Prevalence and incidence of young onset dementia and associations with comorbidities: A study of data from the French national health data system

The epidemiology of dementia in young subjects is currently poorly documented due to the rare nature of this disease. Data from registers or population-based studies have made it possible to obtain initial estimates of its frequency, 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 concern the entire population covered by a health insurance scheme, 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 young subjects. While these data largely underestimate prevalence in elderly subjects, their good match 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 care 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 standardised prevalence and incidence rates were 109.7/100,000 and 24.4/100,000 person-years, respectively. Applied to the general population, these rates allow us to estimate the total number 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 work provides the first robust estimates for the frequency of dementia before the age of 65 in France. These results are consistent with those obtained in a recent meta-analysis^*  based on international data, which reported an age-standardised 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 looked at 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 subjects. Different factors could contribute to explaining the higher incidence of dementia in men than in women: an association between dementia and certain comorbidities that are more frequent in men around mid-life, or the hormonal protection conferred by oestrogen with respect to the risk of dementia in women before the menopause. We were particularly interested in the role of comorbidities in explaining the observed differences in incidence between men and women.

Based on the knowledge acquired in the context of dementia in general, the risk factors we chose to study were cardio- and cerebrovascular diseases and their treatments, metabolic disorders (diabetes, morbid obesity), psychiatric pathologies, antipsychotic treatments, other neurodegenerative diseases (multiple sclerosis, epilepsy, Parkinson's disease) and previous history of head trauma.

Overall, we found strong and significant associations between all the pathologies and treatments studied (except anti-hypertensive treatments) and the incidence of the disease. Cardio- and cerebrovascular disease, neurological disease, psychiatric disease and traumatic brain injury explained more than 55% of the difference in incidence between men and women.

These results show the important contribution of cardio- and neurovascular, metabolic, neurological (other than dementia), psychiatric and traumatic brain injury diseases to the differences in incidence observed between men and women. The higher incidence of dementia in men deserves 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 mid-life (40-55 years) cardiovascular risk factors and addiction-related disorders, as well as the prevention and management of traumatic brain injury, could be studied as strategies to reduce or delay the incidence of YOD. They can be compared with the recommendations of the French High Council for Public Health in their report "Prevention of Alzheimer's disease and related disorders", which stresses the importance of setting up primary prevention actions for age-related dementias from mid-life onwards; in this respect, the prevention of cardio- and neurovascular diseases represents a major issue. 

At Santé publique France, a new approach to prevention and health promotion, entitled "Mid-life action for healthy aging", is being deployed 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 behaviour and environments conducive to health, making it possible to reduce the burden of chronic diseases and to foster healthy ageing by taking action with individuals in mid-life. The novelty of this strategy, which complements the Agency's thematic programmes (nutrition, tobacco, alcohol, etc.), is to propose a combined approach, acting on several determinants simultaneously for a positive synergistic effect on health.

In parallel with prevention actions, epidemiological surveillance of dementia must continue in order to anticipate the burden of this pathology and to organise the 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 frequency indicators. The Constances cohort, for which data on dementia will be available in a few years, will make it possible to feed research in this field. The SNDS and the matching of cohorts to this information system represent the best opportunity to pursue monitoring of this pathology. A project to develop a new algorithm to identify cases of dementia in the elderly, using data matched 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 consists of exhaustive information on all patients seen in memory clinics in France, is an eagerly awaited development for the surveillance and epidemiology of dementia.