Sinonasal Anatomic Variants

Michelle A. Michel, MD

DIFFERENTIAL DIAGNOSIS

Common

Less Common

Rare but Important

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Anatomic variants are rule rather than exception
Multiple variants often present in same patient, so identify and report them!
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

Nasal Septal Deviation
- Most common variant; often associated with previous trauma
  - Deviates from midline or S-shaped
  - Impact on nasal airway patency depends on overall nasal cavity width
- Measure maximum deviation from midline
Agger Nasi Cell
- Present in > 85% (really a variant?); most anterior extramural ethmoid air cell
- Located anterior to frontal recess; at level of lacrimal sac or head of middle turbinate on coronal CT
Nasal Septal Spur
- Nearly always associated with septal deviation; often at ethmoid-vomer junction
- Document direction and length of spur; bony or cartilaginous; contact with lateral nasal wall structures or septum?
Concha Bullosa
- Pneumatization of conchal turn of middle turbinate; may narrow middle meatus
- Can be diseased with mucosal thickening, fluid, retention cysts, osteoma
- Inferior pneumatization is uncommon
Infraorbital Ethmoid (Haller) Cell
- Air cell located along inferior surface of orbital floor (antral roof)
  - ↑ risk of orbital injury during FESS
  - Variable size; often bilateral; can narrow infundibulum and be diseased
Paradoxical Middle Turbinate
- Concavity of turbinate concha directed toward septum
  - Diffuse or focal; variable size; often club-shaped
- Can narrow middle meatus and be diseased

Helpful Clues for Less Common Diagnoses

Pneumatized Anterior Clinoid Process
- Position lateral to optic nerve ↑ risk of injury during sphenoid surgery
Fovea Ethmoidalis, Asymmetric (Low)
- Low position ↑ risk of skull base complication during ethmoidectomy
  - Resulting in CSF leak
  - Encephalocele
  - Parenchymal brain injury
- Report measurement of asymmetry in millimeters in dictation
Pneumatized Uncinate Process
- May narrow either infundibulum or middle meatus
Frontal Cells
- 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
- Type 1: Single cell above agger nasi
- Type 2: Tier of 2 or more cells above agger nasi
- Type 3: Single large cell above agger nasi that extends superiorly into frontal recess
- Type 4: Cell located completely within frontal sinus
Pneumatized Crista Galli
- Drains into 1 of frontal sinuses or frontal recess; mucocele formation and subsequent infection (mucopyocele)
- ↑ risk of anterior cranial fossa infection
Supraorbital Ethmoid Cell
- Cell within orbital plate of frontal bone; posterior to frontal sinus and frontal recess
  - Best delineated on axial imaging
Dehiscent Lamina Papyracea
- Post-traumatic or congenital
- Orbital fat or medial rectus muscle may herniate into ethmoid labyrinth
- ↑ risk for orbital injury during FESS

Helpful Clues for Rare Diagnoses

Sphenoethmoidal (Onodi) Cell
- Pneumatization of posterior ethmoid cell superior to optic nerve
  - Optic nerve at ↑ risk during posterior ethmoidectomy
- Best seen on axial imaging
  - Look for horizontal septation on coronal images between this cell superiorly and sphenoid sinus inferiorly
Carotid Artery, Sphenoid Dehiscence
- Absence of bony covering over internal carotid artery; artery bulges into sphenoid sinus lumen; artery at ↑ risk for injury during sphenoid sinus surgery
- Better delineated on axial imaging

Other Essential Information