A new study indicates that dementia patients tended by family caregivers with depression, anxiety and other symptoms of mental illness typically died sooner than those being looked after by caregivers in good mental health.

According to the study published in the Proceedings of the National Academy of Sciences journal, compared to patients who were cared for by relatives in fairly good mental health, patients tended by family members in poor mental health died, on average, about 14 months sooner.

While the study does not provide definitive causal or directional evidence for the earlier death of patients whose caregivers are in poor mental health, "it highlights the mutual influence both parties' mental and physical states have on one another, and the extraordinarily high stakes that are involved," said study senior author Robert Levenson, a psychology professor at the University of California, Berkeley.

From 2007 until 2016, UC Berkeley researchers tracked the mortality of 176 patients with neurodegenerative diseases that are corrosive to brain function, and measured the mental health of the family members who took care of them.

Of the caregivers, 85 percent were spouses, 8 percent were adult children, and 6 percent were siblings.

A patient's longevity was based on the number of days from their initial assessment at Levenson's Berkeley Psychophysiology Laboratory until their time of death, or, if they had not died, until the study cutoff date in May 2016.

Of the 176 patients, 76 died during the course of the study.

Overall, the results showed that the mortality risk for patients was greater when their caregiver suffered from poor mental health, even when factoring in patients' gender, age, disease severity and mental health.