Discussion
While cognitive symptoms represent one of the core features of major depressive disorder, its persistence after recovery of depressive symptoms calls the attention of many researchers to study the pattern and reason for poor cognitive performance in a patient with MDD. Little is known about the pathophysiological events linking cognitive impairment and depression.
Therefore, our study aimed to evaluate cognitive deficits, in a sample of adult patients with MDD in remission state of their depressive symptoms through a cross-sectional study carried out on persons attending Outpatient Psychiatric Clinics, Suez Canal University Hospital, Ismailia Governorate, Egypt. The mean age of patients was 33± 8.2 years, 77.7 % were female, the average age at onset of MDD was 23.3 ± 6.3 years, the average duration of illness was 8.4 ± 4.99 years and the average number of depressive episodes was 3.87± 1.96. Good matching in our study of the two studied groups ruled out confounding variables like age, sex, and education level (Tables 1 and 2). Patients group with MDD in remission in this study, compared to controls, showed a statistically significant decline in cognitive function regarding visual memory, verbal memory, attention/concentration, and psychomotor speed (Table 3). Also there were statistically significant differences between patients and control group in executive function regarding trial administrated, categories completed and perseverative error (Table 4). This may be due to that cognitive dysfunctions associated with MDD can endure beyond clinical symptom remission, limiting work functioning, and contributing to the overall disability [11].
In agreement with our study, Bortolato et al. studied cognitive in MDD patients and found cognitive dysfunction in several domains, such as executive function, memory, and attention. Therefore, they concluded that cognitive impairment persists during the remission as a residual manifestation in depression [12]. Cognitive impairments in memory, attention, and executive function in our study are consistent with a prospective cohort study that followed patients with MDD until remission. Another study examined subjects with major depressive disorder in remission found impairment in information processing speed and memory, and over half of the subjects had cognitive impairment [13].
Most of the studies of patients with MDD in remission concluded that there are impairment in different cognitive functions, such as sustained attention [3, 14], attention and executive functioning [15], verbal memory and verbal fluency [16, 17], and executive functioning [18,19,20] and thus make great indications that symptom reduction in MDD is not followed by cognitive improvement to a similar degree.
Our findings of memory impairment, attention problems, and executive dysfunction (deficits of conceptualization, abstract ability, and cognitive flexibility) are consistent with the study carried out by Oral et al., which determined that the executive functions, short and long-term memory, working memory, and attention performances of patients with MDD were lower than those of the healthy control group [21]. However, when Memory is studied in terms of explicit and implicit, it is stated that in depression there are no implicit memory problems because it is an automatic processes but the explicit memory affected because requires a recollection [22].
Many evidence reported that cognitive impairment are common both during and residually following depression [23, 24]. Moreover, individuals have reported impairment in cognitive function, even after remission of depressive symptoms, which affect the workplace productivity/performance, quality of life, and the global function [25]. A study found that patients who were currently in a state of remission for MDD experienced persistent cognitive deficits compared to control subjects [24]. The most cognitive deficits that remain after remission of depression are the attention and executive performance [15, 24].
Cognitive impairments persist during periods of remission even after treating MDD [26], which show the discrepancy between emotional and functional improvement. In our study, the relationship between depressive and cognitive profiles revealed that no significant correlations did exist between age at onset of MDD nor the duration of illness with different domains of cognitive function as assessed according to the standard scores of the applied tests except for figural memory (a subtest of verbal memory) of WMS-R and categories completed (reflecting abstract ability) of Wisconsin card sorting test which exhibited statistically high significant correlation with duration of illness (Tables 5 and 6). In agreement with our finding, Karabekiroglu et al. reported that the cumulative duration of depressive episodes and their repetition have effect on the severity of the associated-cognitive impairment [27].
Best et al. concluded that early-onset depression is associated with higher disease severity and with higher levels of recurrence [28]. Also Hasselbalch et al. stated that many factors are involved and affect the neurocognitive course in MDD (number, duration, severity of MDD episodes, age of onset, time passed since the last episode of depression, treatment interventions used, or co-existent psychiatric disorders), which makes the identification of a specific cognitive profile in remitted persons even more complex. The severity of cognitive impairment increases as a function of the cumulative duration of MDEs [29].
