Amyotrophic lateral sclerosis in Sweden, 1991-2005.

Objectives: To investigate the temporal trend of amyotrophic lateral sclerosis (ALS) incidence in Sweden between January 1, 1991, and December 31, 2005, and to explore incidence variations according to major demographic factors.

Design: Population-based study.

Setting: Academic research.

Participants: All incident cases of ALS identified through the Swedish Inpatient Register between January 1, 1991, and December 31, 2005.

Main Outcome Measure: Age-standardized incidence rates were calculated by applying the observed age-specific incidence rates to the age distribution of the Swedish population in 1991. A linear regression model was used to assess the potential trend of the incidence during calendar years. We also followed up the entire population registered in the 1990 Population and Housing Census for incidence of ALS. Relative risk and 95% confidence interval of ALS associated with demographic variables were estimated using Poisson regression models.

Results: The age-standardized incidence rates increased from 2.32 per 100,000 person-years in 1991-1993 to 2.98 per 100,000 person-years in 2003-2005, representing an annual increase of approximately 2% during the 15 years (P value for trend, .002). The age-specific incidence rates increased in all age groups except those younger than 50 years. The observed increase remained significant when restricting the analysis to individuals born in Sweden (P value for trend, <.001). Compared with individuals born from April through June, those born from October through December were at 11% increased risk of ALS (95% confidence interval, 1.01-1.23).

Conclusions: The incidence of ALS has been increasing during the last 15 years in Sweden. Further studies are warranted to explore the underlying reasons for this observed trend.