[Late life depression or prodromal Alzheimer's disease: Which tools for the differential diagnosis?]

INTRODUCTION: Depression and Alzheimer's disease are both very frequent in elderly people. Cognitive deficits are the hallmark of Alzheimer's disease, but they are also common in depressed elderly people who often present cognitive deficits such as memory, attention and executive function problems. On the other hand, people with early Alzheimer's disease demonstrate emotional and behavioral disorders generally encountered in depression such as loss of energy, apathy, mood disorder, and irritability. Thus, in older adults with depression, the presence of cognitive deficits can make it difficult to distinguish cognitive decline that is associated with a depressive illness and the decline encountered in Alzheimer's disease because the clinical picture of the two disorders are similar. However, early distinction between the two disorders is very important from a prognostic and therapeutical point of view.

OBJECTIVE: After a brief description of the relationship between depression and early Alzheimer's disease in elderly people, this paper aims to present an updated literature review of data on differential diagnoses between these disorders.

METHOD: We performed a non-systematical, yet as exhaustive as possible, literature search with Pubmed electronic database, screening studies from 2000 to 2016.

RESULTS: The majority of the studies concerned cognitive aspects, but only a few studies investigated others markers such as cerebral imaging, electroencephalography, cerebrospinal fluid markers. At the neuropsychological examination, a detailed analysis of the mnesic profile revealed a better benefit of semantic cueing in patients with late life depression in comparison to those with prodromal Alzheimer's disease and better performances in recognition memory. Moreover, longitudinal follow-up of patients with depression indicated that deficits in delayed recall memory, but not in executive functions, were associated with the subsequent development of Alzheimer's disease. Several studies showed that tests of executive functions could not differentiate between patients with late life depression and patients with prodromal Alzheimer's disease. A measure of global cognitive decline does not seem to be helpful in differentiating early Alzheimer's disease and depression, unlike an analysis of the neuropsychological profile on several composite scales, such as the Mini Mental State Examination. Furthermore, recent work has investigated the utility of olfactory or gustative markers with promising results and convenient tools for clinical practice. Concerning morphological brain imaging, only detailed volumetric analysis could show differences between the two diseases, but these techniques are not always available for clinical practice. It is the same for other recent techniques, such as quantitative electroencephalography, Near InfraRed Spectroscopy, Single Photon Emission Computed Tomography, or Transcranial Doppler Ultrasonography, which have received little attention so far as differential diagnostic tools. Finally, cerebrospinal fluid analysis could be useful, including beta amyloid levels.

CONCLUSION: Despite numerous efforts in recent years, differential diagnosis of dementia from depression in the elderly remains difficult. Results of this review highlight the necessity of conducting more research in this area, with multi-method studies, using not only cognitive analysis but also cerebral imaging techniques.