Depression and dementia: cause, consequence or coincidence?

The relationship between depression and dementia is complex and still not well understood. A number of different views exist regarding how the two conditions are linked as well as the underlying neurobiological mechanisms at work. This narrative review examined longitudinal and cross sectional studies in the existing literature and determined the evidence supporting depression being a risk factor, a prodrome, a consequence, or an independent comorbidity in dementia. Overall there is convincing evidence to support both the notion that early life depression can act as a risk factor for later life dementia, and that later life depression can be seen as a prodrome to dementia. There is also evidence to support both conditions showing similar neurobiological changes, particularly white matter disease, either indicating shared risk factors or a shared pattern of neuronal damage. These findings highlight the need to examine if effective treatment of depressive episodes has any effect in reducing the prevalence of dementia, as well as clinicians being vigilant for late life depression indicating the incipient development of dementia, and therefore carefully following up these individuals for future cognitive impairment.