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Eur Surg Res. 2016 August 27; 58(1-2): 1–19.
PMCID: PMC5296927

Uniformity of Chronic Pain Assessment after Inguinal Hernia Repair: A Critical Review of the Literature

Abstract

Background

Chronic postoperative inguinal pain (CPIP) is the most common long-term complication of inguinal hernia repair. As such procedures are routinely performed, CPIP can be considered a significant burden to global health care. Therefore, adequate preventative measures relevant to surgical practice are investigated. However, as no gold standard research approach is currently available, study and outcome measures differ between studies. The current review aims to provide a qualitative analysis of the literature to seek out if outcomes of CPIP are valid and comparable, facilitating recommendations on the best approach to preventing CPIP.

Methods

A systematic review of recent studies investigating CPIP was performed, comprising studies published in 2007-2015. Study designs were analyzed regarding the CPIP definitions applied, the use of validated instruments, the availability of a baseline score, and the existence of a minimal follow-up of 12 months.

Results

Eighty eligible studies were included. In 48 studies, 22 different definitions of CPIP were identified, of which the definition provided by the International Association for the Study of Pain was applied most often. Of the studies included, 53 (66%) used 33 different validated instruments to quantify CPIP. There were 32 studies (40%) that assessed both pain intensity (PI) and quality of life (QOL) with validated tools, 41% and 4% had a validated assessment of only PI or QOL, respectively, and 15% lacked a validated assessment. The visual analog scale and the Short Form 36 (SF36) were most commonly used for measuring PI (73%) and QOL (19%). Assessment of CPIP was unclear in 15% of the studies included. A baseline score was assessed in 45% of the studies, and 75% had a follow-up of at least 12 months.

Conclusion

The current literature addressing CPIP after inguinal hernia repair has a variable degree of quality and lacks uniformity in outcome measures. Proper comparison of the study results to provide conclusive recommendations for preventive measures against CPIP therefore remains difficult. These findings reaffirm the need for a uniform and validated assessment with uniform reporting of outcomes to improve the burden that CPIP poses to a significant surgical patient population.

Key Words: Hernia, Chronic pain, Quality of life, Uniform assessment, Outcome definition

Introduction

Chronic postoperative inguinal pain (CPIP) is the most common complication following inguinal hernia repair, occurring in roughly 20% of patients [1,2]. As inguinal hernia repair is a routinely performed surgical procedure, the frequent occurrence of CPIP constitutes a significant burden on surgical care [3]. As a result, CPIP has provided a strong incentive to optimize preventive and therapeutic strategies, yielding a large number of investigative studies over the recent decades. However, subsequent reviews have been faced with significant heterogeneity in study methods and outcomes. The heterogeneity of the available studies is largely attributable to the varying application of definitions of CPIP, the different timing of postoperative assessment utilizing different measurements, and the lack of standardized reporting of outcome results. Such heterogeneous data may be considered insufficient as a basis for consensus, which needs uniform and validated study designs to ensure the adequacy of the scientific evidence for clinical decision-making [4]. As a solution, the Initiative on Methods, Measurement, and Pain Assessment in Clinical Trials (IMMPACT) working group [5] and the International Association for the Study of Pain (IASP) [6] recommended core outcome domains to be considered in the development of studies investigating CPIP. These core domains comprise pain intensity (PI), consequences of chronic pain on physical and emotional functioning, and participants' rating of overall improvement. In addition, these core outcomes should be measured prospectively over a minimal follow-up of 1 year using 2 or more standardized assessment tools.

Furthermore, the National Institute for Health and Clinical Excellence (NICE) has emphasized the importance of utilizing prospective study designs to address these core outcome domains, in order to standardize definitions and assessment methods of pain [7]. The aim of the current review is to investigate whether the recommendations made in 2005 by the IMMPACT, IASP, and NICE have led to improved uniformity and quality in the design of studies focusing on CPIP to a degree that allows the formation of conclusive recommendations to reduce the onset of CPIP.

Methods

Search Strategy

The literature search was performed using several databases, which were MEDLINE in PubMed, Embase, and the Cochrane Library. The following MeSH terms were combined: ‘hernia, inguinal’, ‘chronic pain’, ‘herniorrhaphy’, and ‘Lichtenstein’. To ensure that the search yielded a complete overview of the current literature, the MeSH terms were used in conjunction with free-text word combinations, as this search strategy would also cover papers without appropriate MeSH terms and papers not yet fitted with MeSH terms. The search was restricted to articles published in the English language from 2007 to 2015 to obtain the most recent studies that were relevant to the aim of this review.

Inclusion Criteria

Studies

Prospective studies and study protocols with the Lichtenstein method as the referring technique were included, irrespective of the application or method of randomization. Also, to suit the purpose of this review, studies were included regardless of their sample size, publication status, and whether it concerned single- or multi-center studies.

Patients

As this study focused on the adult patient population, all patients aged 18 years and above were included. All types of hernia (primary or recurrent, uni- or bilateral) that were investigated were included for both adult male and female populations to ensure the broad applicability of our results in a large and diverse patient population in clinical practice.

Interventions

Correction of an inguinal hernia occurred, irrespective of the surgical technique.

Outcomes

CPIP was among the primary or secondary outcomes.

Review Process

The review process was performed in two steps. First, all abstracts were assessed according to the eligibility criteria, consulting the full-text papers in case of doubt about whether the study met these criteria. Next, all full-text papers of the selected abstracts were read and analyzed in full to make a final decision about their inclusion.

Outcomes of Interest

According to the recommendations of the IMMPACT, IASP, and NICE, all included studies were scored for the presence of:

  • 1.
    a formal definition of CPIP;
  • 2.
    validated measurement of both PI and the effects of CPIP on daily functioning or quality of life (QOL);
  • 3.
    a duration of follow-up of at least 12 months;
  • 4.
    a baseline score: preoperative measurement of PI and QOL.

One point was assigned to a study for the availability of each of the abovementioned aspects, so that each study was assigned an overall methodological quality score ranging from 0 to 4.

Results

Using the strategy described above, the search yielded 234 hits (see the PRISMA flowchart in fig. fig.1).1). After applying the search limitations, 109 articles remained eligible for inclusion. Following a critical review of the full texts of these articles, 29 articles were excluded for not meeting the inclusion criteria. The reasons for exclusion at this stage were a retrospective study design, the article being either a review or a comment, or CPIP not being among the primary or secondary outcomes. Also, studies reporting the long-term follow-up results of another included study were considered redundant and were therefore excluded.

Fig. 1
PRISMA flowchart. Overview of the literature search.

Following the critical review, 80 studies fitted the eligibility criteria. Among these studies, 52 articles described RCTs and 38 studies had a comparative study design (table (table1).1). Most studies investigated the Lichtenstein technique using different meshes (n = 10), fixation methods (ProGrip mesh: n = 13; glue: n = 10), analgesics (n = 3), and methods of nerve handling (n = 5). Other studies compared the Lichtenstein technique with pre- or retroperitoneal mesh placement: total extraperitoneal repair (n = 12), Prolene Hernia System repair (n = 4), plug and patch (n = 4), Kugel (n = 2), transinguinal preperitoneal repair (n = 1), and transabdominal preperitoneal repair (n = 5). Seven studies compared Lichtenstein hernioplasty with non-mesh techniques: Moloney's darn repair (n = 2), Shouldice's repair (n = 1), the Desarda technique (n = 1), and suture repair (n = 2). In 55 studies, CPIP was the primary outcome measure; in the remaining studies, CPIP was among the secondary outcome measures.

Table 1
Included studies: study characteristics and methodological quality scores

Definition of CPIP

A definition of CPIP was lacking in 31 (39%) of the studies (table (table2)2) [8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38]. In the other 49 studies (61%), a total of 22 different definitions of CPIP were identified. Almost half (n = 23) of these studies applied the definition provided by the IASP, which is ‘chronic pain is pain that persists beyond three months post-operatively’ [39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62]. The remaining half (n = 26) used multiple definitions of CPIP, which can be categorized and summarized as follows. First, there was heterogeneity in the postoperative time period after which pain was classified as chronic. This ‘chronic’ time frame ranged between 1 and 36 months [63,64,65,66,67]. Second, some studies included the quantitative factor PI in their definition of CPIP, which was either expressed using descriptive terms [68,69,70,71,72,73,74,75], a visual analog scale (VAS) score, or a QOL score [31,76,77,78,79,80,81,82,83,84,85].

Table 2
Overview of the different definitions of CPIP used in the included studies (n = 49; 61%)

In addition to incorporating a definition of CPIP, 31 studies (39%) provided a categorization of pain severity (table (table3).3). In half of the studies, the categorization consisted of reporting the effect of CPIP on daily life using 9 different validated or nonvalidated criteria (table (table3)3) [33,40,48,52,58,61,62,68,73,79,86]. The remaining studies used a categorization of pain severity based on VAS or numerical analog scale measurements [8,10,19,22,23,50,56,71,73,81,87,88,89]. The subsequent categorization of PI was highly heterogeneous (fig. (fig.2).2). Some studies incorporated a minimal PI score to distinguish between clinically relevant or minor CPIP [14,31,71,74,76].

Fig. 2
Categories of CPIP based on VAS scores. Thirty-one studies (39%) provided definitions of the severity of CPIP (table (table3).3). Fifteen studies defined pain severity in terms of PI according to the score on a VAS or numerical analog scale. The ...
Table 3
Overview of the different definitions and categories of pain severity

Assessment of CPIP

Tool(s)

Fifty-three studies (66%) used only validated instruments for the assessment of CPIP. However, among these studies, 33 different validated instruments were identified (tables (tables4,4, ,5,5, ,6)6) [11,13,14,17,19,21,26,28,29,32,36,38,41,43,45,47,48,51,53,56,57,58,59,60,61,62,63,64,65,79,90].

Table 4
Number of studies that uses validated or nonvalidated assessment tools to measure CPIP
Table 5
Tools used to measure CPIP
Table 6
Tools used to assess QOL and/or PI

In 19 studies (24%), nonvalidated instruments were used [10,18,20,27,30,33,34,35,40,42,44,49,50,52,54,65,68,71,77,82]. The majority of these studies described these instrument using nonspecific phrases such as ‘a questionnaire was used’ or ‘patients were interviewed’. Of these studies, 15 utilized a nonvalidated instrument in conjunction with a VAS [10,30,33,34,35,40,42,44,50,52,71,77,82], verbal rating scale [49], inguinal pain questionnaire (IPQ) [54], or functional index score [40].

In 8 studies (10%) there was no information provided about how the data were collected [9,15,16,37,70,72,75].

Validated Assessment of both PI and QOL

Thirty-two studies (40%) had a validated assessment of both PI and QOL [11,14,19,20,22,24,29,31,34,38,40,41,43,45,48,53,54,56,58,60,62,65,67,73,74,76,84,86,87,88,89,90]; in 33 (41%) and 3 (4%) of the studies, respectively, there was only a validated assessment of PI [8,10,13,17,21,23,25,26,28,30,33,35,36,42,44,47,49,50,52,59,61,63,69,71,77,78,80,81,82,83,85] or QOL [46,51,57].

In 12 studies (15%) there was a validated assessment of neither PI nor QOL [9,15,16,18,27,37,55,68,70,72,75].

The majority of the studies investigating PI used VAS measurements, while QOL was most often examined using the Short Form 36 (SF36) questionnaire (tables (tables1,1, ,6).6). Of the instruments that incorporate the assessment of both PI and QOL, the IPQ was used most often.

Duration of Follow-Up of at Least 12 Months

The duration of follow-up ranged from 6 weeks to 96 months. Sixty studies (75%) had a follow-up of 12 months or longer, with a median of 12 months (table (table11).

Availability of a Baseline Score: Preoperative Measurement of PI and Its Consequences for Daily Functioning/QOL

A baseline score was assessed by 45% (36/80) of the studies included (table (table11).

Methodological Quality Score

The full amount of 4 points was scored by 11% of the studies; 26% scored 3 points, 38% scored 2 points, 23% scored 1 point, and 2% scored 0 points (table (table7).7). When comparing the periods of 2007-2010 and 2011 until today there is a significant improvement in methodological quality scores (p = 0.005). The best score was given for the criterion of having a minimum of 12 months of follow-up and the second best for the availability of a definition of CPIP; on the third place was the performance of a baseline measurement, and last a validated assessment of both PI and QOL.

Table 7
Methodological quality score

Discussion

The results from this review demonstrate that the current scientific literature investigating the management of CPIP after inguinal hernia repair is flawed due to a lack of adherence to standards in study methodology and tools as well as the nonexistence of commonly accepted definitions of the primary outcome. We found that although the majority of the studies provided similar definitions of CPIP, the variable interpretation of these definitions does not allow for adequate comparisons, opposes uniformity, and therefore obstructs evidence-based clinical decision-making. Similarly, despite the fact that the majority of the included studies did use a validated assessment tool to quantify CPIP, we found that a total of 33 different tools were used among these studies. Measurements of PI and QOL, which are both included in the recommendations of the IMMPACT and IASP, were performed using nonvalidated tools in a majority of the studies. Furthermore, the greater number of these studies provided no preoperative baseline measurements of CPIP, which clouds the interpretation of their outcome findings.

Despite the efforts put in by the scientific community, it appears that the current scientific literature about CPIP is heterogeneous to a degree that limits meta-analyses. Interestingly, the clinical relevance of this conclusion is not limited to the current state of scientific literature. Similar conclusions were drawn by Kehlet et al. [4], who stated that no proper recommendations for preventing or treating CPIP could be made based on the sparse scientific evidence available over a decade ago. Based on their findings, they issued a call for uniformity and provided recommendations for an optimal study design as a solution for the heterogeneity. A more recent study published in 2007 by van Hanswijck de Jonge et al. [91] concluded that measurements of pain and discomfort scores remained highly heterogeneous as studies evaluated CPIP by different types of instruments of varying quality and accuracy.

Most of the studies included reported CPIP as the primary outcome. At a fundamental scientific level, the primary outcome of a study is the outcome parameter to be measured and compared, either to the control group in a comparative study or to results from the literature in noncomparative studies. Such a comparison to the literature requires the unambiguous definition of the primary endpoint in order to provide conclusions of scientific and clinical value. To further enhance the comparability of scientific literature, it is of great importance to comply with standardized international definitions and to adhere to accepted categorizations of outcome measures. In the case of the current CPIP literature, we found that 39% of the studies lacked a definition of the primary outcome. When a definition was provided, it was often a nonstandardized one, as we were able to identity 22 different CPIP definitions across the remaining 49 studies (table (table2).2). The IASP definition of chronic pain was most frequently used, which states that ‘chronic pain is any pain that persists beyond the normal tissue healing time usually taken to be 3 months’ [39]. The other, nonstandardized definitions diverged with respect to duration, intensity, and severity. It appears that expert opinions differ regarding the cutoff points between acute and chronic pain. This might be expected, considering that the IASP also uses different definitions of chronic pain and persistent postsurgical pain, which is defined as ‘pain that develops after a surgical intervention and lasts at least two months excluding other causes for the pain’ [6]. Aasvang and Kehlet [92] argued that given the possibility of an ongoing inflammatory reaction to a prosthetic mesh, CPIP should be measured at least 6 months postoperatively. Others used a minimum duration of 12 months, based on an earlier article of Kehlet et al. [4].

The definition of CPIP provided by the IASP is based solely on a time factor, as it regards discomfort to be any pain with a VAS above 0. Alternative definitions incorporated a PI factor in their definition of CPIP. For example, such definitions state that a patient needs to express at least a VAS of 2 or 3 on a scale of 10 to be considered in pain. Others added descriptive terms of pain severity in their CPIP definition (table (table2),2), such as discomfort or pain happening once a fortnight, requiring painkillers, or interfering with daily activities. These different and seemingly arbitrary thresholds of severity and duration in the definitions of chronic pain influence incidence and prevalence rates when incorporated into epidemiological studies and hinder comparisons between studies. In a recently published international expert consensus article, CPIP is defined as ‘chronic inguinal post-operative pain that still exists and affects daily life six months post-operatively’ [93]. However, the HerniaSurg Group, working on the World Guidelines for Groin Hernia Management, is now proposing to modify the IASP definition in order to include only chronic pain that is present from 3 months after surgery and which lasts beyond 6 months after surgery.

To generate high-quality evidence for the best preventive and treatment strategies for CPIP, it is imperative to use validated scales. To further enhance the comparability of scientific studies, these scales should ideally be standardized and clearly described in a paper [94]. Since 33 different instruments could be identified among the included studies, it seems that consensus about the optimal instrument for the assessment of CPIP is still lacking.

Several pain assessment tools have been developed to measure different aspects of pain. PI is mostly measured using verbal rating scales, numerical rating scales, and VAS [95]. In this review, we found that the VAS was predominantly used (in 73% of the studies). However, these PI scales only provide a global estimation of a patient's experience of pain, without considering all the relevant aspects and consequences of chronic pain. To elaborate, chronic pain has a major impact on physical, emotional, and cognitive function. Furthermore, chronic pain can negatively affect patients’ social life and their ability to work and secure an income, which also has economic implications that extend beyond health care [96]. The importance of identifying the repercussions of chronic pain as perceived by a patient was demonstrated by Fredheim et al. [97]. They found that patients with non-cancer-related chronic pain reported a QOL that was lower than that of terminal cancer patients. The IMMPACT group [5] and Kehlet et al. [4] therefore emphasized that in order to perform a meaningful assessment of chronic pain, it is required that quantitative measurement tools are utilized in conjunction with multidimensional qualitative tools such as health-related QOL instruments to adequately assess the impact of chronic pain. The Medical Outcome Survey SF36 (MOS SF-36 or SF36) is generally considered to be the gold standard in QOL measurement. The advantage of the generic SF36 is its broad implementation as it is well known by regulatory bodies and physicians. In addition, the SF36 is suitable for comparing changes in QOL between different diseases and treatments. However, some authors including Heniford et al. [98] argue that a disease-specific QOL measure is preferable to assess the impact of CPIP on QOL and patient satisfaction. In this review, 4 hernia-specific QOL measures were identified among 8 studies: the Carolinas Comfort Scale (CCS) [76], the IPQ [48], the Activities Assessment Scale [53], and a questionnaire based on the Danish Hernia Database [99]. Some studies used rating scales such as the VAS to measure QOL [62]. There are also questionnaires available that incorporate the assessment of PI (sensory dimension) and the degree of interference of chronic pain with aspects of daily life (reactive dimension). Examples of such questionnaires are the general McGill Pain Questionnaire, the Short Health Scale, the Brief Pain Inventory [100], and the hernia-specific CCS and IPQ. Beside questionnaires, objective methods such as pain-evoked responses and quantitative sensory testing are gaining popularity but are not yet utilized on a regular basis. Deciding upon the appropriate questionnaire to use will likely remain challenging as long as consensus is lacking.

The majority of the studies reviewed lacked a baseline measurement of PI and QOL. This is unfortunate, as baseline measurements are necessary for a meaningful interpretation of postoperative results. Furthermore, preoperative pain is a known risk factor for developing CPIP and therefore holds clinical relevance that might be undermined when it is not incorporated into studies investigating CPIP [101].

To reiterate, the literature concerning the treatment and prevention of CPIP is highly heterogeneous and inconsistent. Since a consensus measure is the only way to bring about more standardized and comparable results, the CPIP literature will benefit from a common standard. This common standard should include one clear definition of the outcome measure CPIP, incorporating pain duration, PI, and the effects of chronic pain on daily activities. Also, a common study methodology is needed that uses well-defined standard outcome parameters which are evaluated with validated instruments and over a sufficient period of follow-up. Whether certain types of measurement tools should be recommended to further improve the uniformity among studies is open for discussion, for example by the HerniaSurge Group, who are currently designing a global guideline for the management of groin hernia. We recommend an easy-to-use, hernia-specific score incorporating assessments of both PI and QOL, such as the IPQ and CCS. Finally, baseline measurement should become common practice, and follow-ups should be done on standardized time points.

However, without an ambitious implementation plan designed to reach targeted groups, the impact of a common standard could be disappointing. Global recognition and awareness are essential and may be achieved via the worldwide hernia societies.

Disclosure Statement

There are no conflicts of interests and no funding sources to declare.

Acknowledgments

Thanks to Dr. S.F.A. Said for his critical review of the manuscript.

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