## Predisposing Factors in Orbital Cellulitis **Key Point:** Ethmoid sinusitis is the most common source of orbital cellulitis in children, accounting for approximately 50–70% of cases. ### Anatomical Basis The ethmoid sinus has several features that make it a frequent source of orbital spread: 1. **Thin lamina papyracea** — The medial orbital wall is composed of a thin bone (lamina papyracea) that separates the ethmoid air cells from the orbit, allowing rapid bacterial spread. 2. **Rich venous anastomoses** — Ophthalmic veins communicate freely with ethmoid venous drainage, facilitating hematogenous seeding. 3. **Proximity to orbit** — The ethmoid sinus is directly adjacent to the medial orbit, unlike maxillary or frontal sinuses. ### Frequency by Sinus | Sinus | Frequency | Reason | |-------|-----------|--------| | **Ethmoid** | 50–70% | Thin lamina papyracea, direct adjacency | | Maxillary | 10–20% | Thicker bone, less direct communication | | Sphenoid | 5–10% | Posterior location, rarer involvement | | Frontal | 5–10% | Superior location, less common source | **Clinical Pearl:** In children, acute ethmoid sinusitis progressing to orbital cellulitis is a medical emergency requiring urgent imaging (CT orbit) and broad-spectrum antibiotics to prevent vision-threatening complications. **High-Yield:** Remember the "thin lamina papyracea" as the anatomical key — it is the weakest barrier between the ethmoid sinus and the orbit. 
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