شناسایی طعم مورد استفاده قرار گرفته به عنوان یک آزمون افتراقی بالقوه در میان افسردگی و بیماری آلزایمر در افراد مسن؛یک مطالعه پایلوت

Major Depression and Alzheimer's disease (AD) are two diseases in the elderly characterized by an overlap of early symptoms including memory and emotional disorders. The identification of specific markers would facilitate their diagnosis. The aim of this study was to identify such markers by investigating gustatory function in depressed and AD patients. We included 20 patients with unipolar major depressive episodes (MDE), 20 patients with mild to moderate AD and 24 healthy individuals. We investigated the cognitive profile (depression, global cognitive efficiency and social/physical anhedonia) and gustatory function (ability to identify four basic tastes and to judge their intensity and hedonic value) in all participants. We found that AD patients performed worse than healthy participants in the taste identification test (for the analysis of all tastants together); however, this was not the case for depressed patients. We found no significant differences among the three groups in their ability to evaluate the intensity and hedonic value of the four tastes. Overall, our findings suggest that a taste identification test may be useful to distinguish AD and healthy controls but further investigation is required to conclude whether such a test can differentiate AD and depressed patients.

Alzheimer's disease (AD) and major depression (MD) are two common diseases in the elderly. The early stages of these diseases are closely related. For instance, AD patients often experience dysphoria, which is characterized by self-depreciation and sadness. This symptom is similar to apathy that is observed in depressed patients, which is defined as a lack of interests, emotions and motivation. In many cases, diagnosis is based on clinical observations. However, the identification of specific presymptomatic markers of each disease would facilitate the early diagnosis and the clinical care of patients. Recently, it has been suggested that gustatory deficits may be an early marker of AD diagnosis (Steinbach et al., 2010). The impairment in gustatory function has been observed also in MD (Berlin et al., 1998 and Swiecicki et al., 2009). Indeed, if a structured taste test can differentiate between AD and MD, it could be useful procedure in a clinical practice facilitating the diagnosis of these diseases. A sensory test permitting the differentiation between AD and MD has been already used by studying the olfactory identification capabilities (Solomon et al., 1998 and Pentzek et al., 2007a). Comparing to other sensory modalities (olfaction, vision), few studies have examined taste deficits associated with these two diseases. A review of the literature reveals conflicting results about taste deficits in depression and thus underlines the need for more investigation. The taste detection threshold has been reported to be altered in MD (Berlin et al., 1998), whereas taste identification is preserved (Swiecicki et al., 2009). Hedonic responses have been extensively investigated in depression because of the anhedonia (lack of pleasure for stimuli previously considered as pleasant) which is a one of the major symptoms of depression (American Psychiatric Association, 1994). Previous studies found no differences in the ability of depressed patients and healthy controls to evaluate the hedonic value of tastes (Berlin et al., 1998, Swiecicki et al., 2009 and Dichter et al., 2010). However, Amsterdam et al. (1987) suggested that hedonic ratings depend on the concentration of the taste compound. Indeed, high pleasantness ratings were observed only for high concentrations of sucrose solutions (Amsterdam et al., 1987). However, a study comparing unipolar and bipolar patients reported that bipolar patients tend to rate gustatory stimuli as less unpleasant compared to unipolar patients (Swiecicki et al., 2009). Thus, the heterogeneity of results prevents firm conclusions from being drawn. Moreover, most studies have analyzed all tastes together and not one by one, which does not take into account subtle differences. Therefore, the present study aims to avoid this drawback by studying several gustatory parameters of the usual basic tastes. Regarding Alzheimer's disease, some studies suggest that taste deficits concern the tasks involving a more cognitive process (taste identification) and not the gustatory threshold (Broggio et al., 2001). A taste identification test has been used to differentiate AD and MCI (Mild Cognitive Impairment) patients from healthy controls, but this test could not distinguish between AD and MCI (Steinbach et al., 2010). More studies are necessary to investigate this issue given the lack of data. Taken together, previous studies have shown that gustatory function is altered in AD and depressed patients, although the components involved differ between the two diseases. No study has directly compared gustatory function between AD and depressed patients. In the present pilot study we sought to investigate, whether taste can discriminate between these two diseases; therefore, we investigated the ability of AD and depressed patients to identify tastes, and evaluate their hedonicity and intensity. We hypothesized that, identification capacities, are preserved in depression and altered in AD.

3.1. Clinical measures We found a significant difference between the groups for social (K=12.3, p<0.01) and physical (K=6.3, p<0.05) anhedonia scores. For the social anhedonia scale, post-hoc Dunn's multiple comparison tests showed that depressed patients had significantly higher scores than AD patients and healthy controls. However, post-hoc tests demonstrated no difference between the groups regarding the physical anhedonia scale ( Fig. 1). Full-size image (25 K) Fig. 1. Comparison of social and physical anhedonia scores for depressed patients, Alzheimer's disease patients and healthy controls. For each type of anhedonia, values with the same letters are not significantly different at α=0.0167 (significance level Bonferroni corrected) according to the Dunn procedure. Figure options 3.2. Taste identification test When all tastants were analyzed together, there was a significant difference between the groups (K=14.79, p<0.001) in their ability to correctly identify the tastes. The Dunn's multiple comparison showed that AD patients gave significantly more incorrect responses than healthy controls ( Fig. 2). Full-size image (14 K) Fig. 2. Between-group comparison of the mean identification scores (SD) for all taste stimuli. The values with the same letter are not significantly different at α=0.0167 (significance level Bonferroni corrected) according to the Dunn procedure. Figure options When the tastes were analyzed separately, Chi-square test demonstrated no significant differences between the three groups for the sour taste (K=0.11; p=0.95) and water (K=2.12; p=0.35). However, significant differences were found for salty (K=17.57; p<0.001), bitter (K=7.30; p<0.026) and sweet (K=8.68; p=0.013) tastes. The Marascuilo procedure demonstrated that AD patients gave more incorrect answers for these three tastes than healthy controls ( Table 2). Table 2. Comparison of the identification responses (number of correct responses), of the mean (SD) hedonic scores and of the mean (SD) intensity scores of each taste among depressed patients (n=20), AD patients (n=20), and healthy controls (n=24). Type of taste Taste identificationa(number of correct responses) Taste hedonicityb(mean and SD) Taste intensityb(mean and SD) Depressed patients AD patients Healthy controls Depressed patients AD patients Healthy controls Depressed patients AD patients Healthy controls Sour 8 A 9 A 10 A 5.6 (2.1) A 4.2 (2.7) A 5.7 (1.4) A 4.3 (2.7) A 4.6 (2.5) A 3.8 (2.0) A Salty 11 A 5 A 21 B 2.8 (2.0) A 4.5 (1.9) A 4.6 (2.1) A 5.8 (2.9) A 4.0 (1.7) A 5.4 (2.2) A Bitter 10 AB 7A 18 B 3.0 (2.5) A 3.5 (2.2) A 4.4 (1.8) A 5.5 (2.9) A 4.7 (2.9) A 5.4 (2.3) A Sweet 19 AB 15 A 24 B 7.8 (2.8) A 6.5 (2.5) A 7.3 (1.7) A 6.7 (2.8) A 5.1 (1.8) A 6.0 (2.2) A Water 14 A 15 A 21 A For each taste, values with the same letter are not significantly different at α=0.0167 (significance level Bonferroni corrected). a Two-by-two comparisons using the Marascuilo procedure. b Two-by-two comparisons using the Dunn procedure. Table options 3.3. Taste hedonicity and intensity Concerning the evaluation of the hedonicity, a significant difference between the three groups was found only for salty and for sour tastes (salty: K=8.1, p=0.017; sour: K=6.6, p=0.036; bitter: K=1.4, p=0.5 and sweet: K=4.8, p=0.09). However, the Bonferroni corrected post-hoc Dunn's multiple comparison tests demonstrated no difference between the groups for the four tastes ( Table 2). Regarding tastes' intensity scores, the results of the Kruskal-Wallis tests showed a significant difference between the groups for salty and for sweet taste (salty: K=6.3, p=0.044; sweet: K=6.6, p=0.037; sour: K=1.0, p=0.6 and bitter: K=1.5, p=0.5). Moreover, post-hoc tests demonstrated no difference between the three groups concerning all tastes ( Table 2).