[The origin of depression in Alzheimer disease: a systematic review].

AIM: Depression was associated to Alzheimer's disease (AD), even if its role as predictive symptom, risk factor or reactive factor remains unclear. The aim of this review was to investigate the relation between depression and AD. More specifically, we aimed to examine if depression may be a prodrome of AD or an early reaction to cognitive decline.

METHODS: A systematic review based on the electronic bibliographic databases (PsycINFO, PubMed, Scopus, Web of Science, Web of Knowledge) was carried out on the scientific literature from 2010 to 2016. Observational studies and literature reviews were included, searched for predefined inclusion criteria.

RESULTS: A total of 29 studies were included. Most of the reviews reported that depression may be a risk factor for dementia. Moreover, frequency and severity of depressive episodes may increase the risk for dementia. However, the results are contradictory to a possible risk difference between early or late depression in determining the occurrence of AD.

CONCLUSIONS: Depression may be a prodrome of dementia and a risk factor of AD. However, the hypothesis of depression as risk factor is the most accredited one. Finally, there is one evidence suggesting that depression is a reaction to cognitive decline of AD.