Alzheimer’s disease is a well-known form of dementia characterized by the progressive, irrevocable loss of memory and other basic brain functions. According to the National Institute on Aging, more people develop this disease than any other type of dementia. Many mental health professionals view the presence of diagnosable depression as an early dementia symptom when it appears in people with Alzheimer’s. However, evidence gathered over the last few years indicates that depression is actually a risk factor for the eventual onset of Alzheimer’s disease, not a symptom of the disease itself.
Alzheimer’s Disease Basics
Alzheimer’s disease gets its name from Dr. Alois Alzheimer, a German psychiatrist who first identified the brain changes associated with the condition in the early 1900s. Like all forms of dementia, it involves the loss of higher-level functions in the brain such as the ability to make and recall memories, the ability to make logical judgments and the general ability to think clearly. Some forms of dementia produce relatively modest changes in these functions; however, Alzheimer’s disease has a well-deserved reputation for causing major changes that severely disrupt the brain. The processes associated with Alzheimer’s start to alter the brain’s internal environment long before any overt symptoms make their presence known. Specific alterations in this pre-symptomatic stage of the illness include the development of abnormal protein deposits in the brain called amyloid plaques, as well as the development of abnormal protein structures called tau tangles. Damage caused by these deposits and structures degrades the efficiency of the brain’s mainline nerve cells, known as neurons. One of the first areas affected by this degradation is the hippocampus, a structure that plays a critical role in the brain’s ability to create memories. Gradually, the damage spreads and involves more and more of the brain’s tissues, triggering brain shrinkage and robbing the organ of its most basic abilities. In the final stages of Alzheimer’s disease, amyloid plaques, tau tangles, and tissue shrinkage produce fatal degrees of brain deterioration.
Depression is a collective term that mental health professionals use to describe several officially recognized depressive disorders, including major depression, minor depression, and dysthymia. As its name indicates, major depression produces prominent forms of depressive states of mind such as worthlessness, guilt, sadness, and hopelessness, as well as a variety of other severely detrimental physical and mental symptoms. Minor depression produces symptoms that are less severe than major depression, but the unaddressed presence of these symptoms makes the eventual development of major depression more likely. Dysthymia produces non-debilitating depression symptoms that last for at least two consecutive years, and often for much longer periods of time.
Depression as an Alzheimer’s Risk Factor
A variety of modern studies have identified clear symptoms of depression in older people who develop Alzheimer’s disease and still have only mild or moderate dementia. As indicated previously, some researchers believe that depression in these cases functions as an Alzheimer’s symptom, while others believe that the presence of depression in mid- or late life acts as a contributing risk factor for the eventual development of the disorder. In a study published in 2008 in the Archives of General Psychiatry, a team of researchers from Rush University sought to help determine the relationship between the two disorders by following the development of Alzheimer’s disease in over 900 older adults over a period of 13 years. At the beginning of the study, none of the participants had Alzheimer’s, but some had been diagnosed with depression. Over the course of the study, 190 individuals developed Alzheimer’s disease. In those people also affected by depression, depression symptoms did not increase in number or severity during the period of time when dementia-related brain changes had begun but had not yet triggered overt dementia symptoms. In addition, most of the participants affected by both depression and Alzheimer’s did not see a worsening of their depression symptoms after the symptoms of dementia began to manifest. Based on these findings, the authors of the study concluded that depression does function as a risk factor for dementia rather than as a symptom of early Alzheimer’s disease. In a second study, published in 2012 in the Archives of General Psychiatry, another research team examined the relationship between depression and Alzheimer’s disease in 13,500 individuals who were tracked over a period of 36 years. This time interval covered both middle age and old age in the study participants. After accounting for other mental health-related variables, the authors of the study concluded that people with symptoms of depression in old age experience a twofold increase in their risks for Alzheimer’s when compared to people without depression. They also concluded that people with depression symptoms in both middle age and old age experience a threefold increase in their Alzheimer’s risks.