Individuals previously diagnosed with obsessive-compulsive disorder and those whose parents have been diagnosed with the condition may be more likely to develop schizophrenia. This is according to a new study published in the journal JAMA Psychiatry.

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Researchers found that people with a prior diagnosis of OCD and those whose parents have the condition are more likely to develop schizophrenia.

Although obsessive-compulsive disorder (OCD) and schizophrenia are two distinct conditions, past studies have suggested there is an overlap between them.

The research team, led by Sandra M. Meier, PhD, of the Aarhus University in Denmark, notes that both conditions have similar prevalence rates, for example, and onset of both tends to occur in adolescence or early adulthood.

In addition, they say past research has indicated there are significant similarities in the pathophysiology of OCD and schizophrenia.

"Hence," they add, "it is no surprise that obsessive-compulsive and schizophrenic symptoms coexist in a greater proportion of patients than would be expected by chance." They point to a meta-analysis revealing that 23% of patients with schizophrenia also had OCD.

However, the researchers say few studies have investigated the long-term association between these two disorders.

With this in mind, Meier and her team investigated the association between OCD and schizophrenia by analyzing data from Danish national registers involving more than 3 million people born between 1955 and 2006. Follow-up took place from 1995 until 2012.

Phenotypes of OCD, schizophrenia 'more common than currently acknowledged'

Of the 16,231 individuals who developed schizophrenia, the researchers found that 447 (2.75%) had a prior OCD diagnosis. In addition, of the 30,556 people who developed a schizophrenia spectrum disorder - defined as having one or more of the symptoms associated with schizophrenia - 700 (2.29%) had a prior OCD diagnosis.

According to the researchers, their findings suggest that a previous diagnosis of OCD may be linked to an increased risk of developing schizophrenia late in life. Furthermore, the team found there was even an increased risk of schizophrenia among individuals whose parents were diagnosed with OCD.

These findings remained even after controlling for other factors that may influence schizophrenia risk, such as psychiatric history and family history of psychiatric disorders.

But despite the suggestion that OCD shares many etiological factors with schizophrenia and schizophrenia spectrum disorders, the team says this does not necessarily mean the conditions should be combined as one global diagnosis. They add:

"However, given these findings and the fact that OCD and schizophrenia co-occur with one another at a higher rate than would be expected in the general population, the phenotypes of these disorders are potentially more similar than currently acknowledged.

Further research is needed to disentangle which genetic and environmental risk factors are truly common to OCD and schizophrenia or schizophrenia spectrum disorders."

The researchers admit there are some limitations to their study. For example, some individuals may have had symptoms of schizophrenia but had not yet been diagnosed with the condition, which may have affected the results. "In Denmark, on average 1 year passes until patients with schizophrenia receive an adequate treatment," they note.

In addition, they point out that patient diagnosis of OCD, schizophrenia and schizophrenia spectrum disorders was taken solely from routinely acquired clinical information, which could have been flawed.

Medical News Today recently reported on a study claiming that, contrary to popular belief, people with schizophrenia can experience happiness.