Low mood is a sign of depression . The relationship between depression and dementia is a complicated one. In the days before treatment of dementia was available, the medical literature stressed the need to 'rule out' depression before making a dementia diagnosis . A condition known as 'depressive pseudodementia' was discussed commonly. This was a person who looked as though they had Alzheimer's disease , but who responded to treatment with an anti-depressant medication . Whether due to wishful thinking, lack of alternatives, or some other reason, 'depressive pseudodementia' was said to be the case for about one person in ten with dementia. As the treatment of Alzheimer's disease became common, depressive pseudodementia was emphasized less.
As it turns out, however, there appears to be something to the idea of depressive pseudodementia. The relationships are complicated. At some level, the simple explanation that being diagnosed with dementia is depressing is true, and accounts for some of the symptoms of low mood. But it is also the case that with Alzheimer's disease (and vascular dementia, and frontotemporal dementia ) the level of serotonin also falls, as part of the disease process itself. That is why it is sometimes the case that people with mild Alzheimer's disease also benefit from counselling (which itself can raise serotonin levels) and from treatment with an anti-depressant medication.