Paranasal Air Sinus and Frontal Air Sinuses | Anatomy | Functions | Clinical

PARANASAL AIR SINUSES

Paranasal sinuses are a group of four paired air-filled spaces that surround the nasal cavity. The maxillary sinuses are located under the eyes; the frontal sinuses are above the eyes; the ethmoidal sinuses are between the eyes and the sphenoidal sinuses are behind the eyes.

Functions of the paranasal sinuses, the most likely are:

Decrease in the relative mass of the anterior sections of the skull, especially the bones of the facial skull, against the background of their large volume. The shape of the bones of the facial skull is important because the bones are the attachment points for the facial muscles.
Increased voice resonance
Providing a shockproof "buffer" in case of injuries
Isolation of sensitive structures (roots of teeth, eyeballs) from rapid temperature fluctuations in the nasal cavity during inhalation and exhalation.
Humidification and warming of the inhaled air, as a result of slow airflow in the sinuses.
Perform the function of a sensory system of air signals (a baroreceptor organ that responds to changes in environmental pressure).
Development
Paranasal sinuses form developmentally through the excavation of bone by air-filled sacs (pneumatic diverticula) from the nasal cavity. This process begins prenatally (intrauterine life), and continues through the course of an organism's lifetime.

The results of experimental studies suggest that the natural ventilation rate of a sinus with a single sinus ostium (opening) is extremely slow. Such limited ventilation may be protective for the sinus, as it would help prevent drying of its mucosal surface and maintain a near-sterile environment with high carbon dioxide concentrations and minimal pathogen access. Thus the composition of gas content in the maxillary sinus is similar to venous blood, with high carbon dioxide and lower oxygen levels compared to breathing air.

At birth only the maxillary sinus and the ethmoid sinus are developed but not yet pneumatized; only by the age of seven, they are fully aerated. The sphenoid sinus appears at the age of three, and the frontal sinuses first appear at the age of six and fully develop during adulthood.
FRONTAL AIR SINUSES

The frontal sinuses are one of the four pairs of paranasal sinuses that are situated behind the brow ridges. Sinuses are mucosa-lined airspaces within the bones of the face and skull. Each opens into the anterior part of the corresponding middle nasal meatus of the nose through the frontonasal duct which traverses the anterior part of the labyrinth of the ethmoid. These structures than open into the semilunar hiatus in the middle meatus.

STRUCTURE

Frontal sinuses are rarely symmetrical and the septum between them frequently deviates to one or other side of the middle line. Their average measurements are as follows: height 28 mm, breadth 24 mm, depth 20 mm, creating a space of 6-7 ml.

The mucous membrane in this sinus is innervated by the supraorbital nerve, which carries the postganglionic parasympathetic nerve fibers for mucous secretion from the ophthalmic nerve and supplied by the supraorbital artery and anterior ethmoidal artery.

Development
The frontal sinuses are absent at birth, but are generally well developed, and functional between the sixth and eighth years, though they continue to grow slowly until reaching their maximum size after puberty. This is why many children experience suddenly increased mucus production at this age and learn to react accordingly.

Function
Through its copious mucus production, the sinus is an essential part of the immune defense/air filtration carried out by the nose. Nasal and spinal mucosae are ciliated and move mucus to the choanae and finally to the stomach. The thick upper layers of nasal mucus trap bacteria and small particles in tissue abundantly provided with immune cells, antibodies, and antibacterial proteins. The layers beneath are thinner and provide a substrate in which the cilia are able to beat and move the upper layer with its debris through the Ostia toward the choanae.

Clinical significance
Infection of the frontal sinus causing sinusitis can give rise to serious complications, as it is in close proximity to the orbit and cranial cavity (orbital cellulitis, epidural and subdural abscess, meningitis).[2] The endonasal approach into the frontal sinus in children with acute and chronic frontal sinusitis without the usage of surgical optics is not successful, because in this case the operation is performed almost blindly and technically difficult even in adults.

Fractures
Frontal sinus fractures occur from trauma to the part of the frontal bone that overlies the sinus, often from motor vehicle accidents and falls. The hallmark of a frontal sinus fracture is a frontal depression in the anterior table of the bone. Additionally, clear fluid leaking from the nose may indicate that fractures to the posterior table have torn into the dura mater, creating a cerebrospinal fluid leak.

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