WEDNESDAY, March 1, 2017 (HealthDay News) -- People with autoimmune diseases -- conditions that cause a person's immune system to turn against the body -- appear to have an increased risk of developing dementia, a new British study suggests.
Researchers found that 18 out of 25 different autoimmune diseases, such as lupus, psoriasis or multiple sclerosis, "showed a statistically significant association with dementia," said study co-author Dr. Michael Goldacre. He's a professor of public health at the University of Oxford.
But Goldacre and other experts stressed that the study didn't prove that autoimmune diseases cause dementia. The research only showed that these conditions are associated with a higher risk of dementia.
Specifically, the study found that people with multiple sclerosis appeared to have nearly double the risk of dementia. Psoriasis was associated with a 29 percent increased risk of dementia. Lupus was linked to a 46 percent increased risk, and rheumatoid arthritis with a 13 percent increased risk. Crohn's disease was associated with a 10 percent increased risk.
"How do [autoimmune diseases] affect the brain? We don't know, although others have suggested that chronic inflammation, possibly autoimmune effects, or possibly both, may have a role in Alzheimer's," Goldacre said.
For this study, the researchers reviewed information from more than 1.8 million people in England. All had been admitted to a hospital with an autoimmune disease between 1998 and 2012.
Compared with people admitted for other causes, patients admitted for treatment of an autoimmune disorder were 20 percent more likely to wind up back at the hospital later with dementia, the researchers found.
However, when researchers broke down their findings by type of dementia, they found that autoimmune diseases only increased the risk of Alzheimer's disease by about 6 percent.
The autoimmune diseases had a much stronger impact on the risk of vascular dementia. The risk of vascular dementia was 28 percent higher in people with autoimmune diseases. People with vascular dementia experience a decline in their thinking skills due to conditions that block or reduce blood flow to the brain, starving brain cells of oxygen and nutrients.
This apparent increased risk for vascular dementia could be caused by the effect of autoimmune diseases on the circulatory system, the researchers said. The study also found that people with an autoimmune disease were 53 percent more likely to be hospitalized for heart disease. Those with an autoimmune disease were also 46 percent more likely to have a stroke.
The link between vascular dementia and autoimmune diseases is "something new," said James Hendrix. He's the director of global science initiatives for the Alzheimer's Association, based in Chicago.
This link could implicate chronic inflammation as a potential cause of progressive dementia, he said.
Hendrix explained that a person with a sprained ankle experiences inflammation and swelling as the immune system responds to their injury. If the inflammation continues for an extended period, that person could wind up with joint damage and arthritis.
"We are starting to think neuron inflammation is similar," Hendrix said.
Both Hendrix and Goldacre noted that the study is observational, so it couldn't prove a direct cause-and-effect link. In addition, Goldacre said the size of the associations they found was small, and should be taken "more as a message for interested researchers than for interested patients."
Dr. Walter Rocca is a professor of epidemiology and neurology with the Mayo Clinic in Rochester, Minn. He said the findings are "important" but may be limited by the fact that the researchers focused solely on people admitted to the hospital with an autoimmune disorder.
"The concern is that many persons affected by an autoimmune disease may never need to be admitted to a hospital, and many persons affected by dementia may not need to be hospitalized," Rocca said.
"This incomplete capture [of information] may cause a distortion of the findings," he added.
Rocca also pointed out that the 25 autoimmune diseases considered in the study are very different from each other. For example, some attack the joints or the endocrine glands, while others -- like multiple sclerosis -- may directly affect the brain.
Findings from the study were published March 1 in the Journal of Epidemiology & Community Health.
For more about vascular dementia, visit the American Heart Association.