A 7-year-old boy presents with acute onset proptosis, chemosis, and ophthalmoplegia following an upper respiratory tract infection. Blood cultures and imaging suggest orbital cellulitis. What is the most common causative organism in paediatric orbital cellulitis?

Explanation

## Most Common Organism in Paediatric Orbital Cellulitis **Key Point:** Streptococcus pneumoniae is the most common causative organism of orbital cellulitis in children, particularly in the post-Hib vaccination era. ### Epidemiological Context Following widespread Haemophilus influenzae type b (Hib) vaccination, the epidemiology of orbital cellulitis has shifted significantly. While Hib was historically the leading cause in unvaccinated populations, pneumococcal infection now predominates in vaccinated cohorts. ### Organism Comparison in Orbital Cellulitis | Organism | Frequency | Risk Factors | Clinical Notes | |----------|-----------|--------------|----------------| | **S. pneumoniae** | Most common (40–50%) | Post-URTI, sinusitis | Often follows respiratory infection; can cause fulminant disease | | H. influenzae type b | 15–25% | Unvaccinated status | Declining with Hib vaccination programs | | S. aureus | 10–20% | Trauma, MRSA risk | Consider in post-operative or immunocompromised patients | | P. aeruginosa | Rare | Immunocompromised, contact lens use | Uncommon in immunocompetent children | ### Pathogenesis S. pneumoniae typically reaches the orbit via: 1. Haematogenous spread from respiratory tract infection 2. Direct extension from paranasal sinusitis (especially ethmoid) 3. Bacteraemia following otitis media or meningitis **Clinical Pearl:** In a child with orbital cellulitis and recent upper respiratory tract infection, empiric therapy should cover S. pneumoniae with a third-generation cephalosporin (ceftriaxone or cefotaxime) pending culture results. **High-Yield:** The shift from Hib to S. pneumoniae as the leading cause is a direct consequence of successful Hib vaccination programs — a classic example of epidemiological change in response to immunization. **Mnemonic:** POST-HiB ERA = **S. pneumoniae** (Streptococcus pneumoniae) becomes the Primary Organism in the post-Haemophilus influenzae type b vaccination era.