A 35-year-old man presents with acute onset proptosis, chemosis, ophthalmoplegia, and fever for 2 days. Examination reveals restricted eye movements in all directions and a relative afferent pupillary defect. Which investigation is most appropriate to confirm the diagnosis of orbital cellulitis and guide antibiotic therapy?

Question

Accepted Answer

A. Contrast-enhanced CT orbit with coronal and axial sections. ## Imaging Modality Selection in Orbital Cellulitis

### Role of CT in Orbital Cellulitis

**Key Point:** Contrast-enhanced CT (CECT) orbit with both axial and coronal sections is the investigation of choice for suspected orbital cellulitis because it:
- Rapidly identifies orbital inflammation and abscess formation
- Demonstrates orbital fat stranding and globe displacement
- Evaluates the paranasal sinuses (common source of infection)
- Guides decisions about surgical drainage if abscess is present
- Can be performed urgently in acute settings

### Why CT Over Other Modalities

| Investigation | Advantage | Limitation in Orbital Cellulitis |
|---|---|---|
| **CT (CECT)** | Fast, shows bone/sinus involvement, guides drainage | Radiation exposure |
| **MRI** | Superior soft-tissue resolution, no radiation | Slower, less accessible acutely, less useful for bone/sinus detail |
| **Ultrasound B-scan** | Portable, no radiation | Poor visualization of posterior orbit, cannot assess sinuses |
| **FFA** | Evaluates retinal perfusion | Does not image orbital structures, not diagnostic |

### Clinical Context

**High-Yield:** In acute orbital cellulitis with fever, proptosis, and ophthalmoplegia, CT is preferred because:
1. Paranasal sinusitis (especially ethmoid) is the source in ~70% of cases
2. Abscess formation requires urgent drainage — CT shows the cavity and loculation
3. Imaging must be done emergently; CT is faster than MRI
4. Coronal sections specifically show medial rectus displacement and intraconal involvement

**Clinical Pearl:** The presence of RAPD (relative afferent pupillary defect) in this case suggests optic nerve involvement, making imaging urgent to rule out optic neuritis or compressive abscess.

### When MRI Is Preferred

MRI is reserved for:
- Suspected cavernous sinus thrombosis (superior soft-tissue detail)
- Chronic/recurrent cellulitis (better for granulomatous disease)
- When CT is contraindicated (pregnancy, iodine allergy)

[cite:Harrison 21e Ch 428]

![Orbital Cellulitis diagram](https://mmcphlazjonnzmdysowq.supabase.co/storage/v1/object/public/blog-images/explanation/3335.webp)

Answer

B. MRI orbit with fat suppression sequences

Answer

C. Fundus fluorescein angiography

Answer

D. Orbital ultrasound B-scan

Explanation

Imaging Modality Selection in Orbital Cellulitis

Role of CT in Orbital Cellulitis

Why CT Over Other Modalities

Clinical Context

When MRI Is Preferred

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Investigation	Advantage	Limitation in Orbital Cellulitis
CT (CECT)	Fast, shows bone/sinus involvement, guides drainage	Radiation exposure
MRI	Superior soft-tissue resolution, no radiation	Slower, less accessible acutely, less useful for bone/sinus detail
Ultrasound B-scan	Portable, no radiation	Poor visualization of posterior orbit, cannot assess sinuses
FFA	Evaluates retinal perfusion	Does not image orbital structures, not diagnostic