Ethmoid Sinus

The ethmoid sinuses, more accurately called the ethmoidal air cells, are a collection of small air-filled cavities located within the ethmoidal labyrinths of the ethmoid bone. Unlike the frontal, maxillary, and sphenoidal sinuses, the ethmoidal sinus system consists of numerous individual air cells rather than a single large cavity.

The ethmoidal air cells occupy a strategically important position between the nasal cavity and the orbit. They are divided anatomically into anterior, middle, and posterior ethmoidal air cells, with each group having characteristic drainage pathways into the nasal cavity.

Location

The ethmoidal air cells are located within the ethmoidal labyrinths on either side of the nasal cavity.

They lie between the upper lateral wall of the nasal cavity medially and the orbit laterally, immediately inferior to the anterior cranial fossa.

Ethmoid Bone

The ethmoid bone is a complex midline and paired bone contributing to the nasal cavity, medial orbital walls, and anterior cranial base.

Its major components include the cribriform plate, perpendicular plate, crista galli, and paired ethmoidal labyrinths.

Ethmoidal Labyrinth

Each ethmoidal labyrinth is a mass of thin-walled air cells situated lateral to the upper nasal cavity.

The medial surface contributes to the lateral nasal wall and gives rise to the superior and middle nasal conchae, while the lateral surface forms much of the medial wall of the orbit.

Groups of Ethmoidal Air Cells

The ethmoidal air cells are conventionally divided according to their position and drainage into three groups:

• Anterior ethmoidal air cells
• Middle ethmoidal air cells
• Posterior ethmoidal air cells

Drainage Summary

Group	Principal Drainage Site
Anterior ethmoidal air cells	Middle nasal meatus, commonly through the ethmoidal infundibulum
Middle ethmoidal air cells	Middle nasal meatus, commonly on or near the ethmoidal bulla
Posterior ethmoidal air cells	Superior nasal meatus

Anterior Ethmoidal Air Cells

The anterior ethmoidal air cells occupy the anterior part of the ethmoidal labyrinth.

They generally drain into the middle nasal meatus through the ethmoidal infundibulum or closely associated channels.

Ethmoidal Infundibulum

The ethmoidal infundibulum is a funnel-shaped passage in the lateral wall of the middle nasal meatus.

It forms an important drainage pathway for anterior ethmoidal cells and is closely related to the frontal sinus drainage pathway.

Middle Ethmoidal Air Cells

The middle ethmoidal air cells are located between the anterior and posterior groups.

They commonly drain directly into the middle nasal meatus through openings situated on or near the ethmoidal bulla.

Ethmoidal Bulla

The ethmoidal bulla is a rounded elevation on the lateral wall of the middle nasal meatus.

It is produced by underlying middle ethmoidal air cells and is one of the major landmarks of the middle meatal region.

Posterior Ethmoidal Air Cells

The posterior ethmoidal air cells occupy the posterior portion of the ethmoidal labyrinth.

Unlike the anterior and middle groups, they drain into the superior nasal meatus beneath the superior nasal concha.

Relationship to the Middle Nasal Meatus

The anterior and middle ethmoidal air cells have close relationships with the middle nasal meatus.

This region also contains drainage pathways associated with the frontal and maxillary sinuses, making it one of the most important anatomical regions for paranasal sinus drainage.

Relationship to the Superior Nasal Meatus

The superior nasal meatus lies beneath the superior nasal concha.

The posterior ethmoidal air cells open into this meatus, separating their drainage pathway from those of the anterior and middle groups.

Relationship to the Frontal Sinus

The frontal sinus drainage pathway is closely related to the anterior ethmoidal region.

The frontal recess passes through a complex region shaped by adjacent ethmoidal air cells, and variations in these cells can influence frontal sinus drainage.

Frontal Recess

The frontal recess is the drainage pathway extending inferiorly from the frontal sinus toward the middle nasal meatus.

Its boundaries and dimensions vary considerably according to the pattern of surrounding ethmoidal pneumatization.

Relationship to the Maxillary Sinus

The maxillary sinus lies inferior and lateral to much of the ethmoidal labyrinth.

Its natural ostium opens into the middle meatal region, placing its drainage pathway near those of the anterior ethmoidal cells.

Ostiomeatal Complex

The ostiomeatal complex is a clinically important functional region associated with drainage and ventilation of the frontal, maxillary, and anterior ethmoidal sinuses.

Structures commonly considered in this region include the middle meatus, uncinate process, ethmoidal infundibulum, hiatus semilunaris, ethmoidal bulla, and neighboring sinus ostia.

Uncinate Process

The uncinate process is a thin curved projection of the ethmoid bone on the lateral nasal wall.

It forms an important boundary of the ethmoidal infundibulum and hiatus semilunaris and helps define drainage pathways within the middle meatus.

Hiatus Semilunaris

The hiatus semilunaris is a curved cleft located between the ethmoidal bulla and uncinate process.

It is associated with the infundibular region and the drainage pathway of the maxillary sinus.

Relationship to the Orbit

The ethmoidal air cells lie immediately medial to the orbit.

Their lateral boundary is formed by the extremely thin orbital plate of the ethmoid, traditionally called the lamina papyracea.

Orbital Plate of the Ethmoid

The orbital plate forms a substantial portion of the medial orbital wall.

Its thinness creates a close anatomical relationship between the ethmoidal air cells and orbital contents.

Medial Orbital Wall

The medial wall of the orbit separates orbital structures from the ethmoidal labyrinth.

The close relationship is clinically important because disease or trauma involving the ethmoid region can potentially affect neighboring orbital structures.

Relationship to the Anterior Cranial Fossa

The ethmoidal air cells lie inferior to the anterior cranial fossa.

The roof of the ethmoidal labyrinth forms part of the boundary between the paranasal sinus system and the cranial cavity.

Cribriform Plate

The cribriform plate of the ethmoid forms part of the roof of the nasal cavity and floor of the anterior cranial fossa.

Its numerous small foramina transmit olfactory nerve fibers from the nasal mucosa to the olfactory bulbs.

Fovea Ethmoidalis

The fovea ethmoidalis is formed by the orbital part of the frontal bone and contributes to the roof of the ethmoidal labyrinth.

Its height and configuration vary between individuals and may differ between the two sides.

Lateral Lamella

The lateral lamella of the cribriform plate forms part of the superior boundary between the ethmoidal region and anterior cranial fossa.

It is a thin bony structure and an important landmark in surgical anatomy of the ethmoid roof.

Relationship to the Olfactory Region

The superior portion of the nasal cavity contains specialized olfactory mucosa.

The ethmoid bone forms the structural interface through which olfactory nerve fibers pass from the nasal cavity to the cranial cavity.

Relationship to the Sphenoidal Sinus

The posterior ethmoidal cells lie anterior to the sphenoidal sinus.

Variation in posterior ethmoidal pneumatization can alter the anatomical relationship between these structures.

Onodi Cells

An Onodi cell is a posterior ethmoidal air cell that extends posteriorly and often superiorly in close relationship to the sphenoidal sinus.

This variation is clinically important because the optic nerve and internal carotid artery may have close relationships with an extensively pneumatized posterior ethmoidal cell.

Relationship to the Optic Nerve

The optic nerve passes near the posterior ethmoidal and sphenoidal regions on its course toward the optic canal.

In some individuals, posterior ethmoidal pneumatization creates a particularly close relationship between an air cell and the optic nerve.

Relationship to the Internal Carotid Artery

The internal carotid artery has important relationships with the sphenoidal sinus and adjacent posterior paranasal sinus anatomy.

Extensive posterior ethmoidal pneumatization may bring portions of the ethmoidal air-cell system into closer proximity to this region.

Mucosal Lining

The ethmoidal air cells are lined by mucosa continuous with the respiratory mucosa of the nasal cavity.

The lining is generally thin and contains ciliated respiratory epithelium that supports mucociliary transport toward the natural drainage openings.

Mucociliary Clearance

Ciliary activity moves mucus toward the ostia of the ethmoidal air cells.

Effective drainage therefore depends on both patent openings and coordinated mucociliary transport.

Functions

The ethmoidal air cells are components of the paranasal sinus system.

Functions attributed to the paranasal sinuses include contributing to conditioning of inspired air, mucus production and clearance, resonance of the voice, and reduction of the mass of portions of the facial skeleton.

Development

Ethmoidal air cells begin developing early compared with several other paranasal sinuses and are present in an immature form at birth.

They enlarge and undergo further pneumatization during childhood as the nasal cavity and surrounding facial skeleton develop.

Pneumatization

Pneumatization produces a highly variable system of air cells within the ethmoidal labyrinth.

The number, size, shape, and extent of individual cells vary substantially between individuals and often between the two sides of the same person.

Anterior Ethmoidal Artery

The anterior ethmoidal artery is a branch of the ophthalmic artery.

It passes from the orbit through the anterior ethmoidal canal and supplies portions of the ethmoidal air cells, nasal cavity, and adjacent structures.

Posterior Ethmoidal Artery

The posterior ethmoidal artery also arises from the ophthalmic artery.

It traverses the posterior ethmoidal canal and supplies posterior ethmoidal cells and neighboring superior nasal structures.

Blood Supply

The ethmoidal sinus region receives arterial supply primarily from the anterior and posterior ethmoidal arteries, with contributions from branches associated with the nasal arterial network.

The exact distribution reflects the extensive vascular connections of the nasal cavity and paranasal sinuses.

Venous Drainage

Venous drainage generally accompanies the arterial supply and communicates with venous channels of the orbit and nasal cavity.

The close venous relationships of this region reflect its position between the orbit, nasal cavity, and anterior cranial base.

Sensory Innervation

Sensory innervation of the ethmoidal air cells is primarily derived from branches of the trigeminal nerve.

Anterior and posterior ethmoidal nerves, associated with the ophthalmic division through the nasociliary nerve, contribute sensory fibers to the ethmoidal region.

Anterior Ethmoidal Nerve

The anterior ethmoidal nerve arises from the nasociliary nerve within the orbit.

It accompanies the anterior ethmoidal vessels and supplies portions of the anterior ethmoidal region and nasal cavity.

Posterior Ethmoidal Nerve

The posterior ethmoidal nerve is also associated with the nasociliary nerve.

It supplies sensory fibers to posterior ethmoidal and neighboring sphenoidal regions.

Autonomic Innervation

Autonomic fibers regulate glandular secretion and vascular tone in the sinus mucosa.

Parasympathetic secretomotor and sympathetic vasomotor fibers reach the sinonasal mucosa through autonomic pathways associated with the pterygopalatine region and accompanying nerves and vessels.

Ethmoid Sinusitis

Inflammation of the ethmoidal air cells is commonly termed ethmoid sinusitis or ethmoid rhinosinusitis.

Mucosal swelling can narrow the small drainage pathways of individual cells and interfere with ventilation and mucociliary clearance.

Anterior Ethmoidal Disease

Disease involving anterior ethmoidal cells may occur together with inflammation affecting the middle meatus and other sinuses draining through the ostiomeatal region.

The close grouping of these pathways helps explain why obstruction in the middle meatal region can affect multiple sinus compartments.

Posterior Ethmoidal Disease

Posterior ethmoidal disease involves cells draining into the superior nasal meatus.

Their proximity to the sphenoidal sinus, optic nerve, orbit, and skull base gives the posterior ethmoidal region distinct anatomical importance.

Orbital Spread of Infection

The thin orbital plate separates the ethmoidal air cells from the orbit.

This close relationship provides an anatomical basis for potential spread of inflammatory or infectious processes from the ethmoidal region toward orbital tissues.

Skull Base Relationships

The roof of the ethmoidal labyrinth lies adjacent to the anterior cranial fossa.

The thinness and variable configuration of the ethmoid roof make detailed anatomical orientation particularly important when studying disease, trauma, and surgical approaches in this region.

Nasal Endoscopy

Many important landmarks related to the ethmoidal air cells can be examined through nasal endoscopy.

The middle nasal concha, uncinate process, ethmoidal bulla, middle meatus, superior concha, and superior meatus help orient the examiner to different portions of the ethmoidal system.

Endoscopic Sinus Surgery

The ethmoidal labyrinth is a central region in functional endoscopic sinus surgery.

Its variable air-cell pattern and close relationships with the orbit and skull base make precise knowledge of individual anatomy particularly important.

Important Surgical Relationships

• Orbit lies lateral to the ethmoidal labyrinth.
• Anterior cranial fossa lies superiorly.
• Middle nasal meatus lies medially to anterior and middle ethmoidal cells.
• Superior nasal meatus lies medial to posterior ethmoidal cells.
• Sphenoidal sinus lies posterior to the ethmoidal labyrinth.
• Optic nerve may be closely related to posterior ethmoidal cells.

Imaging

Computed tomography provides detailed visualization of the bony anatomy of the ethmoidal labyrinth and neighboring paranasal sinuses.

Imaging can demonstrate the pattern of pneumatization, drainage pathways, anatomical variants, and relationships to the orbit and skull base.

Anatomical Variations

Ethmoidal anatomy is highly variable.

Variations may involve the number and size of air cells, pneumatization of neighboring structures, relationships of the frontal recess, position of the ethmoid roof, and extension of posterior cells toward the sphenoidal sinus.

Agger Nasi Cells

Agger nasi cells are anterior ethmoidal air cells located near the anterior attachment of the middle nasal concha.

They are closely related to the frontal recess and can influence the configuration of the frontal sinus drainage pathway.

Haller Cells

Haller cells are infraorbital ethmoidal air cells that extend along the orbital floor near the maxillary sinus roof.

When prominent, they may alter the anatomy of the infundibular region and narrow adjacent drainage pathways.

Common Ethmoidal Variations

Variation	Anatomical Significance
Agger nasi cell	Anterior ethmoidal cell related to frontal recess
Haller cell	Infraorbital ethmoidal cell near maxillary drainage pathway
Onodi cell	Posterior ethmoidal cell extending near sphenoidal sinus and potentially optic nerve

Comparison with Other Paranasal Sinuses

Sinus	General Location	Principal Drainage Region
Frontal sinus	Frontal bone	Middle meatus through frontal recess region
Ethmoidal air cells	Ethmoidal labyrinth	Middle and superior meatuses depending on group
Maxillary sinus	Maxilla	Middle meatus
Sphenoidal sinus	Body of sphenoid	Sphenoethmoidal recess

Key Anatomical Relationships

• The ethmoid sinus consists of multiple ethmoidal air cells rather than one large cavity.
• The cells occupy the ethmoidal labyrinth between the nasal cavity and orbit.
• They are divided into anterior, middle, and posterior groups.
• Anterior ethmoidal cells generally drain into the middle meatus through the infundibular region.
• Middle ethmoidal cells commonly drain on or near the ethmoidal bulla.
• Posterior ethmoidal cells drain into the superior nasal meatus.
• The orbital plate of the ethmoid separates the air cells from the orbit.
• The anterior cranial fossa lies superior to the ethmoidal labyrinth.
• The frontal recess is closely related to anterior ethmoidal pneumatization.
• The sphenoidal sinus lies posterior to the ethmoidal labyrinth.
• Posterior ethmoidal cells may have close relationships with the optic nerve.
• Agger nasi, Haller, and Onodi cells are clinically important anatomical variations.
• The ethmoidal region is a major anatomical component of endoscopic sinus surgery.

Clinical Importance

The ethmoidal air cells occupy one of the most anatomically complex regions of the paranasal sinus system. Their division into anterior, middle, and posterior groups is particularly important because these groups drain through different regions of the nasal cavity.

The ethmoidal labyrinth also lies at the intersection of several important anatomical compartments. The orbit lies laterally, the nasal cavity medially, the anterior cranial fossa superiorly, and the sphenoidal region posteriorly. These close relationships explain the importance of precise ethmoidal anatomy in imaging, sinus disease, and endoscopic surgery.

Understanding the ethmoid sinus provides the anatomical foundation for studying ethmoidal air cells, paranasal sinus drainage, the ostiomeatal complex, frontal recess, orbital relationships, skull base anatomy, ethmoid sinusitis, and endoscopic sinus surgery.