the appropriate design of larger prospective trials. Furthermore, findings from this study have the potential to guide the clinical practice of implanting physicians with the ultimate aim of improving patient outcomes with SCS.

    Conclusions

    Our findings demonstrate that IPG site pain was a common complication that contributed to surgical revisions, and in some cases was an additional reason for explantation. IPG pain was closely associated with the site of IPG implantation. The posterior chest wall was associated with the lowest rates of IPG site pain and highest rates occurred for IPGs implanted in the abdomen. Type of IPG, gender, initial neuropathic pain scores and age may also play an important role in the etiology of IPG site pain. This study highlights the importance of considering carefully the placement of IPGs in light of the physical attributes of the IPG and the anatomy and baseline characteristics of individual patients. The findings also highlight the importance of systematically recording and reporting SCS associated complications. Future research exploring the prevalence and contributing factors to IPG site pain will improve outcomes for SCS patients.

    Acknowledgments

    We would like to acknowledge Tracey Crowther, Jenny Jennings, Sheila Black, Dudley Bush, John Titterington, Craig Montgomery and the entire pain management team for their contribution to the service.

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    Footnotes

    • Contributors GB conceived the study and led the team. With guidance from GB, BB, TK, NM, CR and BR collected the data and BB performed the statistical analyses. BB and GR wrote the manuscript with support from GB.

    • Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

    • Competing interests GB has consulting agreement with Nevro, Medtronic, Boston Scientific, Nalu Medical and Abbott. GB had educational and research grants from Nevro, Abbott and Boston Scientific. GB has stock options with Nalu Medical. The rest of the authors report no conflicts of interest.

    • Patient consent for publication Not required.

    • Provenance and peer review Not commissioned; externally peer reviewed.

    • Data availability statement All data relevant to the study are included in the article or uploaded as online supplementary information. All relevant data are presented in the article.