Nasal Cavity

The nasal cavity is triangular and is separated in the midline by the nasal septum. The nasal cavity is composed of a cartilaginous portion anteriorly and an osseous portion posteriorly, which is formed by the perpendicular plate of the ethmoid posterosuperiorly and the vomer posteroinferiorly. The opening to each nasal cavity is known as the vestibule, which is bounded medially by the columella and nasal septum and laterally by the nasal alae. The cribriform plate is the roof of the nasal cavity, and the floor is the hard and soft palate. Posteriorly, the nasal cavity communicates with the nasopharynx via the choanae, after rupture of the oronasal membrane during the fetal period. From the lateral wall of the nasal cavity, three pairs of turbinates (superior, middle, and inferior) project into the nasal cavity, each with a corresponding meatus below them. The middle turbinate is attached to the cribriform plate via the vertical lamella and to the lamina papyracea via the basal lamella (Fig. 8-1).

Paranasal Sinuses

The paranasal sinuses form as diverticula from the walls of the nasal cavities and become air-filled extensions in the adjacent bones—maxilla, ethmoid, frontal, and sphenoid. The original openings of the diverticula persist as the ostia of the sinuses that communicate with the nasal cavity (Fig. 8-2, A and B).

The mucosal lining of the nasal cavities is contiguous with the paranasal sinuses and consists of pseudostratified, columnar, ciliated epithelium containing mucinous and serous glands,¹ whereas the nasal septum is lined by squamous mucosa.² In the human embryo, ethmoidal and maxillary sinus budding can be detected at 11 to 12 gestational weeks and at 14 to 15 gestational weeks, respectively.³ In general, only rudimentary maxillary and ethmoid sinuses, which continue to expand until puberty or early adulthood, are present at birth.⁴ The sinuses also are linked to facial growth and dentition and have been studied extensively by different anatomic and imaging methods (Fig. 8-3, A-C).^4–11

Ethmoid Sinuses

At birth the ethmoid sinuses are already developed in number and pneumatized, but they continue to expand, reaching adult proportions at about 12 years of age.¹² The ethmoid sinuses consist of a paired group of a variable number of cells (3 to 18) within the lateral masses of the ethmoid bone, also known as labyrinths. In addition to the lateral masses, the ethmoid bone consists of the cribriform plate superiorly and the perpendicular plate, which is part of the nasal septum (see Fig. 8-3, A). The ethmoid sinus is bordered medially by the nasal cavity and laterally by the lamina papyracea; its roof is formed by the cribriform plate and fovea ethmoidalis (see Fig. 8-1). The anterior and posterior ethmoid air cells are separated by the basal lamella (see Fig. 8-3, C), which is the lateral attachment of the middle turbinate to the lamina papyracea. Drainage of the anterior ethmoid air cells occurs via the ethmoid bulla into the hiatus semilunaris and middle meatus. The posterior ethmoid air cells drain into the superior meatus and then into the sphenoethmoid recess (see Fig. 8-2, A).

The anterior ethmoidal artery arises from the ophthalmic artery in the orbit, pierces the lamina papyracea, and exits via its respective foramina by passing through the ethmoid roof in the superomedial wall of the orbit, 2 to 3 mm behind the anterior wall of the bulla ethmoidalis. Occasionally the anterior ethmoidal artery passes within bony septae of the ethmoid sinuses and is suspended in a mesentery without bony cover (e-Fig. 8-4).¹³

Maxillary Sinuses

The maxillary sinuses are very small and pneumatized at birth, measuring an ellipsoid volume (sinus volume index) of approximately 0.24 ± 0.36 cm³.⁴ During the first years of life, the maxillary sinuses undergo rapid inferolateral expansion, reaching full size between 15 to 18 years of age. The floor of the maxillary sinus usually is seen at the level of the middle meatus during infancy; it reaches the level of the nasal floor by 8 to 9 years of age, and by age 12 years, it is located at the level of the hard palate.⁶ However, variation exists in the final descent of the floor, which lies below the level of the nasal floor in 65% of adults (e-Fig. 8-6, A–D).¹⁶

The maxillary sinus is the largest of the sinuses. Its roof is formed by the orbital floor, which carries the bony canal for the infraorbital nerve; the medial wall is formed by the lateral nasal wall. The posterior wall of the maxillary sinus forms the pterygopalatine fossa (Fig. 8-7, A). The maxillary sinus drains via the maxillary ostium located superomedially into the infundibulum, the hiatus semilunaris, and the middle meatus (Fig. 8-2, A).

Developmental variations of the maxillary sinus include an accessory ostium, isolated unilateral hypoplasia, and prominent septa that appear to compartmentalize the sinus cavity. Occasionally the roots of the maxillary molars impinge on the walls of the sinuses.

Sphenoid Sinus

The sphenoid sinus begins to pneumatize in a ventrodorsal direction from 7 months to 2 years of age. A significant acceleration in sinus expansion occurs between 3 and 8 years of age, with complete pneumatization usually present by age 10 years (e-Fig. 8-8, A–E).⁹ Its border is formed superiorly by the sella turcica, posteriorly by the clivus, anteriorly by the ethmoid sinus, and inferiorly by the nasopharynx (see Fig. 8-2, A and B). This sinus drains anteriorly via the sphenoethmoid recess. Lateral recesses of the sphenoid sinus are formed from pneumatization of the pterygoid process in 44% of people (see Fig. 8-7, A). Benign sphenoid marrow variants, which sometimes are mistaken for lesions, can be seen adjacent to the pneumatized sphenoid sinus (e-Fig. 8-9, A-C).

Important anatomic relationships of the sphenoid sinus include the optic canal and nerve located superolaterally; the foramen rotundum with the maxillary nerve that courses along the inferolateral margin of the sphenoid sinus; the vidian canal, which usually runs along the floor of the sphenoid sinus; and the cavernous portion of the internal carotid artery, which protrudes laterally into the sinus (see Fig. 8-7, B). The sphenoid sinus septum usually is aligned anteriorly with the nasal septum, but the sinus septum can deviate posteriorly, forming unequal sphenoid cavities (see Fig. 8-7, C). Absent pneumatization in a child older than 9 years is usually abnormal and warrants clinical investigation (see Fig. 8-7, D).

Frontal Sinuses

The frontal sinuses are absent at birth and are the last to develop once bone marrow conversion has occurred. The frontal sinus is considered an extension of the anterior ethmoid air cells and pneumatizes between 2 to 8 years, with the most significant growth taking place between ages 1 to 5 years. The frontal sinuses can continue to expand up until the second decade of life.6,11 The frontal sinuses consist of paired, often asymmetric cells. The anterior wall corresponds to the outer cortical table of the frontal bone, and the posterior wall of the sinus separates it from the anterior cranial fossa. Drainage of this sinus occurs via the frontal recess, which is an hourglass-like narrowing between the frontal sinus and the anterior middle meatus (see Fig. 8-2, A). Hypoplasia and aplasia of the frontal sinus can be seen in 4% and 5% of the population, respectively.

Radiography

Although plain radiography is less costly and more widely available than computed tomography (CT), it has significant limitations in evaluating the paranasal sinuses. Specifically, plain radiography often overestimates or underestimates findings, it does not localize pathology well, and it does not provide important anatomic detail.¹⁷ Traditionally, the paranasal sinus series has consisted of four views (Caldwell, Waters, posteroanterior, and lateral) that are technically difficult to perform in children. The Waters view is obtained by angulating the beam in 5-degree increments per year (up to age 6 years) to compensate for the progressive enlargement of the maxillary antra throughout childhood (Fig. 8-10, A and B). Although this approach improves visualization of the maxillary sinuses by projecting them over the petrous pyramids, it also creates double contours that can simulate mucosal thickening. Radiographic density also is critical because overpenetration of the films can completely obscure density differences created by disease, whereas underpenetration can simulate disease. Air fluid levels also can be obscured as a result of beam angulation. Although some have suggested using only the Waters view radiograph, this modality has been shown to have 32% false-negative results and 49.2% false-positive results when compared with CT. Moreover, most of the abnormalities in the ethmoid and sphenoid sinus are not detected in the Waters radiograph.¹⁸ Ethmoid disease on the Caldwell projection can be overdiagnosed or underdiagnosed because of technical and anatomic factors. The lateral view for evaluating the sphenoid sinus is of very little value in children younger than 4 years.^17,19

The American College of Radiology does not recommend the use of plain radiography in diagnosing sinusitis in children. Sinusitis is considered a clinical diagnosis that should not be made on the basis of imaging findings alone.²⁰ Similarly, American Academy of Pediatrics (AAP) guidelines state that plain radiographs are unnecessary for diagnosing sinonasal disease in children younger than 6 years.²¹ Further, plain films do not play a role in evaluating sinonasal masses or complications of sinusitis.

Computed Tomography

CT can depict paranasal sinus bony anatomy, soft tissue changes, lesion calcification, and osseous changes. Coronal CT images best demonstrate the anatomy of the ostiomeatal unit, as well as important anatomic landmarks and variants of the sinuses, effectively providing a road map for functional endoscopic sinus surgery. For these reasons, CT is considered the imaging modality of choice for evaluating inflammatory disease of the paranasal sinuses (i.e., recurrent and chronic sinusitis). CT also plays an important role in detecting orbital and intracranial complications of acute or chronic sinusitis. Along with magnetic resonance imaging (MRI), CT is an excellent tool for evaluating sinonasal masses.

When using nonhelical CT, images of the paranasal sinuses are obtained in a direct coronal plane with the patient in a prone position with the head hyperextended. Multidetector CT with volume isometric imaging allows axial image acquisition perpendicular to the table with the patient in a neutral supine position; images subsequently are reconstructed in coronal and sagittal planes. Slice thickness is usually 2.5 mm, and anatomic coverage extends from the upper teeth to 2 cm above the frontal sinus. If orbital or intracranial complications of sinusitis are suspected clinically, the study should be performed after the intravenous (IV) administration of contrast material, and imaging also should include the brain. Images are reconstructed in both soft tissue and bone algorithms.

In recent years, there has been heightened awareness of and concern about the potentially harmful side effects associated with radiation exposure, particularly in the pediatric population.22,23 For this reason, the practice of the “as low as is reasonably achievable” principle among the radiologic community is critical, with special attention given to CT protocols and parameters.^22,24 In evaluating paranasal sinuses, it is possible to reduce radiation techniques for maxillofacial CT imaging, even to a level comparable to that used for standard radiographic images, without sacrificing diagnostic image quality.²⁵

Magnetic Resonance Imaging

The role of sinus MRI in imaging inflammatory disease is limited, but it is valuable in evaluating complications of sinusitis (e.g., intracranial extension), as well as inflammatory disease associated with sinus or parasinal neoplasm. However, MRI does not depict bony detail and is less sensitive for bony erosions. Other limitations include its availability, higher costs, and the frequent need for sedation in young children.

The normal nasal cycle of vasodilation and mucosal edema followed by vasoconstriction and mucosal shrinkage can produce signal changes that can create significant variability in findings and thus differing interpretations. This cycle varies from 50 minutes to 6 hours, and the signal intensity during the edematous phase is indistinguishable from that of inflammatory change. As with CT and plain radiographs, sinus MRI typically shows a high incidence of findings in asymptomatic persons (13% to 37%) and mucosal thickening of <3 mm that is likely insignificant.² MRI can differentiate mucosal thickening from sinus secretions (Fig. 8-11, A–E).

Along with CT, MRI is very useful in evaluating neoplasms. An estimated 90% to 95% of tumors in the sinuses or nasal cavity exhibit moderately lower signal intensities on T2-weighted images (from hypercellularity) compared with most acute inflammatory diseases (including polyps, mucoceles, and retention cysts), which produce a bright signal on T2-weighted images. However, more mature granulation tissue and fibrosis also have a lower T2 signal, making it difficult to distinguish from tumor. Certain fungal infections, in contrast to other types of acute inflammatory disease, also have a lower T2 signal.

Ostiomeatal Unit

Regarded as an important anatomic region for potential surgical intervention, the ostiomeatal unit is a complex anatomic structure at the crossroads of mucociliary drainage from the frontal, anterior ethmoid, and maxillary sinuses.²⁶ It includes the uncinate process, infundibulum, ethmoid bulla, hiatus semilunaris, and middle meatus (Fig. 8-12, A and B).¹³

The uncinate process arises from the upper medial maxillary wall and defines the wall of the infundibulum.²⁷ The infundibulum is the channel defined laterally by orbit or Haller cells and medially by the uncinate process. The ethmoid bulla is a dominant middle ethmoid air cell that protrudes inferomedially into the infundibulum and hiatus semilunaris. The hiatus semilunaris is a semilunar region between the tip of the uncinate process and ethmoid bulla (see Fig. 8-12, A and B).

Choanal Atresia

Choanal atresia, the most common congenital abnormality of the nasal cavity, is thought to result from failure of rupture of the oronasal membrane during the sixth week of fetal life. It consists of obstruction of the posterior opening of the nasal cavity, which is mixed bony-membranous in approximately 70% of cases and pure bony atresia in 30% of cases. The existence of a purely membranous atresia has been questioned, and evaluation with high-resolution CT often has failed to demonstrate a membranous atresia without a bony component.²⁸ Choanal atresia can be unilateral (in 50% to 60% of cases) or bilateral, and it is more common in girls (the female : male ratio is 2 : 1).