Anatomy of the Sinuses
The nasal cavity is a potential space situated above the oral cavity and hard palate and below the skull base and intracranial compartment. It is separated in the midline by the nasal septum into a right and left side. The nasal septum is composed of cartilage in its front end and bone towards the back of the nose. There are three contributors to the bony septum: the perpendicular plate of the ethmoid bone, vomer bone, and maxilla bone. The nasal septum is often crooked or off-midline, which can result in narrowing of one or both sides of the nasal cavity. The left and right nasal cavities become continuous in the back of the nose via the opening to the nasopharynx, termed the choana. In this area, the nasal cavity transitions into the nasopharynx. The nasopharynx contains a collection of centrally located lymphoid tissue called the adenoids. The Eustachian tubes each open into the sides of the nasopharynx, thus connecting the middle ear with the upper respiratory tracts. Dysfunction of the Eustachian tubes, from swelling, infection, or changes in altitude, among other things, can lead to transient ear ‘popping’ and ‘plugged’ sensation to the ears.
The sidewalls of the nose, or lateral nasal walls, include three structures called turbinates. The turbinates are finger-like projections composed of a bony core and covered with soft tissue and mucosa, and are important because they serve to increase the mucosal surface area of the nasal cavity and regulate nasal airflow. Beneath each turbinate is a cleft, or meatus, named according to the turbinate just above it. The inferior turbinate is the largest of the three paired turbinates, and runs along the entire length of the lateral nasal wall, adjacent to the nasal floor. The nasolacrimal (tear) duct, which collects tears from the eye, drains beneath the inferior turbinate into the inferior meatus, which explains why crying leads to a rapid onset of nasal discharge. The middle turbinate projects into the central nasal cavity and resides next to the nasal septum. It is attached to the lateral nasal wall posteriorly just above the inferior turbinate but behind the maxillary, or cheek, sinus. Superiorly, it inserts along the lateral nasal wall and skull base. The frontal sinus, maxillary sinus, and anterior ethmoid sinus cells drain beneath the middle turbinate into the middle meatus. Sometimes an ethmoid sinus cell can expand within the normally thin walls of the middle turbinate, and form an enlarged structure termed a concha bullosa. The superior turbinate is the smallest of the turbinates. It resides just above and behind the middle turbinate, and also attaches to the skull base superiorly and nasal wall laterally. The sphenoid sinus and posterior ethmoid sinus cells drain into spaces between the nasal septum and superior turbinate called the sphenoethmoid recess.
The entire nasal cavity is lined with a mucosal surface composed of ciliated epithelial cells and many mucus producing glands. The moist surface of the nasal cavity is normally covered in a thin blanket of clear mucus, which serves to trap any particulates, allergens, or microbes and helps to humidify inspired air. . This mucus blanket is continuously propelled along the mucosal surface by millions of cilia that beat rhythmically in order to propel these trapped particles out of the nasal cavity. In this way, the nasal cavity asks much like an environmental filter, warming and purifying inspired air before it passes to the lungs.
The nasal cavity has a diverse blood supply arising from both the internal and external carotid arteries, which arise from the chest and neck. The anterior and posterior ethmoid arteries provide blood to the nose, and are both terminal branches of the internal carotid artery. They supply the lateral and anterior one-third of the nasal cavity and posterior nasal septum, respectively. The internal maxillary artery arises from the external carotid artery and divides into several branches in the head and neck before entering the nasal cavity. The largest branch is the sphenopalatine artery that enters the nasal cavity through a tunnel located along the lateral nasal wall near the back of the middle turbinate. This artery divides into two or more branches to supply the majority of the lateral nasal wall and nasal septum. The second major branch of the internal maxillary artery is the descending palatine artery, which descends to supply the nasal floor and anterior nasal septum, and eventually the mucosa of the hard palate of the mouth.
In addition to this larger vasculature, there is a confluence of the small vessels that supplythe front portion of the nasal septum that is termed Kiesselbach’s plexus. This lattice of veins is a common source of nasal bleeding (epistaxis) due to trauma and dry air exposure, and may require medical attention in many cases.
Sensation is provided to the nasal cavity primarily via branches of the trigeminal nerve, one of the twelve cranial nerves arising from the brain into various structures in the head and neck. The ophthalmic division (V1) of the trigeminal nerve gives off the anterior and posterior ethmoidal nerves, which travel along with their counterpart arteries (noted above), to innervate the upper front and upper back portions of the nasal cavity and septum. The maxillary branch (V2) of the trigeminal nerve exits the brain, ultimately dividing into several smaller branches. The largest of these branches is the sphenopalatine nerve, which travels with the corresponding artery and supplies sensation to the lateral nasal wall and septum. Nasal secretions and mucus production is controlled by autonomic innervation from the brain to the vidian nerve, which then sends special parasympathetic fibers along with branches of the sphenopalatine nerve. Likewise, blood flow to the nasal cavity and nasal mucosa is controlled largely by opposing sympathetic nerve fibers that also travel along with the sphenopalatine nerve. Changes in the balance of autonomic signals in the nasal cavity result in a rhythmic engorgement of nasal blood vessels to the mucosa and turbinates, and result in fluctuations in left and right nasal congestion that some people experience over a 4-6 hour cycle. This diurnal nasal cycle is normal, but can be a source of concern for some patients that may require evaluation.