By Genevra Pittman
NEW YORK (Reuters Health) - People who smoke cigarettes or have smoked in the past may be more likely to develop Lou Gehrig's disease, according to a new study.
The disease slowly kills the neurons that send messages between the brain and the rest of the body, causing patients to lose control of their muscles -- including those that are essential for eating and breathing. Most people who are diagnosed with Lou Gehrig's disease, also known as amyotrophic lateral sclerosis, or ALS, don't survive more than five years after the diagnosis.
About 20,000 to 30,000 people in the United States have ALS, according to the Centers for Disease Control and Prevention. While about 10 percent of those cases are caused by a genetic defect, the rest have no known cause. Some previous studies have suggested that smoking may be one factor that increases a person's risk of getting ALS, but others have found no link.
"Results have been rather conflicting regarding the association between smoking and ALS," said Dr. Fang Fang, who has investigated the question at the Karolinska Institute in Stockholm, Sweden, but was not involved with the current study. Many ALS researchers tend to agree on a positive association between cigarette smoking and ALS risk although some quite-recent studies still report no link, he told Reuters Health in an e-mail.
In the current study, researchers led by Dr. Hao Wang of the Harvard School of Public Health collected data from five long-term studies in the United States that altogether included more than a million adults. In all of the studies, participants had been asked if they currently smoked or had smoked in the past, and if so, how frequently and for how many years. The studies tracked participants for between seven and 28 years, and the researchers were able to determine from a national database of deaths which of the participants ultimately died of ALS.
A total of 832 people across the five studies got ALS according to the results, which are published in the Archives of Neurology. Being a past smoker or a current smoker meant a person was 40 to 45 percent more likely to die of ALS than someone who had never smoked. In absolute terms over a ten-year period, an average non-smoker had about a 0.05 percent chance -- about 1 in 2,000 -- of getting ALS, whereas former or current smokers had a 0.075 percent chance of getting ALS -- about 1 in 1,300.
The researchers also found that starting to smoke at an earlier age meant people were more likely to get the disease, compared to smokers who started when they were older.
Among smokers, however, there was no relationship between how many cigarettes they smoked each day or how much of their lives they smoked and their chances of getting ALS.
That finding is in contrast to the results of some previous studies which have shown that the more people smoke, the higher their risk of getting ALS. That sort of direct link between smoking "dose" and disease risk would be expected if cigarette smoking, specifically the toxic components of cigarette smoke, were able to damage nerves directly, as some researchers believe.
The current study, therefore, cannot show that smoking causes ALS, only that smoking and risk for ALS are somehow linked.
"There is a range of possible mechanisms for smoking to increase the risk of ALS," Dr. Carmel Armon, the head of neurology at the Baystate Medical Center in Springfield, Massachusetts who was not involved in the current research, told Reuters Health. "In this case ... what we're probably seeing is a change that causes the systematic dismantling" of the system of neurons that control muscle movement, he said. Smoking may change the DNA of cells involved in muscle movement -- changes that would then build up over time.
Because this is a report that includes data from multiple studies, it's hard to know if something about the way those studies were done influenced the finding that more cigarettes and a longer duration of smoking didn't increase a smoker's risk of ALS, Armon said.
While Armon said that the link between smoking and ALS has become more and more clear in recent years, what hasn't been studied is whether people exposed to secondhand smoke are also more likely to get ALS than those who aren't exposed. Because of that, researchers might still be underestimating how much of a risk smoking is for ALS, he said.
SOURCE: http://bit.ly/gdU4X2 Archives of Neurology, online February 14, 2011.