2 Pediatric Rhinologic Considerations
Abstract
The developing sinonasal anatomy in children may differ from the adult patient; therefore, the rhinologist must play an important role in the endoscopic endonasal approach to the skull base in the pediatric patient. In this chapter, the developmental anatomy of the nasal cavity and four sets of paired paranasal sinuses are discussed in detail, and functional and comparative anatomy to that of adult sinuses will also be discussed. The location of the facial growth centers inside the nasal cavity should be carefully addressed during this approach to avoid facial growth abnormalities, in particular reference to posterior removal of the nasal septum. Understanding these concepts is integral for performing effective, safe, and appropriate pediatric endoscopic endonasal skull base surgery.
2.1 Sinonasal Embryology and Developmental Anatomy
A clear knowledge of the embryologic and developmental anatomy of the nasal cavity, paranasal sinuses, and surrounding structures is integral to performing safe and appropriate pediatric endoscopic endonasal skull base surgery. In addition, an understanding of the embryologic development of the nasal cavity and paranasal sinuses allows for better comprehension of the spatial involvement of structures addressed during endoscopic approaches. Since the nasal and paranasal sinus structures develop from multiple bones rather than a single bone, the approaches to the targeted site have to be carefully considered in the surgical planning and execution.
The primary bones from which the nasal cavity and paranasal sinus structures develop are the maxillary, ethmoid, sphenoid, and frontal bones, with lesser contributions to paranasal sinus development from the lacrimal and zygomatic bones. The nasal septum develops from four sources: the perpendicular plate of the ethmoid bone, the maxillary bone or crest, the vomer, and the quadrangular cartilage.
2.1.1 Nasal Cavity
The development of the nasal cavity is heralded by the appearance of a series of ridges or folds on the lateral nasal wall between the fourth and eighth weeks of fetal life (▶ Fig. 2.1). During this time, the nasal septum can be seen dividing the right and left sides of the future nasal cavity as the frontonasal and maxillary processes join. The frontonasal process grows over developing forebrain, contributing to nasal olfactory placodes. At 8 weeks’ gestation, the nasal septum arises as a posterior midline growth of the frontonasal process and midline extensions of mesoderm from the maxillary processes that are partially differentiated into cartilage. 1 The descending septum merges with the fused palate to create two distinct nasal cavities.
Beginning at 8 weeks, several ridges that persist throughout fetal development and into later life begin to develop along the lateral nasal wall. 1 These are traditionally called ethmoturbinals. The ethmoturbinals are considered to be ethmoid and maxillary bone in origin and eventually develop into the agger nasi region of the ethmoid sinuses (superior portion of first ethmoturbinal or nasoturbinal), middle turbinate (second ethmoturbinal), superior turbinate (third ethmoturbinal), and supreme turbinate (fourth and fifth ethmoturbinals). 2 , 3 Between 9 and 12 weeks’ gestation, a separate cartilaginous and soft-tissue bud corresponding to the uncinate process forms between the developing middle and inferior turbinates. 1 , 3 At 15 to 16 weeks’ gestation, the inferior, middle, and superior turbinates are clearly formed and easily visible in embryologic sections. 3
Around the same period, three furrows form between the ethmoturbinals and ultimately establish the primordial nasal meatuses and recesses that separate the adult turbinates. 4 , 5 The first furrow forms between the first and second ethmoturbinals. Its descending aspect forms the ethmoidal infundibulum, hiatus semilunaris, and middle meatus in the adult. Its ascending aspect can contribute to the frontal recess. The second furrow forms between the second and third ethmoturbinals developing into the superior meatus, while the third furrow forms between the third and fourth ethmoturbinals developing into the supreme meatus.
2.1.2 Olfactory Mucosa
At 8 weeks’ gestation, a hypercellular mesenchymal capsule forms around the developing nasal structures and olfactory epithelium can be seen in the superior portion of the nasal cavity, with the cribriform plate presenting in a cartilaginous form at 14 to 16 weeks. Postnatally, there is partial regression of the olfactory epithelium such that it occupies the area located in the nasal vault, the upper portion of the nasal septum, the medial surface of the superior turbinate, sectors of the medial surface of the middle turbinate, and the region of the cribriform plate. 6 The overall area averages 1 to 2 cm2 in adults, but covers a much larger region in infants. 7
2.1.3 Paranasal Sinuses
The extent of paranasal sinus pneumatization and development differs greatly from person to person as a result from the extent of invagination and evagination between the developing turbinates and their intervening furrows. 4 All paranasal sinuses are present to varying degrees in the newborn, each one having specific periods of significant growth. The ethmoid sinuses are the first to fully develop, followed in order by maxillary, sphenoid, and frontal sinuses (▶ Fig. 2.2).
Ethmoid Sinuses
As previously mentioned, the ethmoid sinus is the first to develop into detectable pneumatized cells in the fetus. Around the 11 to 12 weeks of fetal life, early anterior ethmoid cells, including the cartilaginous beginnings of the ethmoid bulla, form as a result of budding from the middle meatus. 1 , 5 At 14 to 16 weeks, some anterior ethmoid cells are well formed. 1 Later, the posterior ethmoid buds begin to develop from the superior meatus by 17 to 18 weeks. Ossification of the ethmoid sinuses and lamina papyracea occurs by 20 to 24 weeks’ gestation. 1 , 5
At birth, the ethmoid sinuses are the most developed paranasal sinuses, having a complete number of cells in varying stages for development. 8 They undergo significant growth during the first decade of life, reaching their adult size by 12 years of age. 8 , 9 However, these cells can expand beyond the boundaries of the ethmoid bone to extend into the frontal recess (frontal cells, suprabullar cells, and frontal bullar cells), sphenoid bone (sphenoethmoid [Onodi] cell), and maxillary bone (infraorbital ethmoid [Haller] cell).
It is important to recall that the ethmoid bone contains more than the ethmoid sinuses. Other structures that are derived from the ethmoid bone include the middle turbinate, superior turbinate, supreme turbinate, cribriform plate, and the posterosuperior portion of the nasal septum (perpendicular plate of the ethmoid).
Maxillary Sinuses
The maxilla begins ossification at 11 to 12 weeks’ gestation, as the early anterior ethmoid sinuses are developing. 1 The maxillary infundibulum becomes evident at 14 to 16 weeks of fetal life as an invagination of the maxillary bone, found lateral to the uncinate ridge. However, at this point, there is still no true maxillary sinus cavity. By 17 to 18 weeks’ gestation, an air space is clearly seen lateral to the developing uncinate process, protruding toward the woven bone of the maxilla. 3 The developing maxillary sinus can be differentiated from the nasolacrimal duct at this stage as well. 3 Over the second and third trimesters, the maxillary sinus continues to enlarge from the maxillary infundibulum.
The maxillary sinuses are present at birth, although their small size typically precludes their radiologic appearance. Conspicuous growth in the maxillary sinuses begins by approximately 3 years of age, but inferiorly directed expansion does not occur until eruption of the permanent dentition, when the child is 7 to 8 years of age. The floor of the maxillary sinus approximates the inferior meatus at 8 years of age and reaches the level of the floor of the nose by 12 years of age. Adult size is reached by mid-adolescence.
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