Amyotrophic lateral sclerosis rates may be increasing slightly, according to a paper in the April Archives of Neurology. The results, based on 15 years of data from Sweden, agree with other work suggesting a bit of a rise. Conducted by scientists at the Karolinska Institute in Stockholm, the study found that in 2005 there were 2.98 ALS cases per 100,000 people, up from 2.32 per 100,000 in 1991.
“There is a notion amongst those who see people with ALS that perhaps the disease is rising,” said Eric Sorenson of the Mayo Clinic in Rochester, Minnesota, who was not involved with the current survey. “Out in the trenches where people are providing care, I think more of it is being referred in and more of it is being recognized.”
For the research, first author Fang Fang, principal investigator Weimin Ye, and colleagues took advantage of the thorough data that Sweden’s free, nationalized healthcare system provides. Few patients seek private healthcare, so the researchers were confident that the Swedish Inpatient Register included nearly all ALS cases among everyone who had ever been hospitalized.
Other researchers have reported similar ALS incidence rates: 2.9 cases per 100,000 people annually in northwest Italy from 1995 through 2004 (Chiò et al., 2009); 1.4 per 100,000 in Uruguay between 2002 and 2003 (Vázquez et al., 2008);1.6 per 100,000 in southeastern Italy from 1998-1999 (Logroscino et al., 2005); 2.1 per 100,000 in Ireland from 1995-1997 (Traynor et al., 1999); 1.9 per 100,000 women and 2.1 per 100,000 men in Washington state from 1990-1995 (McGuire et al., 1996).
Therefore, rates are similar across much of the world, with a few exceptions. There are pockets of higher incidence in a few places such as Guam and Japan’s Kii Peninsula, although those rates have declined with the Westernization of those cultures. ALS incidence in Guam had fallen below 3 per 100,000 by 1999 (Plato et al., 2003). Thus far, no explanation for those hotspots has stood up to scrutiny, Sorenson said (Steele et al., 2008).
Several outside factors could interfere with reporting of true disease rates in studies of disease incidence. “We justified our results carefully by taking into account various potential explanations other than a real increasing trend,” Fang wrote in an e-mail to ARF. The Swedish scientists tried eliminating immigrants, in case other regions had different ALS rates than Sweden. They stratified their results by age and gender. ALS is more common in men and typically affects older people; the mean age of diagnosis in the study was 68.
ALS is a disease of aging, and the populations in Sweden and other European countries are aging, with low birthrates and growing life expectancy. Fang and colleagues adjusted their data based on 1991 demographics in Sweden, and they still noted the upward trend. Even so, Stanley Appel of The Methodist Hospital System in Houston, Texas, who did not participate in the current work, believes an aging population remains the most logical reason for the study’s results. “From my perspective, I still think that’s the best explanation,” he said. “I don’t think you can ever definitively rule that out.”
The current work joins previous studies that have found an uptick in ALS incidence: in Scotland, mortality from motor neuron disease (which includes ALS and other conditions) rose from 1.25 per 100,000 in 1968 to 2.1 per 100,000 in 1987 (Swingler et al., 1992); in Norway, deaths from ALS rose from 1.38 per 100,000 to 2.54 per 100,000 from 1961 to 1994 (Seljeseth et al., 2000). “The more people that find it, the more believable it becomes,” Sorenson said.
If ALS is indeed increasing, what could be the cause? Fang points to environmental factors. Thus far, scientists have made only weak links between environment and ALS. Smoking may increase one’s chances of ALS (Nelson et al., 2000). The Veterans Administration has also reported increased ALS rates among Gulf War veterans, although the correlation remains controversial, Sorenson said (Horner et al., 2003).
“We believe that both environment and genes play roles in ALS,” Fang wrote. “Future studies should thus concentrate on both.”—Amber Dance
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