A 7-year-old boy presents with acute proptosis, chemosis, and ophthalmoplegia following an upper respiratory tract infection. Imaging shows orbital cellulitis with involvement of the ethmoid sinus. What is the most common bacterial organism responsible for orbital cellulitis in children?

Explanation

## Microbiology of Orbital Cellulitis in Children **Key Point:** *Staphylococcus aureus* is the most common bacterial organism responsible for orbital cellulitis in children in the current era, particularly following the widespread introduction of Hib vaccination. It is also the predominant organism in post-sinusitis orbital cellulitis across most contemporary series. ### Epidemiology & Risk Factors - **Peak incidence:** School-age children (5–10 years), though any age can be affected - **Route of spread:** Direct extension from paranasal sinusitis (ethmoid > maxillary > frontal > sphenoid) - **Impact of vaccination:** Hib conjugate vaccine (introduced in the 1990s) has dramatically reduced *H. influenzae* type b as a cause; *S. aureus* (including MRSA) has emerged as the leading pathogen ### Organism Frequency in Pediatric Orbital Cellulitis (Contemporary Data) | Organism | Frequency | Key Features | |----------|-----------|---------------| | **Staphylococcus aureus** | **Most common** | Includes MRSA; post-sinusitis, post-traumatic, or skin-source spread | | Streptococcus pneumoniae | Common | Post-sinusitis; incidence varies with PCV uptake | | Streptococcus pyogenes | Less common | Group A Strep; skin/soft tissue source | | Haemophilus influenzae type b | Rare (post-vaccine era) | Historically most common; now uncommon in vaccinated populations | | Pseudomonas aeruginosa | Rare | Immunocompromised or post-surgical patients | **High-Yield (Khurana / AAO):** In the post-Hib vaccine era, *S. aureus* — including community-acquired MRSA — is the most frequently isolated organism in pediatric orbital cellulitis. Empiric therapy must cover *S. aureus*, *S. pneumoniae*, and anaerobes. ### Why Hib Is No Longer the Answer - Prior to widespread Hib vaccination, *H. influenzae* type b accounted for up to 40–50% of cases. - Post-vaccination (post-1990s), its incidence has fallen dramatically to <5% in most series. - Current textbooks (Khurana's Ophthalmology, AAO Basic and Clinical Science Course) list *S. aureus* as the most common organism in the current era. ### Clinical Pearl The classic presentation of orbital cellulitis includes: - Fever and systemic toxicity - Rapid onset of proptosis and chemosis - Ophthalmoplegia (CN III, IV, VI involvement) - Preceding upper respiratory or sinus infection (ethmoid sinusitis most common source) ### Treatment Implications **Empiric antibiotic regimen:** - **Vancomycin** — covers MRSA and penicillin-resistant *S. pneumoniae* - **3rd-generation cephalosporin** (ceftriaxone or cefotaxime) — broad gram-positive and gram-negative coverage - Adjust based on culture and sensitivities; surgical drainage if abscess present **Warning:** Always consider MRSA in children with orbital cellulitis, especially in community-acquired cases without a clear sinusitis source.