Hyaluronic Acid: Perspectives in Upper Aero-Digestive Tract. A Systematic Review

Background To date, topical therapies guarantee a better delivery of high concentrations of pharmacologic agents to the mucosa of the upper aerodigestive tract (UADT). The use of topical drugs, which are able to reduce mucosal inflammation and to improve healing tissues, can represent a relevant therapeutic advance. Topical sodium hyaluronate (SH) has recently been recognized as adjuvant treatment in the chronic inflammatory disease of the UADT. Aims The aim of our work was to review the published literature regarding all the potential therapeutic effects of SH in the chronic inflammatory disease of UADT. Methods Relevant published studies were searched in Pubmed, Google Scholar, Ovid using keywords (“sodium hyaluronate” and “upper airways”) or Medical Subject Headings. Results At the end of our selection process, sixteen publications have been included. Six of them in the post-operative period of nasal-sinus surgery, 2 of them in pediatric patients affected by recurrent upper respiratory tract infections, 4 of them in reducing symptoms and preventing exacerbations of chronic upper airways in adult population, 4 of them in patients with chronic inflammatory disease of UADT, including gastro-esophageal reflux disease (GERD). Conclusions Topical administration of SH plays a pivotkey role in the postoperative phase of patients undergoing FESS and nasal surgery, and positive results are generally observed in all the patients suffering from UADT chronic inflammatory disease.


Introduction
Hyaluronan is a naturally occurring polysaccharide which consists of a linear chain of fragments of d-glucuronic acid and N-acetyl-glucosamine linked by alternating β-3 and-β4 bonds. It is an abundant constituent of the extracellular matrix of connective tissue, synovial fluid, embryonic mesenchyme, vitreous humor, skin, and several other organs and tissues of the human body [1]. Hyaluronan may be an important regulator of inflammatory response: high molecular weight hyaluronan is broken down under the influence of free radicals and enzymes during inflammation. Low molecular weight fragments deliver signal about tissue damage and mobilize immune cells, while high molecular weight form suppresses immune response preventing from excessive exacerbations of inflammation [2].
Depending on molecular weight, hyaluronan can have different effects. Low molecular weight hyaluronan (<300 kD) stimulates cell proliferation and initiates pathways involving inflammation and stimulates ciliary beat frequency (CBF) via RHAMM (receptor for hyaluronic acid mediated motility); instead, high molecular weight hyaluronan (1,000 kD) has no effect on CBF.
Airway's epithelium synthesizes high-molecular-weight hyaluronan, that needs to be degradated by oxidative stress (ROS, reactive oxygen species) induced by allergen challenge in airway lumen in order to signal via RHAMM. The low molecular weight fragments of hyaluronan such generated trigger RHAMM and RON (recepteure d'origine nantais) pathways and increase CBF [2].
CD44 is another crucial HA receptor. It is found on many cell types involved in inflammation, including leukocytes, chondrocytes, fibroblasts, and endothelial and epithelial cells and has been implicated in various processes such as lymphocyte recruitment. Its bound with HA is mediator of different signals depending on molecular weight in a similar manner of RHAMM/RON pathways. In fact low molecular weight HA is shown to stimulate proliferation, whereas high molecular weight fractions were inhibitory. The precise mechanism remains unclear [3]. TLR in another low molecular weight HA protein involved in inflammation signaling. There is no evidence of direct physical binding between TLR4 and HA, but is undoubtable that the both play a role in the same pathway. Its signaling can be CD44 both independent and dependent. In dependent one, it is unclear if CD44 plays a role as competitive or co-operative way [4].
Hyaluronan is also present on the airway surface, being an important component of normal airway secretions, and certainly playing a central role in the homeostasis of physiology in the respiratory apparatus, especially at level of the upper airway [3]. In particular, in the nasal mucosa hyaluronan is primarly involved in the regulation of vasomotor tone and gland secretion and it significantly contributes to mucosal host defense by stimulating ciliary clearance of foreign bodies while simultaneously retaining enzymes which are important for homeostasis in the apical surface [3]. Hyaluronan, which is an hygroscopic macromolecule and its solutions are highly osmotic, forms a scaffold that several sulfur proteoglycans bind to. Such structures can reach large size and are able to trap large quantities of water and ions, providing hydration and tissue turgescence. Data from some studies pointed out that in the oral mucosa this property either enables the control of tissue hydration during inflammation process or it allows the response to tissue injury which results in ulcer formation. As its structure doesn't exhibit species specificity or tissue specificity, its pure form doesn't have any allergizing or immunogenic properties [4,5] Up to date, SH is widely used in several other branches of medicine and neither contraindications nor interactions with drugs are reported [6,7,8,9].
Its very extensive use in the world is a consequence of its high level of safety. SH is widely used in esthetic medicine as a filling material for folds and creases and to enlarge some parts of human body (such as lips, breasts, buttocks, etc.), it is also employed for the treatment of arthritis [7] and a lot of studies on the effect of SH in the lung disease are available in literature, as well [10]. Formulations of SH have been developed for topical administration as coadjutant treatment in clinical cases of acute and chronic pathologies in the upper aero digestive tract (UADT) and in the tissue healing after UADT surgery, based on the large evidence of data available on SH role in UADT, in animals models [11,12] The aim of our work is to systematically review the published literature regarding all the potential therapeutic effects of SH in the chronic inflammatory disease of UADT, treating to better investigate the main application areas of SH in UADT, the root and schedule of SH administration, the main efficacy parameters evaluation of SH in UADT.
Only studies in English, published in peer-reviewed journals, reporting data on the role of the topical administration of sodium hyaluronate in the UADT, in the oral ulcers and mucositis, were included. No studies related to dentistry have been considered.
Literature reviews, technical notes, letters to editors, and instructional course were excluded.
Two authors (CM and MA) independently assessed the full-text version of each publication, by selecting that on the basis of its content and excluding papers without the specific content. The reference lists of the selected articles were fully and accurately reviewed to identify articles not included during the first electronic search.
Taking into account all the journals, articles strictly coherent with the topic were firstly identified, while excluding studies on animal models after a primary selection.
Studies that were based on the use of SH resorbable nasal packing or dressing were not included either. Furthermore, any scientific papers regarding invasive administration of hyaluronic acid, with the exception for those related to the Empty Nose Syndrome, were not considered for the aims of our research, as shown in S1 List of Excluded Studies.
At the end of our study selection process, 16

Results
Sixteen clinical studies, which investigated the role of SH, have been included in the present review, as below reported. Six of them in the post-operative period of nasal-sinus surgery, 2 of them in pediatric patients affected by recurrent upper respiratory tract infections, 4 of them in reducing symptoms and preventing exacerbations of chronic upper airways in adult population, 4 of them in patients with chronic inflammatory disease of UADT, including gastroesophageal reflux disease (GERD). Study design and sample size of each single study are reported in Table 1.

SH and mucosal wound healing after nasal and sinus surgery
Macchi et al. showed that intermittent treatment with SH solution through nasal washes (9 mg vial of SH plus 3 ml saline/daily given for 15 days per months over 3 months) following functional endoscopy sinus surgery (FESS) was associated with significant improvements in symptoms, endoscopic appearance (reduction of nasal edema, crusting and secretions) and ciliary motility, compared to saline nasal washes [15]. Casale et al. examined the potential role of SH (9mg of SH plus 2 ml of sodium chloride 0,9% twice a day for 14 days, trough nasal washes, starting on the first day after surgery) in the radiofrequency surgery that was performed for chronic inferior turbinate hypertrophy, showing a significantly more rapid recovery after surgery especially in the first 2 weeks, with a strong impact on quality of life of these patients in the first weeks after surgery [16]. Gelardi et al. proved the SH efficacy both in clinical/endoscopic parameters and in mucociliary transport in patients undergoing FESS for nasal polyposis (NP) [17], with nasal lavages based on 9 mg of SH nebulized in 3 ml sodium chloride 0,9% twice a day for 30 days, from the second day after surgery. Cantone et al. obtained a faster improvement both in general health and in specific sinonasal status through the use of topical SH, compared to control group only treated with saline solution [18]. Also Soldati et al. achieved a faster mucosal wound healing after nasal and sinus surgery using Rhinogen, a new marketed nasal cream containing hyaluronic acid [19]. The same results both in clinical and functional nasal parameters were obtained, by Gouteva et al. after nasal surgery, with a spray containing SH and Dexpanthenol Hysan Pflegespray (HP) [20].

SH and chronic upper airway inflammation in pediatric population
Macchi et al. demonstrated that intermittent SH treatment with nasal washes was useful in pediatric patients suffering from recurrent upper respiratory tract infections; in particular, the SH group had a significant improvement in ciliary motility, cytological, microbiological, endoscopic and clinical outcomes compared to saline group. The results also showed a significant adenoid size reduction and a lower presence of bacteria and neutrophils, both assessed through  direct evaluation with optical microscopy [21]. Di Cicco et al. reported that SH in association with tobramycin was more effective than hyaluronate alone for the treatment of bacterial rhinosinusitis in patients affected by cystic fibrosis (CF), reducing both upper airway symptoms (hyposmia/anosmia and headache/facial pain) and muco-purulent secretions [22].

SH in adult population
Gelardi et al. noted that in patients with allergic and non allergic rhinitis the addition of SH to intranasal corticosteroids and systemic antihistamines achieved a significant reduction of the neutrophil count observed on nasal cytology with an improvement of the main nasal symptoms and endoscopic parameters compared to what obtained in patients treated with intranasal corticosteroids and systemic antihistamines without SH [23]. Cassandro et al. showed that in patients with chronic respiratory syndrome (CRS) and nasal polyposis (NP) the addition of nebulized SH to intranasal corticosteroid spray, produced a rapid and significant improvement in nasal symptom score, endoscopic parameters score, radiologic score, rhinomanometry and saccharine clearance test with respect to control group [24]. An interesting study performed by Modrzyński on empty nose syndrome (ENS) and atrophic rhinitis, pointed out that submucosal injections of SH into the inferior nasal concha and under the mucous membrane of the septum, achieved both a significant improvement of nasal airflow sensation, and less severe dryness of secretion and crusting [25]. Casale et al., in order to minimize symptoms and prevent exacerbation of CRS tested an alternative therapeutic option to nasal douches with saline solution after FESS; they investigated and proved that intermittent treatment with topical 9 mg SH through nasal washes (during 10 days per month, over 3 months during winter months) both significantly improved quality of life in CRS patients with sino-nasal symptoms and reduced mucosal edema of the middle turbinate and nasal secretions, compared to what observed in patients only treated with saline solution [27].

SH in upper digestive airways
Recent data from literature highlight the use of SH as topical treatment for oral chronic inflammation, as well; in particular some authors reported that topical application of 0.2% SH gel twice daily for 2 weeks seems to be an effective and safe therapy in patients with recurrent aphthous ulcers (RAU) [27] and oral ulcers of Behçet's disease (BD) [7]. Barber C. et al. investigated in this preliminary study the effect of SH on radiotherapy induced oral mucositis (OM) and found that the use of Gelclair (SH-four times during the 24-h period, 30 min before meals) is no more effective than current standard practice (Sucralfate and Mucaine) in relieving general pain, pain on speaking, ability to eat and drink, especially for the most severe cases of OM [28] In the last study we examined in our present review, Palmieri B et al. were the first to evaluate the potential role of SH in adults affected by gastro-esophageal reflux disease (GERD), showing that a treatment with a fixed oral combination of SH and chondroitin-sulphate for 14 days, produced a fast relief of GERD symptoms in adults with symptoms of non-erosive GERD and a low response to PPIs [29].

Discussion
Hyaluronan is a non-sulfated glycosaminoglycan found in the extracellular matrix of all vertebrate tissues, which plays a multifunctional role in scar-free wound healing, while also being essential in the homeostasis of physiology in UADT 5,19) . Literature data emerged from our analysis allow us to affirm that topical SH can be useful as coadjutant treatment in the postoperative period both for nasal and sinus surgery as a supportive treatment for a faster improvement of nasal breathing and to reduce patients' discomfort in the postoperative phase [15][16][17][18][19].
Despite the advances in surgical technique, poor wound healing after endoscopic sinus and nasal surgery still represents a significant problem, indeed. Due to the proximity of denuded surfaces and the presence of clot, the restructured sinus anatomy is prone to subsequent development of synechiae, ostial stenosis, extensive crusts, scar contraction, impairment of ventilation of the para-nasal sinuses due to the presence of secretions, oedema, bleeding, thus frustrating the good results obtained from a well-performed surgical procedure [20]. In order to solve these problems, otolaryngologists have proposed several different solutions although not uniformly driven by evidence-based outcome research: the insertion of physical barriers, such as packing and stents (absorbable and non absorbable); repeated postoperative debridement and irrigation to remove eschars, crusts, and nascent synechiae; modifications of surgical technique, such as resection and medial adhesion of the middle turbinate; postoperative systemic or topical corticosteroids to discourage the development of granulation and scar tissue. Anyway, while these measures haven't often a significant impact in wound healing, a foreign body stent has even the potential to exacerbate. For the above mentioned reasons and given the accessibility of the sinus cavities after endoscopic sinus surgery (ESS), topical therapies are preferred for all patients [30]. Due to its positive action on scar-free wound healing and on the homeostasis of upper airways [19], topical administration of SH following FESS adds further benefits over saline solution given alone. Interesting results show potential benefits of SH when given to not-operated CRS patients as preventive therapy to minimize symptoms and prevent from exacerbations of chronic rhino-sinusitis [19]. Notably, SH was proposed in patients affected by empty nose syndrome (ENS) along with atrophic rhinitis. Both of these clinical conditions are characterized by excessive widening of the nasal meatus and excessive patency, so patients complain about chronic dryness of the nose, dyspnea sensation, headache, and depressive states. The treatment of ENS and the resulting rhinitis is a complicated clinical problem that is difficult to solve [26]. As far as conservative treatment is concerned, several different strategies are recommended such as moisturization of the mucous membrane (drops, gels, and/or inhalations) whereas current surgical therapeutic options give conflicting results. SH, unlike other materials, exhibits better water absorption. This might be an additional advantage for patients suffering from ENS and atrophic rhinitis because it contributes to better moisturize the mucous membrane [25].
It should also be stressed that in pediatric population, SH has been used for recurrent upper respiratory tract infections and, associated with tobramycin, in patients with CF, proving to be effective in both of these clinical conditions by acting on nasal muco-ciliary clearance and on nasal muco-ciliary defense against injury induced by bacterial products [21,25].
Last but not least, it worths pointing out that SH have been used for years in chronic inflammatory mouth disease and recently it was proposed to relief symptoms in patients suffering from GERD, as well [29].
Finally, it should be emphasize that topical therapies offer an improved ability to deliver high concentrations of pharmacologic agents to the mucosa of the aerodigestive tract for their localized, rather than systemic effects [30]. On this pourpose it worths underlining that while particles of diameter greater to 10 μ have been proved not to generally exceed the rhino-pharyngeal cavity, particles with a diameter less than 10 μ more have been shown to easily reach lower airways, as show in Fig 2.

Conclusion
Today SH is widely used in many branches of medicine and the potential applications of SH in the chronic inflammatory upper aero digestive tract are various.
Data deriving from the present review of 16 clinical studies demonstrate that, due to its positive action on tissue repair and wound healing, topical administration of SH plays a pivot role in the postoperative phase of patients undergoing FESS, and positive results are generally observed in all the patients suffering from UADT, with a significant improvement of their quality of life. The strong connection between particle diameter and site of the high concentration of nebulized particles in the upper aero-digestive tract suggests that it should be mandatory to carefully chose the nebulizer device to get better therapeutic results [30]. However, further studies on larger populations are needed to confirm these encouraging results.
Supporting Information S1 List of Excluded Studies. List of excluded studies ordered according to the reason for which they were excluded.