Diagnostic Approach for Microbial Keratitis
The diagnosis of microbial keratitis requires a systematic approach including detailed history, comprehensive slit-lamp biomicroscopy, and appropriate laboratory testing with cultures and smears in specific clinical scenarios. 1
History Taking
- Assess ocular symptoms including pain, redness, discharge, blurred vision, photophobia, and duration of symptoms 1
- Obtain contact lens history including wearing schedule, overnight wear, lens type, solutions used, hygiene protocol, and activities while wearing lenses (swimming, showering) 1
- Review ocular history for risk factors such as HSV keratitis, VZV keratitis, previous bacterial keratitis, trauma, dry eye, and previous ocular surgeries 1
- Evaluate medical history including immune status, rosacea, atopy, diabetes, and history of drug-resistant infections like MRSA 1
- Document current and recently used ocular and systemic medications 1
Physical Examination
Visual Acuity
- Document baseline visual acuity even though it may be compromised by discomfort and inflammation 1
External Examination
- Assess general appearance including skin conditions and overall hygiene 1
- Examine facial features for conditions like rosacea or herpes zoster 1
- Evaluate globe position, eyelids, eyelid closure, and nasolacrimal apparatus 1
- Check conjunctival injection and chemosis 1
- Consider corneal sensation testing when appropriate 1
Slit-Lamp Biomicroscopy
- Examine eyelids for inflammation, ulceration, meibomian gland dysfunction, and abnormalities like trichiasis or lagophthalmos 1
- Assess conjunctiva for discharge, inflammation, morphologic alterations, and foreign bodies 1
- Evaluate cornea for:
- Apply fluorescein or rose bengal/lissamine green staining to identify epithelial defects, distinguishing between staining and pooling in areas of thinning 1
- Check anterior chamber for inflammation, including cell and flare, hypopyon, and fibrin 1
- Examine anterior vitreous for inflammation 1
- Assess the contralateral eye for clues to etiology 1
Laboratory Testing
- Most community-acquired cases of bacterial keratitis can be managed with empiric therapy without cultures 1
- Smears and cultures are specifically indicated when:
- Corneal infiltrate is central, large (≥2mm), or associated with significant stromal involvement or melting 1
- Infection is chronic or unresponsive to broad-spectrum antibiotics 1
- Patient has history of corneal surgeries 1
- Atypical features suggest fungal, amoebic, or mycobacterial keratitis 1
- Multiple infiltrates are present on the cornea 1
- There are ≥1+ cells in the anterior chamber 1
Culture and Staining Recommendations
- For small, peripheral infiltrates without stromal melting: culture and staining are optional 1
- For large, central infiltrates with stromal melting or atypical appearance: obtain cultures and Gram/Giemsa stains 1
- For post-surgical infiltrates: obtain cultures with Gram, potassium hydroxide, Calcofluor white, and acid-fast stains 1
Common Pitfalls and Caveats
- Clinical appearance alone may be insufficient to differentiate bacterial from fungal keratitis, with cornea specialists correctly identifying the causative organism in less than 70% of cases 1
- Initial misdiagnosis of microbial keratitis as a contact lens peripheral ulcer (CLPU) can occur; monitor carefully for expected improvement after lens discontinuation 2
- Hypopyon in bacterial keratitis is usually sterile; aqueous or vitreous taps should not be performed unless there is high suspicion of endophthalmitis 1
- Distinguishing between epithelial staining and pooling of stain in areas of corneal thinning is important; pooling can be wicked away with a cotton swab or irrigation 1
- Prompt diagnosis and treatment are essential for good visual outcomes, as untreated or severe bacterial keratitis may progress rapidly (within 24 hours) to corneal perforation 1, 2