Tuesday, September 1, 2009

Snus Use Lowers Risk for Multiple Sclerosis; Smoking Raises Risk

Multiple sclerosis (MS) is a chronic disease in which the immune system attacks and damages myelin, the substance that insulates nerves and facilitates transmission of electrical signals along the spinal cord, the mode by which the brain communicates with the rest of the body. It is a complicated disease, an excellent summary of which can be found here.

There are no definitive causes of MS; genetic and environmental factors may play a role. Several research studies indicate that smoking increases the risk for the disease by approximately 50% (RR = 1.5, 95% CI = 1.3 – 1.7).

Researchers at the Karolinska Institute of Environmental Medicine in Stockholm have just released a new study of tobacco use and MS, which was published in the September 1 issue of the journal Neurology. They compared tobacco use among persons diagnosed with MS (cases) with a group of controls derived from the Swedish population. This experimental design, called a population-based case-control study, is common in epidemiology.

Readers of this blog are familiar with relative risk (RR), which is used by some epidemiologic studies to measure the association of a risk factor and a disease. The measure of association in the present study was the odds ratio (OR), and its interpretation is essentially identical to the RR and is accompanied by a confidence interval (CI), which is the range within which the RR lies with 95% confidence.

Lead author Anna Hedström and her colleagues report that, compared with nonusers of tobacco, smokers had significantly elevated risks for MS. The OR for male smokers was 1.8 (CI = 1.3 – 2.5), and the OR for smoking women was 1.4 (CI = 1.2 – 1.7). The risk increased with the cumulative dose of smoking (measured in pack-years, that is, packs per day times years of smoking), which adds to the overall validity of the association. For example, compared to nonusers of tobacco, men who had up to 5 pack-years of smoking had an OR = 1.4 (CI = 1.0 – 2.0), while men who had 16+ pack-years of smoking had an OR = 2.9 (CI = 1.7 – 5.1).

In contrast to smoking, the study found that snus users had lower risks for MS than nonusers of tobacco. These lower risks were present among snus users of 5+ package-years who never smoked (OR = 0.4, not statistically significant) and who had smoked (OR = 0.3, CI = 0.1 – 0.9), the latter being statistically significant.

A previous study by the American Cancer Society showed that smokeless tobacco use may be protective for Parkinson’s Disease (a neurologic illness) (RR = 0.22, CI = 0.07 – 0.67). The Karolinska Institute study provides evidence that smokeless tobacco may provide protection for multiple sclerosis as well.

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