Bacterial Keratitis

Bacterial keratitis is a sight threatening infection of the cornea that follows breach of the epithelium. It progresses quickly; Pseudomonas aeruginosa can destroy a cornea in 24 to 48 hours.

• Commonest organisms: Pseudomonas aeruginosa (contact lens wearers), Staphylococcus aureus and coagulase negative staphylococci, Streptococcus pneumoniae, Moraxella, Serratia, and Haemophilus. Nocardia and atypical mycobacteria are seen after trauma or refractive surgery.
• Risk factors: contact lens wear (overnight and extended wear), trauma, dry eye, lagophthalmos, neurotrophic cornea, corneal surgery, topical steroids, and diabetes.

• Rapid onset of pain, redness, photophobia, tearing, and reduced vision with purulent discharge.
• Round or oval grey white stromal infiltrate with an overlying epithelial defect and fuzzy margins, surrounding oedema, ring infiltrate, and hypopyon.
• Pseudomonas: rapidly spreading ulcer with a ground glass or necrotic centre, mucopurulent adherent exudate, and corneal melt; streptococcal ulcers may have an undermined, advancing edge; staphylococcal ulcers are often focal and round.
• Severity assessment: central location, size above 2 mm, depth, hypopyon, and threatened perforation.

• Corneal scraping (before antibiotics) for Gram stain and culture; blood agar, chocolate agar, thioglycollate; sensitivities when growth occurs. Cultures are positive in about half or fewer of cases, and treatment should not be delayed.
• Culture lenses and cases in contact lens wearers.
• Slit lamp documentation, Seidel test, and intraocular pressure measurement when safe.
• Ultrasound B-scan if the view is poor, to exclude endophthalmitis.

• Hospital admission for severe or central ulcers, poor compliance, or when hourly drops cannot be done at home.
• Initial therapy (after scraping):
  • Monotherapy with a fluoroquinolone: moxifloxacin 0.5 percent, levofloxacin 1.5 percent, or ofloxacin 0.3 percent. Loading: 1 drop every 5 to 15 minutes for the first hour, then hourly day and night for 24 to 48 hours, then taper every 2 hours when awake, and then four times daily as healing occurs.
  • Severe ulcers (over 2 mm, central, hypopyon) and when resistance is likely: fortified drops hourly alternating, cefazolin 50 mg/mL (5 percent) with tobramycin or gentamicin 14 mg/mL (1.4 percent); if methicillin resistant Staphylococcus aureus is suspected, vancomycin 25 to 50 mg/mL; ceftazidime 50 mg/mL is an alternative for Gram negative organisms.
  • Modify according to culture, sensitivity, and clinical response at 48 to 72 hours.
• Cycloplegic: cyclopentolate 1 percent twice daily or atropine 1 percent once daily.
• Antiglaucoma therapy if the pressure is raised (timolol 0.5 percent twice daily); avoid prostaglandin analogues.
• Corticosteroids: after 2 to 3 days of effective therapy and clear improvement, prednisolone acetate 1 percent four times daily with slow taper under close observation, only when the organism is sensitive, as a way to reduce scarring; avoid in Nocardia and atypical mycobacterial infection.
• Corneal melt: oral doxycycline 100 mg twice daily and topical ascorbate to reduce collagenase activity; bandage lens and lubrication.
• Perforation or descemetocele: cyanoacrylate glue (up to about 3 mm), amniotic membrane, patch graft, or therapeutic keratoplasty; the latter also for progression despite maximal therapy. Corneal cross linking is an adjunct in some centres.
• Follow up daily until improvement, then at weekly intervals; healing is judged by the epithelial defect, infiltrate size and density, and anterior chamber reaction.
• Prevention: lens hygiene, avoid overnight wear, no tap water exposure, and early review of any painful red eye in a lens wearer.
• Complications: corneal scarring, perforation, endophthalmitis, glaucoma, cataract, and vision loss requiring later keratoplasty.