## Paranasal Sinus Sources of Orbital Cellulitis **Key Point:** The ethmoid sinus is the most common source of orbital cellulitis because of its anatomical proximity to the medial orbit and the thin lamina papyracea separating it from orbital contents. ### Anatomical Basis for Ethmoid Predominance | Sinus | Anatomical Relationship to Orbit | Frequency as Source | Clinical Significance | | --- | --- | --- | --- | | Ethmoid | Medial orbit; thin lamina papyracea | 70–80% | Easiest spread to orbit | | Sphenoid | Posterior orbit; cavernous sinus | 10–15% | Risk of cavernous sinus thrombosis | | Frontal | Superior orbit; forehead | 5–10% | Subperiosteal abscess common | | Maxillary | Inferior orbit; cheek | 5% | Least common source | **High-Yield:** The lamina papyracea is a paper-thin bone (0.2–0.4 mm thick) that forms the medial wall of the orbit. It has no periosteum on the orbital side, allowing rapid spread of infection from ethmoid sinusitis directly into the orbit. ### Pathophysiology of Spread 1. **Direct extension** through lamina papyracea 2. **Valveless orbital veins** allow retrograde thrombophlebitis 3. **Lymphatic spread** from ethmoid to orbital tissues 4. **Osteomyelitis** of lamina papyracea with abscess formation **Clinical Pearl:** Ethmoid sinusitis presenting with orbital cellulitis often manifests as medial rectus palsy (medial rectus is closest to ethmoid sinus) and medial subperiosteal abscess, which may require surgical drainage. **Warning:** Do not confuse ethmoid sinusitis as the source with sphenoid sinusitis, which carries higher risk of cavernous sinus involvement and meningitis due to proximity to the optic nerve and internal carotid artery. 
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