A patient is taking Polymyxin B sulfate/Trimethoprim sulfate eye drops for a bacterial eye infection, what's the next step in their treatment?

Management of Bacterial Eye Infection with Polymyxin B/Trimethoprim Eye Drops

Continue the current polymyxin B/trimethoprim drops at 1 drop four times daily for a total treatment duration of 5-7 days, and reassess the patient at 3-4 days to ensure clinical improvement. 1, 2

Treatment Duration and Monitoring

• Complete the full 5-7 day course as recommended by the American Academy of Ophthalmology for bacterial conjunctivitis, even if symptoms improve earlier 1
• Expect clinical improvement within 24-48 hours, including reduced discharge, pain, and lid edema 1
• Schedule follow-up at 3-4 days if no improvement is observed, as this indicates potential treatment failure requiring alternative therapy 1

When to Escalate Therapy

Switch to a fluoroquinolone (moxifloxacin, levofloxacin, gatifloxacin, or besifloxacin) if:

• No improvement after 48-72 hours of polymyxin B/trimethoprim 1
• The patient is a contact lens wearer (higher risk of Pseudomonas aeruginosa requiring fluoroquinolone coverage) 1
• Moderate to severe infection with copious purulent discharge, significant pain, and marked inflammation 1

Consider fortified antibiotics (cefazolin 50 mg/ml, tobramycin 14 mg/ml, or vancomycin 15-50 mg/ml) if: 3, 4

• Large corneal infiltrate is present
• Hypopyon (layered white blood cells in anterior chamber) develops
• Bacterial keratitis rather than simple conjunctivitis is suspected
• MRSA is suspected (vancomycin essential, as fluoroquinolones poorly effective against MRSA) 4

Critical Limitations of Polymyxin B/Trimethoprim

This combination is insufficient as monotherapy for:

• Gonococcal conjunctivitis: requires systemic ceftriaxone plus topical therapy 1
• Chlamydial conjunctivitis: requires systemic azithromycin or doxycycline 1
• Severe bacterial keratitis: requires fortified antibiotics or fluoroquinolones 3, 4

Dosing Adjustments Based on Severity

• Mild-moderate conjunctivitis: 1 drop four times daily for 5-7 days 1, 2
• Severe keratitis (if polymyxin B/trimethoprim used): loading dose of 1 drop every 5-15 minutes initially, then every hour around the clock for 48 hours, followed by taper based on response 4

Common Pitfalls to Avoid

• Do not discontinue prematurely even if symptoms resolve at 2-3 days, as this increases recurrence risk 1
• Do not use for contact lens-related infections without fluoroquinolone coverage due to Pseudomonas risk 1
• Do not delay culture and sensitivity testing if no improvement by day 3-4, as resistance patterns may necessitate targeted therapy 4
• Do not assume all "red eye" is simple conjunctivitis: examine for corneal involvement (keratitis), which requires more aggressive therapy 3, 4

Special Prophylactic Considerations

If this patient has a Boston keratoprosthesis (artificial cornea), polymyxin B/trimethoprim should be continued as lifelong daily prophylaxis to prevent vision-threatening endophthalmitis 1