Hydration is the process of providing an adequate amount of water to body tissues. Good body hydration has several positive benefits. As regards the voice, better hydration may help prevent vocal fold damage. It has been observed that excessively dry vocal folds are more susceptible to the negative impact of collision occurring during vibration than hydrated vocal folds (1,2). This is because hydration performs like a shock absorber to protect vocal folds from injuries. Hydration also protects the airways because it acts as a physical and biochemical barrier against inhaled particulates and pathogens. In addition, vocal fold hydration improves the functionality of the vocal folds (3). Namely, adequate hydration ensures that the mucosa reaches optimal biomechanical conditions, thus increasing the efficiency of oscillation and mucosal wave (3).

As reported by several authors, in order to promote a better mucosal wave (4,5) and to improve voice quality (2,6), vocal folds need good mobility and pliability (7,8). A good hydration support reduces the viscosity of the mucosal layers of the vocal folds (1,2) meaning that the lower the viscosity, the lower the resistance of the tissues to deformation (4,9). Therefore, the quality of the mucosal wave may be a good indirect indicator of the hydration of the vocal folds (1,10). Even if some devices come close to reading the amplitude of the mucosal wave (e.g. videokymography) (11), there is still no meter that guarantees a precise measure of the mucosal wave’s amplitude. For this reason, several research tools are used to indirectly measure the amplitude of the mucosal wave, as in the case of photoglottography, which measures the glottic space variations, or laryngostroboscopy, that subjectively evaluates vocal fold movements (12,13).

Recent research has focused on hydration induced improvement of vocal fold functionality. Two types of hydration techniques have been proposed: systemic hydration, referring to an oral intake of water that hydrates the whole body; and direct surface hydration of the vocal folds, obtained by inhaling through humidifiers and inhalers (14,15). Overall, several studies outlined that hydrated mucosa requires less subglottic pressure than dry mucosa to begin oscillatory movements, determining a decrease of the phonatory threshold pressure (PTP) and favorable changes in some acoustic microperturbation parameters, such as Shimmer (1,2,4,14,16). The results of these studies suggest a direct relationship between improved vocal fold hydration and the ease and smoothness of glottic vibration.

Systemic hydration has been shown to be the easiest and most cost-effective solution to improve voice quality so far (17). However, though systemic hydration is good for general body hydration, it takes a long time to achieve improved vocal fold hydration. In order to prepare vocal activity in a wide range of conditions factors (e.g. talking for hours, high volumes, dry environments, the moment before a vocal performance, feeling dry throat), the vocal fold mucosa needs a large and immediate supply of hydration as it will face intense and prolonged frictions. As a consequence, a quick and efficient hydration system is required to make the vocal tissues more mobile and deformable (4). Thus, systemic hydration is not sufficient for this purpose since improved vocal hydration is achieved very slowly and unpredictably (the amount of water reaching different areas of the body will vary in accordance with the body’s hydric homeostasis). Furthermore, it should be pointed out that ingestion of an excessive amount of water (i.e., recommended daily amount of water intake – around 1.5 litres of water per day for a sedentary adult (18,19)) – could promote reflux (i.e. in people with reflux or with hiatal hernia) and, consequently, greater laryngeal dryness (20). This which in turn can be counterproductive and start a vicious circle of drinking water/increasing reflux and dryness.

For this reason, direct hydration seems to be a great option. Direct surface hydration by means of humidifiers providing water in microparticles in suspension could be ideal. However, some evidences suggest that this may not be as effective as we expect. The reason for this is that the size of water droplets produced by humidifiers (i.e., aerosols) entering the nose or the mouth is very small and light, around 5 μm. Once these water microparticles, enter the nose or the mouth, they penetrate easily and quickly into the lower respiratory system, drying rapidly before settling (21,22), and they may not penetrate into the vocal folds mucosa. Note that this droplet size produced by the humidifiers is necessary for medical purposes (i.e. drugs delivery in the lower respiratory tract and alveoli) (23,24). On the contrary, larger water droplets (> 10 μm) tend to remain trapped in the nose and throat without drying out completely, permitting longer permanence of the humidity (25). Then, we hypothesized that water droplets bigger that 10µm could remain longer without drying completely in the nose and throat, yielding hydration in the vocal folds. Therefore, they may penetrate the vocal fold tissues thanks to the mucosal permeability, promoting favorable changes in the hydration of the mucosal layers and their viscoelastic properties (7).

Thereby, it would be desirable to aim for direct hydration of the vocal folds by means of water droplets larger than > 10 μm. In the present study, we hypothesize that hydration must follow the respiratory path through the nose and we propose a direct surface hydration method by breathing through the nose wrapped in a damp gauze (26– 29). We hypothesize that the damp gauze could produce water droplets bigger than 10 μm, as the water is in its natural state, and nor broken down by machines into less effective smaller droplets. The gauze is chosen as a vehicle for hydration because of its cross-linked structure, which can retain water very well (i.e., water remains in the gauze, despite some water being inhaled with each breath) (26–29). Hydration through the damp gauze should be done accompanied by vocal warm-up exercises performed for 10 minutes in order to promote water penetration in the vocal folds tissues just like an ointment needs massage to help its absorption by the skin (30,31). Thereby, the present study aims to observe whether direct surface hydration obtained by breathing through the nose wrapped in a damp gauze for 10 minutes combined with vocal warm-up exercises modifies the mucosal wave and the vocal quality in a vocally healthy population. To do so, three groups (Gauze, Exercise and Control) were tested and compared through laryngostroboscopic examinations; acoustic analysis and perceptual evaluations. The tests were carried out before (pre) and after (post) the treatment.