The physiology of normal sinuses is maintained by three components that allow effective and continuous clearance of secretion. Predisposition to bacterial sinusitis occurs when any of the following three components malfunctions:
1. normal sinus secretion
2. properly functioning cilia
3. patent sinus ostia
The sinus mucosa produces intracavitary mucus that is propelled by
the cilia through the ostia into the nasal cavity for drainage to the nasopharynx.
The Mucociliary Apparatus
• It is made of the mucous blanket and ciliated epithelium.
• The mucous blanket consists of two layers:
a) A gel layer, which is superficial viscid fluid that traps particles such as bacteria and debris
b) A serous sol layer, which is the underlying fluid that bathes the cilia, enabling them to beat effectively
• The cilia move quickly in a forward stroke, and return backward in a slow motion.
• The cilia tips touch the gel layer only during their movement forward, thus moving particles in that direction.
• The cilia beat rapidly (1000 strokes/minute) and clearance time of particulate material is approximately 10 minutes.
The Mucociliary Apparatus
The mucous blanket
• It harbors
c) immunoglobulins (IgG and IgA)
• It changes 2 to 3 times per hour and normal mucus does not accumulate in the sinus cavity.
• Changes in the mucus quality can impair the cilia clearance activity. Causes for such changes are cystic fibrosis, infection, dehydration asthma, and abnormal water-electrolyte transport.
Causes of alteration in mucus quality and quantity
(inducing increased viscosity and decreased elasticity)
(decreases or increases)
Abnormal water–electrolyte transport
Exposure to pollutants, irritants, and allergens
Goblet cell metaplasia
The cilia motility and the mucous layer adhesiveness protect the respiratory epithelium from bacterial infection. Alteration in the cilia function can lead to fluid accumulation and bacterial infection. Such alteration can be caused by exposure to pollutants, irritants, and allergens, and goblet cell metaplasia. Viral infection can be cytotoxic to the cilia, which may lead to secondary bacterial invasion.
Causes of alterations in ciliary function
Decrease in beating frequency
Loss of ciliated cells
Exposure to inflammatory mediators (e.g., cytokines)
Cytotoxins produced by viruses and bacteria
Cell death as a result of infection (viruses or bacteria)
Irritants and pollutants
Loss of metachronous coordination
Increase in nasal air flow
Synechiae and scarring
Surgery (removal of tissue)
The ostia role in bacterial sinusitis
• The small size of the ostia (1–6 mm) predisposes them to frequent obstruction.
• They can be blocked by factors that cause mucosal swelling or mechanical obstruction.
The most common causes of ostial obstruction are viral URTI and allergy.
SINUS CYCLES IN HEALTH AND ILLNESS
The healthy sinus cycle depends on
• Normal (composition and quantity) mucosal secretions, fluid-containing antibodies (especially IgA)
• Mucosal absorption and clearance, through an open ostium, of agents that can cause mucosal damage (pollutants, irritants, and organisms)
• Elimination of residual infection by the host defenses
The cycle leading to bacterial sinusitis is initiated by
• Ostial obstruction including mucosal congestion, stagnation of secretions, and decreases in the pH and changes in the mucosal metabolism
• Subsequent damage to the epithelium and cilia, generating favorable conditions for bacterial growth; this causes greater retention of secretions, bacterial infection, and mucosal thickening
Without reversal of ostial blockage, allowing drainage, this cycle can perpetuate, thus leading to chronic bacterial sinusitis (see figure below).