Patients with asthma, particularly older ones, are at increased risk for dementia, a new longitudinal, population-based study suggests.

The findings are “highly reliable” because of the large database, large study sample size, and long follow-up period, but “further study is advised to confirm our findings and explore the underlying pathomechanisms,” the authors conclude.

The study, led by Yi-Hao Peng, Department of Respiratory Therapy, China Medical University Hospital, Taichung, Taiwan, was published onlineSeptember 30 in the Journal of Epidemiology and Community Health.

Using the Longitudinal Health Insurance Database, researchers identified 12,771 patients with asthma newly diagnosed from 2001 to 2003. For each asthmatic case, they randomly selected four people, frequency-matched by age, sex, and index year, for a comparison group of 51,084.

After adjustment for age, sex, and comorbidities, the analysis found that asthmatic patients had a 1.27-fold greater risk (hazard ratio [HR], 1.27; 95% confidence interval [CI], 1.15 – 1.41) for dementia compared with controls.

This result differs from that of a previous study in Sweden that failed to find that asthma increases risk for Alzheimer’s disease (AD). This, said the authors of the current study, may be because that earlier study was based on self-reports instead of physician diagnoses, which makes it less reliable.

Men in the current study had a higher incidence of dementia than women (4.78 vs 4.08 per 1000 person-years), but the dementia risk was lower than for women after adjustment for age, comorbidities, annual outpatient department visits, and oral corticosteroid use (HR, 0.90; 95% CI, 0.83 – 0.97; P < .05).

The elderly had an 11.1-fold higher risk for dementia than those aged 20 to 64 years, a finding that is compatible with those of previous studies and confirms that age is one of the strongest risk factors for dementia.

Patients with stroke had the highest risk for dementia, followed by those with atrial fibrillation, more than 30 annual outpatient department visits, head injury, diabetes, hypertension, and hyperlipidemia.

The data also indicated that the risk for dementia increases with the number of asthma-related emergency department visits and admissions. “This dose–response relationship between asthma severity and dementia risk further strengthened our main finding that asthma increased the subsequent risk of dementia development,” the authors write.

Possible Mechanisms

They outlined several possible mechanisms connecting asthma and dementia. One theory is that both conditions involve inflammatory factors.

“Asthma is a chronic inflammatory disorder of the airways, which involves an array of inflammatory cells and cytokines,” they write. “Similarly, inflammation in the central nervous system has been demonstrated to play a crucial role in the pathogenesis of AD, which accounts for more than two thirds of all dementia cases.”

Other theories are that remodeling of the lung structure in asthmatic patients may lead to chronic hypoxia and then to abnormal synthesis of neurotransmitters, and that corticosteroid use may accelerate hippocampal atrophy thereby increasing dementia risk.

Interestingly, the study showed that patients taking inhaled corticosteroids to control asthma had a significantly lower dementia risk compared with those not taking these medications.

The database didn’t contain lifestyle information on patients, such as education level, occupation, exercise, diet, and cigarette smoking, all of which are potential confounding factors. Another possible limitation of the study is the potential for biases because the information was derived from a retrospective cohort study.

This work was supported by the study projects in China Medical University; Taiwan Ministry of Health and Welfare Clinical Trial and Research Center of Excellence; and health and welfare surcharge of tobacco products, China Medical University Hospital Cancer Research Center of Excellence (Taiwan). The authors have disclosed no relevant financial relationships.

J Epidemiol Community Health. Published online September 30, 2014. Abstract

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