Spinal Cord Stimulation for Intractable Visceral Pain Originating from the Pelvic and Abdominal Region: A Narrative Review on a Possible New Indication for Patients with Therapy-Resistant Pain

Introduction

Visceral pain is characterized by pain that is diffuse and challenging to localize, frequently originating from the midline of the body, the lower sternum or upper abdomen. It is thought that the prevalence of visceral pain exceeds 20% of the global population.¹ In 2016, visceral pain from the chest, abdomen or pelvis accounted for more than 25 million emergency room visits and 2.5 million hospitalizations in the United States.² Visceral pain can be referred, where pain from visceral organs can be perceived in areas other than their source. For example, pain originating from the bladder can radiate to the perineal area and is caused by viscero-somatic convergence in the spinal cord.^3–5 Visceral pain results from activation of nociceptors of the thoracic, pelvic, or abdominal visceral organs and typically originates from inflammation, distention, and ischaemia. Pain is induced when nociceptors surrounding visceral organs are stimulated as a result of inflammation and recurrent distention. When it occurs suddenly, visceral pain is frequently associated with clinical conditions such as appendicitis, cholecystitis, or ulcers. When occurring chronically, it is generally associated with clinical conditions such as endometriosis, pancreatitis or chronic pain originating from the bladder or bowel. Some cases may have no identifiable substrate and are considered idiopathic or functional visceral pain.¹ The sympathetic (thoracolumbar) and parasympathetic (craniosacral) nervous system innervates all thoracic and abdominal organs, other than the pancreas. The viscera are innervated through a complex network of two sets of nerves, either through vagal and spinal nerves, or through two anatomically distinct sets of spinal nerves.⁶

Management of visceral pain presents significant challenges. While prioritizing the resolution of the underlying (chronic) inflammation or distention is crucial, the need for pain management becomes necessary in many cases. The effectiveness of combining paracetamol with Non-Steroidal Anti-Inflammatory Drugs (NSAIDs) for pain treatment in reducing visceral discomfort has been established.^7,8 Opioids may be administered as needed, although their dosage requires caution due to the risk of opioid induced hyperalgesia – namely abnormal sensitivity to pain – which can mimic pain associated with progression of the underlying pathology. Consequently, this can result in the worsening of visceral pain. In addition, the concomitant administration of antidepressants or anti-epileptic medication with opioids may reduce visceral pain.⁸ Finally, neurolytic blocks such as a plexus hypogastricus block, plexus coeliacus block, or splanchnic nerves block, along with radiofrequency ablation, can aid in the reduction of visceral pain.⁹ Despite the implementation of optimal medical interventions, chronic visceral pain has the potential to become refractory, thereby exerting a negative impact on the overall quality of life (QoL) experienced by the affected individuals. A treatment that might offer a solution is Spinal Cord Stimulation (SCS). First introduced in 1967 for chronic intractable cancer pain, SCS is a minimally invasive therapy offering pain relief for a growing number of conditions.¹⁰ In SCS, electric fields are created between metal contacts which are placed in the epidural space. These applied fields can, based on tissue properties near the electrode, change the electrical potential across membranes. The electrodes are typically placed in close proximity to the physiological midline of the dorsal columns. The electrical stimuli activate dorsal column axons, resulting in orthodromic and antidromic transmission of action potentials. These action potentials generate segmental and supraspinal effects, resulting in pain relief. The electrical stimuli can be administered via various wave forms, which can be characterized based on pulse amplitude, frequency, width and electrodes activation sequence.^11,12 For visceral pain, SCS can block the sympathetic pain pathway that carries nociceptive information in small fibres, thereby preventing the pain signal to arrive in the thalamus and cerebral cortex and thus removing the pain.^13,14

In the Netherlands, the number of diagnoses for which SCS is covered by health insurance is limited. Insurance companies cover expenses in cases solely associated with chronic pain caused by Persistent Spinal Pain Syndrome (PSPS) Type 2, Failed Neck Surgery Syndrome (FNSS), Complex Regional Pain Syndrome (CRPS), Diabetic Polyneuropathy with Small Fibre Neuropathy, or medically refractory Chronic Cluster Headache.¹⁵ Although several studies have suggested that SCS is effective in reducing visceral pain, SCS is not yet covered in the Netherlands for this indication by any Dutch health insurer. This because SCS for visceral pain currently does not comply to with the state of research and practice, as determined by the Dutch healthcare institute. Only indications that comply with the determination of the healthcare institute, are covered by healthcare insurance. However, according to the systematic review by Woodroffe et al¹⁶ SCS has been successfully used for pain associated with chronic gastrointestinal (GI) motility disorders such as irritable bowel syndrome (IBS) or gastroparesis and post-surgical visceral hyperalgesia (for example status post Nissen fundoplication, gastric bypass, bowel resection, post-cholecystectomy, among others). For the male patient population, SCS has been successfully applied for chronic orchialgia (testicular pain) which, for example, can be caused by prostate carcinoma or a ruptured epididymis. For the female patient population, it has been successfully applied for chronic pelvic pain that can be caused, among other things, by long-standing endometriosis. The authors noted that recent studies suggest it might be very effective in treating visceral pain. However, scientific evidence supporting this inference is limited.¹⁶

In this narrative review, our primary aim is to provide a comprehensive overview of the existing literature on the efficacy of SCS in treating visceral pain. By analyzing the current literature, we aim to contribute to the understanding of SCS as a potential therapeutic option for various visceral pain conditions and to identify the existing gaps in scientific evidence.

Methods

This narrative review was reported according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA). The narrative review was retrospectively registered in the PROSPERO international prospective register of systematic reviews (CRD42023448103; registration date 13^th of July 2023).

To identify all relevant publications, we conducted systematic searches in the bibliographic databases PubMed, Embase.com and Web of Science (core collection). Articles published between October 1st 1963 until March 7th, 2023 were found. The systematic search was conducted in collaboration with a medical librarian. The references of the identified articles were searched to ensure that no relevant literature was excluded. Duplicate articles were excluded by a medical information specialist using Endnote X20.0.1 (Clarivatetm), following the Amsterdam Efficient Deduplication (AED)-method¹⁷ and the Bramer-method.¹⁸ Full search strategies for all databases can be found in Appendix A.

Three reviewers (MB, JWK and AHA) independently screened all potentially relevant titles and abstracts for eligibility. Full texts were screened when articles were considered eligible for inclusion based on the title and abstract. When there was a difference in judgement between the three reviewers, a consensus procedure was performed by discussing the article, its outcome measures and the reasoning behind the different opinions. This was done until consensus was reached. There are considerable difficulties in studying patients with refractory chronic pain and SCS: the patient group is small and heterogeneous, SCS is expensive and not readily available. These circumstances demand a wide inclusion of studies with various designs. Therefore, reports describing adult patients with all causes of visceral pain employing SCS from 1967 until March 7th, 2023 were included in this narrative review. Clinical trials were ranked highest, followed by cohort studies and finally, case series or reports. Comparative studies and non-comparative proof of concept and feasibility studies were also included. We included articles in Dutch and English. Technical reports, anatomical descriptions, dose finding studies, studies comparing neurostimulator modalities, and studies comparing various approaches were excluded. We focused on the population of patients with chronic refractory abdominal and pelvic visceral pain in whom SCS was employed to treat chronic pain. All articles were reviewed for the following outcomes: effectiveness (pain scores, pain reduction, reduced demand for systemic analgesic drugs, and patient satisfaction); complications (for example nerve injury, lead migration and malfunction, infection); functional recovery (for example gastropareses, bladder function and voiding) and QoL. Other relevant findings (for example medical costs) were also recorded and summarized. (Figure 1).

Figure 1 Results of the literature search and flowchart of all included articles.

Results

The literature search generated a total of 10,854 references. After removing duplicates, 7460 references remained (Flowchart Figure 1). A total of 56 reports were included from the primary search, and 14 were added after thorough examination of the reference lists of included studies. An overview per study can be found in Table 1. The results are summarized according to diagnosis. We identified several randomized controlled trials. However the majority of the included studies were observational studies, case series and case reports, with low to very low quality evidence. Therefore, in addition to the heterogeneity of study outcomes, the results could not be compared quantitatively in a meta-analysis.

Table 1 All IncludedStudies

Table 2 Pancreatitis

Table 3 Pelvic Pain I - Anorectal Pain and Bowel Disorders

Table 4 Pelvic Pain II – Gynaecological

Table 5 Pelvic Pain III – Urological Disorders

Table 6 Pelvic Pain III–Various Causes

Table 7 Visceral Pain – General

Spinal Cord Stimulation for Chronic Pancreatitis, Table 2

Chronic pancreatitis often results in chronic pain, which intensifies during episodes of active inflammation, while normal pancreatic function further deteriorates.⁸⁷ SCS is rarely used in patients with chronic pancreatitis and is currently not recommended. However, studies performed on patients receiving permanent SCS for chronic pancreatitis concluded that pain scores decreased significantly, Pain Disability Indices (PDI) improved, opioid consumption was reduced, and ADLs improved.^{13,19–24,86} Complications that were observed included infection and lead migration, both of which necessitated the revision or removal of the device. Therefore, it could be concluded that when chronic pain from pancreatitis is drug therapy resistant and when management becomes intractable SCS can be a beneficial minimally invasive strategy prior to contemplating a surgical intervention.⁸⁸ The same can be considered for retractable pain associated with Sphincter of Oddi dysfunction, described in a case report by Mamaril-Davis et al.²⁵ They found that pain scores, QoL, sleep and appetite improved after implantation of sacral neuromodulation (SNM), with 90% pain relief at 18 months follow-up. No complications were reported.

Patient selection for treating pancreatitis with SCS is challenging, due to the absence of large RCTs and comprehensive inclusion- and exclusion criteria. Until these criteria are more firmly established, most clinics and studies continue to use a test trial period prior to permanent implantation. The PANACEA trial (NCT03595241), is the first randomized clinical trial evaluating the effectiveness of SCS for patients with chronic pancreatitis.^25,89 They recently reported their four-month follow-up and found significant reduced pain scores when SCS was applied, compared to conservative medical management. However, we are still awaiting their definitive results.⁹⁰

Spinal Cord Stimulation for Pelvic Pain

Anorectal Pain and Bowel Disorders, Table 3

Functional anorectal pain disorder (also known as chronic idiopathic anal pain) is a diagnosis per exclusionem. It is diagnosed only after excluding all other causes of rectal pain with a structural cause such as active ischemia, fissure, inflammatory bowel disease, infection, haemorrhoids and tumours. In the limited studies with small sample sizes that are available, drastic improvement in pain scores and patient global perceived effect (PGPE) have been demonstrated.^28,31,32 However, complications such as infection, lead migration, and discomfort at the stimulator site are frequently observed. The authors of these limited-scale studies acknowledge that this cohort of patients, similar to numerous patients with chronic pain, is not a one-size-fits-all category. Consequently, personalized approaches for patients, such as the use of spinal cord stimulation (SCS), are required when conventional treatment has been proven ineffective.³² The different treatment options were evaluated in a network meta-analysis by Byrnes et al⁹¹ that included 27 studies on the treatment of anorectal pain, of which six studies reported on treatment with sacral neuromodulation (SNM). They concluded that intramuscular injection of triamcinolone and SNM were likely to be clinically effective, with a surface under the cumulative ranking (SUCRA) score of 0.79 and 0.74, respectively. Compared to other treatment methods, SNM showed superior efficacy in reducing pain scores. However, it should be noted that the interpretation of these findings is limited due to the inclusion of low-quality research and the large contribution of the study by Rongqing et al who included 120 patients without implantation of a permanent device.⁹² A case series by Dudding et al permanently implanted three patients for idiopathic anal pain, which was beneficial in only one of their three patients.³³

Bowel disorders such as gastroparesis, irritable bowel syndrome and faecal incontinence might benefit from SCS, as is described in case reports of patients who suffered not only from intractable abdominal pain, but also from debilitating gastrointestinal symptoms.²⁹ All symptoms resolved following initiation of SCS.^30,34 In patients with gastroparesis who were studied prospectively⁷⁹ and retrospectively,⁸⁴ both traditional tonic and 10 kHz subthreshold stimulation was not only effective in improving pain scores but also in reducing the occurrence of nausea and vomiting.^79,84 However, significant improvements in pain scores, nausea and vomiting occurrence were achieved when 10 kHz SCS was used as opposed to low-frequency, traditional tonic stimulation. Pain severity scores in patients stimulated at 10 kHz (n=12) decreased from 8.4±1.2 cm at baseline to 2.6±3.2 cm at the latest patient visit (p<0.001), whereas the traditional SCS group (n=11) scores reduced from 8.9±1.4 cm at baseline to 3.8±3.0 cm (p=0.001). Similarly, the number of ‘nausea days’ per month showed a reduction of over 50% for both study groups, although 10 Hz was significantly better (p=0.035) regarding this metric.⁸⁴

Three studies have reported on the efficacy of SCS for IBS symptoms. Compared to the baseline scores, each study reported (often significant) improvement in pain (measured using VAS, scale 0–10cm), in pain frequency, in anorectal comfort, in IBS-specific symptom scores (measured by the Gastro-intestinal Syndrome Rating Scale), and finally in IBS-specific QoL.^26,27,29 The reported complications included infection, lead migration, pain at the implantation site or loss of device efficacy. Given the current evidence on the “brain-gut axis”,^93,94 it is plausible to propose neurostimulation as a treatment option for severe cases of irritable bowel syndrome, which is considered a dysregulation of the central nervous system.³⁰

Gynaecological, Table 4

Chronic Pelvic Pain (CPP) shares a similarity with the above-mentioned pain syndromes, as it is a diagnosis per exclusionem. Nonetheless, the condition is debilitating. Endometriosis is no diagnosis per exclusionem and is, apart from CPP, additionally associated with symptoms of dysmenorrhea, dyspareunia, dysuria and dyschezia. Because of their experienced pain, patients may undergo a hysterectomy or otherwise invasive surgery, which could be ineffective in removing all pain or might even result in an aggravation of pain.⁹⁵ CPP is a multifactorial condition that affects psychological, psychosocial, cultural, and economic factors. Furthermore, the pelvic region has a complex innervation and chronic pelvic pain may arise due to central sensitization. This condition is marked by the recurrent or extended stimulation of nociceptors, leading to a decreased threshold for activation. As a result, patients may experience pain even when there is no pain substrate,⁹⁶ thereby presenting a significant challenge in terms of treatment.⁹⁷ SCS can be effective in selected patients as it is minimal invasive, making it a viable option to consider before proceeding with a more permanent surgical or neurolytic procedure.^44,46 After implantation of SCS, patients reported an improvement in QoL, pain severity scores, Pain Disability Index (PDI) scores, Beck Depression Inventory (BDI) scores, and functional symptoms. In addition, patients exhibited a high level of satisfaction, with some even expressing a profound level of contentment, in response to SCS treatment.^36,38,40,45

Also of note in this patient cohort there was a high incidence of complications. Some patients underwent explantation of the device due to infection, local pain, device failure, lead migration, a broken lead, excess granulation tissue, the need for an MRI for non-MRI compatible devices, allergy, and gradual loss of effect in the older wave form units. A Finnish national study confirmed this, where they found similar adverse events despite overall positive results. In their study, they determined that women with SCS for CPP related to endometriosis reported superior success rates compared to women with SCS for idiopathic CPP (75% vs 41% resp., P=0.026), with a high percentage of advancement to permanent implantation and strong symptom improvement following implantation. However, the IPG was removed in six of 50 participants (12%) due to loss of efficacy (4/6), pain (1/6) or infection (1/6).⁴² As was already stated in the Cochrane review in 2000:

Given the prevalence and the health care costs associated with chronic pelvic pain in women, randomised controlled trials of other medical, surgical and psychological interventions are urgently required.⁹⁸

Unfortunately, these are currently lacking in the literature.

Five previous studies determined that Sacral Nerve Stimulation (SNS), SCS and/or Peripheral Nerve Stimulation (PNS) were effective in reducing endometriosis-related pain symptoms such as CPP and dyspareunia, anal pain, and intestinal constipation. They also found high patient satisfaction, an improvement of QoL, a reduction in the use of pain medication, an improvement in Oswestry Disability Index (ODI) scores, Hospital Anxiety and Depression Scale (HADS) scores. Finally, they found improvements in social participation where women were able to work again, take part in social activities and travel.^{39,41,43,48,49} The complication most frequently reported was electrode migration. At present, there are three trials recruiting (in France, the Netherlands (NCT05558540) and Turkey) participants that are treated with SCS and suffer either from pelvic pain related to endometriosis or are suffering from CPP in general.

Urological Disorders, Table 5

Despite the number of studies being limited, most prospective studies on SCS for pelvic pain focus on urological disorders and universally conclude that well-selected patients respond favourably to SCS with improved pain severity scores, reduced CPP and improved voiding symptoms.^{52–56,58,60,61,66,69,71,99,100} Their results seem sustainable in the absence of complications, which is substantiated by studies with longer follow-up periods.^62,64,66 Further, when comparing it to a control group with similar disease presentation, SCS improves pain severity scores and overall outcomes.⁷⁰ However, an older study by Elhilali et al demonstrated that not all functional urological problems are successfully treated with SNM and treatment can lose its efficacy over time.⁵⁹ However, this study was published in 2005, at a time when advanced wave forms were unavailable, so application to current clinical realities is questionable. Complication rates vary between studies, with revision rates being as high as 50%. These were primarily attributed to loss of efficacy, local pain at the implant site, painful stimulation, and the necessity for battery revisions. In some studies, the explant rate was as high as 27% and seems to be attributed to a more practical concern, namely the need for an MRI.⁶⁸ This challenge was not exclusive to SCS for urological disorders, but rather a recurring issue encountered in the context of any implanted device.¹⁰¹ However, the significance of this matter has diminished in light of the recent emergence of MRI-compatible technologies. Even though reported revision rates were relatively high, SNM should be considered prior to proceeding with any invasive surgical intervention.^62,102

Aiming to optimize SCS treatment, Everaert et al studied the 1-stage versus 2-stage SNS for bladder dysfunction where a lower rate of treatment failure was demonstrated in the 2-stage implant. Moreover, the trial procedure has been subject to innovation. According to Powell et al (2010) a greater number of patients have a successful trial with quadripolar lead placement compared to a single test lead. However, the benefit of long-term treatment was equal in both groups.^50,63 In their study, Peters et al compared pudendal nerve stimulation to sacral neuromodulation and calculated that 77% of patients (13/17) chose pudendal nerve stimulation following blinded testing. Nevertheless, irrespective of the patients’ preferences, long-term outcomes were comparable in both cohorts.⁵¹ A noteworthy observation is the association between higher pain scores, the Interstitial Cystitis Symptom Problem Index (ICSPI) scores and the levels of urine chemokines.⁵⁷ Follow-up after device implantation demonstrated a decrease in ICSPI with a concomitant decrease in urine chemokines, which implies a role of chemokines in neurophysiological signalling in bladder pain and -function.¹⁰³

A large retrospective cohort study examined the Finnish experience and collected data since the initiation of SCS in Finland in 1996. This study included the application of SCS for three diagnoses: urgency-frequency syndrome, urinary retention, and painful bladder syndrome/ interstitial cystitis (PBS/IC).⁶⁷ Although over 50% of their patients did not receive permanent implantation (74/180), all three groups experienced benefit from SCS with improvement in voiding symptoms, a reduction in the number of catheterizations and in pain severity. The results of the study furthermore suggested that younger patients are more likely to benefit from SCS. For some indications, alternative treatments may be more cost-efficient as financial costs of SCS are significant. However, research comparing health care utilization and cost-effectiveness associated with SCS to other therapies is strongly needed to understand relevant economic issues. For example, Botulinum A toxin is a more cost-effective alternative to SCS for patients with urge incontinence and should therefore be tried first.^67,104

Patients implanted with SCS reported a high rate of satisfaction (90%), despite the limitations and complications of the treatment. Satisfaction with SCS was positively correlated with the ability that the patient could personally adjust the device and whether the patient was still active and/or working. The presence of two or more pelvic floor comorbidities was negatively correlated with the patients’ satisfaction with SCS.⁶⁵ Also, given that chronic pain and mental health are often intertwined, most medical institutions that implement SCS have established a close collaboration with the psychologic or psychiatric departments. For some, the coexistence of pain and depression is considered a contraindication for implantation of a neuromodulator. Nevertheless, the use of SCS has shown potential in improving depression scores,^71,105 raising a complex question regarding causality. Additionally, Killinger et al demonstrated that patients who manifested major depressive symptoms (PHQ-8 scores ≥10) did not display an inferior response to SCS.¹⁰⁶

Various Causes of Pelvic Pain, Table 6

Not all studies make a distinction between different causes of CPP. These studies include various patient cohorts, predominantly those who present cohorts with combined surgical, gynaecological and urological symptoms. Their results were similar to those mentioned in the previous sections: SCS resulted in reduced pain and improved functional symptoms. Patients with chronic pelvic pain following hysterectomy and those with only one pelvic comorbidity reported better results compared to other, or multiple pelvic comorbidities.^73,107 One case report suggests that SCS may be effective in reducing CPP secondary to multiple Tarlov Cysts, which were previously worsened by surgical treatment as well as in reducing the use of analgesics.¹⁰⁸ Another case report suggested that SCS may be effective in reducing CPP secondary to multiple pelvic fractures.⁷⁷ Various pelvic disorders were combined in the study by Hernandez-Hernandez et al who demonstrated substantial improvements in pain severity scores, functional symptoms and QoL. However, complication rates were as high as 40.6%, which resulted in 5/64 patients undergoing device explantation.⁷⁵ This is relatively low compared to the study by Al-Kaisy et al, which identified explant rates of SNM devices of 17.8% after five years and of 25.2% after ten years, where explantation was performed because of various indications.¹⁰⁹ An attempt to develop a management algorithm for patients with CPP was published by Bridger et al in 2021.⁷⁴ Over the course of seven years, 233 patients with CPP were referred, 153 were included in the protocol, and only eleven underwent SCS. These patients experienced neuropathic pain, penile pain, painful bladder syndrome, small fibre neuropathy, and neuralgia. The authors intended to compare the SCS group to the non-SCS group, but patient numbers were too small to allow a comparison. Nevertheless, it appeared that SCS was more effective in treating neuropathic pain compared to other treatment options.⁷⁴

A pilot study across multiple etiologies for chronic pelvic pain (predominantly post-surgical CPP, post-partum CPP, and interstitial cystitis) reported effective use of high frequency SCS³⁷ for patients with CPP, with 10/14 (71%) patients who received a permanent implant reporting ≥50% pain relief at twelve months. This high-frequency 10-kHz therapy is paraesthesia-free, although it requires dedication to uncover the optimal programming. In addition, there frequently is a delay in analgesic onset (12–48 hours or longer), which is distinctly different from low-frequency therapy.^76,110 In their study, Kapural et al studied the efficacy of high-frequency therapy in providing pain relief and compared it to low-frequency spinal cord stimulation (SCS). Their findings indicated that high-frequency therapy may be more successful than low-frequency SCS in alleviating pain, not only in patients with chronic back and/or leg pain, but also in those experiencing chronic abdomen pain. In the examined patient cohort no paraesthesia was reported.^79,111–114 Although high-frequency therapy is not universally implemented in all European countries, it is implemented in the Netherlands. A study from the Netherlands concluded that the use of bilateral stimulation over multiple levels could be more effective because chronic pain can result in recruitment and upregulation of additional fibers.⁷²

Visceral Pain in General, Table 7

In some studies, SCS for visceral pain in general was the terminus a quo, thereby including a wide range of underlying conditions. These conditions included postsurgical pain, post-childbirth, interstitial cystitis, pancreatitis, inflammatory bowel disease, endometriosis, renal calculi, liver pathology, trauma, pudendal neuralgia and post infection chronic pain. Overall, all studies reported improved pain severity scores and PDI, decreased analgesia consumption and improvement in PGIC, QoL, SF-MPQ-2 scores, GAF scores, daily activities and mood.^{35,47,78–80,83–86} The initial prospective SCS study for chronic and refractory abdominal pain that received IDE approval from the FDA was performed by Kapural et al and used a 10 kHz SCS. The authors documented an improvement in pain severity scores (in VAS, scale 0–10cm) of 6cm from baseline, a reduction in PDI and opioid usage and finally a profound improvement in patient satisfaction twelve months after implantation.⁷⁹ Despite these excellent prospects of SCS, no standard approach to chronic abdominal pain is currently governed.¹⁶ Both ventral and dorsal horn implantation were effectively applied. However, ventral stimulation was preferred by four patients with dual leads and was lower for ventral stimulation. This was likely due to the cerebrospinal thickness in the dorsal and ventral space.⁸³ However, recent studies did not confirm this finding. For dorsal column stimulation, a “sweet spot” for treatment of visceral pelvic pain is suggested at the level of T12 due to the segmental suppression of sympathetic outflow to the pelvis via the hypogastric plexus.⁸⁵ Kapural et al also observed that the efficacy of sympathetic nerve blocks may serve as a potential indicator for the success of SCS. Six patients who failed the trial period and one patient who had the device removed because of ineffectiveness shared a poor response to the preceding sympathetic nerve block.⁸⁵ This was supported by the study by Hord et al who treated CRPS with SCS. They found that twelve patients with a positive sympathetic nerve block experienced strong and long-lasting pain relief, whereas only one (33.3%) out of three patients with a negative response to the nerve block reported substantial pain relief.⁸¹ Therefore, caution is advised when implanting patients subsequent to a failed sympathetic nerve block.

There is a potential oversight of patients with visceral pain in our narrative review, since certain cases may be obscured within studies encompassing diverse patient cohorts. Individual patients are often lost in a large group, as no details are provided per subgroup. For example, the series by Kumar et al reported on 410 patients treated with SCS for various underlying chronic pain conditions. Over 200 patients were treated with SCS for Persistent Spinal Pain Syndrome (PSPS) Type 2 in addition to six patients with perirectal pain. Of these patients, four ultimately received a permanent SCS implant with long term success.⁸² No further mention was made regarding this subgroup. The authors strongly supported the use of SCS, as they noted its reversibility, minimal invasiveness, low complication rate, and effectiveness.

Conclusion

This narrative review provides an overview of the current evidence on SCS for visceral pain across a wide range of underlying conditions. Most studies we identified were of low quality, with many retrospective cohorts, case series and -reports. This review highlights the necessity for improved screening and selection criteria to evaluate eligible patients who will benefit most from treatment with SCS. Multidisciplinary involvement is essential for finding the best treatment option for each patient. A noteworthy observation from several studies is the potential of using preceding positive responses to sympathetic nerve block as the basis for selecting patients for SCS. Patients who do not experience a positive effect from sympathetic nerve blocks might not necessarily respond to SCS, even after a positive trial. Some general studies on SCS suggest a positive correlation with younger age and efficacy of SCS for urological disorders. Other studies indicate a negative correlation between a greater number or complex pelvic comorbidities and the efficacy of SCS. Patients with endometriosis might have more favourable results in comparison to individuals with other etiologies of CPP. Furthermore, SCS can successfully treat both chronic pelvic pain and functional symptoms. These include disabling bowel and voiding problems in a selected group of patients. However, high complication rates negatively affect the applicability of SCS, particularly with older units. Steps towards reducing these complication rates should be made, as these directly impact the quality of life of patients. Moreover, some studies suggest that the positive effects of SCS gradually decline over time. However, this concern appears to be mitigated with the introduction of more advanced waveforms. Finally, this review acknowledges that costs are a major concern, as SCS is expensive and not commonly covered by standard health insurance in the Netherlands and on a global level. We stress the need for sound, prospective, possibly randomized and controlled studies, with an adequate number of patients and substantial follow-up, to determine a pain treatment plan including selection criteria for SCS. It is recommended to incorporate cost analyses into future studies.