Research suggests that people who have had a severe infectious disease are at a higher risk of developing dementia later in life.
Medical professionals have long suspected a link between infectious diseases and the risk of dementia. A study analyzing the vast medical records of Finnish citizens showed that people who were hospitalized for severe infectious diseases were at a higher risk of developing dementia later in life.
The role of noninfectious comorbidities in the association between severe infections and risk of dementia in Finland: A nationwide registry study | PLOS Medicine
Severe infections independently amplify the risk of dementia later in life
https://www.psypost.org/severe-infections-independently-amplify-the-risk-of-dementia-later-in-life/
Severe infections can cause persistent inflammation throughout the body, which can affect the blood-brain barrier, the protective barrier that shields the brain from toxins and pathogens in the bloodstream. Damage to the blood-brain barrier allows harmful proteins and inflammatory cells to enter brain tissue, promoting neuroinflammation in the brain and potentially increasing the risk of dementia. Severe infections can also cause problems in the vascular system throughout the body, potentially damaging blood vessels that supply oxygen and nutrients to the brain.
Based on these points, medical professionals have considered whether severe infections may increase the risk of dementia. However, elderly people in their 70s and 80s, who generally experience cognitive decline, often have various physical and mental illnesses, such as diabetes and cardiovascular disease.
Many of these age-related diseases are risk factors for both severe infections and dementia, meaning the same factors may be influencing both. In other words, if a patient develops dementia after infectious pneumonia, it's difficult to determine whether the pneumonia increased the risk of dementia, or whether a pre-existing heart condition caused both the pneumonia and the dementia.
Therefore, a research team led by epidemiologist Piri Sipila of the University of Helsinki investigated the independent impact of infectious diseases on dementia risk. The team accessed Finland's national health registry data and compared 62,555 people aged 65 and older who were diagnosed with dementia between 2017 and 2020 with a control group of 312,772 people without dementia.
In this analysis, one dementia patient was matched with five control subjects who were the same gender, year of birth, and follow-up period. This prevents general variables such as age and time progression from influencing the results, allowing for the identification of variations limited to specific health events.
The research team examined up to 21 years of medical records for each subject and compiled a list of 170 different illnesses and symptoms that led to their hospitalization. By investigating diseases that preceded the diagnosis of dementia, a total of 29 diseases were identified. Of these, 27 were non-infectious diseases, including cardiovascular diseases such as cerebral infarction, metabolic diseases such as type 2 diabetes, mental illnesses such as depression, and trauma such as head injuries.
The remaining two types were cystitis, a type of urinary tract infection, and a common bacterial infection whose site of infection could not be identified. Approximately half of the dementia patients had been hospitalized for at least one of these 29 illnesses in the 20 years prior to the decline in their cognitive function, and many of the subjects had experienced multiple illnesses over many years.
By mapping how these diseases interact, the researchers discovered a network of interconnected diseases, such as 'an early diagnosis of stroke often leads to a diagnosis of a urinary tract infection.' To isolate the impact of infections on dementia risk, the research team adjusted their mathematical model to account for 27 non-communicable diseases.
The results showed that even after considering 27 non-communicable diseases, the association between two types of infections and dementia remained strong. People hospitalized for cystitis were approximately 19% more likely to eventually develop dementia compared to those who were not hospitalized, and a similar increased risk was observed for common bacterial infections.
Furthermore, the research team conducted a similar analysis on early-onset dementia, which develops before the age of 65. They found that in younger individuals, the types of infections associated with an increased risk of early cognitive decline were more diverse, including gastrointestinal disorders, bacterial pneumonia, and severe dental caries. Similar to the case of older adults, the association between infections and early-onset dementia remained unchanged even when other comorbidities were taken into account.
This study is based solely on observational results and does not prove a causal relationship between infectious diseases and dementia. Future research should investigate the impact of infectious disease prevention and treatment on cognitive function, such as analyzing the long-term effects of large-scale vaccination programs.
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