The epidemiology of dementia in young people remains poorly documented to date due to the rarity of this condition. Data from registries or population-based studies have provided initial estimates of its prevalence, but the lack of statistical power in these studies prevents further insights. The use of data from the National Health Data System (SNDS), which is becoming increasingly common in epidemiology, has opened up very interesting prospects for the study of rare diseases. In France, these data cover the entire population enrolled in a health insurance plan, representing 98% of the French population.

Preliminary studies using SNDS data highlighted the potential of this data source for estimating the prevalence of Alzheimer’s disease and other dementias among younger individuals. Indeed, while these data significantly underestimate prevalence among older adults, the good agreement with expected frequencies among individuals under 65 is likely partly due to the severity of the disease at a young age, which leads these patients to seek care.

In the recently published study, based on the SNDS, the prevalence and incidence of dementia among people aged 40 to 64 were estimated by age and sex.

The age-standardized prevalence and incidence rates were 109.7 per 100,000 and 24.4 per 100,000 person-years, respectively. When applied to the general population, these rates allow us to estimate the total number of cases of early-onset dementia at 24,000 people in France in 2016 and the number of new cases at 5,300. Prevalence and incidence increased with age and were higher (by 33% and 39%, respectively) among men than among women.

This study provides the first robust estimates of the prevalence of dementia before age 65 in France. These results are consistent with those obtained in a recent meta-analysis^* based on international data, which reports an age-standardized prevalence of 119.0 per 100,000 inhabitants, with a lower prevalence in low-income countries and younger age groups.

As we have seen, very few studies have examined the risk factors for this disease. The use of the SNDS has allowed us to investigate the contribution of numerous comorbidities to the incidence of dementia in younger individuals. Given that the incidence of dementia is higher among men than among women, various factors could help explain this difference: an association between dementia and certain comorbidities that are more common among men around midlife, or the hormonal protection conferred by estrogen against the risk of dementia in premenopausal women. We focused specifically on the role of comorbidities in the observed differences in incidence between men and women.

Based on knowledge gained in the context of dementia in general, we chose to study, as risk factors, cardiovascular and cerebrovascular diseases and their treatments, metabolic disorders (diabetes, morbid obesity), psychiatric disorders, antipsychotic treatments, other neurodegenerative diseases (multiple sclerosis, epilepsy, Parkinson’s disease), and a history of head trauma.

Overall, we found strong and significant associations between all the conditions and treatments studied (except antihypertensive treatments) and the incidence of the disease. Cardiovascular, cerebrovascular, neurological, and psychiatric diseases, as well as traumatic brain injury, accounted for more than 55% of the difference in incidence between men and women.

These results demonstrate the significant contribution of cardiovascular, cerebrovascular, metabolic, neurological (other than dementia), and psychiatric diseases, as well as traumatic brain injuries, to the observed differences in incidence between men and women. The higher prevalence of dementia among men warrants further investigation to clarify the role of gender-specific risk factors and comorbidities, as well as gender differences in access to care.

Our findings suggest that preventive approaches targeting cardiovascular risk factors, substance use disorders, and the prevention and management of traumatic brain injury in middle age (40–55 years) could be explored as strategies to reduce or delay the incidence of dementia in younger individuals. These findings align with the recommendations of the High Council for Public Health in its report “Prevention of Alzheimer’s Disease and Related Disorders,” which emphasizes the importance of implementing primary prevention measures for age-related dementias starting in midlife; in this regard, the prevention of cardiovascular and neurovascular diseases is a key focus.

At Santé publique France, a new approach to prevention and health promotion, “Taking Action in Midlife for Healthy Aging,” is currently being rolled out with the goal of reducing the risk of loss of independence linked to the accumulation of chronic diseases. The philosophy behind this approach is to promote health-promoting behaviors and environments, thereby reducing the burden of chronic diseases and fostering healthy aging by engaging individuals starting in midlife. The novelty of this strategy, which complements the Agency’s thematic programs (nutrition, tobacco, alcohol, etc.), is that it offers a combined approach, addressing multiple determinants simultaneously to achieve a positive synergistic effect on health.

In parallel with prevention efforts, epidemiological surveillance of dementia must continue to enable anticipation of the burden of this condition and to organize health and medico-social systems accordingly. In France, the baseline data for studying age-related dementia derived from population cohorts (Paquid, Trois-cités, AMI) are no longer sufficient to update prevalence indicators. The Constances cohort, whose data on dementia will be available in a few years, will provide a foundation for research in this field. The SNDS and the linkage of cohorts to this information system represent the best opportunity to continue monitoring this condition. A project to develop a new algorithm for identifying cases of dementia among the elderly using linked data from the Trois-Cités study with the SNDS is underway at Santé publique France, and the first results are expected in the coming year. Finally, the integration into the SNDS of the National Alzheimer’s Bank (BNA), which consists of comprehensive data on all patients seen in memory centers in France, is an anticipated milestone for the surveillance and epidemiology of dementia.