Dementia refers to a series of neurodegenerative diseases, such as Alzheimer's, where a person's memory and other cognitive abilities decline. A recent study may have found a new risk factor that might predispose people to dementia: lung disease.

According to the study by researchers from the University of Minnesota's School of Public Health in Minneapolis in collaboration with colleagues from other academic institutions in the US, people who experience lung disease in midlife may have an increased risk of dementia later on.

The study's findings appear in the American Journal of Respiratory and Critical Care Medicine, a journal of the American Thoracic Society.

The research indicates associations between both restrictive and obstructive lung diseases and dementia or cognitive impairment risk.

Doctors use the term restrictive lung diseases when the lungs are unable to expand. Such diseases include idiopathic pulmonary fibrosis, in which the lungs present scarring, and sarcoidosis, where some lung tissue becomes abnormally swollen.

In obstructive lung diseases, something obstructs the air flow in or out of the lungs. The most common type of obstructive lung disease is chronic obstructive pulmonary disease (COPD).

"Preventing dementia is a public health priority, and previous studies have suggested that poor lung health, which is often preventable, may be linked to a greater risk of developing dementia," notes the study's lead author, Dr. Pamela Lutsey, from the University of Minnesota's School of Public Health.

"In this study," she explains, "we looked at the long-term association between poor lung function and the risk of developing dementia, using high-quality measures."

Restrictive vs obstructive lung diseases

The researchers worked with the data of 14,184 people with an average age of 54 years who participated in the Atherosclerosis Risk in Communities Study (ARIC) in the U.S.

All the participants had undergone spirometry, a standard test that doctors often use to diagnose lung diseases. ARIC researchers also asked the participants about their lung health at baseline, between 1987–1989.

On average, the researchers followed the evolution of these individuals' health for 23 years. Over that time, 1,407 individuals received a diagnosis of dementia.

After analyzing all these data, the authors of the current study found that individuals who had a type of lung disease during midlife were at risk of developing either dementia or mild cognitive impairment, though the risk was higher among people with restrictive lung diseases.

Thus, those with restrictive lung diseases at midlife had a 58% higher risk of dementia or cognitive impairment later in life than people without any lung disease, while those with obstructive lung diseases had a 33% higher risk.

The researchers also found an association between dementia risk and low scores on two spirometry assessments: forced expiratory volume in one second (FEV1), and forced vital capacity (FVC).

FEV1 measures how much air an individual can forcefully exhale in 1 second, while FVC helps determine the size of a person's lungs.

What might explain the link?

Why might cognitive problems be tied to respiratory symptoms? The investigators believe that the explanation may lie in the fact that people with lung disease have low blood oxygen levels.

This fact, they note, may cause abnormal inflammation in the body, and it may be a contributing factor to blood vessel damage in the brain.

The researchers admit that their study did face a series of limitations, including the fact that the participants had their lung function tested only at baseline, and that many of these individuals died before specialists had a chance to assess them for dementia.

Moreover, because this was only an observational study, its findings do not necessarily prove that there is a causal link between lung disease and cognitive impairment or dementia.

If further trials could prove causality, however, this may offer a boost to public initiatives that are trying to improve air quality and help people to give up smoking habits.

"Preventing lung disease is inherently important, [and] if other studies confirm our study's findings, both individuals and policymakers will have an added incentive to make changes that protect lung health, as doing so may also prevent dementia."

—Dr. Pamela Lutsey