People who suffer autoimmune diseases such as multiple sclerosis (MS) may have an elevated risk of developing dementia, a new study suggests.
Researchers have linked autoimmune disease to an increased risk of going on to suffer dementia.
There are more than 80 different types of autoimmune disease, from multiple sclerosis and type 1 diabetes and rheumatoid arthritis.
These conditions are illnesses or disorders that occur when healthy cells get attacked by the body's immune system.
It has been suggested autoimmune and inflammatory activity may have a role in the development of dementia.
In a bid to investigate, the researchers from the University of Oxford, examined 25 different autoimmune diseases.
Of these, 18 showed significant positive associations with dementia, according to the study published in the Journal Of Epidemiology And Community Health.
The authors drew information on hospital admissions data between 1998 to 2012 for England.
During the monitoring period, more than 1.8 million people were admitted with an autoimmune disease, ranging from just over 1,019 people with the rare condition, Goodpasture's syndrome, in which antibodies attack the lungs and kidneys, to more than 316,043 with rheumatoid arthritis.
Overall people admitted to hospital with an autoimmune disease were 20% more likely to have a subsequent admission for dementia than those without an admission for an autoimmune disease.
They found the risk varied between different conditions: People with MS had an almost double risk, those with psoriasis showed a 29% heightened risk and people with Addison's disease showed a 48% increased risk.
The type of dementia was not always documented but, for those recorded, the risk was 6% higher for Alzheimer's disease, and 28% higher for vascular dementia.
People with rheumatoid arthritis showed an elevated risk of vascular dementia but it appeared having the condition seemed to be protective for Alzheimer's disease.
The researchers emphasise the size of the associations they found was small, so further research would be needed to confirm or refute the findings.
"Our findings should be considered as indicative rather than definitive," the authors caution.
But they added: "People admitted to hospital with an autoimmune disease, likely to be those at the severe end of the disease spectrum, do appear to have an elevated risk of dementia.
"This finding is consistent with autoimmune disease predisposing to vascular risk and vascular dementia.
"It is also, separately, consistent with the theory that Alzheimer's disease may have an autoimmune component."
Commenting on the study, Stephen Simpson (pictured), director of research and programmes at Arthritis Research UK, said: "While this study doesn't tell us what causes people with arthritis to be more likely to be diagnosed with dementia, it is a promising step towards better understanding how arthritis affects other aspects of people's health.
"We need more research into these links as part of ongoing work to reduce the pain, isolation and fatigue that arthritis causes for millions of people."
Dr Rosa Sancho, head of research at Alzheimer's Research UK, added: "We are becoming increasingly aware of the important role the immune system plays in dementia, and this new study provides evidence to support this link.
"As this study is observational and based only on hospital admission records, we cannot draw firm conclusions from its findings, but it supports ongoing work into the contribution of the immune system to dementia.
"There are 850,000 people living with dementia in the UK and there are as yet no treatments that can slow or stop damage to the brain in diseases like Alzheimer's.
"Targeting immune and inflammatory responses is a promising approach for researchers working on new dementia treatments."
Imogen Scott-Plummer, head of care and services research at the MS Society, added: "We'd urge people with MS not to worry when they read this research. The study talks about very small differences in the chance of being admitted to hospital for dementia and it doesn't mean that people with MS should be doing anything different.
"MS affects more than 100,000 people in the UK and as part of our Treat Me Right campaign we're calling for people with the condition to get a yearly review with their neurologist. It's only through being seen regularly that a neurologist could pick up any issues with symptoms like cognition."
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