The rheology of nasal mucus: a review

Mucus as a barrier to lipophilic drugs

Mucus is a complex hydrogel, comprising glycoproteins, lipids, salts, DNA, enzymes and cellular debris, covering many epithelial surfaces in the human body. Once secreted, mucin forms a barrier to protect the underlying tissues against the extracellular environment. Mucus can therefore adversely affect the absorption or action of drugs administered by the oral, pulmonary, vaginal, nasal or other routes. Solubility and lipophilicity are key factors determining drug absorption, as a drug has to be soluble in the body fluids at the site of absorption and must also possess enough lipophilicity to permeate the biological membrane. Evidence has accumulated over the past 40 years indicating that poorly soluble drugs will interact with mucus glycoprotein. Studies of the permeability of native or purified mucous gels are important when it comes to understanding the relative importance of hindered diffusion versus drug binding in mucous layers. This review highlights the current understanding of the drug-mucin interaction and also examines briefly the interaction of polymers and particles with the mucus matrix. While the concept of mucoadhesion was thought to provide an intensified and prolonged contact to mucosal absorption sites, mucopenetrating properties are nowadays being discussed for (nano)particulate carriers to overcome the mucus as a barrier and enhance drug delivery through mucus.

Medically unexplained symptoms and somatisation in ENT

Background:

Somatisation has been described as the perception of a physiological event influenced by emotion.

Method:

A review of the medical literature was carried out using the following Medical Subject Headings: somatisation (which identified 357 articles), medically unexplained symptoms (749 articles), unexplained or idiopathic dizziness (142 articles), tinnitus (360 articles), catarrh (1068 articles) and globus pharyngeus (3114 articles).

Results:

Up to 40 per cent of out-patient attendances have medically unexplainable symptoms. In ENT clinics, this includes patients with dizziness, tinnitus, ‘pseudo’ eustachian tube dysfunction, being ‘unable to hear’, catarrh and postnasal drip, atypical facial pain, globus pharyngeus, and functional dysphonia. Medical explanations of these symptoms often differ from patients' perceptions. Demonstrating normal test results and providing reassurance have little effect on patients' doubts and anxieties. Consultations that recognise the symptoms and their impact, and offer a tangible and involving explanation are more likely to satisfy and empower patients.

Conclusion:

The treatment of medically unexplained symptoms has changed in recent years; there is now more emphasis on psychological factors due to an association with anxiety and depression.

A computational study of the hydrodynamics in the nasal region of a hammerhead shark (Sphyrna tudes): implications for olfaction

The hammerhead shark possesses a unique head morphology that is thought to facilitate enhanced olfactory performance. The olfactory chambers, located at the distal ends of the cephalofoil, contain numerous lamellae that increase the surface area for olfaction. Functionally, for the shark to detect chemical stimuli, water-borne odors must reach the olfactory sensory epithelium that lines these lamellae. Thus, odorant transport from the aquatic environment to the sensory epithelium is the first critical step in olfaction. Here we investigate the hydrodynamics of olfaction in Sphyrna tudes based on an anatomically-accurate reconstruction of the head and olfactory chamber from high-resolution micro-CT and MRI scans of a cadaver specimen. Computational fluid dynamics simulations of water flow in the reconstructed model reveal the external and internal hydrodynamics of olfaction during swimming. Computed external flow patterns elucidate the occurrence of flow phenomena that result in high and low pressures at the incurrent and excurrent nostrils, respectively, which induces flow through the olfactory chamber. The major (prenarial) nasal groove along the cephalofoil is shown to facilitate sampling of a large spatial extent (i.e., an extended hydrodynamic "reach") by directing oncoming flow towards the incurrent nostril. Further, both the major and minor nasal grooves redirect some flow away from the incurrent nostril, thereby limiting the amount of fluid that enters the olfactory chamber. Internal hydrodynamic flow patterns are also revealed, where we show that flow rates within the sensory channels between olfactory lamellae are passively regulated by the apical gap, which functions as a partial bypass for flow in the olfactory chamber. Consequently, the hammerhead shark appears to utilize external (major and minor nasal grooves) and internal (apical gap) flow regulation mechanisms to limit water flow between the olfactory lamellae, thus protecting these delicate structures from otherwise high flow rates incurred by sampling a larger area.

Hydrogels in mucosal delivery

The concept of mucoadhesion and the molecular design requirements for the synthesis of mucoadhesive agents are both well understood and, as a result, hydrogel formulations that may be applied to mucosal surfaces are readily accessible. Nanosized hydrogel systems that make use of biological recognition or targeting motifs, by reacting to disease-specific environmental triggers and/or chemical signals to affect drug release, are now emerging as components of a new generation of therapeutics that promise improved residence time, faster response to stimuli and triggered release.

The conversion from sensation to symptom: the case of catarrh, a qualitative study

OBJECTIVE

There is significant variation in symptom tolerance before seeking healthcare advice and it has recently been postulated that there may be a similar variation in the degree to which individuals tolerate deviations in physiological body sensations before considering them symptoms. This study looked to explore this transition from sensation to symptom more closely using the clinical entity of chronic catarrh - a frequently presenting problem which represents a putative alteration of a physiological process.

DESIGN

Qualitative study using semi-structured interviews.

PARTICIPANTS

19 adult patients presenting with chronic catarrh, persistent throat clearing or post-nasal drip.

SETTING

Secondary care institute in North of England.

RESULTS

Subjects' accounts revealed three changes in perception of nasopharyngeal mucus that triggered the transition from sensation to symptom: an apparent change in viscosity, quantity, or constancy. Such changes were invariably deemed to have a consequence (threat to wellbeing, social impact, or source of frustration) and invariably drew a response from the sufferer to limit these consequences.

CONCLUSIONS

Symptoms representing an aberration of normal body sensations likely develop over time in a series of recognizable phases. Discriminatory markers appear to exist which delineate those body sensations accepted and those considered symptoms. These are discussed with the use of a novel symptom evolution pathway diagram.

Nanoscale Particles for Lung Delivery of siRNA

Small interfering RNAs (siRNAs) are potent molecules capable of blocking gene expression after entering cell cytoplasm. Despite their strong efficacy, they need to be carried by nanoscale delivery systems that can protect them against degradation in biological fluids, increase their cellular uptake and favor their subcellular distribution. Several studies have highlighted the potential of local pulmonary delivery of siRNAs for the treatment of lung diseases. For this purpose, nanoscale delivery systems were addressed to target passively or actively the target cell. This review discusses the possibilities of approaching lung delivery of nanoscale particles carrying siRNAs.

Physiology and pathophysiology of respiratory mucosa of the nose and the paranasal sinuses

In this review, anatomy and physiology of the respiratory mucosa of nose and paranasal sinuses are summarized under the aspect of its clinical significance. Basics of endonasal cleaning including mucociliary clearance and nasal reflexes, as well as defence mechanisms are explained. Physiological wound healing, aspects of endonasal topical medical therapy and typical diagnostic procedures to evaluate the respiratory functions are presented. Finally, the pathophysiologies of different subtypes of non-allergic rhinitis are outlined together with treatment recommendations.

Differences between and within human parotid saliva and nasal mucus cAMP and cGMP in normal subjects and in patients with taste and smell dysfunction

BACKGROUND

We previously described some of the moieties in human saliva and nasal mucus including cyclic nucleotides. However, comparison of levels of these latter moieties in saliva and nasal mucus has not been performed and meaning of differences found has not been discussed.

PURPOSE

To compare the levels of cAMP and cGMP in saliva and nasal mucus and to describe the differences in their concentrations and function.

METHODS

cAMP and cGMP in saliva and nasal mucus were compared in normal subjects and patients with taste and smell dysfunction by use of a spectrophotometric colorimetric ELISA.

RESULTS

Both cAMP and cGMP were present in saliva and nasal mucus of normals and patients with levels of both moieties lower in patients than in normals. In normals, cAMP is 6½ times higher in saliva than in nasal mucus whereas cGMP in nasal mucus is 2½ times higher than in saliva. In patients, these differences persist but are less robust. In normals, within saliva, cAMP is 9½ times higher than cGMP whereas within nasal mucus cAMP is half the level of cGMP. In patients, within saliva, these differences persist but at variable differences.

CONCLUSIONS

Both saliva and nasal mucus cAMP and cGMP play roles in taste and smell function, and differences in their concentrations may offer insight into these roles. In nasal mucus, cGMP may be more relevant than cAMP in activity of olfactory epithelial cell function. In saliva, cAMP may be more relevant as a growth factor in taste bud function than cGMP.

Disruption of the mucus barrier by topically applied exogenous particles

The mucus barrier is well established as a formidable barrier to exogenous substances and forms the first line of defense for mucosal surfaces. Drugs and particle systems are known to be significantly hindered via a variety of interactions with mucus, and some efforts have been reported that can mitigate these interactions. We investigated topically applied particulate systems (nano and micro) for their potential to interact with mucus and influence on the diffusion of model drugs across the mucus barrier. Functionalized polystyrene nanoparticles and microparticles and diesel particulate matter were topically applied to established in vitro mucus models. Particle treated mucus was then assessed, compared to controls, for drug permeation rates. The average permeation rate of drugs increased 2-fold following the application of particles to mucus compared to permeation of the same drug through mucus alone. In some cases permeation enhancement of small model drugs was over 5 times that of controls. Assessment of particle physicochemical properties also indicated that significant interactions occurred between mucus and the particles as determined by zeta potential changes and size changes. Collectively this work supports the hypothesis that topically applied particles interact with the mucus barrier causing disruption of this barrier allowing for increased drug permeation. These findings have implications for improved drug delivery and enhanced environmental exposure to exogenous substances.

Biofluid mechanics of special organs and the issue of system control. Sixth International Bio-Fluid Mechanics Symposium and Workshop, March 28-30, 2008 Pasadena, California

In the field of fluid flow within the human body, focus has been placed on the transportation of blood in the systemic circulation since the discovery of that system; but, other fluids and fluid flow phenomena pervade the body. Some of the most fascinating fluid flow phenomena within the human body involve fluids other than blood and a service other than transport--the lymphatic and pulmonary systems are two striking examples. While transport is still involved in both cases, this is not the only service which they provide and blood is not the only fluid involved. In both systems, filtration, extraction, enrichment, and in general some "treatment" of the fluid itself is the primary function. The study of the systemic circulation has also been conventionally limited to treating the system as if it were an open-loop system governed by the laws of fluid mechanics alone, independent of physiological controls and regulations. This implies that system failures can be explained fully in terms of the laws of fluid mechanics, which of course is not the case. In this paper we examine the clinical implications of these issues and of the special biofluid mechanics issues involved in the lymphatic and pulmonary systems.

The effect of viscoelasticity on the stability of a pulmonary airway liquid layer

The lungs consist of a network of bifurcating airways that are lined with a thin liquid film. This film is a bilayer consisting of a mucus layer on top of a periciliary fluid layer. Mucus is a non-Newtonian fluid possessing viscoelastic characteristics. Surface tension induces flows within the layer, which may cause the lung's airways to close due to liquid plug formation if the liquid film is sufficiently thick. The stability of the liquid layer is also influenced by the viscoelastic nature of the liquid, which is modeled using the Oldroyd-B constitutive equation or as a Jeffreys fluid. To examine the role of mucus alone, a single layer of a viscoelastic fluid is considered. A system of nonlinear evolution equations is derived using lubrication theory for the film thickness and the film flow rate. A uniform film is initially perturbed and a normal mode analysis is carried out that shows that the growth rate g for a viscoelastic layer is larger than for a Newtonian fluid with the same viscosity. Closure occurs if the minimum core radius, R(min)(t), reaches zero within one breath. Solutions of the nonlinear evolution equations reveal that R(min) normally decreases to zero faster with increasing relaxation time parameter, the Weissenberg number We. For small values of the dimensionless film thickness parameter epsilon, the closure time, t(c), increases slightly with We, while for moderate values of epsilon, ranging from 14% to 18% of the tube radius, t(c) decreases rapidly with We provided the solvent viscosity is sufficiently small. Viscoelasticity was found to have little effect for epsilon>0.18, indicating the strong influence of surface tension. The film thickness parameter epsilon and the Weissenberg number We also have a significant effect on the maximum shear stress on tube wall, max(tau(w)), and thus, potentially, an impact on cell damage. Max(tau(w)) increases with epsilon for fixed We, and it decreases with increasing We for small We provided the solvent viscosity parameter is sufficiently small. For large epsilon approximately 0.2, there is no significant difference between the Newtonian flow case and the large We cases.

Nanoparticles reveal that human cervicovaginal mucus is riddled with pores larger than viruses

The mechanisms by which mucus helps prevent viruses from infecting mucosal surfaces are not well understood. We engineered non-mucoadhesive nanoparticles of various sizes and used them as probes to determine the spacing between mucin fibers (pore sizes) in fresh undiluted human cervicovaginal mucus (CVM) obtained from volunteers with healthy vaginal microflora. We found that most pores in CVM have diameters significantly larger than human viruses (average pore size 340 +/- 70 nm; range approximately 50-1800 nm). This mesh structure is substantially more open than the 15-100-nm spacing expected assuming mucus consists primarily of a random array of individual mucin fibers. Addition of a nonionic detergent to CVM caused the average pore size to decrease to 130 +/- 50 nm. This suggests hydrophobic interactions between lipid-coated "naked" protein regions on mucins normally cause mucin fibers to self-condense and/or bundle with other fibers, creating mucin "cables" at least three times thicker than individual mucin fibers. Although the native mesh structure is not tight enough to trap most viruses, we found that herpes simplex virus (approximately 180 nm) was strongly trapped in CVM, moving at least 8,000-fold slower than non-mucoadhesive 200-nm nanoparticles. This work provides an accurate measurement of the pore structure of fresh, hydrated ex vivo CVM and demonstrates that mucoadhesion, rather than steric obstruction, may be a critical protective mechanism against a major sexually transmitted virus and perhaps other viruses.

The correlation between nasal symptom and mucociliary clearance in allergic rhinitis

OBJECTIVE/HYPOTHESIS

The Allergic Rhinitis and Its Impact on Asthma (ARIA) classification of allergic rhinitis (AR) is based on the severity and duration of nasal symptoms. Whether the nasal symptoms actually represent underlying nasal inflammation is unclear. The aim of this study was to evaluate the correlation between nasal symptoms and nasal inflammation using mucociliary clearance time (MCCT) in AR.

STUDY DESIGN

A prospective cross-sectional study.

METHODS

73 AR patients were classified according to ARIA class: mild or moderate-severe intermittent AR (MIAR or MSIAR) and mild or moderate-severe persistent AR (MPAR or MSPAR). Each nasal symptom was scored as 1 to 3 on a severity scale (mild-moderate-severe). The sum of the individual nasal symptom scores gave the total symptoms score (TSS). MCCT was determined with the charcoal-saccharin method. MCCTs between ARIA classes were compared and correlations between TSS or days with symptoms per week (DSW) and MCCT were analyzed.

RESULTS

Of the patients, 67.1% were moderate-severe degree. MSPAR had the worst MCCT, followed by MSIAR, MPAR, and MIAR (mean MCCTs of 14.32, 13.87, 11.94, and 10.28 minutes, respectively). TSS was well correlated with MCCT (P = .538, P < .001). DSW was also correlated with MCCT, but did not reach statistical significance (r = 0.217, P = .065). The mean MCCT of overall nasal symptoms increased along with each score step and a significant difference was noted between scores 2 and 3 (P < .001).

CONCLUSIONS

A high percentage of moderate-severe disease and a significant correlation of the severity and MCCTs suggest an important heterogeneity in this disease severity group. Discriminating between moderate and severe rhinitis should help to obtain homogeneous populations and develop improved disease management strategies.

Micro- and macrorheology of mucus

Mucus is a complex biological material that lubricates and protects the human lungs, gastrointestinal (GI) tract, vagina, eyes, and other moist mucosal surfaces. Mucus serves as a physical barrier against foreign particles, including toxins, pathogens, and environmental ultrafine particles, while allowing rapid passage of selected gases, ions, nutrients, and many proteins. Its selective barrier properties are precisely regulated at the biochemical level across vastly different length scales. At the macroscale, mucus behaves as a non-Newtonian gel, distinguished from classical solids and liquids by its response to shear rate and shear stress, while, at the nanoscale, it behaves as a low viscosity fluid. Advances in the rheological characterization of mucus from the macroscopic to nanoscopic levels have contributed critical understanding to mucus physiology, disease pathology, and the development of drug delivery systems designed for use at mucosal surfaces. This article reviews the biochemistry that governs mucus rheology, the macro- and microrheology of human and laboratory animal mucus, rheological techniques applied to mucus, and the importance of an improved understanding of the physical properties of mucus to advancing the field of drug and gene delivery.

Mucus-penetrating nanoparticles for drug and gene delivery to mucosal tissues

Mucus is a viscoelastic and adhesive gel that protects the lung airways, gastrointestinal (GI) tract, vagina, eye and other mucosal surfaces. Most foreign particulates, including conventional particle-based drug delivery systems, are efficiently trapped in human mucus layers by steric obstruction and/or adhesion. Trapped particles are typically removed from the mucosal tissue within seconds to a few hours depending on anatomical location, thereby strongly limiting the duration of sustained drug delivery locally. A number of debilitating diseases could be treated more effectively and with fewer side effects if drugs and genes could be more efficiently delivered to the underlying mucosal tissues in a controlled manner. This review first describes the tenacious mucus barrier properties that have precluded the efficient penetration of therapeutic particles. It then reviews the design and development of new mucus-penetrating particles that may avoid rapid mucus clearance mechanisms, and thereby provide targeted or sustained drug delivery for localized therapies in mucosal tissues.

Rapid transport of large polymeric nanoparticles in fresh undiluted human mucus

Nanoparticles larger than the reported mesh-pore size range (10-200 nm) in mucus have been thought to be much too large to undergo rapid diffusional transport through mucus barriers. However, large nanoparticles are preferred for higher drug encapsulation efficiency and the ability to provide sustained delivery of a wider array of drugs. We used high-speed multiple-particle tracking to quantify transport rates of individual polymeric particles of various sizes and surface chemistries in samples of fresh human cervicovaginal mucus. Both the mucin concentration and viscoelastic properties of these cervicovaginal samples are similar to those in many other human mucus secretions. Unexpectedly, we found that large nanoparticles, 500 and 200 nm in diameter, if coated with polyethylene glycol, diffused through mucus with an effective diffusion coefficient (D(eff)) only 4- and 6-fold lower than that for the same particles in water (at time scale tau = 1 s). In contrast, for smaller but otherwise identical 100-nm coated particles, D(eff) was 200-fold lower in mucus than in water. For uncoated particles 100-500 nm in diameter, D(eff) was 2,400- to 40,000-fold lower in mucus than in water. Much larger fractions of the 100-nm particles were immobilized or otherwise hindered by mucus than the large 200- to 500-nm particles. Thus, in contrast to the prevailing belief, these results demonstrate that large nanoparticles, if properly coated, can rapidly penetrate physiological human mucus, and they offer the prospect that large nanoparticles can be used for mucosal drug delivery.

A blinded, randomized, controlled study on the effect of buffered 0.9% and 3% sodium chloride intranasal sprays on ciliary beat frequency

BACKGROUND

There are conflicting reports regarding the effect of intranasal saline sprays on the various components of the mucociliary clearance system. This study evaluates the effect of normal saline and hypertonic saline on the ciliary beat frequency (CBF).

METHODS

Eight healthy volunteers were randomized to receive 0.9% saline spray to one nostril and 3.0% saline spray to the other. Ciliated cells were collected, and the CBF was calculated using computerized microphotometry.

RESULTS

Although neither solution significantly altered the CBF from baseline measurements, there was a significant difference between the CBFs of the two solutions at 5 minutes postadministration (9.1 Hz with 0.9% saline, 10.1 Hz with 3.0% saline, P < .05). This was a transient effect and was not seen when cells were examined at 60 minutes postadministration.

CONCLUSION

The administration of hypertonic saline results in a significantly faster CBF 5 minutes after administration. This effect is not seen 60 minutes after administration.

Mucous Systems Show a Novel Mechanical Response to Applied Deformation

Mucous secretions have a wide range of biological functions that are intimately linked with their rheological properties. In addition, many mucous secretions are exposed to significant stress and deformation during physiological function. This study has examined the rheological response of three mucous systems, native pig gastric mucus, purified mucin gels, and mucin alginate gels, to increasing applied stress to a level sufficient to induce flow behavior. A novel, frequency-dependent stress hardening was observed in all three systems. This hardening behavior may play a significant role in the ability of mucous systems to resist mechanical disruption in the physiological state.

Mucociliary clearance after external dacryocystorhinostomy

We investigated the nasal mucociliary clearance time in 17 patients who underwent a primary unilateral external dacryocystorhinostomy operation. Mucociliary clearance function in both nasal cavities was assessed with the saccharin test. The results were compared with each other using paired samples t-test. We found significantly worse mucociliary clearance time on the operated side (mean: 6.3 min) than on the non-operated side (mean: 5.5 min) (P = 0.004). External dacryocystorhinostomy negatively affected the mucociliary clearance function in this study.

The nose and paranasal sinuses physiology and anatomy

The paranasal sinuses and nose are much more than two cavities behind a projection on the centre of the face. They humidify, filter, warm, and sense what we breathe. The anatomy and physiology interact forming a dynamic system. The anatomy, airflow, nasal resistance, its turbulence, the nasal cycle - a process by which the turbinates or cushions lining the nose alternatively swell and congest from side to side, can all potentially influence the nasal delivery of drugs. Along with these factors mucus rheology and mucociliary clearance influence the removal of substances delivered to the nose. The health of the nose and its immunological response to what is inhaled, be it pollutants, allergens, drugs or vaccines, all need to be considered. It is a fascinating sensor for the body, not only detecting the potentially harmful substances such as smoke, but its psychosexual aspects have far reaching implications and the olfactory pathway has potential as a pathway for the delivery of drugs.

Altered MUC1 and MUC2 glycoprotein expression in laryngeal cancer

The purpose of this study was to determine whether mucin expression is altered in laryngeal cancer. MUC1 and MUC2 mucin expression was examined in biopsy specimens from 80 patients that comprised 23 laryngeal dysplasias, 36 laryngeal carcinomas, and 21 normal larynx control specimens. High MUC1 expression was found in all 3 groups (P = 0.689, Fisher exact test). However, significantly higher levels of MUC2 expression were detected in carcinomas compared with dysplasias and control specimens (P = 0.009, Fisher exact test). Altered MUC2 expression may be an important step in carcinogenesis in laryngeal cancer.

Nasal mucociliary transport in pregnancy

Using the saccharin method, we performed tests of mucociliary function four times during pregnancy and once one month after delivery in 27 women. As the transport distance for saccharin varied from 37 to 65 mm, we used the transport speed for evaluation. Pregnancy rhinitis affects at least 20% of pregnancies. The mucociliary transport speed was higher in the group of women with pregnancy rhinitis, and was reduced during pregnancy in the group of women without that condition. We found no significant correlation between mucociliary transport speed and objectively registered nasal peak expiratory flow index. The pathophysiology of pregnancy rhinitis is not known, but is possibly multifactorial. The changes occurring in the nasal mucociliary transport system during normal pregnancy and in pregnancy rhinitis need further studies.

The effect of pH of douching solutions on mucociliary clearance

The effect of the pH of hypertonic saline nasal douching solutions on mucociliary clearance was studied in order to investigate the common notion that such solutions should be buffered alkaline. Thirty normal subjects were included in a randomised controlled crossover trial. Mucociliary clearance was measured by the saccharin clearance time. There was no difference in mucociliary clearance after douching with a non-buffered solution and a solution buffered to pH 8. However both solutions significantly improved mucociliary clearance compared to the baseline (P < 0.001) probably on account of their hypertonicity.