A study published this week in the journal Proceedings of the National Academy of Sciences finds that patients with dementia whose caregivers are mentally stressed may die sooner.

Currently, more than 7 million people in the United States live with a neurodegenerative disease.

These conditions include Parkinson's disease, Alzheimer's disease, frontotemporal dementia, Huntington's disease, and multiple sclerosis.

Patients often live for three to 10 years after a diagnosis, depending on the condition and its severity.

During this time, it is typically a family member who cares for the individual as they begin to lose emotional, motor, and cognitive functioning.

Because of the aging population and other factors, these types of conditions are becoming in the U.S. population.

Most research is centered on curing or treating these conditions. However, over recent years more focus has been given to the health of caregivers.

The latest study to investigate this relationship was carried out at the University of California (UC), Berkeley.

Read more: The stages of dementia »

Caring for caregivers

Senior author Robert Levenson, a psychology professor at UC Berkeley, says, "Family members and others who provide critically needed care for these patients do heroic and extremely challenging work.”

He adds, “Soaring rates of dementia and other neurodegenerative diseases constitute one of the most pressing public health challenges of the present era, and this will become even more challenging as the population ages."

The researchers followed 176 patients with neurodegenerative diseases that steadily degrade brain function. They also measured the mental health of the people who took care of them: “85 percent were spouses, 8 percent were adult children, and 6 percent were siblings.”

The participants were all examined at the Berkeley Psychophysiology Laboratory. Researchers gathered information about the caregivers’ social and emotional well-being during follow-up appointments and over the phone.

According to study co-lead author Sandy Lwi, "We were able to meet and speak with a number of caregivers, many of whom talked about the challenges of watching their spouse or loved one slowly lose their sense of self and their ability to live independently."

Out of 176 participants, 76 died during the course of the study (2007 to 2016). Based on the length of time from their initial assessment to their time of death (or the cut-off point in 2016), each participant's longevity was measured.

Longevity decreased for patients whose caregivers had the poorest mental health. This result remained significant even once the data had been adjusted for other factors, such as patient age, gender, disease severity, and mental health.

Overall, the patients whose caregivers had poor mental health died 14 months sooner than those with better mental health.

Caregivers are known to suffer psychologically. In fact, an estimated 40 percent of caregivers have depression, and they often report higher-than-average levels of social isolation, anxiety, and frustration.

Co-lead author Brett Ford says, "These findings make a compelling case that helping preserve the mental health of caregivers may also help the patients in their care."

Read more: A new theory on Alzheimer’s treatment »

How patients are affected

Although the results cannot prove cause and effect, according to Levenson, the findings highlight "the mutual influence both parties' mental and physical states have on one another, and the extraordinarily high stakes that are involved."

There are a number of ways that the patient might be affected negatively by the mental health of their caregiver.

For instance, the level of care might be reduced, which raises the chances of abuse or neglect. Stress on social bonds may also weaken the immune system.

Additionally, negative emotions might be passed between the two parties by so-called emotional contagion, which occurs when one person “absorbs the emotional responses of the other.”

As the number of neurodegenerative disease cases rise – with 8 million U.S. adults estimated to be living with dementia by 2030 – studies such as this become ever more important.

The researchers plan to keep in touch with the surviving caregivers and their patients over the coming years, “to dig deeper into the dynamics” involved.