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  • DS individuals were recruited through public and private organizations working in the field of mental disability. Inclusion criteria were age ≥18y, overall good health, ability to comprehend and perform adequately the test procedure. Exclusion criteria were presence of pathology able to affect olfactory performance (e.g., recent head trauma, rhinitis or chronic sinus infection, diabetes, stroke, thyroid dysfunction, history of smoking/alcohol). The study protocol and the olfactory test procedures were illustrated to DS individuals and/or guardians, and written informed consent obtained. Control subjects were recruited through public advertisement. All investigations were carried out in accordance with the Helsinki Declaration and the protocol was approved by the relevant IRB at the University of Verona (Prot. N. 102, 3rd April 2012, TIT.11/9). Prior to olfactory testing all participants were asked to rate their olfactory function and the answer (better than normal, normal, less than normal) was registered. Olfactory testing was carried out in a well-ventilated room using the “Sniffin’ Sticks” Extended test (Burghart, Wedel, Germany). This is a test of nasal chemosensory performance based on pen-like odor dispensing devices with a felt-tip (length 14 cm, diameter 1.3 cm). Instead of liquid dye, the pen's tampon is filled with 4 ml of a given liquid odorant or odorant dissolved in propylene glycol. The test consists of three subtests namely, threshold (the concentration at which the odor is reliably detected), discrimination (the ability of the subject to distinguish odors) and identification (the subject has to identify 16 different odors from a list of 4 odors each). In order to increase the reliability of the measurements, each subject must give an answer (forced choice paradigm). During each subtest, the experimenter removes the cap of the pens and the pen's tip is held approximately 1 cm under both nostrils for around 3 seconds. Patients are tested blindfolded with a sleeping mask to prevent visual identification of the odorant-containing pens for threshold and discrimination test. The threshold subtest uses n-butanol as target odor (ink pen odor). Threshold is assessed using a staircased, three alternative forced choice procedure (3-AFC procedure). It consists of sixteen dilutions, prepared in a geometric series starting from a 4% n-butanol solution (dilution ratio 1:2 in deionized water as solvent). Three pens are presented in randomized order, with two containing the solvent and one containing the odorant. Subjects are asked to identify the odor-containing pen. Reversal of the staircase (i.e. the presentation of the triplet with the next lower odor concentration) is started when the odor-containing pen is correctly identified in two successive trials. When subjects give an incorrect answer, the triplet with the next higher odor concentration is presented and thus, the staircase is reversed. Testing is completed after seven reversals of the staircase. Odor threshold is defined as the mean of the last four out of seven staircase reversals. The discrimination subtest also uses a 3-AFC procedure: triplets of pens are presented in randomized order, with two containing the same and one a different odorant. Subjects have to determine which of the three pens smells different than the other two pens. For threshold and discrimination subtest the presentation of triplets is separated by 30 seconds. The interval between administrations of individual pens is approximately 3 seconds. The identification subtest is assessed for 16 common odors (e.g., orange, peppermint, coffee, fish, banana). Immediately before the odor is presented, the examiner reads a list of four response options from which the subject has to choose the correct answer. The subject is asked to choose one answer option that s/he feels to be correct after the odor had been presented, having also the possibility to read a paper words list and look at pictures of the four choice options each time, before and after the odor presentation [29, 30]. The interval between odor presentations is approximately 30 seconds. The decision to add to the paper words list odor’s pictures was adopted to give a support in words’ meaning comprehension to the DS subjects [31]. This olfactory task with pictures seems to be particularly appropriate for children and for individuals with mental disability as reported in [23]. We used pictures taken from various sources [freely available on www.google.it/] because a validated olfactory identification test with pictures corresponding to the Burghart identification odors did not exist. Overall, 60 persons with DS were recruited. Four participants were discarded due to a family consent refusal a priori or because of going-on difficulties arisen during the procedure. Data from a final group of 56 DS participants (31 M; 25 F), age range 18-57y were used for analysis. A group of euploid participants (n = 53; M = 23, F = 30, age range: 18-61y) fulfilling the same inclusion and exclusion criteria were used as control. For a subset of DS participants (n = 13; 5M, 8F, mean age: 26.4±4.19y, age range: 18-33y) cognitive evaluation was available. An expert psychologist tested cognition with the Wechsler Adult Intelligence Scale-Revised (WAIS-R). This scale consists of six verbal and five performance subtests. The verbal tests are: Information, Comprehension, Arithmetic, Digit Span, Similarities, and Vocabulary. The Performance subtests are: Picture Arrangement, Picture Completion, Block Design, Object Assembly, and Digit Symbol. Thus, a Verbal Intelligence Quotient (VIQ), Performance Intelligence Quotient (PIQ), and full scale IQ were available (TIQ). Normal distribution of the data was assessed using the Shapiro-Wilk test. The following variables were approximately normally distributed: discrimination, identification, and TDI. Threshold, VIQ, PIQ, and TIQ variables had a non-normal distribution; accordingly, nonparametric statistical methods were adopted in the analysis of these variables. Numerical variables were summarized using mean, standard deviations (SD) or median, Inter-Quartile Range (IQR) as appropriate. Student t test and chi-square (χ2) test were used to compare mean age and the proportion of females in the DS and control group, respectively. Three-way ANOVA was used to investigate the differences in discrimination, identification, and TDI between the 8 groups identified by three binary factors: DS/control, M/F, and age class (young adult, age range 18-29y; older adult, age range 30-61y). The same analysis was performed for threshold using median regression with interactions terms (and sum-to-zero coding for factors). Eta-squared (η2) estimates of effect size were reported for the main effects of each factor, together with F statistics (t statistics for threshold) and P-values. The mean differences in olfactory tests by sex and age were also investigated in control and DS separately using linear regression (median regression for threshold). Wald test on linear and median regression coefficients was performed to evaluate the significance of these differences (t statistic is reported with P-value). Correlation was assessed using the Pearson’s correlation coefficient (r) or the Spearman’s rank correlation coefficient (ρ) where appropriate. Nonparametric estimates of the area under the ROC curve (AUC-ROC) were calculated to assess the ability of olfactory tests in discriminating DS from controls. Differences were considered significant at P≤0.05. Statistical analyses were performed using Stata 13 (StataCorp, College Station, TX).
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