Diagnostic Approach for Microbial Keratitis

History Taking

The American Academy of Ophthalmology recommends assessing 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, 3
Review ocular history for risk factors such as previous bacterial keratitis, trauma, dry eye, and previous ocular surgeries 4
Evaluate medical history including immune status, rosacea, atopy, diabetes, and history of drug-resistant infections like MRSA 5
Document current and recently used ocular and systemic medications 1

Document baseline visual acuity even though it may be compromised by discomfort and inflammation 4, 5
Assess general appearance including skin conditions and overall hygiene 5
Examine facial features for conditions like rosacea or herpes zoster 5
Evaluate globe position, eyelids, eyelid closure, and nasolacrimal apparatus 5
Check conjunctival injection and chemosis 5
Consider corneal sensation testing when appropriate 5
Examine eyelids for inflammation, ulceration, meibomian gland dysfunction, and abnormalities like trichiasis or lagophthalmos 1, 4
Assess conjunctiva for discharge, inflammation, morphologic alterations, and foreign bodies 1
Evaluate cornea for epithelial defects, punctate keratopathy, and edema 1, 2
Evaluate cornea for stromal infiltrates, noting their location, size, shape, depth, and character of margins 4, 5
Evaluate cornea for ulceration, thinning, and perforation 1
Apply fluorescein or rose bengal/lissamine green staining to identify epithelial defects 2
Check anterior chamber for inflammation, including cell and flare, hypopyon, and fibrin 2
Examine anterior vitreous for inflammation 4
Assess the contralateral eye for clues to etiology 4

Most community-acquired cases of bacterial keratitis can be managed with empiric therapy without cultures 2, 6
Smears and cultures are specifically indicated when corneal infiltrate is central, large, or associated with significant stromal involvement or melting 2, 6
Smears and cultures are specifically indicated when infection is chronic or unresponsive to broad-spectrum antibiotics 6
Smears and cultures are specifically indicated when patient has history of corneal surgeries 6
Smears and cultures are specifically indicated when atypical features suggest fungal, amoebic, or mycobacterial keratitis 2, 6
For large, central infiltrates with stromal melting or atypical appearance: obtain cultures and Gram/Giemsa stains 2
For post-surgical infiltrates: obtain cultures with Gram, potassium hydroxide, Calcofluor white, and acid-fast stains 2

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 2
Hypopyon in bacterial keratitis is usually sterile; aqueous or vitreous taps should not be performed unless there is high suspicion of endophthalmitis 2
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 2
Prompt diagnosis and treatment are essential for good visual outcomes, as untreated or severe bacterial keratitis may progress rapidly to corneal perforation 3