gms | German Medical Science

A seeming decline in dementia incidence in Western nations has been a topic of continuous debate resulting in a recently published analysis of data from seven population-based cohort studies [1]. Constructing several nonoverlapping 5-year epochs, the corresponding design and analysis closely follows a framework previously used within the Framingham Heart Study (FHS) cohort. However, we challenged the finding of the FHS cohort on the basis that bias may have resulted from the failure to adequately account for potential disease onset in the period between last observation and death. Re-analyzing the FHS data using spline-based analytic methods, we did not find convincing evidence for a decline in dementia incidence over the epochs [2]. Yet, there is further room for improvement. First, the classification of calendar time into 5-year epochs is both unnecessary and arbitrary; the conclusion of a linear decline in dementia incidence in the FHS data would not have held had e.g., a 4-year follow-up period been used [2]. Second, two separate cohorts (the ‘original’ FHS cohort and a cohort of their offspring) were combined for analysis, which may be inappropriate if they differ markedly.

A more suitable approach for analyzing the question of how dementia incidence has evolved over time in the FHS would be to consider the two cohorts separately, and to dispense with the epoch structure by analyzing age as the time scale. This results in the analysis of separate generations which are ageing over time and being subject to death in greater numbers over time without replenishment from younger participants. If missing dementia cases due to death lead to a bias, the effect of this would therefore be to underestimate the incidence of dementia cases to a greater extent over time. This problem requires the use of statistical methods based on the illness-death multi-state model, such as spline-based penalized likelihood as employed in our earlier study. We will present the findings of the proposed design and analysis strategy, aiming for a realistic quantification of the dementia incidence trend in the Framingham Heart Study cohort.

The authors declare that they have no competing interests.

The authors declare that an ethics committee vote is not required.