Recently, there has been a focus on the sensation of irritation that precedes the motor act of coughing, the UTC sensation, which triggers the cough. It was initially thought that cough could be only generated by a brainstem neural network. Instead, cough can also be initiated by pharyngeal stimulation. This indicates that sensory nerves in the pharynx may be also involved in generation of the cough reflex associated with URTI.
“Because cough is an important mechanism for protection and clearance of the lower respiratory tract, it may be that sensory areas important for physiological initiation of swallowing are equally important and sensitive in generation of cough,” stated Murgia et al (2020).
Upper airway irritatation, caused by mucosal inflammation, reaches the cerebral cortex and elicits voluntary cough. The cortical response to the information from the pharynx is important because tussigenic stimuli in turn increase the UTC.
URTI and other organisms can cause acute cough by irritative stimuli acting on reflexogenic areas of the pharyngeal mucosa. “URTI-associated cough may be determined by inflammatory mediators acting on the sensory terminations of upper airways and enhancing the UTC sensation. Information coming from the pharynx can be encoded into a conscious awareness of airway irritations leading to generation of the UTC, which in turn may facilitate behavioral or evoked coughing finalised to clear the airways,” the authors stated.
Murgia et al (2020) postulate that a new, innovative therapeutic approach should be pursued that protects and hydrates the pharynx mucosa and modulates the sensory feedback from the upper airways to the cortical neural network, to control the UTC.
The authors believe the ideal characteristics of cough syrup can be obtained by a complex mixture of natural substances with lubricant, demulcent, and protective barrier effects. “This ideal compound can act as a humectant and can, thanks to the natural sweetness of a component such as honey, stimulate salivation and send sweet gustatory stimuli to the brain. The sweet taste and the viscous nature of the syrup are fundamental properties of ideal cough medication. In fact, sweet substances stimulate C fibre sensory neurons through transient receptor vanilloid-1 channels that are involved in cough reflex,” they stated.
There can be a number of factors that play a decisive role in triggering UTC, including: mucosal dehydration, and mechanical stimuli, caused by pharyngeal dripping of viscous nasal mucus, viruses, bacteria, inflammatory mediators and irritant substances, play a decisive role in triggering the UTC. A demulcent can form a soothing film over a mucous membrane, reducing pain and inflammation. Up to 85% of the benefit of cough syrups depends on the physical and chemical effects of the syrup that exerts a demulcent action, namely smoothing. These features can be better obtained with complex natural substances such polysaccharides with bioadhesive and demulcent activity. These natural substances adhere to the pharynx mucosa and exert a mechanical barrier effect in addition to the demulcent activity, ultimately protecting the mucosa from irritative stimuli.
An example of an ingredient with these properties is Marshmallow root (Althaea officinalis), that’s been used as a remedy for thousands of years to treat respiratory conditions. Previous studies showed that Althaea officinalis possesses antimicrobial, anti-inflammatory, immunomodulatory, demulcent, soothing, antitussive and many other pharmacological effects. Its therapeutic nature is due in part to the mucilage it contains. The demulcent effects of Althaea officinalis are due to its high content of polysaccharide hydrocolloids, which form a protective coating on the oral and pharyngeal mucosa, soothing local irritation and inflammation. Polysaccharides of Marshmallow exhibited statistically significant cough-suppressing activity (Al-Snafi, 2013).
PROTECTIVE PROPERTIES OF MUCILAGE
Mucilage belongs to a class of compounds that is made up of heterogeneous acidic polysaccharides. The adhesive and protective properties of polysaccharides on mucosae are well known. Mucilage adheres to oral mucosae and accelerates healing of mucosa ulcers through two mechanisms: reinforcement of the resistance barrier and the oxygen radical scavenging activity.
Bioadhesion is a well-known physical phenomenon whereby two materials are held together for an extended time by interfacial forces; if it concerns a mucosal tissue, it is named mucoadhesion.
Aqueous extracts of polysaccharide-rich plants are used for treating cough due to their mucoadhesive properties. This is because these properties allow the formation of a polysaccharide layer on the upper airway mucous membrane. This is because Polysaccharides are too large to be absorbed in the pharynx or to be transported to the bronchial mucosa. This layer creates a barrier and therefore the polysaccharides may indirectly modulate the sensitivity of cough receptors and suppress cough. As a result, their effect, when applied to the pharynx, can be explained simply by a local protective and soothing action.
European Scientific Cooperative on Phytotherapy agrees that the mucilage from marshmallow root and mallow flower can cover the mucosa of the mouth and pharynx, protecting them from local irritation. Such protective effect provides efficacy of polysaccharides use in cough management.
REFERENCES: References available on request.