Describe mucociliary clearance and its disorders.

Mucociliary Clearance

• Mucociliary clearance is the coordinated transport of the mucous blanket by cilia toward the nasopharynx, where it is swallowed.
• It is the principal defence mechanism of the entire respiratory tract, and its failure underlies a large part of sinonasal disease.
• The mucous blanket has two layers, and the distinction is essential: a deep, watery sol (periciliary) layer in which the cilia beat freely; and a superficial, viscous gel layer which floats on it and traps particles
• The cilia beat within the sol layer and their tips engage the gel layer only on the effective stroke, propelling it forward; the recovery stroke is made entirely within the sol layer.
• This explains why dehydration IS SO damaging: if the sol layer thins, the cilia can no longer beat freely and the gel layer becomes immobile — which is exactly what happens in cystic fibrosis, where defective chloride transport dehydrates the periciliary layer
• Cilia beat at 10 to 20 Hz, moving mucus at 1 cm per minute, clearing the nose in 20 to 30 minutes
• The direction of flow is fixed and genetically determined.
• Hence an inferior meatal antrostomy is bypassed: mucus is carried past it to the natural ostium, and if that is obstructed the sinus remains diseased. This is why FESS enlarges the natural ostium rather than creating a new window
• An accessory ostium may cause recirculation — mucus leaving the natural ostium is drawn back in through the accessory one, circulating indefinitely and causing persistent symptoms
• Factors impairing ciliary function — drying and low humidity, the commonest and most correctable; smoking.
• Primary ciliary dyskinesia (immotile cilia syndrome) — an autosomal recessive defect of the dynein arms of the ciliary axoneme, causing chronic rhinosinusitis, otitis media with effusion, bronchiectasis and infertility (immotile sperm tails)
• Kartagener syndrome is the subset with situs inversus, chronic sinusitis and bronchiectasis — the situs inversus arising because embryonic nodal cilia determine laterality, so half of affected patients have their organs reversed by chance
• Diagnosis — very low nasal nitric oxide as a screen; the saccharin test (saccharin placed on the inferior turbinate should be tasted within 20 to 30 minutes); and ciliary biopsy with electron microscopy and high-speed video analysis
• Cystic fibrosis impairs clearance by dehydrating the periciliary layer rather than by a ciliary defect, and must be excluded in any child with nasal polyps