Do musculoskeletal conditions contribute to chronic non-musculoskeletal conditions? The authors of a new paper – inspired by chiropractic thinking, it seems – think so. Their meta-analysis was aimed to investigate whether the most common musculoskeletal conditions, namely neck or back pain or osteoarthritis of the knee or hip, contribute to the development of chronic disease.
The authors searched several electronic databases for cohort studies reporting adjusted estimates of the association between baseline neck or back pain or osteoarthritis of the knee or hip and subsequent diagnosis of a chronic disease (cardiovascular disease , cancer, diabetes, chronic respiratory disease or obesity).
There were 13 cohort studies following 3,086,612 people. In the primary meta-analysis of adjusted estimates, osteoarthritis (n= 8 studies) and back pain (n= 2) were the exposures and cardiovascular disease (n=8), cancer (n= 1) and diabetes (n= 1) were the outcomes. Pooled adjusted estimates from these 10 studies showed that people with a musculoskeletal condition have a 17% increase in the rate of developing a chronic disease compared to people without a musculoskeletal condition.
The authors concluded that musculoskeletal conditions may increase the risk of chronic disease. In particular, osteoarthritis appears to increase the risk of developing cardiovascular disease. Prevention and early
treatment of musculoskeletal conditions and targeting associated chronic disease risk factors in people with long
standing musculoskeletal conditions may play a role in preventing other chronic diseases. However, a greater
understanding about why musculoskeletal conditions may increase the risk of chronic disease is needed.
For the most part, this paper reads as if the authors are trying to establish a causal relationship between musculoskeletal problems and systemic diseases at all costs. Even their aim (to investigate whether the most common musculoskeletal conditions, namely neck or back pain or osteoarthritis of the knee or hip, contribute to the development of chronic disease) clearly points in that direction. And certainly, their conclusion that musculoskeletal conditions may increase the risk of chronic disease confirms this suspicion.
In their discussion, they do concede that causality is not proven: While our review question ultimately sought to assess a causal connection between common musculoskeletal conditions and chronic disease, we cannot draw strong conclusions due to poor adjustment, the analysis methods employed by the included studies, and a lack of studies investigating conditions other than OA and cardiovascular disease…We did not find studies that satisfied all of Bradford Hill’s suggested criteria for casual inference (e.g. none estimated dose–response effects) nor did we find studies that used contemporary causal inference methods for observational data (e.g. a structured identification approach for selection of confounding variables or assessment of the effects of unmeasured or residual confounders. As such, we are unable to infer a strong causal connection between musculoskeletal conditions and chronic diseases.
In all honesty, I would see this a little differently: If their review question ultimately sought to assess a causal connection between common musculoskeletal conditions and chronic disease, it was quite simply daft and unscientific. All they could ever hope is to establish associations. Whether these are causal or not is an entirely different issue which is not answerable on the basis of the data they searched for.
An example might make this clearer: people who have yellow stains on their 2nd and 3rd finger often get lung cancer. The yellow fingers are associated with cancer, yet the link is not causal. The association is due to the fact that smoking stains the fingers and causes cancer. What the authors of this new article seem to suggest is that, if we cut off the stained fingers of smokers, we might reduce the cancer risk. This is clearly silly to the extreme.
So, how might the association between musculoskeletal problems and systemic diseases come about? Of course, the authors might be correct and it might be causal. This would delight chiropractors because DD Palmer, their founding father, said that 95% of all diseases are caused by subluxation of the spine, the rest by subluxations of other joints. But there are several other and more likely explanations for this association. For instance, many people with a systemic disease might have had subclinical problems for years. These problems would prevent them from pursuing a healthy life-style which, in turn, resulted is musculoskeletal problems. If this is so, musculoskeletal conditions would not increase the risk of chronic disease, but chronic diseases would lead to musculoskeletal problems.
Don’t get me wrong, I am not claiming that this reverse causality is the truth; I am simply saying that it is one of several possibilities that need to be considered. The fact that the authors failed to do so, is remarkable and suggests that they were bent on demonstrating what they put in their conclusion. And that, to me, is an unfailing sign of poor science.