Many studies in clinical psychiatry have investigated the concept of subjective memory complaints (SMC) and suggest that the understanding of these complaints is of great relevance. Some studies indicate that the presence of any type of SMC may indicate objective memory impairment or be predictive of future dementia.[2-4] In particular, difficulties following a group conversation or finding one's way around familiar streets may be associated with severe memory impairment.
Recent studies suggest that SMC, in addition to being associated with dementia or cognitive impairment, are associated with depressive states or major depressive disorder (MDD). Fischer et al. studied the presence of SMC among participants with and without MDD and reported that the subgroup of patients with MDD had significantly more subjective memory complaints; however, there were no significant differences between the two groups with respect to objective neuropsychological assessments. The decline in the self-evaluation of patients with MDD or the inhibition of thought may cause them to have SMC. Mowla et al. investigated the cognitive function of patients with MDD and revealed that there was no correlation between SMC and objective memory performance. Therefore, the meaning of SMC among patients with MDD in this study is questionable.
In general, the prevalences of MDD, depressed mood and dementia, or cognitive impairment differ with sex. Previous studies have reported that women are more likely to develop MDD than men.[8-10] Gao et al. investigated the incidence of Alzheimer's disease and reported that the likelihood of women developing Alzheimer's disease relative to men is 1.56 (95% confidence interval, 1.16–2.10). In addition, de Pedro-Cuesta et al. showed that the prevalence of Alzheimer's disease and dementia was higher in women.
Thus, we hypothesized that there would be sex differences associated with SMC. However, no studies have specifically investigated this potential association. In this study, we investigated the association between the presence of SMC, cognitive impairment and depressed mood in a community-dwelling population and investigated the differential effect of sex on this association.
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The results of this study showed that SMC are differentially associated with cognitive impairment or depressive symptoms on the basis of sex. In male participants, SMC were associated with actual cognitive impairment, while in female participants, SMC were associated with depressive states. This is the first study to show a sex difference with regard to the clinical meaning of SMC. These findings indicate that when patients have SMC, clinicians should pay more attention to cognitive impairment in men and depressive status in women. Odds ratios (OR) of the significant variables were relatively low, but sex showed the highest OR (1.870) in all participants. Thus, we should particularly consider the sex of patients who have SMC.
Previous studies have reported that SMC are related to actual cognitive impairment.[3, 4] Clarnette et al. indicated that cognitive function, as assessed using MMSE scores, in subjects with SMC (n = 97) was significantly lower than that in controls (n = 38). In addition, Benito-Leon et al. investigated SMC in a large sample (1073 vs 1073) and reported that some measures of cognitive function were significantly different between groups, while other measures of cognitive function were not different between subjects with and without SMC. However, there was no information regarding the impact of sex on these differences. In this study, there was no significant association between SMC and actual cognitive impairment in the sample population.
An association between SMC and depressive mood has been reported in a previous small study. Scores on several SMC measures, although not all measures, for depressed patients were significantly higher than those for controls. However, this study did not include a description of the effect of sex on this difference or the number of male and/or female subjects. In the present study, there was a significant association between SMC and CES-D scores, not only in female participants but also in the entire sample population. This result may have been influenced by the fact that most of the subjects with SMC in this study were female.
We are unable to explain why the meaning of SMC differs by sex; however, sex differences in character or temperament may influence the results. Men may objectively recognize their actual cognitive impairment and thus accurately report SMC. On the other hand, women may interpret minor forgetfulness as severe symptoms of dementia and may become depressed, or women with depressive symptoms may be more likely to have SMC than men.
The MMSE scores of female participants were significantly higher than those of male participants. The scores of MMSE subscale 1 (P = 0.045) and 4 (P = 0.020) of female participants were significantly higher than those of male participants. A lower subscale 1 score indicates temporal orientation disturbance and a lower subscale 4 score indicates attention and calculation disturbance. The difference of lifestyle between men and women might lead to a difference in the score of the subscale. In the Japanese countryside community, men go to work every day and their lifestyle tends to be the same as usual. Women do housework and grocery shopping, so they have to calculate frequently and be aware of the dates of the garbage days. Such habits might lead to differences in the MMSE subscales, but it is unclear whether these lead to SMC.
Mild cognitive impairment (MCI) is one of the most important themes in neuropsychiatric research that is related to dementia and depression. Modrego and Ferrandez reported that MDD increases the likelihood of the development of Alzheimer's disease in patients with MCI. Although MDD is an independent risk factor for dementia,[20, 21] depressive symptoms are the prodromal symptoms of cognitive decline.[22, 23] Palmer et al. reported that among individuals with MCI and no dementia, 11% remained stable and 25% improved compared to baseline after 3 years of follow-up; in particular, the group demonstrating improvement may have consisted of individuals with MDD who subsequently recovered from the condition. Therefore, the presence of MDD should be considered for patients who present with MCI.
There are some limitations to the current study. First, we used only one question (‘Have you been distressed by forgetfulness?’) to assess the presence of SMC. Some studies have used a subjective memory complaints scale to evaluate the presence of SMC;[2, 25, 26] however, the present study did not include an analysis of the quality of SMC. Second, we studied a general, community-dwelling population. It is unclear whether our data are generalizable to patients visiting hospitals or clinics for SMC. Further studies are needed to further clarify the influence of sex on subjective memory complaints in patients. Third, this study was conducted at only one site; therefore, our results could have been influenced by the culture of this region of Japan. Further research in other countries or of other races is needed to confirm our preliminary results. Fourth, there is no information about the physical diseases and the impairment of activities of daily living. These could influence the cognitive function and lead to depressive symptoms, and such information might change the results of the present study.
In conclusion, this study showed that SMC are differentially associated with cognitive impairment or depressive symptoms on the basis of sex. In male participants, SMC were associated with actual cognitive impairment, while in female participants, SMC were associated with depressive states. Further studies are needed to confirm our preliminary results.