No! I was not born then… I’m much older than that. No, in this longitudinal cohort study, participants recruited from the general public in Sweden were surveyed five times: at inception in 1995, again in 1998, 2003, 2007 and 2016. The article I’m reporting on included all respondents who had completed information on at least 3 of the 5 time points, a total of 1858 people! That’s a decent-sized study.
Longitudinal studies are really hard to do but offer us so much information about what happens over time to a group of people and it’s something we need to do more often. The problems with longitudinal studies are that people drop out, get lost to the researchers, they cost a lot to run, and research changes over time so the research questions may change, the measurement instruments may change, and it’s difficult to predict the variables that might be relevant at the beginning of the study. So, hats off to Aili, Campbell, Michaleff, Strauss, Jordan, Bremander et al (2021) who carried out this study!
Like many of these longitudinal studies, the authors developed an analysis in which groups of people who share similar characteristics in their persistent pain were identified. By following these groups over time, researchers are able to identify who changes, who doesn’t, and to identify some of the predictive factors that might put a person in the “at risk” group for developing increasingly poor outcomes. The “so what” factor for this type of study is that by identifying “at risk” people early on in their trajectory with pain, it may be possible to develop interventions that could help reduce this risk and that has to be both a human and economic gain for a community. In this instance, in order to interpret the findings, it’s important to remember that Sweden and other countries in the world don’t all look alike, have different healthcare systems, different genetic factors, and different social structures that can influence the process. So bear that in mind as I proceed.
The question asked about pain was this: “have you experienced pain lasting more than 3 months during the last 12 months?” Participants were then asked to indicate the location of the pain using a manikin with 18 predefined bodily regions in the musculoskeletal system; head and abdomen were not included (that latter one is a bit of a problem for me, tbh – given abdominal pain is a common experience for many women…). Participants were then identified as either having no chronic pain, or chronic widespread pain or chronic regional pain.
Sleep, health status, socio-economic status, treatment-seeking (for pain), lifestyle factors (alcohol, tobacco use), immigrant status and social support were also analysed.
Now for the statistics! Latent class growth analysis was used to identify common patterns or trajectories of pain over the time of the study. This analysis clusters participants according to their pain status over time, with each cluster representing a certain pattern of pain over time – participants were placed in a group where they had the highest probability of belonging based on their individual pattern of pain over time.
At the beginning of the study, nearly 13% of participants reported chronic widespread pain, with just over a quarter (25.3%) reporting chronic regional pain. Using the analysis and various confirmatory statistical processes, the authors identified a 4-cluster model, and a 5-cluster model. Both models included a group that had NO ongoing pain at all, as well as a group with ongoing widespread pain. The preferred model was the 4-cluster model, as this had four different but clinically meaningful patterns – persistent no pain; persistent chronic widespread pain; those moving from chronic regional pain to chronic widespread pain, and a change trajectory: eg no chronic pain to chronic regional pain or chronic widespread pain. The cluster identified as including those who typically migrated from NCP to CRP or CWP was the smallest (5% of the sample), and the least reliably detected.
The trajectories are really interesting: (1) 47% of people never reported persistent pain at all over the 21 years. 5% pf people reporting no pain initially, but then moved to chronic regional pain or chronic widespread pain over time. (2) 22% of people reported chronic pain initially, or moved from chronic pain to no pain over time – the authors argue that the cluster of people may have chronic regional pain initially and migrate between chronic regional pain and no pain over time. (3) 10% of the group moved from chronic regional pain to chronic widespread pain and this grew more likely over time. (4) The final group were a small group (6%) of people who continued to report chronic widespread pain over the whole 21 years
The predictors for those in group (3) were being female, seeking care for pain over the preceding 12 months, lack of social support, poor physical function, poor vitality, and poor mental health. Being a manual worker nearly made the cut, and in an age-adjusted analysis, did for the group moving to widespread pain. Age (middle-age!), poor sleep, smoking and being an immigrant also tended towards having a poorer prognosis.
What on earth does this mean?
Well, one exciting thing is that a large group of people never experience persistent pain. This might imply that these people are generally more healthy, and it can also help explain why some people living with pain feel so alienated from the rest of their community. If most people around you don’t develop pain, then they most likely don’t understand what it might be like to live with pain 24/7.
When we look at the factors that predicted moving from no pain to chronic pain, there were several factors that can be modified to reduce the risk: sleep problems, poor physical and mental health, poor vitality, seeking treatment, and limited social support. Perhaps by addressing these factors, some people might find their move towards greater pain could be reversed, or at least halted. My question is, however, whether treatment seeking might reflect the fact the person wasn’t feel great either physically or mentally, and for some people, being unwell might mean withdrawing from social interaction, so it’s not entirely simple to interpret.
The authors point out that “The highest risk of belonging to the group developing CRP or CWP is seen in age groups below 50 years. Previously published figures from baseline data in this study show a prevalence of over 20% of chronic pain already among 20 to 29 year olds. The overall image is that long-term patterns of pain are already becoming established by the time adulthood is reached, and rising age in adulthood increases the likelihood of stable patterns over time.” [italics mine] They also point out that women who develop pain have a greater risk for developing long-term and increasingly widespread pain. Other important factors for this group were those who were older, had poor sleep, worked in a manual job, drank less alcohol, were immigrants, had less education, generally poor health, more care seeking and low social support.
To me, this study shows the impact of living on the fringes of our society – the women (in particular) who work manual jobs like cleaning, who left school early, perhaps moved to a new country and have few friends, and were probably quite stressed – leading to poor sleep, and poorer health, with consequent treatment seeking. What are we doing to help these women? Here in New Zealand, we can probably add Māori, Pasifika, refugees, and people who don’t speak English terribly well. How well are our current pain management services working for these people?
Aili, K., Campbell, P., Michaleff, Z. A., Strauss, V. Y., Jordan, K. P., Bremander, A., Croft, P., & Bergman, S. (2021, May 1). Long-term trajectories of chronic musculoskeletal pain: a 21-year prospective cohort latent class analysis. Pain, 162(5), 1511-1520. https://doi.org/10.1097/j.pain.0000000000002137