Clinical Anatomy of the Nose and Sinuses1

The external nose—the nasal pyramid—is formed by bones, cartilages, and soft tissue. The superior third of the pyramid is boney, being made up of the paired nasal bones, which articulate medially with their counterpart and laterally with the frontal process of maxilla. Superiorly, the boney pyramid is attached to the frontal bone.

The nasal cavity extends from the nostrils to the choanae, which communicate with the nasopharynx, and is bound superiorly by the cribriform plate of the ethmoid bone and inferiorly by the hard palate (maxillary and palatine bones) (Fig. 1.1). The nasal septum splits the nasal cavity in two and provides integral structural support to the nasal pyramid. The four pairs of paranasal sinuses all drain into the nasal cavity at various locations, with all mucus eventually draining into the nasopharynx.

The frontonasal process develops a pair of ectodermal thickenings called nasal placodes, which invaginate to form the nasal pits surrounded by the lateral and medial nasal processes. The lateral nasal process fuses with the maxillary process to form the nasolacrimal duct. The medial nasal process fuses with the maxillary process to form the central upper lip—the philtrum and the premaxilla (primary palate). The nasal pits deepen to become nasal sacs, and the epithelial plug (nasobuccal membrane) that fills the nasal cavity becomes resorbed around the 24th week allowing communication between the nose and the nasopharynx.

There is a membrane that separates the nasal pits from the primitive mouth—the oronasal membrane that breaks down between 5th and 6th week of gestation, as the palate develops from the maxilla and forms the floor of the nose.

The structural components of the lateral nasal wall are key to understanding nasal and paranasal sinus anatomy. While the inferior turbinate develops as a separate bone, other lateral nasal wall structures develop from the ethmoid bone. Growths from the ethmoid bone—the ethmoturbinals—are 5 in number.

The sinuses develop as diverticulae from the walls of the nasal cavity and are rudimentary at birth, apart from the frontal sinus, which appears around the age of 2 years. Maxillary, ethmoidal, sphenoid sinuses enlarge throughout early life, but undergo a rapid spurt around the age of 6 years, which coincides with the eruption of secondary dentition, and then again after puberty. The frontal sinus develops as the most anterior ethmoidal air cells grow into the frontal bone, and become apparent on X-ray and computerized tomography (CT) imaging around the age of 7 years.

Throughout the mucosa, inflammatory cells can be found that will elicit a rapid release of inflammatory mediators from mast cells, basophils, and other leukocytes in response to microbial or chemical insult. Keratinizing squamous epithelium is always abnormal.

The mucous blanket created by the secreting glands and goblet cells is made up of a thick Gel layer superficially, and a thin Sol layer inferiorly in contact with ciliated cells. The cilia are continually beating, thus providing a constant motion to the mucus blanket within the nose and sinuses. The Gel layer of mucus acts as a filtering and cleaning system for the upper respiratory tract as well as helps to maintain the moisture content within the nose and sinuses.

The lateral nasal wall has its surface area enlarged by the presence of the inferior, middle, superior, and occasionally the supreme turbinates. The space inferior to the turbinate is called the meatus, and is labeled according to the turbinate it is inferior to: inferior meatus lies inferior to the inferior turbinate. The turbinates are lined by mucoperiosteum with a unique submucosal vascular arrangement. The turbinates have capacitance vessels—the sinusoids—connecting capillaries to venules. This arrangement allows congestion and engorgement of the turbinates to take place in one nostril, while the other side is decongested. The sides alternate every 3–6 hours and this process is called the nasal cycle.

The inferior turbinate is a separate bone attached to the frontal process of maxilla anteriorly, the ethmoid bone, and the palatine bone posteriorly. The nasolacrimal duct drains the nasolacrimal sac into the inferior meatus via the Hasner's valve.

The middle, superior, and supreme turbinates arise from the ethmoid bone. The MT anatomy is key to the understanding of normal sinus drainage pathways. The MT has a complex three-dimensional attachment, which is in the sagittal/vertical plane anteriorly—attaching to the base of skull, coronal/vertical attachment to the lateral wall (the paper-thin lamina papyracea) in its middle segment, and horizontal in its posterior attachment to the lateral wall. Anterior to the coronal/vertical part (the ground lamella of the MT)—the remnant of the 2nd ethmoturbinal—lie the anterior ethmoidal air cells. Posterior to this structure lie the posterior ethmoidal air cells. The anterior ethmoidal air cells, thus, drain into the middle meatus, while the posterior ethmoidal cells drain postero-superior to the MT, into the superior meatus.

Arteries: The nose is supplied by branches of the ICAs and ECAs. The external nasal pyramid is supplied by the dorsal nasal artery (terminal branch of ophthalmic artery, ICA) at the base, and lower by the external nasal artery (branch of anterior ethmoidal, ICA) and by the lateral nasal and septal branches of the facial and its superior branch, respectively (ECA). The nasal cavity is supplied superiorly by the anterior and posterior ethmoidal arteries (branches of ophthalmic, ICA). The lateral nasal wall is supplied by conchal branches of the sphenopalatine artery (terminal branch of maxillary artery, ECA). The septum is supplied posteriorly by the posterior septal artery (branch of the Sphenopalatine artery), the palatine artery anteriorly (branch of maxillary, ECA), and the nasal septal branch of superior labial artery (branch of facial artery, ECA). On the anterior septum, a rich plexus—the Kiesselbach's plexus (also known as Little's area)—thus forms (greater palatine artery, nasal septal artery, ant ethmoidal artery, posterior septal artery). A second—Woodruff's plexus—forms on the posterior-inferior lateral wall (SPA branches, posterior nasal artery, pharyngeal and ascending pharyngeal arteries).

Veins: Veins accompany the arteries and drain in various directions. The pterygoid venous plexus is reached by the sphenopalatine foramen vessels, the internal jugular vein is reached by the facial vein, and the cavernous sinus is reached via the ethmoidal veins draining into the inferior ophthalmic vein.

Lymphatics: Lymphatics run with veins and drain into the submandibular, superior deep cervical, and retropharyngeal nodes.

The olfactory filaments (approximately 20 in number) pierce the cribriform plate to supply the vault of the nasal cavity, the superior turbinate, and the corresponding part of the septum. The fibers are bipolar cells with cell bodies in the mucosa and synapse with mitral cells in the olfactory bulb located in the anterior cranial fossa (ACF).

The general sensory supply includes the branches of the infraorbital nerve in the anterior vestibule, the anterior and posterior ethmoidal nerves (branches of nasociliary nerve, V1) superiorly, posterior superior lateral and posterior inferior lateral nasal nerves (V2) supplying the posterior lateral wall, branch of superior anterior alveolar nerve (V2) supplying anterior lateral wall, and nasopalatine nerve (V2) supplying the septum in combination with greater palatine nerve (V2) via the incisor foramen.

The parasympathetic innervation controls the volume, consistency, and viscosity of nasal secretions as well as causes vasodilatation. The innervation arises from the superior salivary nucleus in the pons, and initially travels via nervus intermedius that subsequently joins the facial nerve. At the geniculate ganglion, the parasympathetic fibers separate as the greater superficial petrosal nerve (GSPN), join the deep petrosal nerve (sympathetic fibers) at the foramen lacerum, and enter the pterygopalatine fossa via the vidian canal as the vidian nerve. Here the parasympathetic fibers synapse at the pterygopalatine ganglion. The fibers then follow the branches of maxillary division of trigeminal nerve (V2) into the nose and paranasal sinuses (Fig. 1.2).

The sympathetic innervation causes vasoconstriction and also alters the nature of nasal secretions. The fibers arise from the thoracic spinal cord, and after synapsing at the superior cervical ganglion, join the internal carotid and follow it as the internal carotid sympathetic plexus intracranially. The fibers separate as the deep petrosal nerve and join the GSPN to form the vidian nerve at the foramen lacerum. The sympathetic fibers follow the arteries out of the pterygopalatine fossa into the nose and paranasal sinuses.

The drainage pathway of the frontal sinus—the frontal recess—drains anterior to the anterior ethmoidal air cells, and thus also empties into the middle meatus.

The opening of the maxillary sinus—maxillary sinus ostium—is overlapped by a hook-like process of the ethmoid bone called the uncinate process. Thus, on direct examination of the nose, one cannot see inside the maxillary sinus, but the contents of the sinus drain into the middle meatus.

The contents of the sphenoid sinuses drain directly via a sphenoid sinus ostia into the sphenoethmoidal recess, and then join the posterior ethmoid sinus drainage pathway in the superior meatus.

Thus, the contents of the frontal, maxillary, and anterior ethmoidal sinuses empty into the middle meatus and flow posteriorly into the nasopharynx anteroinferiorly to the opening of the Eustachian tube.

This common drainage space within the middle meatus is referred to as the osteomeatal complex.

Hence, the osteomeatal complex is a three-dimensional space lateral to the MT.

The contents of the posterior ethmoid and sphenoid sinuses empty into the superior meatus and flow posteriorly into the nasopharynx supero-posterior to the Eustachian tube opening. Sphenoid sinuses drain medial to the superior or supreme turbinate via an independent ostia, which in turn communicates with the sphenoethmoid recess.

Hiatus semilunaris inferior opens into a three-dimensional space—ethmoidal infundibulum.

Infundibulum continues superiorly into frontal recess in about 14% cases.

Other 86% infundibulum ends in recess terminalis.

It is bounded:

Medially by the uncinate process, and laterally by the lamina papyracea. The frontal process of the maxilla and lacrimal bone form its anterior-superior boundary. It measures 4 cm in length and is 5–12 mm deep.

Superiorly by the frontal recess (depends on attachment of uncinate process)—14% of them communicate with the frontal recess.

Ground lamella is well developed. It divides ethmoid into anterior and posterior parts.

In radiologic evaluation of the sinus scans, the common variability of these cells makes preoperative imaging essential to clarify a patient's individual anatomy.

Skull base evaluation is important in preoperative evaluation of the patients undergoing sinus surgery.

The following acronym is useful when evaluation CT images of the paranasal sinuses.