Sinonasal Anatomic Variants
Michelle A. Michel, MD
DIFFERENTIAL DIAGNOSIS
Common
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Nasal Septal Deviation
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Agger Nasi Cell
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Nasal Septal Spur
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Concha Bullosa
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Infraorbital Ethmoid (Haller) Cell
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Paradoxical Middle Turbinate
Less Common
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Pneumatized Anterior Clinoid Process
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Fovea Ethmoidalis, Asymmetric (Low)
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Pneumatized Uncinate Process
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Frontal Cells
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Pneumatized Crista Galli
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Supraorbital Ethmoid Cell
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Dehiscent Lamina Papyracea
Rare but Important
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Sphenoethmoidal (Onodi) Cell
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Carotid Artery, Sphenoid Dehiscence
ESSENTIAL INFORMATION
Key Differential Diagnosis Issues
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Anatomic variants are rule rather than exception
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Multiple variants often present in same patient, so identify and report them!
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Variants may make patients prone to recurrent inflammatory disease and may increase risk of complications during functional endoscopic sinus surgery (FESS)
Helpful Clues for Common Diagnoses
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Nasal Septal Deviation
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Most common variant; often associated with previous trauma
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Deviates from midline or S-shaped
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Impact on nasal airway patency depends on overall nasal cavity width
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Measure maximum deviation from midline
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Agger Nasi Cell
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Present in > 85% (really a variant?); most anterior extramural ethmoid air cell
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Located anterior to frontal recess; at level of lacrimal sac or head of middle turbinate on coronal CT
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Nasal Septal Spur
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Nearly always associated with septal deviation; often at ethmoid-vomer junction
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Document direction and length of spur; bony or cartilaginous; contact with lateral nasal wall structures or septum?
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Concha Bullosa
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Pneumatization of conchal turn of middle turbinate; may narrow middle meatus
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Can be diseased with mucosal thickening, fluid, retention cysts, osteoma
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Inferior pneumatization is uncommon
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Infraorbital Ethmoid (Haller) Cell
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Air cell located along inferior surface of orbital floor (antral roof)
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↑ risk of orbital injury during FESS
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Variable size; often bilateral; can narrow infundibulum and be diseased
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Paradoxical Middle Turbinate
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Concavity of turbinate concha directed toward septum
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Diffuse or focal; variable size; often club-shaped
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Can narrow middle meatus and be diseased
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Helpful Clues for Less Common Diagnoses
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Pneumatized Anterior Clinoid Process
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Position lateral to optic nerve ↑ risk of injury during sphenoid surgery
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Fovea Ethmoidalis, Asymmetric (Low)
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Low position ↑ risk of skull base complication during ethmoidectomy
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Resulting in CSF leak
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Encephalocele
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Parenchymal brain injury
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Report measurement of asymmetry in millimeters in dictation
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Pneumatized Uncinate Process
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May narrow either infundibulum or middle meatus
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Frontal Cells
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Located anterior to frontal recess (above agger nasi cell) or within frontal sinus; 4 types (Bent classification); ↑ incidence of these cells with other variants (concha bullosa); types 3 and 4 may be associated with ↑ disease in frontal sinus
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Type 1: Single cell above agger nasi
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Type 2: Tier of 2 or more cells above agger nasi
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Type 3: Single large cell above agger nasi that extends superiorly into frontal recess
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Type 4: Cell located completely within frontal sinus
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Pneumatized Crista Galli
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Drains into 1 of frontal sinuses or frontal recess; mucocele formation and subsequent infection (mucopyocele)
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↑ risk of anterior cranial fossa infection
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Supraorbital Ethmoid Cell
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Cell within orbital plate of frontal bone; posterior to frontal sinus and frontal recess
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Best delineated on axial imaging
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Dehiscent Lamina Papyracea
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Post-traumatic or congenital
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Orbital fat or medial rectus muscle may herniate into ethmoid labyrinth
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↑ risk for orbital injury during FESS
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Helpful Clues for Rare Diagnoses
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Sphenoethmoidal (Onodi) Cell
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Pneumatization of posterior ethmoid cell superior to optic nerve
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Optic nerve at ↑ risk during posterior ethmoidectomy
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Best seen on axial imaging
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Look for horizontal septation on coronal images between this cell superiorly and sphenoid sinus inferiorly
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Carotid Artery, Sphenoid Dehiscence
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Absence of bony covering over internal carotid artery; artery bulges into sphenoid sinus lumen; artery at ↑ risk for injury during sphenoid sinus surgery
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Better delineated on axial imaging
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Other Essential Information
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Anatomic variations can also be categorized based on location or anatomic structure involved
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Frontal region variants
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Ethmoid region variants
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Middle turbinate variants
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Uncinate variants
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Sphenoethmoidal region variants
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Nasal septal variants
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Additional variants not mentioned above
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Variable pneumatization
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Aplasia, hypoplasia, hyperpneumatization
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Intersinus septal cell
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Located within septum between frontal sinuses
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Fusion of uncinate to middle turbinate, lamina papyracea, or skull base
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Pneumatization of vertical lamella of middle turbinate
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Septal recess
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Cell within posterior nasal septum
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Sphenoid sinus septations inserting on carotid canal
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Pneumatization of dorsum sella
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Image Gallery
![]() Coronal bone CT shows deviation of the nasal septum to the right with a small bony spur
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