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Prevalence of dementia by the time of Medicare eligibility was significantly elevated among individuals with schizophrenia, a retrospective cohort study found.

Using data from a national sample of Medicare recipients, the prevalence of diagnosed dementia by age 66 was 25 times greater for patients with schizophrenia, at 27.9% versus 1.3% in individuals without a history of serious mental illness (SMI), reported T. Scott Stroup, MD, MPH, of Columbia University in New York City, and colleagues.

By age 80, prevalence in these groups increased to 70.2% and 11.3%, respectively, according to the findings in .

In those without a prior history of dementia, the annual incidence of dementia diagnoses at age 66 was 52.5 per 1,000 person-years (95% CI 50.1-54.9) among the schizophrenia group, but only 4.5 per 1,000 person-years (95% CI 4.4-4.6) among those without a past SMI. By 80, incidence of dementia diagnosis increased to 216.2 per 1,000 person-years (95% CI 179.9-252.6) for those with schizophrenia and to 32.3 per 1,000 person-years (95% CI 32.0-32.6) for the comparator group.

"When aging and other risk factors for dementia start accumulating, [individuals with schizophrenia] might cross a threshold for dementia diagnosis much sooner," Stroup told MedPage Today.

Stroup's group acknowledged that the study's most significant limitation was that clinical diagnoses could not be individually confirmed or validated.

That concern was shared by both Jean-Pierre Lindenmayer, MD, of NYU Langone Health in New York City, and Dilip Jeste, MD, of the University of California San Diego, who were not involved in the research.

"You never quite know how psychiatrists and medical practitioners diagnose these people and how accurate that is," Lindenmayer told MedPage Today. He added that findings from much smaller studies, though, have demonstrated that older patients with schizophrenia have indeed formed early forms of dementia.

"I would even suggest that they may be underestimating the prevalence of the two diagnoses combined," Lindenmayer said. "Psychiatrists tend to underdiagnose dementia because they are just not that familiar with it."

"We don't know what criteria [clinicians] used for diagnosing dementia," added Jeste, an expert on studying accelerated biological aging in patients with schizophrenia, adding that it's difficult to draw conclusions without detailed examinations.

But Jeste said he was not surprised by the study's findings, noting that individuals with schizophrenia typically develop various diseases at younger ages than the general population, including cognitive diseases.

"This research is a real starting point for us to understand that much more," said Stroup. "The next steps will be to try to figure out if we can come up with specific preventive efforts, and then, once they have schizophrenia and dementia together, how to best help them treat those problems."

The study used medical data from January 2007 to December 2017 on over 8 million people in fee-for-service Medicare and Part D, including 74,170 individuals diagnosed with schizophrenia and 7,937,603 individuals without a diagnosis of an SMI.

Vascular dementia accounted for 5% of diagnoses in the schizophrenia group by age 66 and 15.6% by age 80. In the comparator group, vascular dementia accounted for 0.2% of diagnoses by age 66 and for 1.5% by age 80.

Prevalence of Alzheimer's disease was 8.2% by age 66 in the schizophrenia group and 37.2% by age 80. In the non-SMI group, prevalence was 0.4% and 5.1% at ages 66 and 80, respectively.

The schizophrenia cohort included a higher proportion of men than the comparator group (43.4% vs 36.5%), Black individuals (20.5% vs 7.8%), and a lower percentage of women (56.6% vs 63.5%) and non-Hispanic whites (68.6% vs 81.4%).

In order to be eligible, schizophrenic adults had to have at least two stays in an outpatient facility or one stay in an inpatient facility during the qualifying year. Those from the control group could not have had a diagnosis of schizophrenia, bipolar disorder, or recurrent major depressive disorder during the qualifying year.

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