General introduction 29 1 ‘Depression’: Overall, depression is a mood disorder that persists for more than two weeks. The main symptoms of depression are persistent feelings of sadness and loss of interest. In addition to these two symptoms, of which at least one should be present, there are seven additional symptoms. A total of five or more symptoms should be present for most of the day. One of these criteria is difficulty in thinking, concentrating, making decisions, and remembering things. It is consistent that cognitive impairment is common in depression and even more so in late-life depression. Up to 90% of depressed patients experience, According to the STAR*D study, some kind of cognitive difficulties (Rush et al., 2006). This includes verbal processing, attention, learning, memory, and several aspects of executive functioning, including set-shifting, working memory, and response inhibition. It is debated whether cognitive impairment persists in patients with remitted depression (Grützner et al., 2019; Semkovska et al., 2019). Most of the cognitive impairment occurs during an episode and improves after patients recover from their depression (Roca et al., 2015; Ahern and Semkovska, 2017; Grützner et al., 2019). Although not all domains recover equally or fully, there seems to be a relationship with the number of episodes that someone has suffered (Roca et al., 2015; Ahern and Semkovska, 2017; Riddle et al., 2017; Semkovska et al., 2019). Some studies have reported a less significant recovery (Ahern and Semkovska, 2017). For unipolar depression, deficits in selective attention, working memory, and long-term memory persist after remission and worsen with repeated episodes (Semkovska et al., 2019). However, it is argued, especially in unipolar depression, that this is due to the inconsistency in clinical remission in these studies (Grützner et al., 2019). Among older depressed patients, up to 50% meet the criteria for MCI (O’Brien et al., 2004). In the case of late-onset depression, it is suggested that it can be a prodromal sign of dementia (Lenoir et al., 2011) and it is considered a risk factor for dementia. Patients with MCI and depression have a higher conversion rate than patients with MCI who are not depressed (Ma, 2020; Mukku et al., 2021). However, the opposite seems to be true as well. Dementia is considered a risk factor for developing depression, and up to 50% of patients with MCI and dementia have depression (Lyketsos et al., 2002; Zhao et al., 2016; Eikelboom et al., 2021). Although the literature is not consistent with this prevalence, it is clear that it is higher than noncognitive impairment (Ma, 2020).
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