Intravitreal Antibiotic Therapy for Exogenous Endophthalmitis
For bacterial exogenous endophthalmitis, intravitreal vancomycin 1 mg/0.1 mL combined with either ceftazidime 2.25 mg/0.1 mL or amikacin 0.4 mg/0.1 mL should be injected directly into the vitreous as the primary treatment. 1, 2
Initial Antibiotic Selection
The standard intravitreal antibiotic regimen consists of:
- Vancomycin 1 mg in 0.1 mL - covers gram-positive organisms including methicillin-resistant strains 1, 2
- Plus either:
This combination provides broad-spectrum coverage against the full range of bacteria causing exogenous endophthalmitis 2. The vancomycin-ceftazidime combination is increasingly preferred as a reasonable alternative to vancomycin-amikacin, particularly given emerging resistance patterns 2.
Rationale for Intravitreal Administration
Intravitreal injection is the most important component of treatment because it achieves extremely high intraocular antibiotic concentrations that cannot be reached by systemic administration alone 3. Direct injection into the vitreous ensures therapeutic drug levels at the site of infection 1, 4.
The high intravitreal concentrations achieved by direct injection mean that antimicrobial synergy between antibiotics is probably less critical than ensuring adequate coverage of both gram-positive and gram-negative organisms 2.
Safety Considerations
Vancomycin has been extensively studied and doses up to 2 mg are nontoxic in both phakic and aphakic-vitrectomized eyes 1. The recommended 1 mg dose provides an excellent safety margin while maintaining efficacy 1, 2.
Antibiotic clearance varies by eye anatomy: phakic eyes clear antibiotics most slowly, aphakic-vitrectomized eyes without intact lens capsules clear most rapidly, and aphakic-vitrectomized eyes with intact capsules show intermediate clearance 1. This may necessitate repeat injections in certain clinical scenarios.
Adjunctive Surgical Management
Vitrectomy should be performed in severe cases to remove inflammatory debris, infectious organisms, and improve antibiotic penetration 4, 3. Pars plana vitrectomy is a sight-saving procedure that mechanically debulks the infection and allows better visualization of the retina 5, 3.
Role of Systemic Antibiotics
The role of systemic antibiotics in exogenous endophthalmitis remains controversial 4. In purely exogenous cases, infection is confined to the eye with no fever or significant peripheral leukocytosis, making systemic therapy less critical than intravitreal treatment 4. However, systemic antibiotics are clearly indicated in endogenous endophthalmitis where bacteremia is the source 4, 3.
Critical Timing
Endophthalmitis is a medical emergency requiring immediate treatment 4, 3. Delay in diagnosis and treatment may result in permanent vision loss 4, 3. Prompt intravitreal antibiotic injection is essential for preserving vision 3.
Diagnostic Vitreous Tap
Before initiating treatment, obtain vitreous and/or aqueous humor specimens for culture to identify the causative organism 5, 3. This allows for targeted antibiotic adjustment if needed, though empiric broad-spectrum coverage should not be delayed while awaiting culture results 3.
Common Pitfalls to Avoid
- Do not rely on subconjunctival injections alone - they are generally ineffective and cannot replace intravitreal administration 5
- Do not delay treatment waiting for culture results - begin empiric intravitreal antibiotics immediately 4, 3
- Do not use systemic antibiotics as monotherapy for exogenous endophthalmitis - intravitreal injection is essential 4, 3