The epidemiology of dementia in young people is currently poorly documented due to the rarity of this disease. Data from registries or population-based studies have made it possible to obtain initial estimates of its prevalence, but the low statistical power of these studies prevents further insight. The use of data from the National Health Data System (Système national des données de santé, SNDS), which is becoming increasingly common in epidemiology, has revealed some very interesting perspectives for the study of rare diseases. In France, these data cover the entire population enrolled in a health insurance plan, i.e., 98% of the French population.

Preliminary work using SNDS data highlighted the potential of this data source for estimating the prevalence of Alzheimer’s disease and other dementias in younger individuals. While these data largely underestimate prevalence in older adults, their good alignment with expected frequencies in individuals under the age of 65 may partly be due to the severity of the disease at a young age bringing these patients into contact with the healthcare system.

In the newly published study based on the SNDS, the prevalence and incidence of dementia in people aged 40–64 were estimated by age and sex.
The standardized prevalence and incidence rates were 109.7 per 100,000 and 24.4 per 100,000 person-years, respectively. Applied to the general population, these rates allow us to estimate the total number of YOD cases at 24,000 people in France in 2016, and the number of incident cases at 5,300. Prevalence and incidence increased with age and were higher (by 33% and 39%, respectively) in men than in 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 reported an age-standardized prevalence of 119.0 per 100,000 inhabitants, with a lower burden 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 enabled us to study the contribution of numerous comorbidities to the incidence of dementia among young individuals. Various factors may help explain the higher incidence of dementia in men compared to women: an association between dementia and certain comorbidities that are more common in men around midlife, or the protective effect of estrogen against the risk of dementia in premenopausal women. We were particularly interested in the role of comorbidities in explaining the observed differences in incidence between men and women.

Based on existing knowledge regarding dementia in general, the risk factors we selected for study included cardiovascular and cerebrovascular diseases and their treatments, metabolic disorders (diabetes, morbid obesity), psychiatric conditions, 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 and cerebrovascular disease, neurological disease, psychiatric disease, and 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 and cerebrovascular, metabolic, neurological (other than dementia), psychiatric, and traumatic brain injury conditions to the observed differences in incidence between men and women. The higher incidence of dementia in men warrants further study to clarify the role of gender-related risk factors and comorbidities, as well as gender differences in access to care.

Our results suggest that preventive approaches targeting cardiovascular risk factors and addiction-related disorders in mid-life (ages 40–55), as well as the prevention and management of traumatic brain injury, could be explored as strategies to reduce or delay the incidence of YOD. These findings align with the recommendations of the French 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 mid-life; in this regard, the prevention of cardiovascular and cerebrovascular diseases is a key priority.

At Santé publique France, a new approach to prevention and health promotion, entitled "Mid-life action for healthy aging," is being implemented with the aim of reducing the risk of loss of autonomy caused by the accumulation of chronic diseases. The philosophy of this approach is to promote behaviors and environments conducive to health, making it possible to reduce the burden of chronic diseases and to foster healthy aging by taking action with individuals in mid-life. The novelty of this strategy, which complements the Agency’s thematic programs (nutrition, tobacco, alcohol, etc.), lies in proposing a combined approach that addresses multiple determinants simultaneously to achieve a positive synergistic effect on health.

In parallel with prevention efforts, epidemiological surveillance of dementia must continue in order to anticipate the burden of this condition and to organize health and medico-social systems accordingly. In France, the reference data for the study of age-related dementia from population-based cohorts (Paquid, Trois-cités, AMI) no longer allow for the updating of prevalence indicators. The Constances cohort, for which data on dementia will be available in a few years, will provide valuable input 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 to identify cases of dementia in the elderly, using data linked 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 Database (Banque Nationale Alzheimer, BNA), which contains comprehensive information on all patients seen in memory clinics in France, is a highly anticipated development for the surveillance and epidemiology of dementia.