The transition from acute to chronic post-surgical pain – A prospective cohort study

Background: Chronic pain is pain which persists beyond the normal physiological healing timeframe. It affects up to 20% of the Australian and New Zealand populations. Chronic pain is also costly, not only in terms of healthcare costs, but also in lost work days and decreased productivity. Surgery is one of the leading causes of chronic pain, highlighting the need to understand and prevent the transition from acute to chronic post-surgical pain. This study has documented the onset and development of subacute pain and the development of chronic pain. The undertaking of this study may have added to the arsenal of tools used to predict which patients will go on to develop chronic pain. By predicting chronic pain development in advance, interventions may be developed to prevent the development of persistent post-surgical neuropathic pain. Considering the rising costs of healthcare, and the growing socioeconomic burden of chronic pain, strategies to prevent chronic post-surgical pain need to be carefully considered. Hypothesis: That one or more of the proposed factors measured perioperatively will predict continued pain at six weeks and three months after gynaecological surgery. Aims: 1) To determine the prevalence of acute persistent pain at 6 weeks following surgery 2) To determine the prevalence of chronic pain at 3 months following surgery 3) To determine the extent to which (if at all) the perioperative factors measured predict continued pain at six weeks and three months. Methods: Patients were assessed at the Christchurch Women’s Hospital Pre-Admission clinic and approached for written informed consent. Those agreeing to participate were given validated questionnaires to complete in order to document their physical, emotional and functional state pre-surgery. In addition a cold pressor test was carried out to determine pain threshold and tolerance. The cold pressor test involves placing a hand in cold water containing ice. The pain slowly builds until the participant can bear it no longer, at which point it is voluntarily removed from the stimulus. Intraoperative factors (anaesthetic techniques, surgical techniques, and analgesic use) were measured, as well as patient-controlled analgesia (PCA) use, and medications prescribed on discharge. At 6 weeks, participants were telephoned, and validated questionnaires completed. At 3 months, all participants were again telephoned, and validated questionnaires were completed. Results and conclusions: Of the 54 participants 15.7% were deemed to be experiencing significant pain at 6 weeks post-operatively; 8.2% of participants were deemed to be experiencing significant pain at 3 months postoperatively. The psychometric questionnaires used often found differences between those experiencing pain and those not experiencing pain at given observation points, but only the Brief Illness Perception Questionnaire (BIPQ) appears to be predictive of developing prolonged postoperative pain. The mean difference (7.4 on a 0-50) scale may even be enough to see it used clinically alongside other predictive measures. Many of the demographic factors correlated with the experience of pain at a given time point(s), but none were predictive of the development of prolonged pain to significant levels. The cold pressor test did not show any significant differences between those in pain at baseline, and those that are not in pain at baseline. However, pain threshold as measured by this test was shown to predict prolonged pain outcomes. Pain tolerance and pain endurance followed the same trend, but were not statistically significant. No surgical approach or group was significantly more likely to develop a prolonged pain state than the others. However, it did seem as though laparotomy may be associated with poorer pain outcomes, which is supported by the literature. Intraoperatively, those who would later develop prolonged pain states were less likely to receive opioids, but possibly likely to receive greater doses if they did. Perhaps worryingly, those who would later develop prolonged pain states were significantly more likely to receive tramadol intraoperatively. There were no statistically significant trends in postoperative inpatient or discharge prescribing, however non-significant trends were noted across both in terms of prescription of specific non-steroidal anti-inflammatory drugs (NSAIDs) and weak opioids. Introduction to this thesis: Chapters One, Two, and Four of this thesis review the relevant literature for the following: the pathophysiology and epidemiology of pain (Chapter One); the tools currently available for the management of pain (Chapter Two); the tools used in this study to measure pain; and the tools used in this study to measure pain related behaviour (Chapter Four). Chapter Three gives a brief overview of the relevant aspects of the gynaecological surgical types used in this study. Chapter Five of this thesis outlines how the protocol for this study was designed and tested, while Chapter Six outlines the hypothesis, specific aims, and methods used in data collection and analysis. Chapter Seven details the results discovered using the aforementioned methods. Chapter Eight discusses the results found in Chapter Seven, the limitations and strengths of the present study, and draws conclusions based on these. From this, Chapter Eight makes appropriate recommendations for future research and notes potential influences on clinical practice