Paranasal Sinuses Definition

Four sets of hollow chambers in the face as well as cranial bones make up the paranasal sinuses. They are situated around the nose in the middle of the face (paranasal). The paranasal sinuses are air-warming cells that serve to protect the airway from infections. Other purposes mentioned in theories include lowering overall skull weight and modulating voice tone, albeit these are not fundamental functions.

What are the Paranasal Sinuses?

The frontal, maxillary, ethmoid, and sphenoid bones all include four paired hollow spaces called paranasal sinuses. Air cavities are a term used to describe sinuses. The four paranasal sinuses are coated by mucus-producing cells and epithelium and surround the nasal cavity. The nasal cavity is accessed by all paranasal sinuses.

Air is drawn into the nasal cavity when we breathe through our nostrils. A part of this air reaches the cavity of the paranasal sinuses.

There are four pairs of paranasal sinuses in humans:

• The frontal sinuses extend from the nasal bridge to the lower forehead.
• The maxillary sinus is a fan-shaped sinus that extends through the inner eye corners to the cheeks.
• The ethmoidal sinus is next to the lower eye socket and over the nasal cavity.
• The sphenoidal sinus is located near the pituitary gland in the rear of the nasal cavity.

Paranasal Sinuses: Anatomy

The architecture of the paranasal sinuses is straightforward: they are bone portions that have been hollowed out and covered with a layer of cells.

A layer of pseudostratified (columnar) ciliated epithelium covers the surrounding bone; this strip of cells includes goblet cells as well (mucus-producing cells). Mucus is moved into the nasal cavity by the cilia.

The development of the paranasal sinuses occurs at various periods. Human infants are born with paired maxillary and ethmoid sinuses. Typically, the frontal sinus is developed by age five. Although, it does not completely mature until adolescence, up to 10% of individuals are born without one. Adolescence is when the sphenoid sinus develops.

The nose is the outflow for all paranasal sinuses. This may happen via ostia (small pores), clefts, or gaps between bone compartments. The ostium links the maxillary as well as sphenoid sinuses to the nasal cavity. The ethmoid sinuses have a number of tiny air compartments (air cells) that flow through the gaps between them. The frontal notch, an inverted funnel-shaped duct, empties the frontal sinus.

Although all animals possess paranasal sinuses, not all mammals possess four. In dogs, there are just three paranasal sinuses: the frontal and sphenoidal sinuses, as well as the maxillary recess. Rabbits have a dorsal conchal sinus, maxillary sinus, and sphenoidal sinus instead of frontal and ethmoidal sinuses.

What Are the Functions of Paranasal Sinuses?

The function of the paranasal sinuses is still up for dispute. Numerous ideas have been proposed because of their closeness to the brain, eye, nose, and mouth. These include things like making the skull lighter; filtering, humidifying, and warming inspired air; immune-boosting structures; and even a technique to make our voices louder.

The vast majority of paranasal activities are negative. They’re connected to the spread of the SARS-CoV-2 virus, creating the optimal conditions for the infection to thrive. New viruses have more time to grow due to the mucous membranes of the paranasal sinuses becoming sticky. They are meant to trap infections and draw in white blood cells to eliminate them. COVID-19 seems to be in this category.

Observing people who don’t have paranasal sinuses is one approach to learning about their functioning. Total paranasal sinus aplasia, a rare condition in which all four paired sinuses are missing, reveals the primary function of healthy sinuses.

Two individuals having entire paranasal sinus aplasia are being treated by doctors. In this uncommon case, the symptoms seemed to have minimal physical symptoms — one simply described the first experiencing a feeling of heaviness in the face, while the second suffered recurring head pain, and discomfort while eating. Both had sinus infections on a regular basis.

Paranasal Sinuses: Immunity

Both the innate and adaptive immune systems are strongly linked to the paranasal sinuses. However, as we’ve shown, the few people who were investigated with complete paranasal sinus aplasia did not suffer recurring infections. The nasal cavity may be capable of performing this role on its own, with the paranasal sinuses acting as a backup.

The immunity that we are born with is known as innate immunity. The skin, stomach acid, tears, blood-brain barrier, and mucus are all components of the innate immune system. This immediate response to an attack consists of processes such as inflammation caused by damaged cells and the body’s little battalion of trash removal units (phagocytes, natural killer cells, mast cells, innate lymphoid cells, eosinophils, and basophils).

Normally, there is no gap between the cells of the paranasal epithelium, which are aligned in rows. This presents an impediment. The connections between these cells are looser in people with chronic rhinosinusitis (a persistent infection of the nasal cavity and paranasal sinuses). Pathogens that get beyond the protective epithelium cause particular kinds of white blood cells to react. Inflammatory cytokines are released by these cells.

Our innate immunity includes not just the epithelium, but also the mucus generated by goblet cells. Mucus collects antigens (foreign particles). Sneezing or blowing your nose will get rid of them.

Another function of the paranasal sinus epithelium is the adaptive immune system, also known as the acquired immune system. Antibodies are produced during acquired immunity. T cells and B cells, for example, study to detect certain alien particles and generate or assist in the production of antibodies. Antibodies bind to infections and identify them as targets for elimination.

The nasal cavity and paranasal sinuses work together to protect the breath against viral, bacterial, and fungal infections. Nasal dendritic cells are found in the epithelium and absorb foreign microorganisms before presenting them to T cells, triggering an immune response.

Air Warmth and Humidity

The typical individual breathes in roughly seventeen kilos (14,000 litres) of air every 24 hours while going about their regular activities. Energy is expended by the body to warm and humidify the air, which should be close to body temperature and contain roughly 40 grammes of water per kilogramme when it reaches the lungs. The lungs may be injured by being overly cold or heated.

A person will need roughly 250 calories over the course of twelve hours to heat and humidify dry air on a warm (but not hot) day. The body consumes up to 10 times this amount at very cold conditions of -22 °F (-30 °C). As a result, the paranasal sinuses conserve a great deal of energy.

In adult men, approximately 90 cm³ is the total volume among all four paired paranasal sinuses. Whereas in females it is around 70 cm3. The typical amount of air we inhale (tidal volume) is around 500 cm³. So, when we breathe in cold, dry air, good paranasal sinuses warm and humidify around 15% of the air we inhale, sparing calories which may otherwise be used to restore the respiratory system back to body temperature.

Skull Weight

According to several sources, the paranasal sinuses of the head assist in reducing bone mass. However, research indicates that the bone mass loss owing to these hollow places is insignificant—between two and three ounces (55 to 85 grammes).

Nitric Oxide Production

In the last decade, researchers have found that the epithelium of the paranasal sinuses may produce nitric oxide gas. Nitric oxide destroys bacteria while also widening blood vessels. Chronic rhinosinusitis patients have been reported to have low levels of paranasal sinus nitric oxide.

Nitric oxide expands local blood arteries and enhances oxygen intake when it reaches the lungs.

Vocal Resonance

The influence of the paranasal sinuses on voice resonance has been extensively studied. The intensity of the voice is referred to as resonance. Tests on cadavers (who cannot talk or sing) refute this notion. According to other research, the maxillary sinuses have little impact on voice tone.

Paranasal Sinuses: Disease

Sinusitis and mucoceles are the two kinds of paranasal sinus illness.

The most frequent cause of sinusitis (rhinosinusitis or sinus infection) is a cold. Viruses that enter the nose cause tissue edoema (inflammatory response). Inflammation may obstruct the drainage of one or multiple sinuses into the nasal cavity. Mucus fills the afflicted cavities, causing pressure headaches.

Mucoceles might be indicated by mucosal thickening in the paranasal sinuses. Sinus obstructions generate mucoceles, which are mucus-filled cysts. Mucoceles, commonly known as sinus cysts, develop slowly and might harm the surrounding bone. A mucocele should be removed endoscopically since it will continue to expand in size.

Paranasal Sinuses: Cancer

The maxillary sinus is the most likely location for paranasal sinus malignancy.

The bone, sinus lining cells, and all cells that defend the body’s paranasal sinuses possess the potential to transform into cancerous tumours.

Squamous cell carcinomas account for the majority of paranasal sinus cancer cases (of squamous epithelial cells). Adenocarcinomas may develop mutations of epithelial cells that produce mucus (goblet cells). White blood cells that protect our body from infections may develop into paranasal sinus lymphomas. The ensuing tumour is a sarcoma if the bone cells around the sinus cavity mutate.

Thankfully, paranasal sinus carcinoma is uncommon. This condition affects around 2,000 people in the United States each year, the majority of whom are men over the age of 55.

References

• Navarro JAC. (2012). The Nasal Cavity and Paranasal Sinuses: Surgical Anatomy. New York, Springer-Verlag.
• Biggs NL, Blanton PL. The role of paranasal sinuses as weight reducers of the head determined by electromyography of postural neck muscles. The Journal of Biomechanics. 1970 May;3(3):255-62. doi: 10.1016/0021-9290(70)90027-8. PMID: 5521543.
• Jaskuluska E. (2014). Adaptation to Cold Climate in the Nasal Cavity Skeleton: A comparison of Archaelogical Crania from Different Climatic Zones. Unpublished doctoral dissertation. University of Warsaw, Poland.