A 35-year-old man with a history of chronic sinusitis presents with acute orbital cellulitis. Which paranasal sinus is the most common primary source of orbital cellulitis?

Explanation

## Paranasal Sinus Sources of Orbital Cellulitis **Key Point:** The ethmoid sinus is the most common primary source of orbital cellulitis, accounting for 70–80% of cases. Its anatomical proximity to the orbit and thin lamina papyracea make it the most frequent route of spread. ### Anatomical Basis for Ethmoid Predominance | Sinus | Frequency | Anatomical Relationship to Orbit | |-------|-----------|----------------------------------| | **Ethmoid** | **70–80%** | Lateral wall of orbit; thin lamina papyracea; direct extension | | Maxillary | 10–15% | Inferior orbit; less direct communication | | Frontal | 5–10% | Superior orbit; less common source | | Sphenoid | <5% | Posterior orbit; rare source | **High-Yield:** The ethmoid sinus borders the medial orbit via the lamina papyracea (a thin bony plate). Infection readily breaches this barrier, making ethmoid sinusitis the leading cause of orbital cellulitis. ### Pathophysiology of Spread 1. **Acute ethmoiditis** → inflammation and infection of ethmoid air cells 2. **Lamina papyracea erosion or inflammation** → loss of anatomical barrier 3. **Direct extension** → pus and inflammatory cells enter the orbital space 4. **Orbital cellulitis** → proptosis, chemosis, ophthalmoplegia, vision loss ### Clinical Presentation Clues - **Medial proptosis** (not axial) suggests ethmoid source - **Medial orbital swelling** and chemosis - **Preceding upper respiratory infection** or acute sinusitis - **Fever, systemic toxicity** **Clinical Pearl:** A child with orbital cellulitis and recent rhinitis or upper respiratory infection should raise suspicion for ethmoid sinusitis as the primary source. Imaging (CT) will show air-fluid levels in the ethmoid sinus. ### Why Other Sinuses Are Less Common **Maxillary sinus:** - Located below the orbit; less direct anatomical communication - Accounts for only 10–15% of cases - More likely to cause periorbital cellulitis than true orbital cellulitis **Frontal sinus:** - Superior location; less frequent involvement - When involved, often in conjunction with ethmoid disease - Accounts for 5–10% of cases **Sphenoid sinus:** - Posterior and medial location; rare source (<5%) - When involved, may cause cavernous sinus thrombosis - Usually occurs with other sinus involvement **Mnemonic:** **EMFS** (Ethmoid > Maxillary > Frontal > Sphenoid) — order of frequency in orbital cellulitis. ### Imaging & Diagnosis - **CT orbits with contrast:** Shows air-fluid levels in ethmoid sinus, orbital edema, and abscess if present - **MRI:** Better for soft tissue delineation and cavernous sinus assessment - **Blood cultures:** Positive in 50% of cases - **Sinus aspirate culture:** Guides antibiotic therapy