29 Regional differences in perceived oral dryness INTRODUCTION Saliva is a multi-functional fluid which provides mucosal lubrication and moistening, and protection of the teeth and oral mucosa surface, and plays an important role in digestion, protecting oral tissues, swallowing, taste, and speaking [1, 2]. Therefore, an adequate saliva flow is important for the maintenance of oral health [3, 4]. Saliva flow can be impaired due to many factors. A reduction in saliva secretion rate can be the result of xerogenic medications, radiotherapy of the head and neck, or systemic diseases such as Sjögren’s syndrome [5–7]. Patients suffering from a reduced salivary flow rate may complain about taste alterations, swallowing difficulties, and a burning sensation in the mouth. Other oral complications include increased risk of ulcerations, caries, gingivitis, periodontitis, and oral Candida spp. infections [8, 9]. A reduced salivary flow rate is known as hyposalivation and can objectively be determined by sialometry. Hyposalivation is defined as a salivary flow rate is < 0.1 mL/min at rest or < 0.7 mL/min upon stimulation [8]. In contrast, the subjective sensation of a dry mouth experienced by the patient is called xerostomia [9, 10], which can only be determined with self-reported questionnaires [11–15]. Over the past decades, several questionnaires have been developed to quantify the overall feeling of a dry mouth [11–15]. For example, the Xerostomia Inventory (XI) is an internationally validated and frequently used questionnaire with 11 items on a 5-point Likert scale to quantify the severity of the xerostomia [11]. The sensation of a dry mouth is not solitarily related to the reduction in salivary secretion rate changes but might also be related to the unequal thickness of the saliva film on both soft and hard oral tissue surfaces [16]. To exemplify, the salivary film that remains in the oral cavity after swallowing is the thickest at the dorsal area of the tongue and the thinnest at the hard palate [17–21]. In addition, differences in salivary composition have also been implicated in the perception of dry mouth [19–21]; the salivary mucin MUC5B retains large amounts of water and contributes to the generation of a hydrophilic gel essential for lubrication of the oral epithelium [22–24]. Moreover, MUC5B is the main component that determines the viscoelasticity of saliva [24]. Local variations in the MUC5B concentration have been reported with higher intensity on the hard palate than other oral surfaces [18]. In light of these local variations [17, 18, 21], the palate may be more frequently related to xerostomia complaints than other areas, e.g., the tongue [19]. 2
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