About 50% of individuals with Alzheimer’s disease develop symptoms of psychosis, such as delusions or hallucinations, but these symptoms have been difficult to manage or treat because studies have been inconclusive as to their causes.

Now, a study published Jan, 5 in the Journal of Alzheimer’s Disease has found that cerebrovascular disease, a group of conditions that restrict the circulation of blood to the brain, is a major determinant of the psychosis in people who have Alzheimer’s disease.

Toronto psychiatrist and researcher Corinne Fischer, M.D., and colleagues analyzed autopsy data from 1,073 people using data collected from 29 Alzheimer’s disease centers in the United States from 2005 to 2012. Of the 890 people in whom Alzheimer’s disease had been diagnosed while they were alive, the people most likely to have psychosis were those whose autopsies showed that they had more physical signs of Alzheimer’s disease, such as protein deposits and twisted fibers inside their brain cells. When the researchers examined data related to the 728 people whose autopsies confirmed they had Alzheimer’s, however, those with psychosis did not show increased physical evidence of Alzheimer’s disease.

In both groups of people, psychosis correlated significantly with Lewy bodies, the abnormal protein aggregates that are found in the nerve cells of those with Parkinson’s disease. This finding was not unexpected, the researchers said, because psychosis is prominent when dementia accompanies Parkinson’s disease. What they did not expect, however, was that vascular risk factors — hypertension, diabetes, age when someone quit smoking — and cerebral injuries related to small vessel disease play a prominent role in psychosis.