What is the recommended dose of polymyxin B (antibiotic) for a 3-year-old pediatric patient with conjunctivitis and no significant past medical history?

Polymyxin B Dosing for Pediatric Conjunctivitis

For a 3-year-old child with bacterial conjunctivitis, administer polymyxin B combination eye drops (polymyxin B/trimethoprim) 1 drop in the affected eye(s) every 3 hours while awake (maximum 6 doses per day) for 7-10 days. 1

Dosing Regimen

• Standard dosing: Instill 1 drop every 3 hours (maximum 6 doses daily) for 7-10 days 1
• This translates to approximately 4-6 times daily dosing in practical outpatient settings 2, 3
• Treatment duration should be 7 days minimum, with clinical reassessment at 3-4 days 4, 5

Evidence Supporting Polymyxin-Based Therapy

Polymyxin B combinations (polymyxin-bacitracin or polymyxin-trimethoprim) are well-established alternatives to fluoroquinolones for pediatric bacterial conjunctivitis:

• Clinical efficacy: In a randomized controlled trial of 102 children, polymyxin-bacitracin achieved 62% clinical cure by days 3-5 versus 28% with placebo (P<0.02), and 91% cure by days 8-10 2
• Bacterial eradication: Polymyxin-bacitracin eradicated bacterial pathogens in 71% by day 3-5 and 79% by day 8-10, compared to only 19% and 31% with placebo (P<0.001) 2
• Broad-spectrum coverage: Polymyxin B/trimethoprim provides both gram-positive and gram-negative activity against the most common pediatric pathogens (Haemophilus influenzae and Streptococcus pneumoniae) 3
• Real-world effectiveness: A survey of 472 children treated with polymyxin B/trimethoprim showed 95% were cured or improved within 7 days, with excellent or good efficacy rated in 96% of cases 3

Important Clinical Context

While fluoroquinolones (moxifloxacin, levofloxacin) are considered first-line therapy for children >12 months 4, 5, polymyxin B combinations remain highly effective second-line options when:

• Fluoroquinolones are unavailable or cost-prohibitive 4
• There are concerns about fluoroquinolone resistance patterns 6
• Parents prefer older, well-established medications 3

Critical Red Flags Requiring Immediate Ophthalmology Referral

Do NOT treat with topical antibiotics alone if any of the following are present:

• Visual loss or moderate-to-severe pain 4, 5
• Corneal involvement (infiltrate, ulcer, or opacity) 4, 5
• Severe purulent discharge suggesting gonococcal infection—requires systemic ceftriaxone 125 mg IM plus topical therapy 4, 5
• Age <1 month (neonatal conjunctivitis requires systemic treatment) 6, 5
• No improvement after 3-4 days of appropriate topical therapy 4, 5

Special Considerations for Gonococcal and Chlamydial Infections

If gonococcal or chlamydial conjunctivitis is suspected (severe purulent discharge, sexual abuse history):

• Gonococcal: Requires ceftriaxone 125 mg IM (for children <45 kg) plus azithromycin 1g PO, with daily follow-up until resolution 4, 5
• Chlamydial: Requires systemic erythromycin 50 mg/kg/day divided into 4 doses for 14 days (children <45 kg) 4, 5
• Mandatory: Consider and report suspected sexual abuse in any child with these infections 4, 6, 5

Follow-Up and Patient Education

• Reassess at 3-4 days if no improvement; consider resistant organisms or alternative diagnoses 4, 5
• Hand hygiene is critical to prevent transmission 4, 5
• Return to school/daycare is permitted after 24 hours of treatment when symptoms begin improving 4
• Avoid sharing towels or close contact during the contagious period 5

Common Pitfall to Avoid

Do not use oral antibiotics (such as cefixime) for routine bacterial conjunctivitis—a randomized trial showed oral cefixime was not superior to topical polymyxin-bacitracin for clinical cure or prevention of acute otitis media 7. Systemic antibiotics are reserved exclusively for gonococcal and chlamydial infections 6.