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Odds of dementia strongly linked to number of co-existing mental illnesses, study finds
The odds of developing dementia double with one psychiatric disorder and rise to 11 times higher with four or more, new research has revealed.
The study tracked 3,688 patients aged 45 and over diagnosed with one or more common psychiatric disorders – depression, anxiety, psychosis, substance misuse, personality disorder and bipolar disorder – and assessed their risk of developing dementia or cognitive impairment.
In particular, patients with both mood and anxiety disorders had odds of dementia of up to 90 per cent, the findings show.
Researchers at Bicêtre Hospital in Paris analysed cases recorded between August 2009 and October 2023.
The average age of the group was 67 – 70 for those who developed dementia and 66 for those who did not.
On average, dementia was diagnosed 18 months after the first psychiatric disorder, though in some cases the gap was between seven and 13 years.
Of the patients, nearly 71 per cent (2,608) had one disorder, 21.5 per cent (789) had two, 6 per cent (226) had three, and 2 per cent (65) had four or more.
After adjusting for age, sex and cardiovascular risk, dementia odds rose in line with the number of psychiatric disorders.
Those with two disorders were twice as likely to develop dementia, those with three more than four times as likely, and those with four or more 11 times as likely.
The effect appeared specific to dementia, as checks for links with other diseases – in this case renal failure – found no association.
The study was observational, so it cannot prove cause and effect.
The researchers also noted that dementia diagnoses may have been delayed or misclassified, and that the single psychiatric department studied may not reflect all patients. Data on other factors, including socioeconomic status, family history, duration of psychiatric disorder, treatment and brain scans, was unavailable.
The authors said: “The findings of this study highlight the strong association between the co-occurrence of psychiatric disorders and an increased posterior probability of developing dementia, particularly for patterns with anxiety and mood disorders.
“Recent advancements in dementia diagnoses, such as the development of biomarkers in blood and cerebrospinal fluid, as well as the use of positron emission tomography, encourage earlier and more accurate detection of dementia.
“Integrating these tools into clinical practice for high-risk individuals, especially those with specific psychiatric comorbidities identified in this study, could significantly enhance their care management, given the recent advancements in dementia treatment.”