1. Descriptive epidemiology of chronic pain
Epidemiological studies have demonstrated that, within the past month, around half us will have experienced an episode of pain which has lasted at least 1 day, with the most common sites reported, in a United Kingdom population study, being the low back (30%), hip (25%), neck and shoulder (25%) and knee (24%). Using a more stringent definition (suffered pain for 6 months, experienced pain in the last month and several times during the last week), a pan-European study reported a prevalence of 19%.
Prevalence of chronic pain increases through adult life, reaching a peak around the seventh decade (eg, Ref. 5). Pain at some regional sites, particularly in the lower limb, increase in prevalence across the age range (eg, Ref. 12), whereas some such as low back pain decrease at older ages. Such a decrease has been hypothesised to be related to changes in pain perception, expectation of pain at older ages and comorbidities. Chronic pain may be related to premature mortality; a review of studies showed a relationship with cancer and cardiovascular death which may partly be related to lifestyle factors such as low levels of physical activity and poor quality diet.
2. Early life factors in the aetiology of chronic pain
Early life factors have been linked to chronic pain in adulthood. Although retrospective studies are subject to recall bias, which has been shown to occur, long-term prospective studies demonstrate an increased risk of chronic pain in adulthood related to social environment in childhood (raised in care, death of a parent) as well as physically traumatic events such as premature birth, very low birth weight, and hospitalisation for a motor vehicle accident at young ages.
3. Mechanisms mediating the relationship between early life factors and adult chronic pain
Clinical studies have demonstrated that physical stresses (eg, preterm birth) result in altered function of the stress-response axis and such perturbations are still present at age 18 months. Amongst preterm infants, neonatal procedure-related stress predicted cortisol levels at age 7 years. This gave rise to the hypothesis that among persons experiencing “stressful” early life events, altered function of the Hypothalamic–Pituitary–Adrenal axis may be an important mediator of pain onset, and this has been confirmed in a prospective population-based study. Risk is likely to involve both genetic and environmental effects although it is likely that many genes, all with small effects, will be involved.
4. Predicting outcome of an episode pain
A key issue for epidemiologists is identifying, amongst those with a new episode of pain, in whom the pain is likely to become chronic. Focussing early management on these people is likely to optimise cost effectiveness of management approaches. A review of factors preceding transition from acute to chronic pain identified clinical factors, older age, and mood as the factors with strongest evidence for long-term disability or work absence, and tools such as the Orebro Screening Questionnaire seek to capture such factors (and wider psychosocial factors) in terms of predicting risk of poor outcome.