What are the common causes of orbital cellulitis?

Causes of Orbital Cellulitis

Orbital cellulitis is almost always a complication of sinusitis (particularly ethmoid sinusitis), with the most common bacterial pathogens being Streptococcus pneumoniae, nontypeable Haemophilus influenzae, Streptococcus pyogenes, Moraxella species, anaerobic bacteria, and in immunocompromised patients, fungal organisms including Aspergillus species and zygomycetes. 1

Primary Etiology: Sinusitis

• Sinusitis is the source in the vast majority of orbital cellulitis cases, with infection spreading from adjacent paranasal sinuses (especially ethmoid sinuses) through the thin medial orbital wall, valveless veins, or orbital foramina 1, 2
• The anatomical proximity and lack of lymphatic drainage facilitate direct extension of sinus infections into the orbital space 2

Common Bacterial Pathogens

Most Frequent Organisms:

• Streptococcus pneumoniae - one of the most common causative agents 1, 3
• Nontypeable Haemophilus influenzae - remains common despite vaccination programs 1
• Streptococcus pyogenes (Group A Streptococcus) - increasingly recognized, with rapid progression to abscess formation 1, 4, 3
• Staphylococcus aureus (including MRSA) - common pathogen, particularly in trauma-related cases 1, 3, 5
• Moraxella species - frequently isolated from sinus-related infections 1
• Anaerobic bacteria - present in polymicrobial infections, especially in adults 1, 2

Age-Related Patterns:

• Children and young adults are most commonly affected 2, 6
• Polymicrobial infections are more likely in individuals older than 15 years 2

Fungal Causes (High-Risk Populations)

• Aspergillus species - in immunocompromised patients 1
• Zygomycetes (mucormycosis) - particularly in diabetic or immunosuppressed patients 1
• Maintain high index of suspicion for invasive fungal infection in immunocompromised patients due to high morbidity and mortality 1

Alternative Routes of Infection

Less Common Causes:

• Direct trauma to the orbit with percutaneous introduction of pathogens 1, 2, 5
• Orbital foreign bodies 2
• Dental infections or dental procedures 2
• Maxillofacial surgeries including ORIF procedures 2
• Retinal buckling procedures 2
• Hematogenous spread (rare) 1

Important Clinical Distinctions

Orbital cellulitis must be distinguished from preseptal (periorbital) cellulitis, which has different causative patterns:

• Preseptal cellulitis is typically caused by Staphylococcus aureus or Streptococcus pyogenes following trauma or skin breaks 1, 7
• Preseptal cellulitis lacks proptosis, ophthalmoplegia, or visual impairment that characterize orbital cellulitis 7

Critical Pitfalls to Avoid

• Do not overlook fungal causes in diabetic or immunocompromised patients - these require urgent MRI evaluation and aggressive management 1
• Blood and skin cultures are frequently negative (as demonstrated in clinical series), so empiric broad-spectrum coverage is essential 3
• Post-vaccination era changes: With widespread H. influenzae type B and S. pneumoniae vaccination, other pathogens like S. pyogenes are emerging as more prominent causes 4
• Polymicrobial infections are common, particularly in adults, requiring broad-spectrum antibiotic coverage 2