Ethmoid Sinus (Ethmoid Air Cells)

What Is the Ethmoid Sinus?

The ethmoid sinus is the collective name for the ethmoid air cells, many small air-filled spaces inside the ethmoid bone, a light, box-shaped bone at the center of the face behind the bridge of the nose. Instead of one large cavity, each side holds a cluster of cells separated by very thin bony walls, so the region is often called the ethmoid labyrinth.

It is one of the four pairs of paranasal sinuses, along with the frontal, maxillary and sphenoid sinuses. It also sits at the crossroads of the others: the frontal and maxillary sinuses drain through channels that pass among the anterior ethmoid cells, so the ethmoid region acts as the gateway for sinus drainage.

Explained simply: between your eyes, behind the top of your nose, the bone looks like a tiny sponge or honeycomb. Each little hole in that honeycomb is an air cell, and all of them together make up the ethmoid sinus.

Where Is the Ethmoid Sinus Located?

The ethmoid sinus is located between the eyes, in the two lateral masses of the ethmoid bone on either side of the upper nasal cavity. Each cluster of cells lies between the nasal cavity on its inner side and the orbit on its outer side, just below the floor of the front part of the skull.

The bone around the cells is thin on every side, as the table below shows. The outer wall, the lamina papyracea or orbital plate, is so thin that its name means paper layer. The roof, the fovea ethmoidalis, is formed mainly by the frontal bone and separates the cells from the anterior cranial fossa. The inner wall carries the superior and middle nasal conchae, which are themselves parts of the ethmoid bone.

Between the two ethmoid labyrinths lie the upper nasal septum and, above it, the cribriform plate, which carries the nerve fibers serving the sense of smell.

Ethmoid Sinus Anatomy: Anterior and Posterior Ethmoid Cells

The ethmoid air cells are divided into anterior and posterior groups by the basal lamella of the middle concha, a thin sheet of bone where the middle turbinate attaches to the side wall of the nose. Some textbooks also describe a middle group, which today is usually counted with the anterior cells.

The number and size of the cells vary widely between people. Anterior cells tend to be smaller and more numerous, while posterior cells are fewer and larger. Several cells are distinctive enough to have their own names, such as the agger nasi cell at the front, the ethmoid bulla in the middle meatus and the Onodi cell at the back; the second table lists them with their positions.

Lining, vessels and nerves

The cells are lined by thin ciliated respiratory epithelium, continuous with the lining of the nose. Blood comes mainly from the anterior and posterior ethmoidal arteries, branches of the ophthalmic artery that cross the roof, with help from the sphenopalatine artery. Sensation travels in the anterior and posterior ethmoidal nerves, from the ophthalmic division of the trigeminal nerve, and in small orbital branches linked to the maxillary nerve.

Where Do the Ethmoid Air Cells Drain?

The anterior ethmoid cells drain into the middle meatus, and the posterior ethmoid cells drain into the superior meatus. Each cell opens into a neighbor or directly into the nose through small openings, rather than through one large ostium.

• Anterior cells: into the ethmoidal infundibulum and semilunar hiatus, or onto the ethmoid bulla, in the middle meatus.
• Middle cells, when described: on or above the ethmoid bulla, also in the middle meatus.
• Posterior cells: into the superior meatus under the superior concha, and sometimes into a small supreme meatus above it.

The anterior ethmoid region is part of the ostiomeatal complex, the narrow area of the middle meatus where the frontal, maxillary and anterior ethmoid sinuses all empty. Because these channels are so close together, swelling of the ethmoid lining can block the drainage of the larger sinuses as well. Mucus from every cell is swept by the mucociliary escalator toward the nasopharynx; the nasal meatus page shows where each channel opens.

What Is the Function of the Ethmoid Sinuses?

The ethmoid sinuses lighten the center of the face, add mucus and moisture to the nasal lining and help form the resonating spaces of the voice, the general roles of all paranasal sinuses described on the paranasal sinuses page.

Ethmoid sinus function in the respiratory system

In the respiratory system, the ethmoid cells provide a large lined surface right beside the upper nasal airway. Their mucus drains into the middle and superior meatuses, adding to the film that traps particles and moistens inhaled air on its way to the pharynx and the lungs. The honeycomb of thin walls also keeps the bone between the eyes light while bracing it, much like the cells of a beehive.

The ethmoid bone that holds the cells also carries the superior and middle conchae, which shape airflow through the upper nose, and it lies just below the olfactory area in the roof of the nose. The air cells themselves do not detect odors.

Ethmoid Sinus Anatomy on CT and Radiology

On CT, the ethmoid sinus appears as a honeycomb of small dark air cells separated by thin white bony walls, lying between the orbits on coronal images. CT is the standard way to study ethmoid anatomy, because plain X-rays show the overlapping cells poorly; on a front-facing Caldwell view they look like a mottled dark area between the eyes.

Radiologists check several landmarks on a sinus CT:

• whether the lamina papyracea is intact and in its usual position;
• the course of the anterior ethmoidal artery across the roof;
• Haller cells, Onodi cells and concha bullosa, which narrow drainage paths or lie near the optic nerve;
• the depth of the olfactory fossa, graded with the Keros classification.

Keros classification

The Keros classification describes how far the cribriform plate sits below the ethmoid roof, measured as the height of the thin lateral lamella of the cribriform plate. Type I is shallow, about 1 to 3 millimeters; type II is medium, about 4 to 7 millimeters; and type III is deep, about 8 to 16 millimeters. A deeper olfactory fossa means a longer stretch of very thin bone at the skull base, which surgeons take into account.

How Do the Ethmoid Sinuses Develop?

The ethmoid sinuses are present at birth, making them, with the maxillary sinuses, the earliest paranasal sinuses to form. They begin before birth as outpouchings of the nasal lining into the developing ethmoid bone, and a newborn already has a set of small cells on each side.

The cells enlarge and multiply through childhood and reach close to their adult size around the early teens. Their early presence is one reason the ethmoid sinuses are so often involved when young children have sinus inflammation, while the frontal sinus, which grows up from the anterior ethmoid region, appears only later. The overall timeline is covered on the development of the respiratory system page.

Common Conditions That Affect the Ethmoid Sinus

The ethmoid sinus is involved in many sinus conditions because it sits at the center of sinus drainage. These are listed for education only; the respiratory diseases page gives wider context.

• Ethmoid sinusitis: inflammation of the ethmoid lining, often felt as pressure between the eyes or at the bridge of the nose. See sinusitis.
• Chronic rhinosinusitis and nasal polyps: long-lasting inflammation often centers on the ethmoid cells, and nasal polyps commonly arise from the ethmoid region.
• Orbital complications: because the lamina papyracea is so thin, ethmoid inflammation can, uncommonly, spread to the tissues around the eye, especially in children.
• Mucocele: a mucus-filled sac from a blocked cell; the ethmoid cells are the second most common site after the frontal sinus.
• Tumors: rare growths of the ethmoid lining, which can involve the orbit or the skull base.

Swelling or redness around the eye, changes in vision or a severe headache are reasons to see a doctor without delay.