What is the recommended imaging protocol, specifically CT (computed tomography) with or without contrast, for a patient presenting with orbital cellulitis, considering factors such as symptom severity, history of contrast allergy, and impaired renal function?

CT Imaging Protocol for Orbital Cellulitis

CT orbits with intravenous contrast is the recommended initial imaging modality for suspected orbital cellulitis in the emergent setting. 1, 2

Primary Imaging Recommendation

Obtain CT orbits with IV contrast immediately as the first-line study for all patients with suspected orbital cellulitis. 1, 2 This protocol provides critical diagnostic information including:

• Detection of subperiosteal or orbital abscesses requiring surgical intervention 1
• Evaluation of bone erosion using Chandler criteria to guide surgical decision-making 1
• Assessment of paranasal sinus involvement as the most common source of infection 1
• Identification of intra-orbital extension of inflammatory disease 1

Precontrast imaging is not necessary in the acute evaluation of orbital cellulitis, as it does not add significant diagnostic information and delays definitive imaging. 1, 3

When to Add CT Head with Contrast

Add CT head with IV contrast when intracranial complications are suspected based on clinical presentation: 1, 2, 3

• Altered mental status or severe headache suggesting meningitis or brain abscess 1, 2
• Bilateral eye findings or cranial nerve palsies suggesting cavernous sinus thrombosis 1
• Frontal bone tenderness suggesting osteomyelitis (Pott's puffy tumor) 3

Role of MRI

MRI orbits and head with and without IV contrast serves as complementary imaging in specific clinical scenarios: 1, 2

• When CT findings are equivocal but clinical suspicion for orbital cellulitis remains high 4
• For detailed assessment of intra-orbital spread when surgical planning requires more precise soft tissue delineation 1
• When intracranial complications are suspected on CT or clinical grounds, as MRI provides superior soft-tissue resolution for cavernous sinus thrombosis, subdural empyema, or cerebritis 1, 5
• In immunocompromised patients where invasive fungal infection is a concern, given the high mortality (50-80%) of this condition 1, 6

Although contrast is preferred for MRI, noncontrast orbital MRI may provide useful information in patients who cannot receive contrast due to severe renal impairment or contrast allergy. 1

Clinical Decision Algorithm for Imaging

Order CT orbits with IV contrast immediately if ANY of the following are present: 1, 6, 2, 3

• Proptosis (exophthalmos) 1, 6, 2
• Impaired or painful extraocular movements 1, 6, 2
• Ophthalmoplegia 1, 6, 2
• Decreased visual acuity 1, 6, 2, 3
• Gross periorbital edema 1, 6, 3
• Age >3 years with significant periorbital swelling 1, 6
• High neutrophil count or previous antibiotic therapy without improvement 1, 6

Special Considerations for Contrast Contraindications

In patients with contrast allergy:

• Premedicate with corticosteroids and antihistamines if the allergy history suggests non-severe reaction, as the diagnostic benefit of contrast outweighs the risk in this sight-threatening and potentially life-threatening condition 1
• Consider noncontrast CT orbits followed by MRI orbits without and with contrast if contrast CT is absolutely contraindicated, though this delays definitive imaging 1

In patients with impaired renal function:

• Assess risk-benefit ratio carefully, as orbital cellulitis carries risk of blindness, cavernous sinus thrombosis, and death 1, 6, 2
• Proceed with contrast CT if eGFR >30 mL/min/1.73m² given the emergent nature and high morbidity of untreated disease 1
• Consider MRI orbits without and with contrast as an alternative if renal function prohibits iodinated contrast, though this may delay diagnosis 1, 5

Critical Pitfalls to Avoid

Do not rely on clinical examination alone to distinguish preseptal from orbital cellulitis, as physical findings are insufficient to exclude postseptal involvement or abscess formation. 6, 3

Do not obtain plain radiographs, as they are insufficient to detect orbital pathology and have been completely supplanted by CT imaging. 1, 3

Do not delay CT imaging when any orbital signs are present (proptosis, visual changes, ophthalmoplegia), as failure to recognize intracranial extension can result in cavernous sinus thrombosis, meningitis, brain abscess, or death. 6, 2

Do not assume all periorbital swelling is infectious—idiopathic orbital inflammatory syndrome (IOIS) and IgG4-related orbital disease can mimic infection but require corticosteroids rather than antibiotics. 1, 6

Advanced Vascular Imaging

Add CTA or MRA to routine imaging only when vascular complications are specifically suspected: 1

• Cavernous sinus thrombosis based on bilateral findings or cranial nerve involvement 1
• Superior ophthalmic vein thrombosis 1
• Invasive fungal disease in immunocompromised patients 1

Contrast-enhanced MRA provides additional information beyond traditional noncontrast MRA in the setting of suspected cavernous sinus thrombosis. 1