41 Regional differences in perceived oral dryness does not only apply to healthy subjects but is also applicable for dry-mouth patients [18, 20, 21]. The current study found intra-oral differences in perceived mouth dryness, in line with previous research finding different saliva film thickness at different intra-oral locations. This study found that the posterior palate was experienced as most dry, whereas other studies indicated that the anterior hard palate had the thinnest saliva coating [17, 18, 21]. The latter region is comparable with the anterior palate in this study. A possible explanation for this difference could be that patients find it hard to distinguish between two directly adjacent regions: the anterior part (up to the rugae) and posterior part (from the rugae to the end of the soft palate) of the palate and the posterior palate and the pharynx. In both cases, these regions have higher correlations than those of non-adjacent regions. Another study reported the whole hard palate as having the thinnest saliva film without making a distinction between the anterior and posterior part [19]. Our results are in line with this study, as the schematic illustration of the posterior palate in the RODI is a combination of the hard palate and soft palate, which partly resembles the area studied by DiSabato-Mordarski and co-workers. Wolff and co-workers concluded that mostly hyposalivation patients had lower saliva film thickness at the posterior palate about 5-mm palatal to the second molars [20]. This could indicate that these patients could experience more dryness at the soft palate which is a part of the posterior palate in the present study. In our study, the floor of the mouth was the wettest of all intra-oral regions. This finding is in line with previous studies [19, 20]. Another study also showed that the CODS item, no saliva pooling in the floor of the mouth, was only scored positively in the most severe hyposalivation patients [28]. However, three other studies only indicated the dorsal surface of the tongue as most wet [17, 18, 21]. These differences can be explained by the fact that these studies only measured the saliva thickness at the tongue and did not investigate the floor of the mouth. The salivary flow rates had only negligible correlations with the perceived oral dryness at the nine regions. This supports the hypothesis that flow rates and severity of xerostomia do not have to be correlated [16, 23, 34]. Pai and coworkers explored self-reported dryness at four locations (lips, mouth, tongue, and throat) with a Visual Analogue Scale (VAS). They also found that the VAS scores showed little or no significant correlations with salivary flow rates [35]. Although the XI has been developed to quantify the overall feeling of mouth dryness, it contains some items referring to the dryness at different parts of the 2
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