Wednesday April 9 2008
“Depression may increase the risk of developing Alzheimer’s”, The Daily Telegraph says. It reports on a study that followed more than 900 Catholic clergy for up to 13 years. The study found that those who developed the disease had more symptoms of depression at the beginning of the study.
The main aim of the research was to look at changes in depressive symptoms in the early stages of Alzheimer’s. There is a known association between dementia and depression. However, there are different theories as to whether depression causes Alzheimer’s or whether they both develop because of a separate cause. By investigating changes in the severity of depression around the time that dementia develops, the researchers hoped to shed some light on the debate.
Their study found no increase in depressive symptoms before Alzheimer’s became evident. This suggests that depression is not an early sign of the same processes that cause dementia. The researchers say that this therefore implies that depressive symptoms are a risk factor for Alzheimer’s.
This study challenges the theory that depression and dementia are caused by another factor. It therefore adds weight to, but does not prove, the theory that depression is a risk factor for dementia. However, this study has shortcomings, and further research that is free of these should provide a clearer picture. Until more is known, depression sufferers should not be overly worried that they will develop dementia.
Where did the story come from?
Dr Robert Wilson and colleagues from the Rush University Medical Center, Chicago, and the Center for Neurobiology and Behaviour at the University of Pennsylvania carried out the research. The study was funded by the National Institute of Aging. It was published in Archives of General Psychiatry, a peer-reviewed medical journal.
What kind of scientific study was this?
This was a cohort study designed to investigate the theory that depressive symptoms increase during the early stages of Alzheimer’s.
The researchers used participants from the Religious Orders Study, which has been investigating ageing and Alzheimer’s in a group of Catholic nuns, priests and brothers since 1994. The researchers excluded those who already had dementia by giving the participants a clinical assessment to identify those with mild cognitive impairment or Alzheimer’s.
The researchers then identified those with depression using a recognised scale and gave them a score that related to the number of symptoms reported. They also asked about certain personality characteristics and looked at past medical history.
Each year, the participants completed a depression scale to score their symptoms, and underwent a complete neurological examination to identify any mild cognitive impairment or onset of dementia.
When the researchers analysed their results, 917 people were available who had been in the study for an average of eight years. Alzheimer’s was the only form of dementia that the researchers were interested in, so people who developed other types of dementia were excluded.
The researchers were particularly interested in how depressive symptoms changed once Alzheimer’s had developed while taking into account other factors that could affect depression, such as age, sex, level of education, personality and vascular conditions. They also considered whether the number of symptoms at the start of the study was associated with an increased risk of Alzheimer’s disease.
What were the results of the study?
The main finding from this study was that depressive symptoms did not change prior to the diagnosis of Alzheimer’s disease being made, or following diagnosis.
During follow up, 190 participants went on to develop Alzheimer’s after an average of four years of follow up. They tended to be older and had poorer mental state scores as well as greater problems with memory and cognition at the beginning of the study.
The researchers confirmed the findings of previous studies by noting an association (not necessarily causal) between the measure of depression at the start of the study and incidence of Alzheimer’s disease. Those who developed Alzheimer’s were also older, had lower levels of cognitive function, were more concerned about their memory and had different personalities.
What interpretations did the researchers draw from these results?
The authors conclude that there is no increase in depressive symptoms during the early stages of Alzheimer’s disease. They say that these results do not support the ‘reverse causality’ theory about depression and Alzheimer’s, i.e. that depression is an early sign of the processes leading to dementia. The study therefore implies that depression may be a risk factor for Alzheimer’s disease.
What does the NHS Knowledge Service make of this study?
This study was set up to investigate whether symptoms of depression increased prior to dementia becoming established. It was carefully conducted and included a large number of medical assessments using recognised clinical criteria for diagnosing disease.
However, it should be noted that the participants were all older members of a religious order whose lifestyle and health behaviour may differ significantly from the general population. The participants also reported their symptoms themselves. Self reporting can introduce some error, particularly in people with cognitive impairment. Additionally, despite being a relatively large study, the number of people who went on to develop Alzheimer’s was quite small. Much larger numbers would be useful to draw more meaningful results. Finally, although the researchers tried to account for factors associated with the condition, such as age and family history, it is unclear whether their analysis has accomplished this fully.
Rather than investigating whether depression causes Alzheimer’s, this study was actually set up to investigate the theory that depression is an early indicator of the processes that cause dementia. It did not find evidence to support this theory.
It is often difficult to unpick the complexities of causation and association. Studies such as this add to the body of evidence behind the different theories. With the current level of knowledge, sufferers of depression should not be overly worried that they are at an increased risk of developing Alzheimer’s.