4.0 Anatomy and Physiology of the Nose and Paranasal Sinuses

The external nose consists of the nasal pyramid (frontal process of the maxilla), with the paired nasal bones forming the dorsum and meeting the frontal bone superiorly at the glabella. Inferiorly are the upper lateral cartilages and lower lateral (alar) cartilages, which contribute to the nasal tip and nasal valves ( Fig. 4.1 ).

The internal nasal and sinus anatomy is complex and variable. The midline nasal septum is composed of the quadrangular cartilage, the perpendicular plate of the ethmoid bone, the vomer bone, and the palatine bone, with an overlying mucosal covering. There are four paired sinuses: the maxillary sinus, frontal sinus, ethmoidal air cells, and sphenoidal sinus ( Fig. 4.2 ). The lateral nasal wall consists of the inferior, middle, and superior conchae (turbinates); below each concha is its corresponding meatus. The nasolacrimal duct opens into the inferior meatus. The frontal and maxillary sinuses and anterior ethmoidal air cells drain via the middle meatus. The posterior ethmoidal air cells drain via the superior meatus. The sphenoidal ostia are near the level of the superior meatus on the anterior wall of the sphenoidal sinus. Clinically, the relation of the paranasal sinuses to adjacent anatomic structures is important, as it relates to the potential for the spread of infection or an iatrogenic injury. Specifically, the ethmoid roof may be an extremely thin bone along the lateral lamella of the cribriform plate and may vary in its height considerably; intracranial contents lie superiorly. The lamina papyracea is, as its name implies, a paper-thin sheet of bone that separates the orbit from the ethmoidal air cells.

The sphenoid sinus is bounded by the internal carotid artery, optic nerves, cavernous sinus, and sella turcica; an overriding posterior ethmoidal (Onodi) cell may risk critical structures. Dehiscence of the bone covering the internal carotid artery within the sphenoidal sinus is relatively common and should be routinely assessed for on a preoperative computed tomography (CT) scan. The frontal sinus is bounded by the orbit and the anterior cranial fossa and may also be a source of spread of rhinogenic infection. A Haller cell is an anterior ethmoidal cell that pneumatizes laterally at the orbital floor and can contribute to maxillary sinus drainage problems. Agar nasi cells are anterior ethmoidal cells that pneumatize superiorly and can contribute to frontal sinus drainage problems.

Blood Supply

There is abundant external and internal carotid supply ( Fig. 4.3 ). Kiesselbach′s area (also known as Little′s area) of the anterior septum provides superficial anastomoses. External carotid branches supply the nose externally (via the facial artery), and the maxillary artery, including the sphenopalatine artery, does so internally. Internal carotid branches are supplied via the ophthalmic artery to the anterior and posterior ethmoidal arteries. Venous drainage occurs via facial veins as well as ophthalmic veins, which have valveless intracranial connections to the cavernous sinus and therefore relate to intracranial hematogenous spread of infection. Epistaxis (nosebleed) is discussed in Chapter 4.1.5.

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