The epidemiology of chronic pain after inguinal hernia surgery is well documented. Indeed, the volume of literature reporting CPSP after hernia surgery has increased dramatically in the last two decades. Many studies have been specifically designed to investigate persistent pain as an outcome rather than the traditional outcomes of hernia recurrence, wound infection, or return to work. Guidance on laparoscopic and open hernia surgery published by the UK National Institute for Clinical Excellence (NICE) acknowledged chronic pain as a common postoperative adverse event and recommended that future studies assess persistent pain along with recurrence and other adverse outcomes.59
Two systematic reviews of the epidemiology of chronic pain after inguinal herniorrhaphy have been published since 2003. First, Poobalan et al.7 reviewed data on chronic pain from 40 experimental and epidemiological studies published up to the year 2000. Using strict inclusion criteria, with chronic pain defined as that at or beyond three months, 40 of 101 potential studies were eligible for inclusion. The frequency of chronic pain after herniorrhaphy ranged from 0 to 53 percent depending upon timing and method of follow up, with moderate to severe pain in about 10 percent of patients. An updated systematic review only included studies published between 2000 and 2004, with sample sizes of at least 100 patients and a rigorous definition of chronicity of pain at or beyond six months of surgery.60 Despite these strict limiting criteria, the search still yielded over 100 potential studies, although many were rejected because of lack of follow up beyond six months. Chronic pain was the primary outcome in 16 studies. Both reviews found higher pain prevalence rates where chronic pain was the primary outcome of interest, with rates being three times higher compared to studies where pain was the secondary out-come.7,60 These systematic reviews indicate the increase in volume of literature, from both epidemiological and experimental studies.
Hernia surgery provides a good model for studying the mechanisms of chronic pain, given it is a common elective procedure for a benign condition in a relatively healthy and active population. The condition can be uncomplicated by extensive comorbidity or by confounding treatment variables, such as chemotherapy and radiotherapy. It is a common surgical procedure, with approximately 70,000 procedures being conducted in England per annum and 700,000 in the USA.59,61 Therefore, the potential burden of illness is high given the population exposed to surgery and given that 10 percent will be expected to develop chronic pain after surgery.
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