The effect of biofeedback interventions on pain, overall symptoms, quality of life and physiological parameters in patients with pelvic pain

Background Biofeedback is recognized as an effective additive method for treating certain phenotypes of chronic pelvic pain syndrome and is a therapeutic option in other pelvic pain conditions. This review aims to evaluate evidence from the literature with a focus on the effect of biofeedback on pain reduction, overall symptom relief, physiological parameters and quality of life. Methods A systematic literature search was conducted using the databases PubMed, MEDLINE, Embase, Cochrane Library and PEDro from inception to July 2020. Data were tabulated and a narrative synthesis was carried out, since data heterogeneity did not allow a meta-analysis. The PEDro scale and the McMaster Critical Review Form—Quantitative Studies were applied to assess risk of bias. Results Out of 651 studies, 37 quantitative studies of primary research evaluating pelvic pain conditions in male and female adults and children were included. They covered biofeedback interventions on anorectal disorders, chronic prostatitis, female chronic pelvic pain conditions, urologic phenotypes in children and adults and a single study on low back pain. For anorectal disorders, several landmark studies demonstrate the efficacy of biofeedback. For other subtypes of chronic pelvic pain conditions there is tentative evidence that biofeedback-assisted training has a positive effect on pain reduction, overall symptoms relief and quality of life. Certain factors have been identified that might be relevant in improving treatment success. Conclusions For certain indications, biofeedback has been confirmed to be an effective treatment. For other phenotypes, promising findings should be further investigated in robust and well-designed randomized controlled trials.


Introduction
Biofeedback therapy is an instrument-based learning process employing operant conditioning. Autonomic and neuromuscular activity is measured and visual, acoustic and verbal feedback is provided to promote the acquisition of self-control over physiological processes, which are otherwise outside awareness or under less voluntary control [1].
Pelvic pain is perceived in pelvis-related structures and organs of either men or women and may be acute or chronic. In terms of chronic pelvic pain (CPP), there is no generally accepted definition. It can be subdivided into conditions with well-defined classical pathology and those with no obvious pathology-the chronic pelvic pain syndromes (CPPS). The European Association of Urology (EAU) describes CPPS as the occurrence of CPP with no proven infection or other obvious local pathology accounting for the pain, continuous or recurrent for at least 6 months. It is often associated with symptoms suggestive of lower urinary tract, sexual, bowel, gynecological or pelvic floor dysfunction and with negative cognitive, behavioral, sexual or emotional consequences [2].
Up to 85% of women with CPP have dysfunction of the musculoskeletal system, including spasm of the levator ani muscle [7]. Myofascial pelvic pain is a major component of CPP which is not always properly identified by healthcare providers [8]. It may be a primary or contributing source of CPP [8]. Its hallmark diagnostic indicators are myofascial trigger points in the pelvic floor musculature that refer pain to adjacent sites [8]. They are thought to occur in response to acute and chronic physical or psychosocial stress or trauma [9].
The pathophysiology of CPP is not well understood. Treatment is therefore often unsatisfactory and limited to symptom relief [7]. Several nonsurgical strategies exist that include medical, psychological, cognitive, behavioral, complementary and physical therapy [5,7,10]. In the case of myofascial pelvic pain in particular, a multidisciplinary team of specialists [8] and a multimodal treatment strategy are warranted. In a large proportion of patients, treatment does not necessarily result in pain relief. CPP therefore carries a significant physical, mental, and social burden for patients and puts a heavy burden on healthcare systems worldwide. Increased medical attention to identify and test effective treatment strategies is warranted [5,7,10,11].
Biofeedback seems to be a promising adjuvant tool in the cognitive-behavioral treatment of somatoform disorders because it aims to enhance control over the psychophysiological processes that may be involved in these conditions [1]. Biofeedback is also one of several effective physical therapy techniques used to treat myofascial pelvic pain [8]. The recent EAU guidelines 2019 on CPP state that biofeedback is the preferred treatment for chronic anal pain and can improve the outcome of myofascial therapy as an adjuvant to muscle exercises in patients with hypertonic pelvic floor dysfunction [2]. It is considered a treatment option in type III chronic prostatitis according to the National Institutes of Health (NIH) classification [10]. Previous systematic reviews have evaluated the evidence of physiotherapy interventions in general in the management of CPP [5,12,13]. One review focused on the effect of biofeedback on improving symptoms of pelvic floor dysfunction in 2008 [14]. The primary aim of our review was to evaluate the effect of biofeedback interventions on subjective outcome pain, overall symptom improvement and quality of life in patients with acute or chronic pelvic pain conditions. A secondary aim was to investigate whether biofeedback interventions improved physiological parameters indicative of pelvic floor muscle tone and/or general relaxation. S12 The effect of biofeedback interventions on pain, overall symptoms, quality of life and physiological. . . EMG-based BFB can produce alleviation of idiopathic rectal pain. Outcome was significantly improved in patients who completed the treatment schedule compared to those who self-discharged. Outcome was not influenced by patients' ages, duration of symptoms or prior history of surgery and was not significantly related to the presence of paradoxical puborectalis contraction (EMG or defecography) Grimaud et al. 1991 [20] France Non-RCT, singlegroup (prospective) + crosssectional 12 (24) 12 healthy 0/12 Chronic idiopathic anal pain ± constipation (n = 9/12) Chronic idiopathic anal pain is associated with abnormal anorectal manometric profiles (↑ anal canal resting pressure), probably resulting from a dysfunctioning of the striated external anal sphincter. BFB is an effective treatment for chronic idiopathic anal pain. Anorectal pain disappeared after a mean of 8 BFB sessions

Constipation, dyssynergic defecation
Chiarioni et al. 2006 [21] Italy RCT ( Patients with PFD are likely to have continued benefit from BFB, whereas its effects on slow-transit constipation seems to be maximal in short-term course. Anorectal manometric variables remained unchanged (apart from sensation threshold ↓ in PFD group, maximum rectal tolerable volume ↓ in slow-transit group) Wang et al.
2003 [25] Italy Non-RCT, singlegroup (prospective) 24 (24) n.a. Chronic idiopathic constipation (Rome II criteria) BFB has a long-term effect with no side effects for most patients (62.5%) with chronic idiopathic constipation unresponsive to traditional treatment. Patients with slow/ normal transit, with/without paradoxical PF contraction benefited equally. The psychological status rather than anorectal test could predict outcome. The efficacy of the two modes of BFB was similar K The effect of biofeedback interventions on pain, overall symptoms, quality of life and physiological. . . S13 review article   Individualized movement retraining using motion-sensor biofeedback resulted in significant and sustained improvements in low back pain. This pilot trial also refined the procedures and sample size requirements for a fully powered RCT BFB biofeedback, CPPS chronic pelvic pain syndrome, CP/CPPS chronic prostatitis/chronic pelvic pain syndrome, DSM-5 criteria diagnostic and statistical manual of mental disorders, EGS electrogalvanic stimulation, EMG electromyography, FSFI female sexual function index [56], f/u follow-up, IG intervention goup, LUTSS lower urinary tract symptom score [57], MSQ menstural Symptom questionnaire (MSQ) [58], NIH National Institute of Health, NIH-CPSI National Institute of Health -chronic prostatitis symptom index score, PF pelvic floor, PFD pelvic floor dyssynergia, ↓ significant decrease, ↑ significant increase, RCT randomized controlled trial, vs. versus a missing outcome data: n = 31/52 for American Urological Association (AUA) symptom score [59], n = 39/52 for visual analog scale (VAS) [60]) symptom severity/ effect daily life

Protocol and registration
A systematic review of the existing scientific literature was conducted, based on the guidelines recommended by the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) statement [15]. The review protocol was registered (PROSPERO registration number: CRD42020201751).

Identification and selection of studies
The search included the electronic databases PubMed, Medline, Embase, PEDro and Cochrane Library. Trials with the keywords "pelvic pain AND biofeedback" were extracted and considered for inclusion. No filters were used. No restrictions were placed on the year of publication. A systematic literature search was independently performed by two researchers (BW, MS) and disagreements in selection were resolved through discussion. The process was supervised by an experienced senior researcher (RC). Table 1 presents the inclusion and exclusion criteria regarding study design, participants, interventions and outcome evaluation. Any quantitative study type of primary research (with the exception of case studies/case series less than 10 participants) was included to present a comprehensive overview of the current literature. This approach goes along with previous reviews [5,13] which stated that including randomized controlled trials (RCTs) only was not feasible in reviewing physiotherapy interventions in patients with CPP. We considered males and females of all ages with either acute or chronic pelvic pain conditions as listed in the EAU guidelines [2], including both specific disease-associated pelvic pain and pelvic pain syndromes. Interventions were judged eligible if biofeedback was administered as a sole intervention or significant component of a multimodal or multidisciplinary intervention (including mechanical or electrical devices), as multidisciplinary management of CPP is considered optimal [13]. S16 The effect of biofeedback interventions on pain, overall symptoms, quality of life and physiological. . .

Data collection and analysis
For eligible papers, the following data were extracted: study characteristics (author and year of publication, country site, study design, sample sizes, drop-out rate, diagnoses investigated, author's conclusions), patient characteristics (sample characteristics, mean duration of symptoms, sex, mean age), intervention characteristics (interventions administered, time points of follow-up, biofeedback devices, training extent, intervention setting, adverse events) (Tables 2, 3 and 4). Primary outcome parameters were pain, overall symptom improvement and quality of life. Secondarily, physiological parameters were assessed. When certain data were not given in the respective studies the information was stated as "not available". Outcome data were presented by means of the mean difference within a study group or between groups and their statistical significance (Tables 5, 6 and 7). Few studies provided effect sizes or the corresponding interval estimates (e.g. the confidence intervals) for the mean differences. These values were calculated by the authors if studies provided the relevant data to do so. The criteria for determining effect sizes according to Cohen [61] are listed in the legend of Tables 5 and 7.

Data synthesis
An attempt was made to bundle data for a meta-analysis; however, due to the substantial heterogeneity of study designs, patient characteristics, interventions and effect measures, a meta-analysis was not possible as results are considered unreliable when a small number of heterogeneous studies are assessed [70]. Rather, a narrative synthesis of study results was performed [71], and findings were juxtaposed in the respective tables to provide a comprehensive overview of the current literature.

Quality assessment
As trials differed in their study design, the McMaster Critical Review Form-Quantitative Studies [72,73] was chosen for assessing the methodological quality of all studies included. This critical appraisal tool allows comparisons across different types of quantitative study designs due to its generic composition [5,74,75]. It comprises 15 items that evaluate method rigor and bias and has a guideline for completing the questionnaire that facilitates consistency in interpretation and application [72,74]. In its original form, the tool did not provide a numerical summation. Based on previous reviews [5,74,75], for better comparability between included studies, a sum score of the respective subdomains was established. Each question is rated with either "yes" (1 point), "no", "not addressed" or "not applicable (N/A)" (0 points). In this arbitrary scoring system, higher scores indicate higher methodological quality, resulting in a possible total score of 14 points [5].
In addition, studies with an RCT design were evaluated using the Physiotherapy Evidence Database-PEDro score, a valid and reliable tool for assessing the methodological quality and completeness of statistical reporting of randomized and quasi randomized controlled trials in physiotherapy [12,[76][77][78][79][80][81]. The tool evaluates internal validity and interpretability [82]. Eleven items are rated yes or no (1 or 0 points) according to whether the criterion is clearly satisfied in the study. A total PEDro score is achieved by adding the ratings of items 2-11 for a total score between 0 and 10. Higher scores indicate superior methodological quality. Studies with 9-10 points are considered excellent, 6-8 good, 4-5 fair and < 4 poor quality [80].

Study selection
A total of 651 studies published between 1978 and 29 July 2020 were found and screened for eligibility by title and abstract. After eliminating duplicates, 389 studies were rejected as non-includable, 83 studies were selected for full-text analysis and 37 articles corresponded to the inclusion criteria. Details on the systematic literature search and the selection process are presented in Fig. 1.

Study characteristics
Quality assessment Table 8 shows the quality assessment using the Mc Master Critical Review Form-Quantitative Studies Tool for assessing the risk of bias of all studies included.
All studies but one were judged to have clearly stated the purpose of the study [38] and to have reviewed the relevant background literature [36]. The majority of the studies (29/37) gave enough detail on important sample characteristics. Only 10/37 studies stated how they arrived at the sample size. A minority of studies explicitly stated to have used reliable [21,29,34,40,45,48,55] and valid [21,25,26,34,35,37,40,47,[53][54][55] outcome measures. For several tools, however, the psychometric properties are described in the literature. If at least one main outcome tool was used that is described in the literature, studies were marked with an asterisk. The majority of the studies (30/37) was judged to have described the intervention in detail. Where applicable/where addressed, most studies (13/14) were assessed to have avoided contamination through inadvertent treatment but not to have avoided co-interventions (17/23) as in many cases, subjects were taking medication during the study period (e.g. analgesics, laxatives in anorectal disorders). Most studies (34/37) reported results in terms of statistical significance, chose analysis methods appropriate for the study and the outcomes (32/35) and reported on drop-outs (26/37). All studies (37/37) were assessed to have discussed the relevance of the results to clinical practice and the majority of the studies (32/37) were judged to draw appropriate conclusions, given the study methods and results. The arbitrary sum score ranged between 5 and 13 (mean 9.2). Table 9 shows the quality assessment of the 9 RCTs according to the PEDro scale [81], resulting in 2 studies of fair [26,55] and 7 studies of good [16,21,42,44,45,47,48] quality. The mean PEDro score of these studies was 6 (range 5-8). All studies were randomized (9/9), analyzed the between-group difference (9/9), reported point estimate and variability (9/9) and had similar groups at baseline (9/9). Some studies had a concealed allocation (4/9), 4 out of 9 studies reported adequate follow-up. The majority of the studies did not have blinded participants (8/9), blinded therapists (9/9) or blinded assessors (5/9). In 7 out of 9 studies all subjects for whom outcome measures were available received the treatment or control condition as allocated or, if this was not possible, data for at least one key outcome were analyzed by intention to treat [81]. Table 2 gives an overview of the characteristics of the included studies, additionally outlining study design, comparison characteristics and sample sizes.
Age ranged between 11 and 96 years in studies mainly enrolling adults. The mean age for trials involving children was 8.4 years [51-53] and 16.5 years for the study investigating adolescents [54].
Anorectal manometric systems and surface EMG techniques were the commonly applied anorectal physiological assessment tools in studies dealing with anorectal disorders. Male chronic pelvic pain syndromes used EMG-guided training [32,[34][35][36]. In urogenital phenotypes in children and adolescents, both urodynamics and perineal EMG were used. In female chronic pelvic pain syndromes, most studies used pelvic floor EMG to evaluate pelvic floor function. Three studies on patients with dysmenorrhea [43][44][45] aimed at increasing general relaxation by using heart rate variability training, skin temperature training and EMG of the frontalis and lower abdominal muscles.
Overall, the biofeedback training extent was largely heterogeneous, 2-30 sessions were administered, lasting between 10 and 60 min, for up to 6 months. Most designs applied biofeedback weekly, less often sessions were scheduled twice or three times a week or once every 2 weeks. Biofeedback in a home-based setting was applied daily in 3 studies on gynecological disorders [38,39,45]. Treating anorectal disorders, four large trials by Chiarioni et al. [16,21,28] and Ba-Bai-Ke-Re et al. [26] proved 5 weekly biofeedback sessions of 30min to be successful (Table 4).

Outcome
Primarily evaluated outcomes: pain intensity, overall symptom improvement, quality of life Heterogeneous assessment methods were used to evaluate primary outcome measures within a certain phenotype (Tables 5 and 7). Pain was assessed using either visual S18 The effect of biofeedback interventions on pain, overall symptoms, quality of life and physiological. . . He et al. 2010 [36] n.a. >3 months 100% b 0% n.a.

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The effect of biofeedback interventions on pain, overall symptoms, quality of life and physiological. . . S19 review article  [60].
Outcome tools together with references of the respective questionnaires are outlined in Tables 5 and 7.
Secondarily evaluated outcomes: physiological parameters Pelvic floor function was assessed using manometric devices, urodynamic devices as well as surface EMG techniques and digital examination. One study observed general relaxation through heart rate variability measures [45]. Table 5 presents the effect of biofeedback-assisted interventions on pain and overall symptom improve-S20 The effect of biofeedback interventions on pain, overall symptoms, quality of life and physiological. . .     Table 2.

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Only three [16,17,19] out of five studies evaluating anorectal pain syndrome provided p-values for pain outcomes. Significant anorectal pain relief could be shown, whereby patients who finished had superior results compared to those who discharged themselves before completion of treatment [19]. A large RCT of good quality by Chiarioni et al. 2010 found biofeedback to be superior to electrogalvanic stimulation and local massage therapy both in the short and long term, whereby these differences were only significant in patients with a highly likely levator ani syndrome (tenderness of the levator ani muscle on the rectal examination) [16].
Eleven studies investigated patients with constipation: 2 RCTs of adequate sample size studied patients with dyssynergic defecation [21,26] and found that biofeedback significantly decreased abdominal pain compared to laxatives (polyethylene glycol) [21,26] with long-term effects and huge effect sizes significantly different from zero [21]. The same two RCTs found biofeedback superior to laxatives in terms of constipation symptom improvement with very large effect sizes [26].
Several of the remaining nine non-RCTs found pain [22-24, 27, 29] and constipation symptoms [22,23,25,27,29,31] improved after biofeedback, at least for certain subgroups. Studies showed contradictory results regarding the question of whether biofeedback only benefited patients with PF dyssynergia or also patients with prolonged transit time. Some studies found that biofeedback improved (long term) symptoms for pelvic floor dyssynergia [22,24,28] but not for slow transit constipation [24,28], others found that both phenotypes benefited equally from treatment [23,25,27].

Effect of biofeedback interventions on quality of life
Nine studies used biofeedback to improve pelvic floor function and found a significant improvement in the S24 The effect of biofeedback interventions on pain, overall symptoms, quality of life and physiological. . .          quality of life postintervention in eight trials ( Table 6). The findings came along with small [29,35] to huge [26,36] effect sizes, with 5 studies showing a significant effect for at least some outcomes [26,29,35,36,45]. A home-based heart rate variability training failed to significantly improve quality of life compared to standard care [45]. Table 7 presents significant changes in physiological outcome assessment following biofeedback interventions. Biofeedback training could significantly improve at least some manometric values in 9 [16, 20-22, 24-26, 28, 31] out of 10 studies on anorectal dysfunction. In Heah et al. [17] manometric values did not significantly change posttreatment. Six [16,17,21,22,24,28] out of 9 studies did not improve resting anal canal pressures. Studies on constipation and dyssynergic defecation found that paradoxical contraction on evacuation [21,22,24,28,31] and the ability to defecate a balloon [21,28,31] could be improved. Patients with dyssynergic defecation could improve more manometric values than patients with slow transit constipation [28]. A landmark trial on anal pain syndrome showed that patients with a tenderness of the levator ani muscle on digital palpation could improve more manometric values than patients without tenderness on the rectal examination [16].

Effect of biofeedback interventions on physiological parameters
In female chronic pelvic pain, four studies did improve EMG values of the pelvic floor or lower abdomen [38,39,44,52] whereas two (mostly) failed to do so [40,42].

Quality
This systematic review included 37 quantitative studies and found tentative evidence that biofeedback-assisted training interventions can improve the primarily evaluated outcomes pain, overall symptoms, and quality of life. Results should be considered with caution due to quality issues of many of the included trials. Only 9 studies had an RCT design, out of which 7 were judged to be of good quality according to PEDro assessment. Many studies were likely underpowered and did not provide a sample size calculation.
Biofeedback is a modality to improve self-efficacy and learning based on operant conditioning [83]. Biofeedback is not used as an intervention on its own but is rather an adjunctive tool to other standard interventions (e.g. pelvic floor exercises, education, lifestyle modification [84]). At times, studies applied biofeedback together with additional physical modalities. Besides, patients were often under medication during the study period for symptom control. Therefore, the single effect of biofeedback intervention is difficult to extract. Biofeedback protocols are difficult to compare between institutions as treatment protocols, biofeedback devices and training amount varied considerably.
Most studies compare improvements within an intervention group which reduces the strength of evidence. Most authors drew their conclusions based on the statistical significance, only two papers [45,48] reported on effect sizes and confidence intervals of pain and symptom outcomes. Ten studies [21,26,29,35,36,[40][41][42][43][44] provided data to calculate effect sizes and confidence intervals to evaluate the clinical relevance of the results [85]. The majority of the studies did not perform a post hoc analysis or a correction S32 The effect of biofeedback interventions on pain, overall symptoms, quality of life and physiological. . .    [61,63]; criteria for determining effect sizes not calculated by the authors are stated in the respective studies; effect sizes and confidence intervals which were calculated by the review authors are not marked, those effect sizes and confidence intervals that are stated in respective paper are marked with ( c ) BFB biofeedback, CG control group, CI confidence interval, IG intervention group, MD mean difference, n.a. data not available, NIH-CPSI National Institutes of Health Chronic Prostatitis Symptom Index [66], NMES neuromuscular electrical stimulation, PEMF pulsed electromagnetic field therapy, QoL quality of life, SF-12 Short Form-12 [68], SF-36 Short Form 36 [67], UCPPS Urological Chronic Pelvic Pain Syndrome, VAS visual analog scale; wk/wks week(s) a t-test, b Wilcoxon for multiple testing. Some studies used nonvalidated outcomes to evaluate pain and overall improvement. The impossibility to fulfil certain quality requirements such as blinding of participants or the administration of placebo treatment, which are standard in pharmacological studies, is immanent to the biofeedback training method and setting.

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Given these limitations, the statements that were drawn conducting this review should be understood as tentative evidence and should be considered with caution. Three RCTs of above-average quality with respect to sample size, study design, and reporting [16,21,45] are given special attention in the subsequent discussion.

Efficacy of biofeedback in certain phenotypes and existing recommendations of guidelines
For anorectal disorders, such as dyssynergic defecation and levator ani syndrome, guidelines exist that state that biofeedback is the preferred treatment for chronic anal pain syndrome (level of evidence IA), [2] and is considered useful in the short-term treatment of levator ani syndrome with dyssynergic defecation (level of evidence IIB) [83]. Biofeedback is recommended for the short-term and long-term treatment of constipation with dyssynergic defecation (level of evidence IA), which is the most common defecation disorder, affecting about 40% of patients with chronic constipation [83]. Biofeedback seems to benefit patients with dyssynergic defecation above other types of constipation [24,28,83,86]. In PF dyssynergia, a landmark trial by Chiarioni et al. [21] found biofeedback to be superior to laxatives (polyethylene glycol), two other RCTs [87,88] (not considered in this review) also considered it superior to alternative treatments (diazepam), placebo, sham feedback and standard treatment [86]. The pathophysiology of levator ani syndrome seems to be similar to that of dyssynergic defecation, thus similar techniques and protocols have been used [16]. Both EMG and pressurebased biofeedback therapy protocols appear to be efficacious in restoring a normal pattern of defecation, but larger comparative trials are lacking [83]. Surface EMG probes are cheaper, more durable and usually provide a one or two-channel display [83]. Manometric systems are more expensive, have a multiple channel display and can facilitate rectoanal coordination and sensory training because they have a balloon and rectal sensor [83,86].
In patients with vulvar vestibulitis syndrome (vulvodynia, dyspareunia), preliminary evidence has suggested that altered muscle abnormalities (as shown by altered EMG activity such as elevated resting activity, reduced muscle contraction strength, muscle instability) are present and EMG biofeedback muscle rehabilitation, therefore, is beneficial [1,38]. According to Mariani, biofeedback should be used as a firstline treatment in moderate to severe vulvar vestibulitis (together with antidepressants and psychological counseling) [89]. Two uncontrolled studies by Glazer et al. and McKay at al. using portable EMG biofeedback devices showed promising results with this indication [38,39]. Bergeron       review) applied the home-based Glazer protocol in an RCT design. They confirmed that EMG biofeedback as well as cognitive-behavioral therapy and vestibulectomy, could improve sexual function and reduce pain (greatest pain reduction in the vestibulectomy group [1,90,91]) in the short and long term.

Pros and cons of biofeedback in pelvic pain conditions and criteria to improve treatment success
Biofeedback is a safe method, which has not shown any significant adverse effects. This might make biofeedback an attractive treatment option even in indications with a smaller success rate. As biofeedback is a labor-intensive approach [83] and quite time-consuming for both therapist and patient, it is important to preselect those patients who have a high chance of benefitting from the intervention. The use of biofeedback to treat pelvic pain is based on the idea that these pain conditions may result from, or are associated with, pelvic floor muscle dysfunction. Digital palpation of pelvic floor muscles should be integrated into routine examination to identify myofascial pain as a primary or contributing source of pelvic pain condition [2,8]. In anorectal pain conditions, tenderness on rectal examination has shown to be a valid criterion of treatment success [16]. Shoskes et al. identified and grouped six clinical phenotypes (urinary, psychosocial, organ-specific, infection, neurologic, tenderness of skeletal muscles) in the UPOINT classification in patients with urologic CPPS [2,92,93]. This classification was implemented to help direction therapy according to phenotypes, thereby improving outcomes [93]. Thus, patients with a musculoskeletal phenotype can be selected who most likely benefit from biofeedback interventions.

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Another criterion of success might be a center's capacities to administer a certain amount of training sessions and the patient's willingness to complete the course of therapy as suggested by the therapist [19]. In patients with chronic constipation and dyssynergic defecation, consensus guidelines on biofeedback therapy [83] recommend 4-6 biofeedback sessions to manage dyssynergic defecation accordingly: 3 sessions [31] achieved a symptom improvement of only 45.3% compared to e.g. 80% achieved by 5 sessions in Chiarioni et al. [21], hence following existing consensus recommendations improves outcome. As biofeedback requires commitment on the patient's part to take responsibility for their own health, the patient's motivation and adequate encouragement to complete the course of therapy through the therapist are other important requirements for the therapeutic success [29,36]. Cognitive impairment in the older population might lead to slower learning and the need for a higher number of treatment sessions [30]. Medical staff should be capable of demonstrating and explaining the method according to the patient's comprehension and education levels [36]. Similarly, counteracting problems of comprehension by using appro- Included priate explanations and psychological approaches are important in the work with children [54]. The effectiveness of pelvic floor biofeedback training also depends in part on the skills and experience of the biofeedback therapist and the particular techniques that are used to perform the training [28], which is why it is recommended to follow existing consensus guidelines [83].
As the access to biofeedback remains limited in many areas [31] and only a few centers offer biofeedback therapy, home-based self-training program is desirable and is a promising approach in anorectal and gynecologic (vulvar vestibulitis syndrome) disorders [38,39,83], at least to continue training after initial training at a center.
With somatoform disorders and related syndromes, the etiology is still not fully understood but evidence supports an interaction of physiological, psychological and interpersonal factors [1]. Therefore, a multimodal treatment strategy can be promoted, using biofeedback, relaxation training and stress management to address physiological and emotional arousal as well as cognitive techniques, psychoeducation and attention training to alter cognitive-perceptual factors, a modification of illness behavior and graded McMaster CRF: 15 items; total score = 14; study design item does not contribute to total score; codes: yes = 1, no = 0, N/A (not applicable) = 0, N.add not addressed (no information provided in the study) = 0 [5]; a not addressed in respective paper, but at least one main outcome tool was judged to be valid/reliable by review authors (described in the literature) Total score: higher scores indicate higher methodological quality, resulting in a possible total score of 14 points. Table 9 Methodological quality assessment, evaluating the included randomized controlled trials (n = 9): Physiotherapy Evidence Database (PEDro) scale [81] activity [1]. Multidisciplinary management, which is a common approach to many chronic conditions, is still not commonly available in gynecology because of cost factors and limited availability of interested specialists [7]. Yet multidisciplinary, multimodal and phenotype-oriented approaches have been increasingly proposed to deal with gynecologic phenotypes, such as provoked vestibulodynia and myofascial pain as well as with other chronic pelvic pain conditions such as bladder or prostate pain syndrome [2,8,90,93]. Hence, biofeedback is not a complementary or alternative but an additive method for both diagnostic and therapeutic purposes. It should be used in addition to standard care, based on a state of the art concept, if the physician in charge gives the indication.

Limitations of the present review
The authors decided to include any quantitative study type of primary research to present a comprehensive overview of the current literature. This reduces the methodological quality of the trials and thereby the significance of the results.
The search term "pelvic pain" is wide-ranging, yet there are many terms used in literature to describe pain syndromes which are perceived in a certain organ [94] and specific pathologies that cause pelvic pain. Therefore, our pragmatic and generalized search strategy carries the risk of missing relevant articles. Studies evaluating biofeedback on constipated patients were included, yet constipation was not the primary focus of this paper as the pain component is not paramount; however, this phenotype has been researched in depth, and our search term did not reveal all relevant studies available in the literature. As with the phenotype of dyspareunia, the reader is referred to the respective relevant literature [14,83,86,90].
Due to language restrictions, studies that would have otherwise fulfilled the inclusion criteria could not be included.

Conclusion
Several landmark studies demonstrated the efficacy of biofeedback for anorectal disorders. For other phenotypes of chronic pelvic pain, there is tentative evidence that biofeedback-assisted training interventions can improve the outcomes on pain, overall symptoms, and quality of life. Clinical improvements came along with improvements in certain physiological parameters in several studies. Many trials were characterized by methodological limitations, such as a very small sample size, nonvalidated outcomes and S44 The effect of biofeedback interventions on pain, overall symptoms, quality of life and physiological. . . K review article a lack of control group. The preliminary positive findings should be investigated further in robust and well-designed randomized controlled trials. Certain factors have been identified that might be relevant for improving biofeedback treatment success.

Implications for future research
Future studies should aim to: conduct a systematic literature review using MeSH terms that more thoroughly evaluate the effect of biofeedback therapy in a certain phenotype (e.g. anorectal disorders, urological chronic pelvic pain syndrome, bladder pain syndrome, gynecologic pelvic pain conditions); list the term "pelvic pain" in the keywords of studies on certain pelvic pain subtypes so that these trials are detected by a literature search on the umbrella term (as chronic pelvic pain comprises many phenotypes); improve the quality of future studies, e.g. by choosing an RCT study design that is based on a sample size calculation, performing a post hoc analysis or a correction for multiple testing; report on the effect size and an estimate of their precision such as the confidence interval to describe the clinical relevance of results; conduct future trials with more homogeneous outcome assessment (to allow future meta-analysis). Ideally, validated questionnaires or pain scales should be used to measure outcome. For stating success rates, an international consensus on the graduation of these rating scales would be beneficial to standardize outcome and improve comparability between study results; continue to evaluate the optimum type and extent of biofeedback interventions for the certain phenotypes; evaluate changes in quality of life and psychological parameters, such as anxiety and depression (as psychological disorders are common comorbidities in patients with chronic pain conditions). Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permis-sion directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.