Risk for developing more than one mental health disorder revealed

Professor McGrath and colleagues looked at Danish health data across a record 5.9 million individuals, providing the largest and most comprehensive study of comorbidity -- the development of two or more mental health disorders.

This long time frame enabled researchers to track a person's mental health for extended periods of time, and resulted in the startling finding that risk of a second mental health diagnosis remains high, even fifteen years after an initial diagnosis.

"In the first six months after initial diagnosis, the risk of a second mental health diagnosis is very high: so for example, people diagnosed with mood disorders like depression have a very high risk of being diagnosed with a neurotic disorder like anxiety in the first six months since diagnosis," Professor McGrath said.

"After the first year this risk drops substantially, but it then stabilises and remains 2 or 3 times higher than those without a prior disorder, even fifteen years after initial diagnosis."

"Importantly, the result was pervasive -- we found it across all pairs of disorders that we studied."

The findings also suggest that for some pairs of disorders, it did not matter which one came first -- there was an increased risk of developing the other disorders. For example, an initial diagnosis of depression was just as likely to be followed by a second diagnosis of anxiety, and vice versa.

Importantly, people who were younger than 20 at the onset of their first mental disorder were significantly more at risk of developing a second disorderover the next 10-15 years, and this vulnerability was found across most pairs of disorders.

"We found that in younger people, mental health disorders were more 'pluripotent', meaning an observed change or disruption in mental health was more flexible in the direction it manifested as a disorder," Professor McGrath said.

Not only is the study the largest and most comprehensive in its field, but its innovative presentation of results has set a new standard for epidemiology (the study of health and disease data), said Professor McGrath.

"The paper is open access, with all data available and code free to download, and results presented in an interactive website with educational videos," he said.

"It's far more user-friendly than the traditional method of presenting results purely in an academic paper."

"We've provided a detailed, comprehensive atlas of the ways that mental disorders cluster together available openly for public use, and we hope that this informs better practice, better clinical care, and future research."

The next step for Professor McGrath and his colleagues is to extend the study to identify the risks between mental disorders and the subsequent onset of general medical conditions, such as epilepsy, migraine, heart attacks, and diabetes.

The paper was published in JAMA Psychiatry and supported by the Danish National Research Foundation.