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Causation revisited again
A paper published recently in The Medical Journal of the Islamic Republic of Iran (“X-ray radiation and the risk of multiple sclerosis: Do the site and dose of exposure matter?” Motamed et al.) explores the possibility that X-rays are a risk factor for multiple sclerosis (MS). (Do we routinely read this journal? No. Ken sent me a pdf of the paper, and when I asked him where he'd gotten it, he said he thought I'd sent it to him. Which I had not. On looking back at the email, he finds that it contained no actual message, just the pdf, and not even an identifiable sender. Creepy spam? I guess we'll find out. But until our computers are taken over by bots, despite its iffy provenience, the paper does bring up some interesting questions.)
From the paper abstract:
Methods: This case-control study was conducted on 150 individuals including 65 MS patients and 85 age- and sex-matched healthy controls enrolled using non-probability convenient sampling. Any history of previous Xray radiation consisted of job-related X-ray exposure, radiotherapy, radiographic evaluations including chest Xray, lumbosacral X-ray, skull X-ray, paranasal sinuses (PNS) X-ray, gastrointestinal (GI) series, foot X-ray and brain CT scanning were recorded and compared between two groups. Statistical analysis was performed using independent t test, Chi square and receiver operating characteristics (ROC) curve methods through SPSS software.
Results: History of both diagnostic [OR=3.06 (95% CI: 1.32-7.06)] and therapeutic [OR=7.54 (95% CI: 1.59-35.76) X-ray radiations were significantly higher among MS group. Mean number of skull X-rays [0.4 (SD=0.6) vs. 0.1 (SD=0.3), p=0.004] and brain CT scanning [0.9 (SD=0.8) vs. 0.5 (SD=0.7), p=0.005] was higher in MS group as well as mean of the cumulative X-ray radiation dosage [1.84 (SD=1.70) mSv vs. 1.11 (SD=1.54) mSv; p=0.008].
So, it was a very small study, but the odds ratios were quite significant, particularly for therapeutic X-ray, for which dosage is likely to be higher than for diagnostic X-ray.
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| Chest x-ray; Wikipedia |
And this isn't the only study, in fact, that has found an association between X-ray and MS. Axelson et al. find a similar link in Sweden, described here, also in a very small study. But, this about exhausts the reports of such a link. The problem is that the risk of MS is small (the National Multiple Sclerosis Society estimates that there are 400,000 people in the US with MS, or about 1/1000) relative to the number of people getting X-ray, therapeutic or diagnostic. This means that even if X-ray is causal, and the odds ratios (relative risk) in these small studies fairly large, the actual (absolute) risk is minuscule.
The cause of MS is unknown. Many hypotheses have been considered — it may be immune-related, or viral, or genetic, or perhaps environmental, and some believe lack of vitamin D is a prime candidate. But as with other complex diseases, with the kind of varying and complex phenotype that is seen with MS, it's possible that there are numerous causes, and/or numerous triggers, rather than a single one. So, if X-ray really is causal, perhaps it causes some tissue irritation that stimulates immune response, triggering some over-response that contributes to MS risk. Thus, it's conceivably possible that X-ray is in fact contributory in some cases. But it's possible to imagine many such possible explanations.
But this again raises a larger question, one we've been blogging about off and on, well, forever, but recently, including last week, and again yesterday with respect to the new dietary recommendations, that no longer include cautions against eating foods high in cholesterol. Why is it so hard to determine the cause of so many diseases? Why don't we yet know the cause of MS, or heart disease, or obesity, or many other common diseases? Essentially, it comes down to the fact that our methods for determining causation just aren't good enough when every case is different.
In the near future, we'll write about this issue in the context of how epidemiology is done these days.
Source: http://ecodevoevo.blogspot.com/2015/02/causation-revisited-again.html
