Polymyxin B Dosing for Bacterial Conjunctivitis
For bacterial conjunctivitis, instill one drop of polymyxin B (typically in combination with trimethoprim or bacitracin) in the affected eye(s) every 3 hours (maximum 6 doses per day) for 7-10 days in mild to moderate infections. 1
Standard Dosing Regimen
- Polymyxin B/trimethoprim combination: 1 drop every 3 hours while awake (maximum 6 doses daily) for 7-10 days 1
- Polymyxin B/bacitracin ointment: Apply 4 times daily for 7 days 2
- The FDA-approved dosing for polymyxin B ophthalmic solution specifically states every 3 hours with a maximum of 6 applications per day for 7-10 days in mild to moderate infections 1
Clinical Context and Efficacy
Polymyxin B combinations are effective broad-spectrum options for bacterial conjunctivitis, though newer fluoroquinolones may achieve faster clinical resolution. 3
- Polymyxin B/trimethoprim achieved clinical cure or improvement in 92% of patients (47% cured, 45% improved) by 3-6 days, with 84% cured by 2-7 days after therapy completion 4
- Polymyxin B/bacitracin demonstrated 62% clinical cure by days 3-5 and 91% cure by days 8-10, significantly better than placebo (28% and 72% respectively) 2
- However, moxifloxacin 0.5% three times daily showed superior speed of resolution compared to polymyxin/trimethoprim four times daily, with 81% complete resolution at 48 hours versus 44% (P=0.001) 3
Spectrum of Activity
Polymyxin B is primarily effective against gram-negative organisms and requires combination therapy for adequate gram-positive coverage. 5, 6
- Polymyxin B alone is ineffective against Staphylococcus aureus keratitis 5
- Combination with neomycin (1.75-3.50 mg neomycin plus 3,000-6,000 units polymyxin B per gram) provides the most effective coverage 5
- Trimethoprim-polymyxin B combination is equally effective as trimethoprim-sulfacetamide-polymyxin B, suggesting the sulfacetamide component is unnecessary 6
Treatment Duration and Follow-up
A 5-7 day course is typically sufficient for uncomplicated bacterial conjunctivitis, with patients advised to return if no improvement occurs within 3-4 days. 7
- Topical antibiotics reduce disease duration and transmissibility, allowing earlier return to school for children 7
- For more severe presentations, more frequent application may be warranted 7
- Bacterial conjunctivitis is self-limited, but antibiotic therapy shortens clinical disease duration and enhances bacterial eradication 2
Important Caveats
- Polymyxin B combinations are not first-line for gonococcal or chlamydial conjunctivitis, which require specific systemic therapy (ceftriaxone plus azithromycin for gonorrhea; azithromycin or doxycycline for chlamydia) 8
- For contact lens wearers with purulent conjunctivitis, consider broader coverage due to risk of Pseudomonas and other resistant organisms 8
- Ointment formulations may be useful at bedtime but have limited corneal penetration compared to solutions 8