"History of BD [bipolar disorder] is associated with significantly higher risk of dementia in older adults."
So said the systematic review and meta-analysis published by Breno Diniz and colleagues  taking in the accumulated peer-reviewed literature on this topic. Including data for some 3000 individuals diagnosed with bipolar disorder and nearly 200,000 controls (without bipolar disorder), authors calculated something of a significantly higher risk of dementia in those with a documented history of bipolar disorder. They note that there is more research to do in this area, specifically on mechanisms and "to evaluate interventions that may reduce the risk of dementia in this population."
Outside of the literature included in the Diniz study, similar findings have been reported in the science literature. The paper by Almeida and colleagues  noted that: "Bipolar disorder in later life is associated with increased risk of dementia" based on their analysis of ~38,000 older men (65-85 years old) and their "13-year risk of dementia." Perhaps more worryingly were their findings that: "Bipolar disorder was also associated with increased mortality" in relation to "death by suicide, accidents, pneumonia or influenza, and diseases of the liver and digestive system." Other data looking more generally at clinical depression paints a similar picture  suggesting something of a connection between various types of depression and risk of various types of dementia: "depressive symptomatology is associated with pathological mechanisms associated with neurodegeneration."
I've tackled the topic of dementia a couple of times on this blog; most recently in relation to how incidental vitamin D deficiency *could be* something important when it comes to at least some cases of dementia (see here). Minus any sweeping generalisations and accepting that there may be many different roads leading to dementia and/or bipolar disorder, I am intrigued at the possibility that the sunshine vitamin might be something to consider as a 'connector' between elements of the depression and dementia spectrums as per other findings (see here for example). At the very least, it invites lots more targeted investigation, including whether vitamin D might indeed be a nootropic of choice for some (see here)...
 Diniz BS. et al. History of Bipolar Disorder and the Risk of Dementia: A Systematic Review and Meta-Analysis. The American Journal of Geriatric Psychiatry. 2017. Jan 4.
 Almeida OP. et al. Risk of dementia and death in community-dwelling older men with bipolar disorder. Br J Psychiatry. 2016 Aug;209(2):121-6.
 Cherbuin N. et al. Dementia risk estimates associated with measures of depression: a systematic review and meta-analysis. BMJ Open. 2015 Dec 21;5(12):e008853.
Diniz BS, Teixeira AL, Cao F, Gildengers A, Soares JC, Butters MA, & Reynolds CF 3rd (2017). History of Bipolar Disorder and the Risk of Dementia: A Systematic Review and Meta-Analysis. The American journal of geriatric psychiatry : official journal of the American Association for Geriatric Psychiatry PMID: 28161155