Depression is unusually common among people with dementia, but no one is certain why. Coping with the loss of memory and other cognitive skills is a profoundly upsetting experience, so it could simply be that depression is a reaction to the onset of dementia. But experts contend that this explanation is unlikely.
Research has shown that depressed people often have reduced volume in the hippocampus, a region of the brain associated with memory loss. And some studies have found that people with depression have a heightened risk for Alzheimer’s disease (AD) and other forms of dementia. For example, a study in the Archives of Neurology found that people who experienced symptoms of depression within one year before the onset of Alzheimer’s were significantly more likely to develop AD than those whose depression had occurred earlier. Interestingly, however, even people whose first symptoms occurred more than 25 years before the onset of AD had at least a modestly elevated risk of developing Alzheimer’s.
Others theorize that depression may be an early symptom of AD and other forms of dementia— for example, that brain damage caused by AD interferes with the healthy signaling of neurotransmitters that govern mood. This scenario seems like an especially plausible explanation for depression that occurs near the onset of cognitive impairment.
Another possibility is that some common biological phenomenon links dementia and depression. A 2017 study of more than 10,000 adults in the United Kingdom found that while depression during middle age did not increase the risk of dementia later in life, people who were diagnosed with dementia had had an increase in depressive symptoms 10 years before the onset of dementia symptoms. The authors theorized that this increase in depressive symptoms is either an early aspect of the dementia or a sign that depression and dementia have a common cause.
For instance, scientists report that people who are depressed have an increased risk of cardiovascular and cerebrovascular disease—blockages in the arteries that impede blood flow to the heart and brain, respectively. Reduced blood flow to the brain, which starves tissue of oxygen and nutrients, is also at the core of vascular dementia, the second most common form of the disease, after AD. Results from a 2013 analysis of 23 studies are striking and lend support to this idea: While depression was associated with an 85 percent increased risk for dementia from all causes, the risk for vascular dementia was 250 percent.
Another theory: Runaway inflammation may be the link between depression and dementia. Blood tests show that depressed people tend to have high circulating levels of inflammation-promoting proteins called cytokines. These factors have also been implicated in Alzheimer’s disease.
In all likelihood, there is no single or simple explanation for the link between dementia and depression. Rather, a complex intertwining of multiple factors is likely to be responsible.