What is the significance of evaluating anatomical variations of the nose and paranasal sinus on computed tomography (CT) scan in an adult patient with a history of chronic rhinosinusitis and recurrent sinus infections?

Anatomical Variations on CT in Chronic Rhinosinusitis

CT scan of the paranasal sinuses is essential for evaluating anatomical variations in patients with chronic rhinosinusitis, particularly when planning functional endoscopic sinus surgery, as it identifies critical anatomic variants that increase surgical risk and guides treatment decisions. 1

Primary Role of CT Imaging

Multidetector CT without IV contrast is the imaging modality of choice for evaluating anatomical variations in chronic rhinosinusitis patients. 1 The excellent bony detail provided by CT allows precise identification of:

• Anatomic variants that narrow sinus drainage pathways 1
• Structures that increase risk for intracranial, intraorbital, and vascular injury during surgery 1
• Variations that predispose to CSF leak 1

Critical Surgical Planning Function

CT is critical for surgical planning before functional endoscopic sinus surgery, which is now the standard of care for restoring patency of paranasal sinus outflow tracts, with postoperative improvement in symptoms and quality of life reported in over 75% of patients. 1 The imaging specifically identifies:

• Sphenoethmoidal (Onodi) air cells, which increase risk of injury to optic nerves or carotid arteries 1
• Cribriform plates, lamina papyracea, and anterior ethmoidal artery canal - structures critical to avoid during surgery 1
• Complex ethmoidal anatomy and ostiomeatal unit variations 1

Important caveat: Low-dose CT techniques have been shown to be limited in visualizing these surgically relevant anatomical structures, particularly in patients with nasal polyps and prior sinus surgery. 1 A standard sinus CT protocol that can be used by image guidance systems is recommended. 1

Common Anatomical Variations Identified

The most frequently encountered anatomical variations on CT include:

• Agger nasi cells (49-80%) - most common variation in multiple studies 2, 3
• Deviated nasal septum (61-88%) - highly prevalent but limited correlation with symptoms 2, 3, 4
• Concha bullosa (47-76%) - pneumatization of middle turbinate 2, 3, 4
• Haller cells (32%) - infraorbital ethmoid cells 3, 5
• Onodi cells (21%) - posterior ethmoid cells extending over sphenoid 3
• Uncinate process variations (17%) 3
• Paradoxical middle turbinate (9-12%) 3, 4

Clinically Significant Correlations

Statistically significant associations exist between specific anatomical variations and sinus disease:

• Ipsilateral agger nasi cells correlate with frontal sinusitis (p<0.001) 5
• Ipsilateral Haller cells and concha bullosa correlate with maxillary sinusitis (p<0.001) 5
• Onodi cells correlate with sphenoid sinusitis (p<0.001) 5
• Common variations (septal deviation, bilateral concha bullosa, medial deviation of uncinate process, Haller cells, hypertrophic ethmoidal bulla, agger nasi cells) show statistically significant association with sinus mucosal disease (p<0.05) 6

Important limitation: No obvious statistical correlation has been noted between deviated nasal septum and ipsilateral maxillary sinusitis, despite its high prevalence. 5

Appropriate Timing for CT Imaging

CT should be obtained when:

• Diagnosing or evaluating chronic rhinosinusitis or recurrent acute rhinosinusitis 1
• Planning surgical intervention 1, 7
• Direct visualization via nasal endoscopy is inconclusive or not feasible 7
• Objective findings suggestive of chronic rhinosinusitis are present on endoscopy 7

CT should NOT be performed:

• Solely for evaluation of septal deviation - clinical anterior rhinoscopy and endoscopic examination is the reference standard, as CT has limited correlation with physical examination and may underestimate nasal obstruction 1
• For patients without cardinal symptoms of chronic rhinosinusitis (nasal congestion, facial pain/pressure, purulent nasal drainage) 7, 8
• As a substitute for direct visualization - anterior rhinoscopy or nasal endoscopy should be performed first 7

Diagnostic Confirmation Role

CT provides objective confirmation of chronic rhinosinusitis through identification of:

• Mucosal thickening 1
• Sinus opacification 1
• Polyps or retention cysts 1
• Sclerosis and thickening of sinus walls 1

Critical caveat: CT findings do not necessarily correlate with symptom severity, though correlation may be higher in patients with associated nasal polyps. 1 The Lund-Mackay scoring system shows variable correlation with disease severity and surgical outcomes. 1

Additional Diagnostic Capabilities

CT can exclude aggressive pathology that may mimic chronic rhinosinusitis:

• Osseous destruction, extrasinus extension, and local invasion suggest malignancy - if noted, MRI should be performed to differentiate benign obstructed secretions from tumor and assess for intracranial spread 1
• Silent sinus syndrome - CT better delineates osseous thinning, obstruction of the infundibulum, and lateralization of the uncinate process compared to MRI 1

Technical Considerations

Contrast-enhanced CT is not necessary for demonstrating findings of chronic rhinosinusitis or for surgical planning of paranasal sinus inflammatory disease. 1 Cone beam CT (CBCT) can confirm diagnosis and identify anatomic variants for presurgical planning with lower radiation exposure, but is limited in evaluating soft-tissue structures and is not the modality of choice if extrasinus disease is suspected. 1