Natural course of long-term postherniorrhaphy pain in a population-based cohort

1 Introduction

Although the treatment of immediate postoperative pain has improved considerably in recent years, the problem of long-term pain remains a major problem. It has been found to be a very common, previously neglected, iatrogenic cause of reduced quality of life following thoracotomy (Kalso et al., 2001; Meyerson et al., 2001), breast surgery (Stevens et al., 1995) as well as hernia repair (Callesen et al., 1999; Fränneby et al., 2006). The pain is generally considered to be persistent if it lasts at least 2 months after surgery (MacRae, 2001, 2008), although the natural course varies considerably. Postherniorrhaphy pain has been explored in several previous studies (Bay-Nielsen et al., 2001; Hallén et al., 2008; Massaron et al., 2008; V Veen et al., 2007). There is a general consensus that persistent pain after hernia repair is a considerable problem, although the prevalence in these studies has varied from 1% to 32% depending on the criteria used for defining persistent pain (Bay-Nielsen et al., 2004; Condon, 2001; Poobalan et al., 2001; Vironen et al., 2006). More than 50% of those suffering from persistent postherniorrhaphy pain have been found to have neuropathic pain (Kalliomäki et al., 2009; Mikkelsen et al., 2004), which may be very resistant to all forms of therapy. Consolation for these patients would be that the pain may gradually decrease with time. Provision of such information, however, implies that we must have reliable knowledge of the natural course of persistent pain following hernia surgery. Although no mathematical model can provide a full understanding of such a complex process as the natural course of postoperative pain, assuming an exponential course may help to analyse the course the first years after surgery, enable comparisons with other studies and give a base for exploring factors that influence the duration of the postoperative pain.

The aim of the present study was to explore the natural course of pain persisting after hernia repair in a population-based cohort and identify risk factors for prolonged duration of pain.

2 Methods

3 Results

Altogether 2834 hernia repairs were performed 1998–2004 in the county of Uppsala. Two operations on the same side were recorded in 229 patients, 11 patients were operated three times, and 227 patients were operated bilaterally. A total of 2583 patients were thus operated. In the Swedish Population Register 162 patients who died after surgery were identified and excluded from the study. The remaining 2421 were sent the IPQ by mail in May 2005. Non-responders were sent a reminder in September the same year. A total of 1763 patients filled in and returned the questionnaire but 19 responders could not be traced back to the Hernia Register. Altogether 1744 answers were thus included in the final analyses. The final response rate reached 72.0%. The time interval from operation to questionnaire ranged from 6 months to 7 years. The cohort of responders consisted of 1611 (92.5%) males and 133 (7.5%) females, between the ages of 18 and 95 years (mean 61 years). Altogether 426 (24.4%) stated pain to some extent in the item “pain right now” and 519 (29.7%) in the item “worst pain last week”. The halving time for the prevalence of pain was found to be 6.7 years for the item “pain right now” and 6.8 years for the item “worst pain past week” (Fig. 1). The corresponding figures if laparoscopic repair was excluded were 6.4 years for “pain right now” and 6.4 years for “worst pain past week”. The sample was not sufficient enough to do the same estimate including only patients after laparoscopic repair. Spearman’s rank correlation test did not show any correlation between pain intensity in those with persisting pain and time elapsed since surgery. Choice of anaesthesia did not correlate with duration of pain (Fig. 2).

Figure 1

Prevalence of postoperative pain by time from surgery. Error bars indicate 95% confidence interval.

Figure 2

Prevalence of pain by method of anaesthesia. Error bars indicate 95% confidence interval.

In a multivariate analysis (Table 1), low age, postoperative complication and open method of repair were found to predict an increased risk for pain persistence exceeding 1 month in the question “When did the pain cease?”. The impact of these factors on the postoperative decline of pain is shown in Figs. 3,4,5.

4 Discussion

The halving time of 6.7–6.8 years found in our study shows that long-term pain is a common and severe problem following hernia surgery, although it is not permanent in all patients. Whereas efforts to treat persistent postoperative pain, in particular neuropathic pain, are often fruitless, this group can at least rely on the hope that the pain, for some of the patients, gradually decreases with time. Young age, open methods of repair and postoperative complications were found to be associated with prolonged pain persistence in the immediate postoperative period. The same factors have also been found to be associated with a higher prevalence of pain in a cross-sectional analysis (Kalliomäki et al., 2008).

Figure 3

Prevalence of pain by age. Error bars indicate 95% confidence interval.

Figure 4

Prevalence of pain by postoperative complications. Error bars indicate 95% confidence interval.

Figure 5

Prevalence of pain by method of repair. Error bars indicate 95% confidence interval.

In two repeated questionnaire studies from the Danish Hernia Database, it was found that the prevalence of postoperative pain decreased from 11% 1 year after surgery to 6% 6.5 years postoperatively (Aasvang et al., 2006). This yields a halving time of 6.3 years, i.e. very close to our findings. In the Swedish SMIL study, the prevalence of discomfort in the groin was found to decrease from 12.7% 1 year after TAPP to 8.5% 5 years postoperatively (Berndsen et al., 2007), corresponding to a halving time of 6.9 years. In the same study, patients randomised to Shouldice repair perceived discomfort in 13.9% cases 1 year after surgery and in 11.4% cases 5 years after surgery, i.e. a halving time of 14.0 years. A similar outcome for the laparoscopic group was also seen in a trial performed by the Medical Research Council Laparoscopic Groin Hernia Trial Group (Grant et al. (Grant et al., 2004)). They found that the prevalence of persistent pain decreased from 27.7% 1 year after surgery to 18.1% 5 years after surgery in the laparoscopic arm (halving time 6.5 years). Somewhat surprisingly, however, they found that it decreased from 35.6% to 20.1% in the open arm, i.e. a halving time of 4.8 years.

Whereas the halving time was relatively stable, regardless of method of repair, there were several factors affecting the time until the pain ceased when recalled retrospectively. As shown in Figs. 3,4,5, low age, occurrence of postoperative complications and open method of repair were each associated with a more protracted course. It has also been suggested that the method of anaesthesia could influence postoperative pain (Nienhuijs et al., 2008). Theoretically, the effect of local anaesthesia could persist postoperatively and lead to a faster decline of the pain than after general or regional anaesthesia (Nordin et al., 2003). However, we were not able to find such an association (Fig. 2).

As the patient group was assembled over a long period of time and the follow-up time was defined retrospectively, it cannot be ruled out that the association between the prevalence of pain and the time elapsed since surgery may be explained by changes in management of hernias during the period of the study. In particular, the introduction of laparoscopic methods of repair towards the end of the 1990s may be a major cause of confounding. The halving time, however, did not change considerably when patients operated upon with laparoscopic methods were excluded from the analysis. Nevertheless, there may be other sources of confounding during the period that may have biased the results.

Determining a halving time assumes an exponential course for pain persistence. Needless to say, this is a crude simplification of a complex biological and psychological process. There are several factors that influence the level of pain over time, making an assumption of a linear or exponential course very uncertain. An exponential model is only fully adequate if the percentage of patients experiencing a cease of the pain is constant over time, which is a very uncertain hypothesis. Furthermore, we do not know the course after the first 7 years. There may be group of patients with pain that does not decrease at all, no matter how long they are followed up. Assuming an exponential course in such a case would lead to underestimation of the percentage of patients with persisting pain after 7 years. The pain may also follow a more linear course, in which case the percentage with persisting pain would be overestimated after 7 years. The true course is probable dependent on a number of factors that vary with time, including chronic inflammation, nerve entrapment, nerve regeneration, psychological assimilation and decreased demands on physical performance with age. Each of these factors is unevenly distributed in the population, giving a varying influence during the course of pain development. Thus, a mathematical model cannot give a full description of the course, but only an approximation. Nevertheless, the halving time as found in this study gives an understanding of the long-term natural course of postoperative pain which corroborates with the findings of other studies with a few minor exceptions. Although persistent postoperative pain has so far been very treatment-resistant, in many cases it does eventually recede. On the other hand, 14% still suffered from various degrees of chronic pain 7 years after hernia surgery. Further studies are required to evaluate the prevalence of pain beyond that.