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Galen described the “porosity of the skull” for the first time in the second century A.D., but it was only later in 1489, with the work of Leonardo da Vinci, that we had a clear description of paranasal sinuses. In his anatomical tables Leonardo represented the frontal sinuses and maxillary ostium.1

The role of paranasal sinuses is largely unknown nowadays, even if a lot of interesting theories have been suggested. All the hypotheses that have been made, however, can be grouped under three main theories: the structural, the evolutionary, and the functional.

Structural Theory

Vesalio (1542) and Falloppio (1600) hypothesized that sinuses are just a way created by nature to make the bone structure lighter and thus diminish the work of the muscles of the neck. At the end of the 19th century Braune and Clasen² disproved that idea, because if sinuses had been full of cancellous bone, the total weight of the cranium would have increased by only about 1%. According to another hypothesis³ belonging to the structural theory, paranasal sinuses contribute to the maintenance of equilibrium and the position of the head lightening the anterior portium of the cranium, but during the 1960s, this idea was proved not to be reliable through electromyographic studies of the muscles of the neck.

According to Flottes et al, paranasal sinuses would be the result of the development of the facial bones,⁴ whereas, according to Proetz, they could have a function in remodeling facial bones.⁵ However, changes in the morphology of sinuses do not seem to be related to changes of physiognomy.

Evolutionary Theory

The most innovative theory considers paranasal sinuses as the evolutionary response of anthropomorphic monkeys to the shift from the terrestrial environment to the aquatic one.

According to Hardy,⁶ Australopithecus represents the link between Homo habilis and Homo sapiens and anthropomorphic monkeys. Our old ancestor would have gotten specific characteristics, such as bipedal gait, loss of down, the presence of subcutaneous fat, and a different production of sweat and tears to live in an aquatic environment.

About 6.5 millions years ago the African environment, where the Australopithecus lived, was invaded by the sea, which dissociated a piece of mainland that became an island. The monkeys that remained on the island, because they could not reach the mainland, had to learn to live in a different way, looking for food in the waters surrounding the island. Natural selection let them adapt to their new environmental condition by developing a floating mechanism as in the case of some aquatic amphibians and reptiles which have developed sacs of air to allow the floating of the cephalic part and the maintenance of the nasal cavities out of water. In the same way paranasal cavities, through hydrodynamic push, support the muscles of the neck and would have allowed Australopithecus to maintain airways out of the water. (It is interesting to underline how man, unlike other mammals, has another characteristic of aquatic reptiles, the membrane of Shrapnell, which diminishes the transmission of acoustic waves in water.7)

According to the evolutionary theory, paranasal sinuses are now not only useless, they also represent a biological disadvantage, especially considering the diseases they frequently develop.

Functional Theory

Paranasal sinuses seem to have different functions. Bartholini⁸ was the first to consider these cavities as important organs of resonance, and Howell⁹ noted that the Maori of New Zealand have a particularly metallic voice because they do not have wide enough paranasal cavities. This theory was disproved by Proetz,¹⁰ however, who stressed how animals with a strong voice, such as lions, do not have big sinuses. According to Proetz, paranasal sinuses work as a barrier for the vital organs against thermic and acoustic insults.

Cloquet¹¹ thought that in paranasal sinuses there was an olfactive epithelium. Even if this has been shown not to be completely true, however, the mucosa of paranasal sinuses represents in carnivorous mammals the residual aspects of the olfactory organ.

In our opinion, the most important function of paranasal sinuses is to improve the nasal function. The embryological origin as an invagination from the nose and the histological continuity of the respiratory mucosa covering the nasal fossae and the sinuses make these cavities an efficient mechanism in strengthening the defense function through the additional secretion of lythic enzymes and secretory immunoglobulins. The strict relationship of the paranasal sinuses with the nasal cavities influences the functionality of the paranasal sinuses, and the center of the whole paranasal physiology is the patency of the ostia through which the sinuses ventilate, drain, and secrete mucus.

Ventilatory Function

We define sinusal ventilatory function as all gaseous exchanges between the nose and the paranasal sinuses and between the sinuses and the bloodstream through the mucosa.¹² The gaseous exchanges are determined by active phenomena, depending on nasal respiratory activity, and passive phenomena of passive diffusion.

During the act of breathing, the nasal valve transforms air into a laminar flow that, through the nasal cavities, spreads over the middle meatus, where it changes in microturbulences that lead to a uniform mixing of air. The alternation in expiration of every cycle creates a gradient of pressure between nasal and paranasal cavities, which causes airflow entering the sinuses at the end of every inspiration and at the beginning of the following expiration when a situation of hyperpressure exists inside the nasal cavities. This is inverted at the end of every expiration and at the beginning of the following inspiration when endonasal pressure reaches negative values (Fig. 2–1).

Only 1/1000 of the air volume of the sinuses is exchanged through a single respiratory act. (Actually, the exchange is faster, as demonstrated through O₂ micro-electrodes placed in the maxillary sinus: 5 minutes is enough time to renew 90% of the gaseous content there.¹³) Therefore, even though the paranasal air exchange is 2 times faster during nasal versus oral breathing, the alternations of endonasal pressure do not influence the ventilation of paranasal sinuses very much. Pressure changes in nasal cavities contribute to ventilation of paranasal sinuses supplying only 1/10 of the total air. The other 9/10 depend on the processes of passive gaseous diffusion through the ostium.¹⁴ In particular, the diameter of the ostium is affected by the speed of oxygenation in physiological conditions and by the efficiency of the drainage in pathological situations.¹⁵

All the studies on the patency of the ostia have been performed on the maxillary sinus, which is the most accessible one. Anatomical dimensions of the ostium are not related to the sinus volume and do not change because of gender. The anatomical diameter is different in every subject (mean values from 2 to 4 mm), reaching its final size and remaining stable at the end of the period of maxillofacial bone development. Because the mucosa that covers the wall of the ostium is rich with blood vessels and has the same histological characteristics as the nasal cavities, the diameter of the ostium can change following physiological events, such as the passage from standing to the supine position, physical exercise, and the phases of the nasal cycle. For example, when changing from standing to the supine position, the diameter is almost the same until the subject’s inclination with the horizontal plane corresponds to an angle of 30 degrees., Then it rapidly decreases, reaching 70% of the starting value when in the supine position, because of the increase in hydrostatic pressure on the neck.

The patency of the ostium has been evaluated through the resistance values that the maxillary antrum has toward air introduced into it.16 Through the evaluation of these resistances we can distinguish (1) subjects with patent ostium, (2) subjects with partially patent ostium, (3) and subjects with an obstructed ostium.¹⁶

The level of O₂ in the first group is 16% of the gaseous content, 14% in the second and 11% in the third. Moreover, the ostium resistance is influenced by the presence of secretions showing a 20% decrease after irrigation.

The decrease of O₂

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