In healthy individuals the oral tissues are constantly bathed with saliva. The constant flow of saliva eliminates food debris and bacteria by swallowing. When stimulated by chewing or by moderate strength citric acid, the whole saliva flow rate increases from the resting value of around 0.3–0.65 ml/min to around 1.5–6.0 ml/min [1, 2] with great individual variations.

Saliva has an important role in oral health by providing immunological protection, and its role as a reservoir of ions which aid remineralization of the calcified tissues [3]. The secretions of salivary glands are important for protecting the mucosa from feeling dry and according to Naito et al this oral dryness is contributing to reduce health related quality of life [4]. The perception of oral dryness (xerostomia) was found to be related to the rate of secretion of minor palatal glands [5, 6]. The feeling of dryness was evident when the rate of that secretion was below 3 μL/cm³/min [7]. This suggests that the sensation of a dry mouth is perceived when there is insufficient mucosal wetting, especially of the palate. Others considered that the subjective feeling of oral dryness might be a result of salivary gland hypofunction [8, 9]. Hence, from the clinical point of view there will be a considerable difficulty in making a decision as to whether a given patient has salivary gland hypofunction, a condition that requires additional salivary gland evaluation. Associating saliva flow rate with other clinical features such as dryness of the lip, dryness of the oral mucosa and difficulty with speech and swallowing may aid in such decision making process [10, 11].

Other than salivary hypofunction, a dry mouth can be caused by variety of reasons: dehydration, mouth breathing, medications and radiotherapy [12].

Although numerous studies discussed functional disturbance of salivary gland in relation to xerostomia in elderly people [13–15], it appears that very few studies included children and adolescents [16, 17]. The prevalence of oral dryness, among different age groups despite its effect on oral health, has not been elucidated in many populations, including Saudi Arabians.

Many studies have estimated values of stimulated salivary flow and found them to be normal in subjects complaining of xerostomia [18]. However, since the resting saliva is actually the flow that occurs continuously in the patients mouth, this flow would be more relevant and may correlate better with patients symptoms, especially in cases where the gland can be stimulated to produce enough saliva. The present study was planned to investigate whether resting or stimulated flow correlate better with symptoms and signs of oral dryness.

This study aimed to determine: (i) the prevalence of signs of oral dryness among 3 groups of children, adolescents and adults; (ii) whether signs of oral dryness are related to the salivary variables namely flow rate, pH, and buffer capacity; (iii) whether salivary flow rate is correlated to subjective complaints of oral dryness.

Upon clinical examination, the prevalence of dry lip in the total sample was estimated to be 37.5% (table 1). Dry lip was observed in 33.3, 44.4 and 35.3% of children, adolescent and adults respectively. The difference between the three age groups was not statistically significant. The sex difference was statistically significant in the adults group, where dry lip occurred in 51.2% of the males in contrast to 23.2% of the females.

Dry mucosa was less evident among the study groups than dry lip, it was prevalent in 3.2% of the sample with no age or sex differences.

Xerostomia was defined as the subjective sensation of dry mouth, while hypo salivation was known as the objective finding of a reduced salivary flow rate [23]. In the present study 37.5% of the samples had dry lip and 3.2% of them had dry mucosa.

Dry lip was the most prevalent sign of oral dryness in this study probably because it is the site of initial contact with inspired air. It was reported that the anterior part of the palate and anterior dorsum of the tongue are the sites where xerostomia symptoms more pronounced [7] because saliva is very susceptible to evaporation from those areas during mouth-breathing. In Saudi Arabia, especially in Jeddah, the very hot weather could be the reason behind excessive dryness of the lips. Since veil constitutes an important component of dress for women in Saudi Arabia, it is expected to see females with less lip dryness than males.

It was reported that physiological pH range increases as the flow rate increases and vice versa, thereby maintaining a saliva pH within a range of minimum 6.6 for resting saliva to 7.4 for stimulated saliva was recorded as acceptable [24]. The present study investigated signs of oral dryness in relation to salivary flow rate, pH and buffering capacity and found that subjects with lip dryness exhibited reduction in resting salivary flow rate more often than did subjects who had normal lip. However, changes of pH and buffering capacity were not of significant values. Wolff and Klienberg [6] found that saliva flow rate and pH were both directly related to mucosal wetness. In this study data on dry mucosa which was recorded only in ten subjects could not be conclusive.

The present study used self administered questionnaire that included questions which are predictive of reduced saliva output. Question number one related to the resting saliva production because it focused on the patient's general feeling of oral dryness. The rest of the questions were mainly oriented at the stimulated saliva production to assist in chewing and swallowing. Complaints of dry mouth were reported by 19% of the sample. This result is comparable to that reported earlier among another group from central region of Saudi Arabia [25]. Studies of xerostomia complaints in elders reported prevalence that varied from 28 to 63% [26–29]. Most of those studies used subjects residing in hospitals and community dwelling. One study examined 529 elders out patients and reported that 29% of the entire group had complaints of oral dryness [12].

In the present study, complaints of dry mouth are lower than previous studies which did not indicate the proportion of subjects who had an underlying disease. Although the disagreement might reflect racial, social and culture differences between the samples, it may also be attributed to the difference in sample inclusion criteria, since individuals with systemic diseases were excluded from present study.

Atkinson and his co-authors [29] suggested that complaints of dry mouth may not reflect reduced salivary function and may instead reflect dehydration or other systemic condition. Therefore, authors considered that examining complaints of dry mouth will not reflect true risk for oral disease in the population. Similar results were reported among Iranian menopausal women with and without oral dryness feeling [30].

The present study showed that the subjects who had complaints had significantly less stimulated salivary flow rate than those who reported no complaint. The resting salivary rate was reduced in subjects with complaints but the severity of reduction was minor relative to stimulated flow rate. These findings are in agreement with that of Loesche et al [26] who found stimulated flow rate to be lower in those complaining of oral dryness. In contrast, a positive answer to the feeling of dry mouth according to Beck et al [31] and Bardow et al [19] seemed to be valuable for identifying patients with low resting salivary flow rates. Some authors found that dry mouth was not always associated with reduction in whole salivary output and therefore, insufficient mucosal wetting and changes in salivary composition have been regarded as factors that influence the perception of dry mouth [5, 6]. These findings are supported by Dawes and Odlum [32] who found the mean residual volume of saliva is only reduced by 29% in patients with sever hyposalivation although they reported the feeling of a very dry mouth.