Age-period-cohort analysis of trends in amyotrophic lateral sclerosis incidence.

Amyotrophic Lateral Sclerosis (ALS) is a progressive neurodegenerative disease with an unknown cause. Studies have reported that the incidence rate of ALS might be changing. As ALS is an age related disease, crude incidence could increase as population structure changes and overall life expectancy improves. Age-period-cohort (APC) models are frequently used to investigate trends in demographic rates such as incidence. Age-specific incidence rate for ALS from 1996 to 2014 were taken from a population-based ALS register in Ireland. To circumvent the well-known identifiability issue in APC models, we apply the method of Partial Least Squares Regression to separate the effects of Age, Period and Cohort on ALS incidence over time. This APC analysis shows no cohort effect and the initial signs of a period effect; increasing incidence of ALS in the most recently diagnosed group. As further years of data accrue to the Irish register it will become clear if this effect emerges as a strong trend in the incidence of ALS in Ireland and replication of these analyses in other populations will show if our findings on temporal patterns in ALS incidence are shared elsewhere.