For a detailed discussion of sinonasal development, see Chapter 1 of this book. The paranasal sinuses and nasal cavity are lined by the ciliated columnar epithelium, which contains both mucinous and serous glands. The common drainage pathway for the frontal sinuses, maxillary sinuses, and anterior ethmoid air cells is through the ostiomeatal complex.1,2 The ostiomeatal unit consists of the maxillary sinus ostium, the infundibulum, the hiatus semilunaris, and the middle meatus ( Fig. 3.1A,B ). This drainage conduit is centered around the uncinate process, an osseous extension of the lateral nasal wall. Secretions that accumulate within the maxillary sinuses circulate toward the maxillary sinus ostium propelled by ciliated mucosa.1,3 From the maxillary ostium, mucus passes through the infundibulum, located lateral to the uncinate process. Secretions progress through the hiatus semilunaris, the air space between the posterior edge of the uncinate process and the anterior and inferior surface of the ethmoidal bulla (the largest ethmoid air cell), and then pass into the middle meatus, the nasal cavity, and ultimately into the nasopharynx where they are swallowed.1,3
The frontal sinuses drain inferiorly via the frontal ethmoidal recess into the middle meatus, which is the common drainage site also for the anterior ethmoid air cells.1
The anteriormost of the ethmoid air cells is the agger nasi, but other variants of ethmoid pneumatization may be present, including frontal cells, supraorbital ethmoid cells, and suprabullar cells.4 Infraorbital ethmoid cells (also known as Haller cells or maxilloethmoidal cells) are located along the anterosuperior maxillary surface just inferior to the orbital floor, typically in close proximity to the maxillary sinus ostium. Present in less than 10 to 18% of imaged patients, infraorbital ethmoid cells are important because they may encroach or obstruct mucociliary clearance from the maxillary sinus and may contribute to sinonasal inflammatory disease.4
The posterior ethmoid air cells are located posterior to the basal lamella of the middle turbinate, and secretions originating from the posterior ethmoid sinus drain through the superior meatus and/or the supreme meatus into the sphenoethmoidal recess, the nasal cavity, and finally, into the nasopharynx ( Fig. 3.1C,D ). The sphenoid ostium is located medial to the superior turbinate and drains directly into the sphenoethmoid recess. Cilia are necessary for drainage of the sphenoid sinuses as secretions must be propelled to the sphenoid ostia located superior to the sinus floor.
The anterior and inferior nasal septum is made up of cartilage. The posterior portion of the nasal septum is osseous. The superior posterior osseous portion is the perpendicular plate of the ethmoid bone, whereas the inferior posterior osseous portion is the vomer. The nasolacrimal duct runs from the lacrimal sac at the medial canthus, along the anterior and lateral nasal wall, and drains into the inferior meatus. The three sets of turbinates in the nasal cavity include the superior, middle, and inferior turbinates. Occasionally, there may be a supreme turbinate located posterior and superior to the superior turbinate. Concha bullosae (aerated middle turbinates) are present in up to 50% of patients.5,6 A large or opacified concha bullosa may obstruct the ostiomeatal complex. In most individuals, there is normal cyclical passive congestion and decongestion that alternates between each side of the nasal cavity, as dictated by the nasal cycle. These periodic fluctuations in blood flow may result in apparent relative hypertrophy of intranasal structures.
Blood supply to the sinonasal structures comes from the internal and external carotid arteries. The arterial supply to the frontal sinuses is from supraorbital and supratrochlear branches of the ophthalmic artery, whereas venous drainage is through the superior ophthalmic veins. The ethmoid air cells and sphenoid sinus also receive blood supply from branches of the sphenopalatine artery (arising from the external carotid circulation) as well as ethmoidal branches of the ophthalmic artery (arising from the internal carotid circulation). Branches of the internal maxillary artery that arise from the external carotid artery predominantly supply the maxillary sinuses.
The venous drainage pattern of the paranasal sinuses (ultimately communicating with the cavernous sinus and pterygoid venous plexus) is responsible for the potential intracranial complications of sinusitis, including meningitis, subdural empyema, and venous thrombosis. The venous drainage is through nasal veins and/or ethmoidal veins that drain into the ophthalmic veins, which subsequently drain into the cavernous sinus. The maxillary sinuses drain through facial and maxillary veins, the latter communicating with the pterygoid venous plexus.