Nose and Sinonasal Cavities

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 ( Figs. 8.2A 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, whereas the nasal septum is lined by squamous mucosa. 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. In general, only rudimentary maxillary and ethmoid sinuses, which continue to expand until puberty or early adulthood, are present at birth. The sinuses also are linked to facial growth and dentition.

Ethmoid Sinuses

The ethmoid sinuses are already developed in number and pneumatized at birth but continue to expand, reaching adult proportions at about 12 years of age ( Fig. 8.3 ). The ethmoid sinuses consist of a paired group of a variable number of cells (3–18) within the lateral masses of the ethmoid bone. The ethmoid bone also consists of the cribriform plate superiorly and the perpendicular plate, which is part of the nasal septum. 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. 8.3C ), 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. 8.2A ).

The anterior ethmoidal artery arises from the ophthalmic artery in the orbit, pierces the lamina papyracea, and exits 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 the bony septae of the ethmoid sinuses and is suspended in a mesentery without a bony cover ( e-Fig. 8.4 ).

Anatomic Variants

Any asymmetry in the height of the ethmoid roof should be documented by the radiologist due to risk of surgical penetration during endoscopic surgery on the side where the roof is lower.

Concha bullosa refers to the pneumatization of the middle turbinate as a result of intramural extension of posterior ethmoid air cells ( Fig. 8.5B ). A large concha bullosa can eventually cause obstruction of the nasal cavity.

Extramural expansion of ethmoid air cells can result in anatomic variants, including the following: Agger nasi cells ( Fig. 8.5A ), which are the most anterior and inferior cells involving the lacrimal bone or maxilla; Haller cells, which are middle ethmoid air cells extending into the inferomedial floor of the orbit ( Fig. 8.5C ); and Onodi cells , which are posterior sphenoethmoid air cells with prominent superolateral pneumatization in close relationship to the optic nerve canal.

Maxillary Sinuses

The maxillary sinuses are very small and pneumatized at birth and undergo rapid inferolateral expansion during the first years of life, reaching full size between 15 to 18 years of age. The floor of the maxillary sinus is seen usually 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. The maxillary floor lies below the level of the nasal floor in 65% of adults ( e-Fig. 8.6 ).

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. 8.7A ). The maxillary sinus drains via the maxillary ostium located superomedially into the infundibulum, the hiatus semilunaris, and the middle meatus (see Fig. 8.2A ).

Developmental variations of the maxillary sinus include an accessory ostium, unilateral hypoplasia, and septations that can compartmentalize the sinus cavity. Maxillary molar roots may impinge on the walls of the sinuses.

Sphenoid Sinus

The sphenoid sinus pneumatizes in a ventrodorsal direction from 7 months to 2 years of age. Significant sinus expansion occurs between 3 and 8 years of age, with complete pneumatization usually present by age 10 years ( e-Fig. 8.8 ). Its superior border is formed by the sella turcica, posteriorly by the clivus, anteriorly by the ethmoid sinus, and inferiorly by the nasopharynx (see Figs. 8.2A 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. 8.7A ). Benign sphenoid marrow variants, which sometimes are mistaken for lesions, can be seen adjacent to the pneumatized sphenoid sinus ( e-Fig. 8.9 ).

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. 8.7B ). The sphenoid sinus septum usually is aligned anteriorly with the nasal septum but can deviate posteriorly, forming unequal sphenoid cavities (see Fig. 8.7C ). Absent pneumatization in a child older than 9 years is usually abnormal and warrants clinical investigation (see Fig. 8.7D ).

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, pneumatizes between 2 to 8 years, and continues to expand until the second decade of life. The frontal sinuses consist of paired, often asymmetric cells. The anterior wall corresponds to the outer cortical table of the frontal bone, and its posterior wall separates this sinus from the anterior cranial fossa. This sinus drains via the frontal recess, which is an hourglass-like narrowing between the frontal sinus and the anterior middle meatus (see Fig. 8.2A ). 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. Plain radiography often overestimates or underestimates findings, does not localize pathology well, and does not provide important anatomic detail. The paranasal sinus series traditionally consists 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 ). This approach improves visualization of the maxillary sinuses by projecting them over the petrous pyramids but can create double contours that simulate mucosal thickening. Air fluid levels also can be obscured as a result of beam angulation. Using only the Waters view radiograph has been shown to have 32% false-negative results and 49.2% false-positive results when compared with CT. Abnormalities in the ethmoid and sphenoid sinuses are not detected on a Waters radiograph. Ethmoid disease on the Caldwell projection is limited, and the lateral view for evaluating the sphenoid sinus is of little value in children younger than 4 years.

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

Computed Tomography

CT nicely demonstrates paranasal sinus bony anatomy, soft tissue changes, lesion calcification, and osseous changes. Coronal images best demonstrate the anatomy of the ostiomeatal unit, as well as important anatomic landmarks and variants, 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 plays an important role in detecting orbital and intracranial complications of sinusitis. Along with magnetic resonance imaging (MRI), CT is an excellent tool for evaluating sinonasal masses.

Currently, multidetector CT with volume isometric imaging allows axial image acquisition with the patient in a neutral supine position. Images are subsequently reformatted in the 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 of the brain should be considered. Images are displayed in both soft tissue and bone algorithms.

Awareness of and concern about the potentially harmful radiation side effects is particularly important in children. The practice of the “as low as is reasonably achievable” (ALARA) principle among the radiologic community is critical, with special attention given to CT protocols and parameters. 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.

Magnetic Resonance Imaging

MRI is valuable in evaluating complications of sinusitis (e.g., intracranial extension), as well as inflammatory disease associated with sinus or parasinal neoplasm. Although MRI detects bone marrow involvement, it is less sensitive for bony erosions than CT. 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, varies from 50 minutes to 6 hours. 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%–37%) and mucosal thickening of less than 3 mm that is likely insignificant. MRI can differentiate mucosal thickening from sinus secretions ( Fig. 8.11 ).

MRI is very useful in evaluating neoplasms. An estimated 90% to 95% of tumors in the sinuses or nasal cavity exhibit moderately lower signal intensity 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. It includes the uncinate process, infundibulum, ethmoid bulla, hiatus semilunaris, and middle meatus ( Fig. 8.12 ).

The uncinate process arises from the upper medial maxillary wall and defines the wall of the infundibulum. 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.

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 is questionable. Choanal atresia can be unilateral (in 50%–60% of cases) or bilateral, and it is twice as common in girls.