Embryology and Anatomy

In children, the precise anatomy of the paranasal sinuses is complex and quite variable, however, some consistencies do exist. There are four groups of paranasal sinuses: maxillary, ethmoid, frontal, and sphenoid. All the paranasal sinuses develop as nasal chamber outpouchings, eventually extending into their respective bony vaults to varying degrees. The paired maxillary sinus develops early during the second trimester of fetal life and is fully developed by 3 years of age, up to which time the respective cavities are growing in width and height.3'6 Ultimately, the floor of the maxillary sinus is determined by the eruption of teeth.

The ethmoid sinus develops during the fourth gestational month, although matures at a slower pace than the maxillary sinus. The ethmoid sinus exists as paired groupings of individual cells or compartments subdivided anatomically on the basis of their locations, usually with 5 to 15 cells per side.3 Aeration of ethmoid cells is variable, producing a honeycombed radiographic appearance. The thin lateral walls of the ethmoid labyrinth are intimately related to the medial orbit.

Development of the typically paired frontal sinus is quite variable. After 4 years of age, the frontal sinus is in a supraorbital position; however, it is radiologically indistinguishable from the ethmoid sinus until 6 to 8 years of age. Growth then continues for another 8 to 10 years before reaching full adult development.3 The sphenoid sinus is present at 3 years of age and is generally fully developed by 12 years of age. Its slow growth and relative isolation in the skull base may preserve it from frequent infection, as isolated sphenoid sinusitis is uncommon in children.3

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