The Nose and Paranasal Sinuses
The External Nose and Nasal Skeleton
Externally, the nose is a midline pyramid-shaped structure that projects anteriorly from the face. Superiorly, it joins with the forehead at the glabella (the area between the eyebrows). Inferiorly, it joins with the upper lip. Laterally, the nasolabial folds are formed by the junction of the nose with the midface (cheek). The external nose is divided into nine aesthetic subunits, which are important to consider when undertaking reconstructive procedures of the nose. The nasal segments include the nasal dorsum (the bridge of the nose), the two lateral nasal wall segments (on either side of the dorsum), the lobule (the tip of the nose), the two alar segments (the dome-shaped soft tissue that forms the lateral walls of the nostrils), the two soft tissue triangle segments (located between the ala and the columella), and the columellar segment (the soft tissue dividing the two nostrils).
In addition, several anatomic landmarks are often referred to when discussing nasal anatomy. The nasion refers to the nasofrontal suture, where the nasal bones join with the frontal bones. The radix encompasses the nasion and refers to the region of the nasofrontal angle; it is also called the nasal root. The rhinion refers to the junction where the inferior aspect of the nasal bones meets the superior aspect of the upper lateral cartilages of the nose.
There is tremendous individual variability in the contour and dimensions of the nose, including nasal width, length, and projection, owing to differences in the nasal skeleton. The skeleton of the nose is divided into thirds, with the upper third composed of bone (bony vault) and the middle and lower thirds composed of cartilage (upper and lower cartilaginous vaults, respectively). The bony vault is an arch-shaped structure composed of the paired nasal bones and the nasal processes of the maxilla. At its inferior border, it meets and overlaps with the paired upper lateral cartilages (ULCs) at the rhinion. This region of overlap is also referred to as the keystone area. The upper lateral cartilages are fused laterally to the pyriform aperture, or the midline opening in the skull where the nasal bones and maxilla terminate. Medially, the ULCs are attached to the nasal septum, forming a 15 degree angle. The space between the caudal ULCs and the septum is referred to as the internal nasal valve and is usually the narrowest segment of the airway. The internal nasal valve ends at the anterior face of the inferior turbinate; the region caudal to this is termed the external nasal valve.
Inferior to the upper lateral cartilages are the C-shaped lower lateral, or alar, cartilages (LLCs). The LLCs are more mobile than the ULCs, with only loose fibromuscular attachments to the septum. The LLCs are further subdivided into medial, intermediate, and lateral crura. The paired medial crura abut in the midline and form the structure of the columella. The lateral crura extend laterally, partially forming the curvature of the nasal ala; however, the alar shape is primarily a result of the soft tissue rather than the cartilage. There are additional accessory (sesamoid) cartilages lateral to the lateral crura of the LLCs.
The integrity of the nasal cartilages and their attachments to each other as well as the septum serve as important mechanisms for maintaining support of the nasal tip. Deficits in these tip support mechanisms can result in tip ptosis and possible resulting nasal obstruction.
There are four major muscle groups of the nose: elevators, depressors, compressor, and dilators. These muscles are connected by an aponeurosis called the nasal superficial musculoaponeurotic system (SMAS).
→Blood Supply and Innervation
The majority of the external nose is supplied by branches of the facial artery (a branch of the external carotid artery). The superior labial artery supplies the columella and lateral nasal wall, while the angular artery supplies the nasal tip and dorsum. Venous drainage of the external nose is via the angular vein and anterior facial vein (eventually emptying into the internal jugular vein). Importantly, these veins form anastomoses with valveless veins draining to the cavernous sinus. For this reason, the region encompassing the nose and upper lip is sometimes referred to as the danger triangle of the face. It is possible, though rare, for superficial infections in this region to spread intracranially in a retrograde fashion via the venous anastomoses. Sensation to the external nose is provided by branches of the trigeminal nerve (cranial nerve V). These include the external nasal nerve (originates from ophthalmic division of trigeminal, V1) and the infraorbital nerve (originates from maxillary division of trigeminal, V2).
The Nasal Septum
The nasal septum is a midline bony and cartilaginous structure that divides the nasal cavity into left and right chambers. It is responsible for providing substantial structural support to the nose. Injury or abnormalities of the septum can result in significant cosmetic deformity, including nasal dorsal depression (saddle nose) and nasal tip collapse. Additionally, deviations of the septum from midline can produce both aesthetic and functional complaints. A detailed discussion of septal abnormalities can be found in Nasal Septal Abnormalities. The posterior portion of the septum is bony and consists of contributions from several separate bones of the skull, including the nasal, ethmoid, vomer, palatine, maxilla, and frontal bones. Anteriorly, the septum is composed of the quadrangular (quadrilateral) cartilage.
→ Blood Supply and Innervation
The septum is primarily supplied by branches of the internal maxillary artery (IMA, a branch of the external carotid) as well as the ethmoid arteries (arising from the ophthalmic artery, a branch of the internal carotid). There is a confluence of vessels on the anterior septum known as Kiesselbach’s plexus or Little’s area; this region is frequently the source of nasal bleeding (epistaxis, covered in greater detail in Epistaxis). As with the external nose, sensory innervation of the septum is via the ophthalmic and maxillary branches of the trigeminal nerve.
The Nasal Cavity and Lateral Nasal Wall
The nasal cavities extend superiorly to the cribriform plate and inferiorly to the hard palate. Anteriorly, the nasal cavities terminate at the nares (nostril openings). Posteriorly, the nasal cavities join with the nasopharynx at the choanae. The nasal cavities are lined with ciliated respiratory mucosa. The lateral walls of the nasal cavities contain structural elements called turbinates (concha), which are protuberances composed of thin bone. The turbinates function to direct nasal airflow, as well as to heat and humidify inhaled air. There are three turbinates: inferior, middle , and superior. Some individuals also have a rudimentary fourth turbinate called the supreme turbinate. The turbinates are attached to the lateral nasal wall, and the middle and superior turbinates also attach superiorly to skull base. Beneath the attachment points, there is a space between the lateral nasal wall and the lateral surface of the turbinate. This space is called a meatus, and each meatus corresponds to the space beneath the turbinate. The inferior meatus drains the nasolacrimal duct; the middle meatus drains the maxillary, anterior ethmoid, and frontal sinuses; and the superior meatus drains the posterior ethmoid sinuses.
→Blood Supply and Innervation
The blood supply of the nasal cavity derives from branches of the internal maxillary artery and the anterior and posterior ethmoid arteries. The sphenopalatine artery (SPA) is a branch of the IMA. It enters the nasal cavity at the sphenopalatine foramen, which is located on the lateral nasal wall, near the inferior attachment of the middle turbinate. Branches of the SPA supply the lateral nasal wall and septum. The SPA and its branches are frequently the source of posterior epistaxis. Venous drainage of the nasal cavities is via the pterygoid plexus and ophthalmic plexus. The pterygoid plexus eventually drains into the external jugular vein, while the ophthalmic plexus drains into the cavernous sinus; however, like the venous drainage of the external nose, multiple anastomoses exist between the plexuses. Sensory innervation of the nasal cavities is via the ophthalmic and maxillary divisions of the trigeminal nerve. In addition, the nasal mucosa receives parasympathetic innervation via secretomotor nerve fibers to mucous glands. Efferent parasympathetic signals generated in the superior salivatory nucleus of the brainstem travel in the facial nerve before branching off as the greater superficial petrosal nerve (GSPN). The GSPN is joined by fibers of the deep petrosal nerve to form the vidian nerve. This nerve synapses in the sphenopalatine (pterygopalatine) ganglion, which is located in the pterygopalatine fossa of the skull. From here, postganglionic nerve fibers travel with branches of the sphenopalatine nerve to reach the nasal mucosa. Sympathetic innervation of the nasal mucosa begins in the thoracic spinal nerves. These nerve fibers synapse in the superior cervical sympathetic ganglion. Postganglionic sympathetic nerve fibers travel superiorly with the internal carotid artery before branching off as the deep petrosal nerve and joining with the GSPN to form the vidian nerve. Sympathetic vasomotor fibers innervate the blood vessels of the nasal mucosa.
The Paranasal Sinuses
There are four major paranasal sinuses: maxillary, ethmoid, sphenoid, and frontal. The physiologic function of the paranasal sinuses is unknown, but hypotheses include protecting the brain and orbits from injury (by serving as crumple zones), reducing the weight of the skull, and improving voice quality. The maxillary sinus is located within the body of the maxilla. The natural drainage portal for the maxillary sinus is the maxillary sinus ostium; it is located superiorly on the medial wall of the sinus. The maxillary sinus ostium drains into the ethmoid infundibulum, which is a funnel-shaped space that serves as a common drainage collection site for the maxillary sinus, anterior ethmoid sinuses, and occasionally the frontal sinus. The ethmoid infundibulum opens into the hiatus semilunaris, which is a two-dimensional space in the middle meatus. It lies between the largest anterior ethmoid air cell (ethmoid bulla) and the uncinate process (a hook-shaped projection of the ethmoid bone).
The ethmoid sinuses are composed of multiple air cells within the ethmoid bone separated by bony trabeculae. The air cells are divided into anterior and posterior compartments. The ethmoid sinuses are bound laterally by the medial orbital wall (lamina papyracea) and medially by the turbinates. Superiorly, the ethmoid roof is formed by the frontal bone and cribriform plate. Posteriorly, the ethmoid air cells abut the face of the sphenoid. The ethmoid bulla is the largest anterior ethmoid air cell, situated on the lamina papyracea. Drainage of the anterior ethmoid sinuses is into the middle meatus, whereas the posterior ethmoid air cells drain into the superior meatus.
The sphenoid sinus is a midline cavity located posteriorly at the skull base. It is often divided into left and right compartments by an intersinus septum. This septum is often attached to the internal carotid artery on one side. Radiographically, the sphenoid sinus may appear to have an additional superior compartment; this actually represents the most posterosuperior ethmoid cell (Onodi cell). Dissection within this cell during sphenoidectomy should be avoided, as injury to the optic nerve can occur. The sphenoid sinus drains into the sphenoethmoid recess, which is a space above the superior turbinate (between the superior turbinate and the skull base).
The frontal sinuses are pyramid-shaped cavities formed as a result of pneumatization of the frontal bone. There is considerable individual variability in the degree of pneumatization; some individuals may have no frontal sinuses at all. The frontal sinuses separate the anterior table of the skull (which forms the forehead) from the posterior table (posterior to which lies the cranial cavity and brain). Drainage of the frontal sinuses is via the frontal recess, a potential space located where the frontal sinus meets the anterosuperior ethmoid. The anterior-most ethmoid cell is termed the Agger nasi cell; it forms the anterior border of the frontal recess. The frontal recess drains into the middle meatus. This drainage tract is sometimes referred to as the nasofrontal duct, although it is not a true duct.
→Blood Supply and Innervation
The vascular supply of the paranasal sinuses is via the IMA (maxillary and ethmoid sinuses), ethmoid arteries (ethmoid and sphenoid sinuses), and ophthalmic artery branches (frontal sinus). Innervation is primarily via the ophthalmic and maxillary divisions of the trigeminal nerve.
In addition to respiratory epithelium, the nasal cavity contains specialized sensory epithelium for olfaction. This olfactory neuroepithelium lines the olfactory cleft, which is located posterosuperiorly between the septum and the middle and superior turbinates. The neuroepithelium contains olfactory receptor neurons that have ciliated knobs, or tips. Odorant molecules enter the nose and are directed to the olfactory cleft by nasal airflow. Within the cleft, the odorants become trapped in the mucus overlying the olfactory epithelium. The odorant molecules then bind to olfactory receptors on the neuronal cilia, causing depolarization of the neuron. This allows for transduction of the signal into electrical action potentials. The axons of the olfactory neurons pass through the cribriform plate to enter the anterior cranial fossa, where they synapse onto secondary neurons in the olfactory bulb, located at the base of the frontal cortex. From the olfactory bulb, projections are sent to multiple brain areas, including the piriform cortex (sometimes referred to as the olfactory cortex), amygdala, and entorhinal cortex. These connections explain why smell is often associated with intense emotions and memories.
Mucociliary Clearance and Paranasal Sinus Drainage
The mucosal lining of the posterior nasal cavity and sinus cavities is composed of pseudostratified columnar epithelium that contains both ciliated and nonciliated cells. The cilia beat at a rate of 10-20 times per second in a coordinated fashion to direct mucus toward natural drainage pathways. In the sinuses, mucus is propelled at a rate of 3-25 mm per minute in the direction of the natural sinus ostia. From there, the mucus is transported to the nasopharynx and eventually swallowed. The ability of the respiratory epithelium to direct mucus drainage is referred to as mucociliary clearance. This phenomenon explains why opening a second sinus ostium is an ineffective method of treating sinus blockage—mucus will continue to be directed toward the obstructed natural ostium rather than the newly created drainage pathway. A more effective approach, therefore, is to enlarge the existing natural ostium. Functional endoscopic sinus surgery (FESS), which can be used to treat cases of sinus obstruction, is discussed further in Functional Endoscopic Sinus Surgery.