What is the pathophysiology of postnasal drip?

Pathophysiology of Postnasal Drip

The pathophysiology of postnasal drip (now termed Upper Airway Cough Syndrome) remains incompletely understood, but current evidence suggests it involves increased viscosity of nasal secretions, impaired mucociliary clearance, and heightened nasopharyngeal sensitivity rather than simply increased secretion volume. 1, 2

Key Pathophysiological Mechanisms

Altered Secretion Properties

• Increased viscosity of nasal secretions is the most significant pathophysiological factor, with patients demonstrating significantly higher viscosity compared to healthy controls, and this abnormality reverses during symptom-free intervals 2
• Contrary to traditional concepts, the actual volume of nasal secretions is not increased in postnasal drip patients compared to healthy individuals 2
• The sensation of postnasal drip does not correlate with the presence or viscosity of secretions in chronic rhinitis patients, suggesting alternative mechanisms beyond simple mechanical drainage 3

Mucociliary Dysfunction

• Mucociliary clearance is prolonged in patients with postnasal drip, contributing to accumulation and altered movement of secretions 2
• This impaired clearance mechanism works in conjunction with increased viscosity to create the sensation of secretions in the throat 2

Altered Sensory Perception

• Nasopharyngeal sensitivity is significantly reduced in postnasal drip patients, representing a paradoxical hyposensitivity despite increased symptom awareness 2
• This altered sensory function may represent mucosal inflammation causing heightened cough or irritant throat sensory dysfunction rather than true mechanical drainage 3
• Chronic rhinitis patients fail to detect artificially increased viscosity of secretions that healthy subjects readily perceive, suggesting fundamental differences in sensory processing 3

Underlying Inflammatory Processes

Rhinitis-Related Inflammation

• In allergic rhinitis, mucosal inflammation is characterized by tissue infiltration of CD4+ T-lymphocytes and CD25+ T-cells in the submucosa and epithelium, with mast cells, basophils, and eosinophils playing defining roles 1
• A Th2/Th1 cell imbalance favoring Th2 responses drives IgE synthesis and cell recruitment at sites of allergic inflammation 1
• Non-allergic rhinitis produces similar symptoms through different inflammatory pathways, though the exact mechanisms remain less well-defined 1

Rhinosinusitis Contribution

• Bacterial sinusitis, allergic fungal sinusitis, and chronic rhinosinusitis can all trigger postnasal drip through inflammatory mediator release and altered secretion properties 1
• The presence of Staphylococcus aureus nasal colonization worsens outcomes and may contribute to persistent inflammation 4

Controversial Mechanisms

The "Drip" Concept Itself

• The traditional concept of postnasal drip as physical drainage of secretions from nose to pharynx is now questioned, as the actual mechanism may involve direct irritation or inflammation of upper airway cough receptors rather than mechanical drainage 1
• This uncertainty led to the recommendation that "Upper Airway Cough Syndrome" (UACS) be used instead of "postnasal drip syndrome" to more accurately reflect the pathophysiology 1

Proposed but Unlikely Mechanisms

• The naso-bronchial neural reflex has been proposed but lacks strong supporting evidence 1
• Post-nasal drip of inflammatory cells and mediators into the lower airway is considered "most unlikely" as a mechanism 1
• Systemic absorption of inflammatory mediators from the nose into circulation and subsequent bronchial deposition remains unproven 1

Clinical Implications

Diagnostic Challenges

• No objective test exists to quantify postnasal drip or directly prove causation of symptoms 1
• Physical findings like cobblestoning of the oropharyngeal mucosa and mucoid secretions are suggestive but not pathognomonic 1
• Response to specific therapy remains the pivotal factor in confirming the diagnosis, making empiric treatment both diagnostic and therapeutic 1

"Silent" Postnasal Drip

• Approximately 20% of patients have no obvious symptoms of drainage yet still respond to treatment directed at upper airway conditions 5
• This phenomenon further supports the concept that direct mucosal inflammation and altered sensory function, rather than mechanical drainage, drive the pathophysiology 1, 5