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A new study published in The American Journal of Medicine highlights another reason not to light up that cigarette — smokers (current and former) are more likely to suffer from low back pain than people who have never smoked . Although the association is moderate, it is strongest for chronic back pain and for adolescents.
By now, the vast majority of us know that smoking is bad for you. A number of health risks are associated with smoking. Indeed, many women are not aware that smoking is a risk factor for breast cancer . However, there are other conditions associated with smoking besides the key conditions of cancer and heart disease. Previous research has looked at the link between the experience of low back pain and the potential risk factor of smoking [3-6]. The experience of back pain is widespread . “Chronic” back pain is often of particular interest as it is associated with days lost from work and healthcare costs, in addition to the impact on the patient’s quality of life. In the UK, “persistent” back pain is that which has lasted more than 6 weeks . In the US, “chronic” back pain is pain lasting more than 3 months . The causes of back pain are often complex and unclear. In the present study, the association between back pain and smoking was assessed.
Often, the same topic is explored by multiple researchers who may find slightly different results when exploring the association between two things. In these cases, a systematic literature review is often helpful. This means that researchers locate, report, evaluate and collate results from all the identifiable studies that address the key question. A really useful tool then to make sense of research is the “meta-analysis”: the results from multiple studies are combined using statistical methods to come up with the strength of an association across many studies. In this case, researchers searched for studies that examined the link between smoking and back pain; 40 different research studies were included in their statistical analysis. The advantage of this method is clear then — we can put all the research together and find out what the general picture is. The researchers identified studies from all over the world to include in this meta-analysis.
The association between smoking and low back pain was found to be significant (not just caused by chance). The researchers used a statistic known as an “odds ratio” or “OR”. This statistic allows us to understand how big the association is between two things.
The OR for chronic back pain was 1.79 (95% Confidence interval = 1.27 — 2.50) for current smokers. As the OR is greater than 1 (and the confidence interval does not include 1) we can say there is a significant association between smoking and the experience of chronic back pain. In English, this means that a current smoker has around 1.79 times the odds of having chronic back pain than a non-smoker. A really interesting result of the study was that the link between smoking and back pain was strongest for chronic low back pain, which is the most disabling form of back pain. For pain in the last month, the last 12 months and chronic pain, former smokers had a lower prevalence of back pain. Thus, the association between having ever smoked and back pain is smaller than the association between still smoking and back pain.
So does this mean that smoking causes back pain? Well, no. This type of analysis cannot clearly say if something causes something else. Investigating and establishing cause is highly complex and all too often we attribute cause to things that are merely related. For example, one possibility is that people with back pain are more stressed, so they take up smoking. We could further explore this link by looking at time: if our sample of people report significantly more back pain after some of them take up smoking (on their own, of course, we could not encourage this!) but those who do not take up smoking do not report more back pain, the data would suggest that smoking is, in some way, causing back pain.
Generally speaking, we are not all that great at understanding probability and risk (and that appears to include many academic researchers). Several articles have been written that discuss the use and misuse of statistics like the “OR” . Odds ratios are far less informative in situations where the health condition is more common — as back pain is. Therefore, it is really important to be aware of some of the limitations of this sort of research when trying to understand health information. Media reports that state that “Murders in New York Caused by Higher Ice-Cream Sales” are much more likely to sell papers than the more accurate “As Ice-Cream Sales Increase, So Do the Number of Murders in New York” (a commonly used example in statistics classes). Being aware of these issues helps us to better understand what health research actually does identify. We can also investigate a “causal mechanism”: how would smoking cause back pain? Suggestions include the possibility that smoking increases pro-inflammatory substances in the body, that smoking affects back pain as it increases the risk of osteoporosis, and that smoking reduces blood circulation leading to disc damage, which then leads to pain. All of these claims need to be further researched and the degree to which they really contribute to the causes of back pain explored before we can confidently state that “smoking causes back pain”.
- Shiri et al. The association between smoking and low back pain: a meta-analysis. Am J Med. 2010 Jan;123(1):87.e7-35. DOI: 10.1016/j.amjmed.2009.05.028
- Peacey et al. Low levels of breast cancer risk awareness in young women: an international survey. Eur J Cancer. 2006 Oct;42(15):2585-9. Epub 2006 Jul 7.
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- Feldman et al. Smoking. A risk factor for development of low back pain in adolescents. Spine 1999 Dec 1;24(23):2492-6.
- Goldberg et al. A review of the association between cigarette smoking and the development of nonspecific back pain and related outcomes. Spine 2000 Apr 15;25(8):995-1014.
- Alkherayf and Agbi. Cigarette smoking and chronic low back pain in the adult population. Clin Invest Med. 2009 Oct 1;32(5):E360-7.
- Strine and Hootman. US national prevalence and correlates of low back and neck pain among adults. Arthritis Rheum. 2007 May 15;57(4):656-65.
- National Institute for Health and Clinical Excellence: Early management of persistent non-specific low back pain. NICE Clinical Guideline 88. 2009 May.
- Back Pain Information Page. National Institute of Neurological Disorders and Stroke. Accessed Feb 18, 2010.
- Westergren et al. Information Point: Odds Ratios. Journal of Clinical Nursing. 2001 10,257-269.