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Martin J. Citardi, M.D.
The paranasal sinuses are air-filled pockets located within the bones of the face and around the nasal cavity. Each sinus is name for the bone in which it is located:
- Maxillary (one sinus located in each cheek)
- Ethmoid (approximately 6-12 small sinuses per side, located between the eyes)
- Frontal (one sinus per side, located in the forehead)
- Sphenoid (one sinus per side, located behind the ethmoid sinuses, near the middle of the skull)
Each of these pockets has an opening that connects to the nose. This opening is called an ostium.
Of course, the nose and sinuses are closely related. The nasal septum divides the nose into two nasal cavities. The side wall of the nose (the lateral nasal wall) has three important structures, which are known as the superior, middle and inferior turbinates. Each turbinate is a rounded projection that extends the length of the nasal cavity. The space between each turbinate is called a meatus, and each meatus is named for the meatus above it.
The inferior turbinate, which is larger than the other turbinates, runs parallel to the floor of the nose. The nasolacrimal duct drains tears into the inferior meatus. (This explains why one develops nasal congestion when one cries.)
The middle turbinate is located above the inferior turbinate. The anterior (or front) ethmoid cells open into the middle meatus. The term "frontal recess" refers "ante-chamber" just below the frontal sinus ostium. Therefore, the frontal sinus drains into the middle meatus. The frontal recess contains a variable number of ethmoid cells.
The superior turbinate, which is the smallest turbinate, is above the middle turbinate. The posterior (or back) ethmoid cells drain into the superior meatus. The space between the superior turbinate, the septum and the sphenoid sinus front wall is known as the sphenoethmoid recess. The sphenoid drains here.
The paranasal sinuses are covered with a special lining (or epithelium). The lining secretes mucus, a complex substance that keeps the nose and sinuses moist. The sinus epithelium is ciliated; that is, each cell on its surface has a cilium, which is a relatively long structure that has the capacity to push sinus mucus. This movement of mucus (which is known as mucociliary clearance) is not random; rather, it is programmed so that the mucus moves along in a specific pattern. The sinus do not 'drain' by gravity-it is an active process.
Figure 1. Lateral Nasal Wall Dissection
The lateral nasal wall (or sidewall) of the human nose has a very complex structure. These photos were taken during a dissection of a human cadaver. The tip of the nose is on the left.
In Figure 1A, the inferior, middle, and superior turbinates can be seen. Note the locations of the frontal and sphenoid sinuses. Only a thin layer of bone separates the sinuses from the brain.
In Figure 1B, the middle turbinate has been folded upon itself. Now, the structures of the middle meatus can be seen. The uncinate process is a thin sickle-shaped structure. It covers the entrance to the maxillary sinus. The ethmoidal bulla is the ethmoidal cell that is located closest to the front of the middle meatus.
In Figure 1C, the uncinate process has been removed. Now, the entrance to the maxillary sinus can be seen. In addition, the opening to the frontal sinus (known as the frontal ostium) is also visible. The blue line is drawn over the area where the uncinate process was attached.
Patients with persistent sinusitis after antibiotics may be candidates for sinus surgery. The initial step during this type for this surgery often is removal of the uncinate process. This can be done with special surgical instruments through the nostril. In this way, the blockage for sinus drainage can be removed.
Figure 2. Axial CT Scan
Computed tomography (CT scans or CAT scans) can provide detailed information about sinus and nasal structure. Each CT scan image is a picture of the anatomy in a given slice. This image is an axial CT scan. The ethmoid and sphenoid sinuses are well seen. The bone that separates the ethmoid sinuses from the orbits (eye sockets) is thinner than eggshell. The sphenoid sinus is located deep in the head. Rarely sphenoid sinus infections can spread to the brain.
Figure 3. Coronal CT Scan
Coronal CT scans are also routinely used. These scans are slices that are oriented approximately perpendicular to the palate.
In Figure 3A, many important structures can be seen. Both the inferior and middle turbinates are seen. Note the large uncinate process (also seen in Figure 1). The nasal septum divides the nose into two separate cavities.
In Figure 3B, additional structures can been. This slice is located behind the slice shown in Figure 3A. The maxillary sinuses are rather large. The inferior, middle and superior turbinates are all seen; the superior turbinate is the smallest of the three, and the inferior turbinate is the largest. The ethmoid cells are also shown.
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