Paranasal Sinus Anatomy

The paranasal sinuses are air-filled spaces located within the bones of the skull and facial bones. They are centered on the nasal cavity and have various functions, including lightening the weight of the head, humidifying and heating inhaled air, increasing the resonance of speech, and serving as a crumple zone to protect vital structures in the event of facial trauma. ^{[1, 2]} Four sets of paired sinuses are recognized: maxillary, frontal, sphenoid, and ethmoid (see the image below).

Maxillary sinus

The maxillary sinus is the largest paranasal sinus and lies inferior to the eyes in the maxillary bone. It is the first sinus to develop and is filled with fluid at birth. It grows according to a biphasic pattern, in which the first phase occurs during years 0-3 and the second during years 6-12. The earliest phase of pneumatization is directed horizontally and posteriorly, whereas the later phase proceeds inferiorly toward the maxillary teeth.

This development places the floor of the sinus well below the floor of the nasal cavity. The shape of the sinus is a pyramid, with the base along the nasal wall and the apex pointing laterally toward the zygoma. The natural ostium of the maxillary sinus is located in the superior portion of the medial wall.

The anterior maxillary sinus wall houses the infraorbital nerve, which runs through the infraorbital canal along the roof of the sinus and sends branches to the soft tissues of the cheek. The thinnest portion of the anterior wall is above the canine tooth, called the canine fossa, which is an ideal entry site for addressing various disease processes of the maxillary sinus.

The roof of the maxillary sinus is the floor of the orbit. Behind the posteromedial wall of the maxillary sinus lies the pterygopalatine fossa, a small inverted space that houses several important neurovascular structures and communicates with several skull base foramina. The infratemporal fossa lies behind the posterolateral wall of the maxillary sinus.

The maxillary sinus is supplied by branches of the internal maxillary artery, which include the infraorbital, alveolar, greater palatine, and sphenopalatine arteries. It is innervated by branches of the second division of the trigeminal nerve, the infraorbital nerve, and the greater palatine nerves.

Frontal sinus

The frontal sinus is housed in the frontal bone superior to the eyes in the forehead. It is formed by the upward movement of anterior ethmoid cells after the age of 2. Developmentally, this is the last sinus to pneumatize. Growth increases at age 6 years and continues until the late teenage years. ^[3] The frontal sinuses are funnel-shaped structures with their ostia located in the most dependent portion of the cavities. The posterior wall of the frontal sinus, which separates the sinus from the anterior cranial fossa, is much thinner than its anterior wall.

The frontal sinus is supplied by the supraorbital and supratrochlear arteries of the ophthalmic artery. It is innervated by the supraorbital and supratrochlear nerves of the first division of the trigeminal nerve.

Sphenoid sinus

The sphenoid sinus originates in the sphenoid bone at the center of the head. It arises not from an outpouching of the nasal cavity but from the nasal embryonic lining. The sinus reaches its full size by the late teenage years. The sphenoid sinus is variably pneumatized and may extend as far as the foramen magnum in some patients.

The thickness of the walls of the sphenoid sinus is variable, with the anterosuperior wall and the roof of the sphenoid sinus (the planum sphenoidale) being the thinnest bones. The sphenoid sinus ostium is located on the anterosuperior surface of the sphenoid face, usually medial to the superior turbinate.

The sphenoid sinus is supplied by the sphenopalatine artery, except for the planum sphenoidale, which is supplied by the posterior ethmoidal artery. Innervation of the sphenoid sinus comes from branches of the first and second divisions of the trigeminal nerve.

Ethmoid sinus

The ethmoid sinuses arise in the ethmoid bone, forming several distinct air cells between the eyes. They are a collection of fluid-filled cells at birth that grow and pneumatize until the age of 12. The ethmoid cells are shaped like pyramids and are divided by thin septa. They are bordered by the middle turbinate medially and the medial orbital wall laterally. The ethmoid labyrinth may extend above the orbit, lateral and superior to the sphenoid, above the frontal sinus, and into the roof of the maxillary sinus.

The ethmoid sinuses are supplied by the anterior and posterior ethmoidal arteries from the ophthalmic artery (internal carotid system), as well as by the sphenopalatine artery from the terminal branches of the internal maxillary artery (external carotid system).

Ethmoid bone

The medial portion of the ethmoid bone is a cruciate membranous bone that is composed of the crista galli, the cribriform plate, and the superior portion of the nasal septum. The crista galli is a thick piece of bone, shaped like a cock’s comb, that projects superiorly into the cranial cavity and serves as an attachment of the falx cerebri. If pneumatized, the crista galli air cell drains into either the left or right frontal sinus.

The cribriform plate contains numerous perforations that transmit olfactory fibers to the superior turbinate and the superior portions of the nasal septum and middle turbinate. The perpendicular plate of the ethmoid connects with the quadrangular septal cartilage anteroinferiorly and the vomer posteroinferiorly to form the nasal septum.

Ethmoid roof (fovea ethmoidalis)

The vertical lamella of the middle turbinate divides the anterior skull base into the cribriform plate medially and the roof of the ethmoid, or fovea ethmoidalis, laterally. The ethmoid labyrinth of air cells lies lateral to the middle turbinate and terminates at the paper-thin bone forming the medial orbital wall, called the lamina papyracea.

The fovea ethmoidalis slopes inferiorly when travelling in an anterior-to-posterior or lateral-to-medial direction along the skull base. Understanding this orientation is important for preventing inadvertent entry into the skull base during endoscopic sinonasal procedures.

The roof of the ethmoid is composed of a thicker horizontal portion, called the orbital plate of the frontal bone, and a thinner vertical portion, called the lateral cribriform plate lamella (LCPL). The orbital plate comprises most of the ethmoid roof, with the LCPL forming a small medial portion. The height of the LCPL defines the depth of the olfactory cleft, where dura is closely adherent to the bone. The bony thickness of the LCPL ranges from 0.05 mm to 0.2 mm and provides little resistance to injury. ^[3]

In addition, increasing depths of the olfactory cleft correlate with a greater risk of inadvertent injury during surgery. Injury may often result in a cerebrospinal fluid leak, pneumocephalus, or intracranial bleeding. The Keros classification divides the ethmoid roof into 3 configurations: shallow type I (1-3 mm), medium type II (4-7 mm), and deep type III (8-16 mm). ^[4] The type III configuration, being the deepest, is at greatest risk for complications during endoscopic endonasal surgery.

One additional anatomic variant exists in which the orbital plate is thinner and the LCPL runs in a more horizontal direction. An even greater risk of inadvertent injury exists if the surgeon perceives the thinner orbital plate to be part of a superior ethmoid cell, rather than the actual skull base. ^[5]

Lateral nasal wall

Turbinates

Three (sometimes 4) laminar projections, called turbinates or conchae, emanate from the lateral nasal wall: the inferior, middle, superior, and supreme turbinates. These structures are composed of bone, covered by an inner submucosal layer and an outer respiratory epithelium. The turbinates function to warm, humidify, and purify the air before it enters the lower airway. They also aid in olfaction by directing nasal air flow to the olfactory cleft. Each turbinate extends the length of the nasal cavity and has a space associated with it, called a meatus. Each meatus is named for the turbinate above it and contains important structures. 

The inferior turbinate is the largest of these and has a different embryologic origin than the other turbinates. It is derived from the maxilloturbinal, whereas the middle, superior, and supreme turbinates are all derived from the ethmoturbinal. Within the submucosa of the inferior turbinate are a large network of capacitance vessels that become alternately engorged and constricted during phases of the nasal cycle. Anteriorly, the inferior meatus bears the inferior opening of the nasolacrimal duct. Surgical approaches to the maxillary sinus through the inferior meatus should be carried out through the thinner bone in the posterior portion of this space, due to reduced risk of damage to the nasolacrimal duct.

The middle turbinate is a prominent, easily visualized structure that serves as an important anatomic landmark for endoscopic sinus and skull base surgery. Structural variations of the middle turbinate include the paradoxical middle turbinate, in which the bone is convex in curvature, and concha bullosa, in which the bone becomes pneumatized. Normal middle turbinate anatomy comprises the body, the anterior buttress, the posterior buttress, the horizontal lamella, and the vertical lamella.

The anterior buttress is a point of attachment of the turbinate to the lateral nasal wall in the agger nasi region. The posterior buttress is a point of attachment to the lateral nasal wall near the posterior end of the middle turbinate. The vertical lamella attaches to the LCPL and marks the boundary between the cribriform plate and the ethmoid roof. The horizontal lamella (also called the basal lamella or ground lamella of the middle turbinate) attaches to the lateral nasal wall and marks the division between the anterior and posterior ethmoid air cells.

Preserving the anterior buttress, posterior buttresses, and vertical lamella of the turbinate during surgery is important to prevent lateralization of the turbinate and consequent obstruction of sinus outflow. Preservation of the inferior portion of the horizontal lamella further decreases the risk of turbinate lateralization.

The middle meatus contains numerous important structures and ostia, including the uncinate process, ethmoid infundibulum, ethmoid bulla, semilunar hiatus, the maxillary ostium, and the agger nasi cell. The superior turbinate is situated posterior to and in continuity with the middle turbinate. Olfactory epithelium lines the medial surface of the superior turbinate to a variable degree; therefore, aggressive resection of this structure may result in hyposmia. The superior meatus houses the ostia of the sphenoid sinus and the posterior ethmoid sinuses. 

Uncinate process and ethmoid infundibulum

The uncinate process is a saber-shaped ethmoid bone that attaches to the lateral nasal wall through multiple bony and fibrous attachments. The uncinate lies lateral to the middle turbinate and has a free edge that runs from the maxillary sinus ostium inferiorly to below the frontal recess superiorly. The uncinate forms the medial wall of the ethmoid infundibulum, a 3-dimensional space that accepts drainage from the maxillary, frontal, and anterior ethmoid sinuses.

The infundibulum is bordered medially by the uncinate process, laterally by the medial orbital wall (lamina papyracea), superiorly by the frontal recess, and inferiorly by the maxillary sinus ostium.

The uncinate process usually attaches to the medial orbital wall superiorly but may attach to the skull base or the middle turbinate. The site of attachment of the uncinate influences the drainage pattern of the frontal sinus. When attached to the medial orbital wall, the frontal sinus drains into the middle meatus, defined by the space under the middle turbinate. If the uncinate process is attached to the skull base or the middle turbinate superiorly, the frontal sinus drains directly into the infundibulum. Rarely, the uncinate is pneumatized, which can lead to outflow obstruction at the infundibulum. 

Ethmoid bulla and semilunar hiatus

The ethmoid bulla is a constant landmark during endoscopic surgery and is usually the largest of the anterior ethmoidal cells. It lies posterior to the uncinate process, superior to the infundibulum, and anterior to the basal lamella. The ethmoid bulla often extends all the way to the skull base but sometimes is attached to the skull base via the bulla lamella.

The space between the lateral/inferior surface of the ethmoidal bulla and the superior surface of the uncinate process is called the hiatus semilunaris (semilunar hiatus). This 2-dimensional space connects the infundibulum to the middle meatus.

If the ethmoidal bulla has a posterior surface, the space between the bulla and the basal lamella is called the retrobullar recess or sinus lateralis. The 2-dimensional opening into the retrobullar recess is called the hiatus semilunaris posterior (posterior semilunar hiatus). The retrobullar recess may communicate with the suprabullar recess, which is a cleft that forms between the superior surface of the ethmoidal bulla and the fovea ethmoidalis.

The basal lamella represents the division between the anterior and posterior ethmoidal cells. Anterior ethmoidal cells drain into the middle meatus via the ethmoid infundibulum. Posterior ethmoidal cells, which are fewer in number, drain into the superior meatus.

Ostiomeatal complex

The ostiomeatal complex (OMC) represents a functional 3-dimensional space that is bordered by the lamina papyracea laterally, the middle turbinate medially, the frontal recess superiorly, and the maxillary sinus ostium inferiorly. This space includes the uncinate process, ethmoid infundibulum, semilunar hiatus, and clefts between uncinate and middle turbinate and between ethmoid bulla and middle turbinate. Chronic inflammation and edema of the OMC causes anatomic and functional obstruction, leading to chronic inflammation of sinuses draining into the area.

Haller (infraorbital ethmoid) cell

The Haller cell, or infraorbital cell, is an anterior ethmoidal cell that pneumatizes into the maxillary sinus ostium just below the inferior orbital wall. The presence of a Haller cell may contribute to persistent maxillary sinus disease in some cases of chronic sinusitis. This is typically secondary to mucosal inflammation of the common wall between the Haller cell and the maxillary sinus ostium. The presence of a Haller cell often increases the surgical complexity of maxillary antrostomy. The Haller cell may be mistaken for the orbital floor and thus not exteriorized, resulting in persistent disease. If the orbital floor is mistaken as a Haller cell, orbital injury can result. 

Accessory ostia

The lateral nasal wall has 2 areas deficient in bone that lie anterior and posterior to the uncinate process, and are termed fontanelles. When the thin bone and overlying mucosa of these areas becomes dehiscent, accessory ostia form and are sometimes confused with the natural maxillary sinus ostium. Accessory ostia are believed to be found in 10-28% of the population. ^[6] Because the mucociliary clearance pattern of the maxillary sinus flows to the natural ostium, these accessory ostia are rarely functional. They are believed to be associated with recurrent acute sinusitis. ^[7]  

Frontal recess, frontal sinus, and associated cells

In broad anatomic terms, the frontal recess is bordered posteriorly by the sloping anterior skull base and anteriorly by the nasofrontal beak, a prominence seen at the confluence of the frontal processes and the nasal bones. The anatomy of the frontal recess is made complex by the wide variety of anterior ethmoidal cells that populate the space. Some of the more common frontal recess cells include the agger nasi cell, supraorbital ethmoid cell, interfrontal sinus septal cell, frontal bulla cell, suprabullar cell, and 4 types of frontal cells (types I-IV). The most prominent cells include the agger nasi cell, supraorbital cell, and 4 types of frontal cells.

Although the exact configuration and pathway of the frontal recess vary, this space is usually bounded anteriorly by the posterior wall of the agger nasi cell, superiorly by the frontal sinus, medially by the lateral cribriform plate lamella, laterally by the lamina papyracea, and posteriorly by the anterior wall of the ethmoidal bulla, or suprabullar recess.

Surgical dissection in this area is challenging and carries numerous risks. Damage to the olfactory fossa may result in CSF leak or hyposmia. Laceration of the anterior ethmoidal artery carries the potential for vision loss, and direct injury to the orbit can also occur.

Agger nasi cell

The agger (from Latin, meaning mound) nasi cell is the most anterior of all the ethmoidal cells found in the lacrimal bone and is located anterior and superior to the anterior buttress of the middle turbinate. As the most prominent and constant ethmoidal cell, the agger nasi is characterized as a bulge in the lateral nasal wall and is evidenced in over 90% of CT scans. ^[3]  Developmentally, it is the first anterior ethmoidal cell to undergo pneumatization.

The medial and posterior walls of the agger nasi cell often lie in close association with the vertical lamella of the middle turbinate and the skull base, respectively. The posterior wall usually represents the anterior face of the frontal recess. The superior surface (or cap) of the agger nasi cell, if left in place during frontal recess surgery, may contribute to iatrogenic frontal sinus obstruction.

Supraorbital ethmoid cell

The supraorbital ethmoid cell is an anterior ethmoidal cell that pneumatizes into the orbital plate of the frontal bone. This cell extends over the orbit and sometimes pneumatizes all the way to the lateral wall. When extensively pneumatized, this cell may be mistaken for the frontal sinus or a septate frontal sinus. Often, this cell may be completely missed by the endoscopic sinus surgeon, especially when it is hidden behind the bulla lamella.

Frontal sinus

The floor of the frontal sinus corresponds to the roof of the orbit. The frontal sinus is defined anteriorly by the thicker anterior table of the frontal bone and the thinner posterior table which separates the frontal sinus from the anterior horns of the frontal lobes of the brain. The pneumatization patterns of the frontal sinus widely vary. An intersinus septation generally divides the sinus, albeit asymmetrically. In some patients, the frontal sinus may exhibit developmental hypoplasia that can be unilateral or bilateral. In approximately 5-15% of adults, the frontal sinus is completely aplastic. ^[8]

The inferior portion of the frontal sinus is often considered to take on an hour-glass shape, with the thinnest part of the neck corresponding to the frontal sinus ostium. Superior to this, the frontal sinus is drained by the frontal infundibulum, a funnel-shaped space. The frontal sinus ostium opens inferiorly into the frontal sinus recess. 

According to the Kuhn classification, four types of frontal cells pneumatize above the agger nasi cell, contributing to the complexity of frontal recess anatomy. A type I frontal sinus cell lies above the agger nasi cell. A type II frontal sinus cell is a configuration of two cells stacked above the agger nasi cell. A type III cell is a large frontal sinus cell that pneumatizes into the frontal sinus and occupies nearly 50% of the sinus. Finally, a type IV frontal sinus cell is a single isolated cell that exists completely within the frontal sinus and has no connection to the frontal recess. ^[9]   

Sphenoid bone

The sphenoid is a butterfly-shaped bone that is divided into 4 main parts: the body of the sphenoid centrally, 2 sets of greater and lesser wings laterally, and 2 sets of pterygoid processes inferiorly. Medial and lateral pterygoid plates arise from each pterygoid process; the space contained is called the pterygopalatine fossa. 

The lesser wing and the planum sphenoidale (the roof of the sphenoid sinus) form the medial portion of the anterior cranial fossa. The medial portion of the middle cranial base is formed by the body of the sphenoid bone, the tuberculum sella, the pituitary fossa, the middle and posterior clinoid processes, and the dorsum sellae. The lateral portion of the middle cranial base is formed by the lesser and greater wings of the sphenoid bone.

Various foramina contained within the sphenoid transmit numerous important neurovascular structures. The supraorbital fissure is located at the junction of the greater and lesser wings of the sphenoid and carries cranial nerves III, IV, V₁, and VI, as well as sympathetic fibers from the cavernous sinus to the orbit. The foramen rotundum transmits the maxillary branch (V₂) of cranial nerve V, which exits into the pterygopalatine fossa. Running through the median pterygoid plate, the pterygoid (Vidian) canal carries the Vidian nerve into the pterygopalatine fossa. The foramen ovale is located posteriorly to the lateral pterygoid plate and allows passage of cranial nerve V₃. Sitting in the infratemporal surface of the greater wing of the sphenoid, the foramen spinosum transmits the middle meningeal artery.

Sphenoid sinus

The sphenoid sinuses are a pair of large paranasal sinuses located posterior to the ethmoid sinuses. These paired sinuses develop separately from the nasal capsule of the embryonic nose, often divided by a single vertical intersinus septation. However, multiple complete and incomplete bony septations, with vertical, horizontal, or oblique orientations, may further subdivide the sinus.

In one radiologic study, 80% of sphenoid sinuses were found to have a single sphenoid septation, and 20% were found to have a double septation. ^[10] These septations often localize to the carotid artery, underscoring the importance of atraumatic dissection to avoid a catastrophic vascular injury.

In a study of 54 sphenoid sinuses, 27 sphenoid sinuses were examined with high-resolution CT scanning, and the other half were examined in fresh frozen cadaveric heads. Of the radiographically examined sinuses, 85% had at least 1 sphenoid septation, and 41% had at least 2 septations inserted into the internal carotid arteries. Of the sinuses in the cadaveric group, 89% had 1 septation, and 48% had 2 septations. Only 13% of specimens had an isolated midline septation. ^[11]

Pneumatization patterns

Pneumatization of the sphenoid sinus is highly variable and can extend as far as the clivus inferiorly, the sphenoid wings laterally, and the foramen magnum inferiorly. Pneumatization of the vast majority of sinuses reaches the sella turcica by age 7. Three major pneumatization patterns for sphenoid sinus have been noted: sellar (80%), presellar (17%), and conchal (3%). ^[12]

A sellar sphenoid sinus has pneumatization anterior and inferior to the sellar prominence. A presellar sphenoid sinus has pneumatization only anterior to the sella. A conchal sphenoid sinus has minimal to no pneumatization. A conchal configuration poses the greatest anatomic challenge to the endoscopic management of sphenoid, pituitary, or anterior skull base pathology.

In addition to the aforementioned configurations, a postsellar type is occasionally seen. This configuration consists of presellar pneumatization followed by bone and a smaller pneumatization located posterior to the sella.

These various configurations highlight the critical importance of obtaining preoperative imaging prior to attempting surgical dissection in this area. 

Endoscopic anatomy

The endoscopic anatomy of the sphenoid sinus has several critical anatomic landmarks that are important for the endonasal endoscopic surgeon. The midline posterior wall of the sphenoid sinus reveals the sellar protuberance. Inferior to this lies the clivus, which is separated by the sellar-clival junction. Superior and anterior to the sella lies the planum sphenoidale, separated by a thick ridge of bone called the tuberculum sellae, which corresponds to the chiasmatic sulcus intracranially.

The lateral wall of the sphenoid sinus reveals 4 prominences and 3 depressions. The 4 prominences, from superior to inferior, are the optic nerve, the parasellar internal carotid artery, the maxillary division, and the mandibular division of the trigeminal nerve. The 3 bony depressions of the lateral sphenoid sinus wall are the lateral opticocarotid recess, the depression between the cavernous sinus apex and the maxillary nerve, and the depression between the maxillary and mandibular divisions of the trigeminal nerve. ^[13]

In addition, the space medial to the junction between the optic nerve and the carotid arteries, called the medial opticocarotid recess, has been labeled the anatomic keyhole in endonasal skull base surgery. ^[14] Nearly 25% of patients may have a bony dehiscence over critical structures such as the optic nerve and the carotid artery. Caution must be exercised in the removal of intersinus septations in order to avoid visual or vascular injury. ^[12]

Sphenoid ostium and sphenoethmoid recess

The natural sphenoid sinus ostium is located 1.5 cm superior to the posterior choanae on the anterior sphenoid sinus wall. The natural ostium is elliptical in shape and is found in close association with the superior turbinate in the sphenoethmoid recess. In 83% of cases, the ostium is located medial to the superior turbinate and may be visualized with gentle lateralization of the superior turbinate. ^[15]

Another anatomic landmark for identifying the sphenoid ostium involves dividing the superior turbinate into thirds. The natural ostium lies at the junction of the inferior and middle thirds of the superior turbinate.

The sphenoethmoid recess is the narrow vertical corridor enclosed by the septum medially and the superior turbinate laterally. It is defined superiorly by the cribriform plate and inferiorly by the floor of the nasal cavity. The natural sphenoid ostium drains into this corridor, as do the posterior ethmoid cells.

Parallelogram box theory

The sphenoid sinus can be entered not only through the natural sphenoid sinus ostium but also through the posterior ethmoid cells. The so-called parallelogram box concept provides a safe mechanism for entering the sphenoid sinus through the posterior ethmoid complex.

The parallelogram box is defined by the medial orbital wall laterally, the vertical lamella of the superior turbinate medially, the fovea ethmoidalis superiorly, and the horizontal lamella inferiorly. An oblique line drawn from the superior medial corner of the box to the inferior lateral corner divides the parallelogram into 2 equal halves. The safest route of entry into the sphenoid sinus is through the medial-inferior triangle of the sphenoid face because the optic nerve and carotid artery lie in the superior-lateral triangle of the box. ^[16]

Onodi cell

The Onodi cell (named for the Hungarian otolaryngologist Adolf Onodi), or sphenoethmoid cell, is a posterior ethmoidal cell that pneumatizes posterior, lateral, and superior to the sphenoid face. It is present in 7-25% of patients and nearly 50% of patients from East Asia.  ^[12]

Recognizing the presence of this cell before and during endoscopic sinus or skull base surgery is important. An Onodi cell may often encompass the optic nerve laterally in the posterior ethmoid sinus, making it potentially vulnerable to injury. Also, the presence of an Onodi cell places the sphenoid sinus in a more medial and inferior position, thereby increasing the risk of intracranial penetration if the surgeon expects the sinus to be behind the last posterior ethmoid cell.

Vascular supply

The nose and the paranasal sinuses are supplied by the internal and external carotid arteries. The anterior ethmoidal artery (AEA) and posterior ethmoidal artery (PEA) arise from the ophthalmic artery, the first branch of the supraclinoid internal carotid artery. These ethmoidal arteries traverse the orbit and pierce the lamina papyracea to supply the nose and paranasal sinuses.

The AEA crosses the medial rectus, penetrates the lamina papyracea, and runs across the anterior fovea ethmoidalis before branching and supplying the cribriform plate and anterior and superior nasal septum. The AEA runs at a 45 º angle from lateral to medial along the skull base. It usually travels at the base of the frontal recess or behind the ethmoid bulla. If the bulla lamella (attachment of the ethmoidal bulla to the skull base) exists, the AEA runs in it or just posterior to it.

Usually the AEA is flush with the skull base, but 14-43% of the time it lies in a mesentery hanging from the skull base, thereby exposing it to a greater risk of injury during surgery. ^[17]

The PEA crosses the medial rectus, penetrates the lamina papyracea, and courses through the posterior ethmoid cells near the anterior face of the sphenoid in close association with the skull base. This artery supplies the posterior ethmoidal sinuses, superior posterior septum, and portions of the superior and middle turbinates. The location of the posterior ethmoidal artery is parallel to the optic nerve near the orbital vertex.

Either the AEA or the PEA may be dehiscent, and caution is necessary to prevent injury and retraction of these arteries into the orbit. Orbital hematoma or vision loss may result from damage to these vessels. 

The sphenopalatine artery, a terminal branch of the internal maxillary artery, provides blood to the posterior nasal cavity, as well as to portions of the maxillary, ethmoid, and sphenoid sinuses. It passes through the pterygopalatine fossa through the sphenopalatine foramen and branches into the posterior septal artery and the posterior lateral nasal artery.

The sphenopalatine artery is located at the posterior edge of the maxillary sinus ostium and is transmitted between the orbital processes of the palatine bone. It is visualized after reflection of the mucosa of the maxillary sinus laterally and the nasal mucosa medially. Removal of the bone that remains leads to the pterygopalatine and infratemporal fossa.

The nasal and paranasal sinuses are lined with respiratory mucosa composed of pseudostratified ciliated columnar epithelium. The underlying lamina propria contains a rich capillary network and is firmly adherent to the periosteum or perichondrium of adjacent bone or cartilage. 

Four types of cells are found in the sinonasal mucosa: ciliated columnar epithelial cells, nonciliated columnar cells, goblet cells, and basal cells.

The ciliated columnar epithelial cells make up 20-50% of the epithelium and have 50-200 cilia per cell, beating at 700-800 times per minute. These cilia move the 10-15 micron thick mucous blanket at an approximate rate of 9 mm/min. Nonciliated cells comprise about 70% of the epithelium and have microvilli on the apical surfaces that enhance humidification and warming of air before it enters the lower airway.

The goblet cells secrete glycoproteins, which influence the character of the mucus. The parasympathetic input to these glands produces thicker mucus, whereas the sympathetic innervation produces thinner, watery mucus. The function of the basal cells is unknown. ^{[18, 19]}