13 General introduction, aim and outline of the thesis (such as agenesis of the salivary glands), metabolic disorders (such as dehydration), infectious disorders (such as HIV and COVID) and other diseases (such as fibromyalgia) are associated with impaired salivary function [16]. In addition, salivary gland dysfunction is common in patients who have received radiotherapy of the head and neck region. In these patients, the development of salivary gland dysfunction depends on the cumulative dose of radiation and the volume of salivary gland tissue included in the field of radiation [15]. Additionally, various other factors can inhibit the secretion of saliva, such as tobacco smoking, using alcohol, the wearing of complete dentures, heavy snoring, and mouth breathing due to functional impairment of the upper airways [11, 19]. Also, periods of acute anxiety and stress can induce transient oral dryness due to predominant activation of the sympathetic stimulation [9]. Finally, during the physiological process of aging unfavorable changes in the number of secretory cells (acini) within the salivary glands may develop which also induce reduced secretion and/or a dry mouth. This problemmainly affects menopausal women and individuals older than 65 years [11]. However, the literature also depicts that these dry-mouth symptoms are not solely explained by the physiological processes of aging, but can also be caused by age-related increase in the number of medications used [11]. 1
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