Introduction

• Keratitis is inflammation of the cornea.
• It may result from infective or non-infective causes
• Infectious keratitis can be classified as microbial keratitis (bacterial, fungi or protozoal) or viral keratitis (herpes viruses).
• Bacterial keratitis represents the most common form of microbial keratitis
• Keratitis requires urgent ophthalmological review as delay in treatment may result in loss of sight.¹
• The following article aims to provide an overview of microbial keratitis.

Risk Factors

• Contact lens use
• Trauma
• Ocular surface diseases (e.g. dry eyes, corneal anaesthesia, corneal exposure from lagophthalmos)
• Lid disease (e.g. blepharitis, entropion, trichiasis)
• Systemic conditions including immunosuppression

Causes

Bacterial keratitis

Pathogens: Staphylococcus, Streptococcus and Pseudomonas species. Pseudomonas aeruginosa is the most common cause in contact lens wearers

Fungal keratitis

Pathogens: Fusarium and Aspergillus species (filamentary fungi) often from ocular trauma; Candida species (yeasts) in the immunocompromised

Protozoal keratitis

Pathogen: Acanthamoeba

A very rare cause of keratitis and often difficult to treat

Characteristically, pain is severe and disproportionate to clinical findings

Viral keratitis

Pathogen: Herpes Simplex

-Primary infection: direct contact with infected secretions or lesions

-Reactivation/ recurrence: systemic illness, trauma, sunlight, stress

Clinical Features

• Pain
• Redness
• Reduced visual acuity
• Photophobia
• Discharge

Clinical Examination

• Take a detailed ocular history – any previous eye trauma/surgery or eye disease
• Contact lens history – type of lens used, how long they continuously wear, cleaning regime, do they sleep/shower/swim in them?
• Visual acuity testing (+ pinhole)
• Anterior segment examination (ideally with slit lamp)

Typical clinical findings may include:

• Conjunctival injection
• Cornea: a yellow-white opacity which represents the area of infection (infiltrate). The overlying epithelial defect (ulcer) will stain green upon application of 2% fluorescein drops when viewed with a blue light
• Any anterior chamber reaction (eg hypopyon: inflammatory cells in anterior chamber)

Diagnosis is based on clinical history and slit lamp examination showing the presence of a corneal infiltrate. Fluorescein should be used to highlight areas of epithelial cell loss.²

Investigation

• Corneal scrapes (if ulcer larger than 1mm) – take these before commencing topical antibiotics, send for Gram Stain and bacterial culture
• Acanthamoeba and Viral PCR swabs. If contact lenses wearer – send lenses, solution and cases for culture

Management

• Contact lens use should be stopped immediately
• Topical Cycloplegia – helps with photosensitivity
• Initial broad-spectrum topical antibiotics until culture results available
• In viral infections- topical/ systemic antiviral can be used; steroids are contraindicated in active epithelial disease
• In fungal infections – topical/systemic antifungals can be used
• Acanthamoeba infection if suspected is treated with a combination of epithelial debridement, topical biguanides and diamidines
• Penetrating keratoplasty (full-thickness corneal transplant): considered in cases with severe corneal scarring or extensive necrosis. Recurrence may occur in grafted tissue. ^3,4

Complications

• Corneal scarring
• Corneal perforation
• Endophthalmitis

Key Points

• A red eye with a corneal infiltrate is suspicious of microbial keratitis.
• Patients who wear contact lenses are at greatest risk of developing microbial keratitis.
• Management includes urgent referral for ophthalmic review in a hospital emergency department or eye clinic

References

1. Eye Infection, Treatment Summaries. BNF, NICE Guidelines; 2023.
2. Bacterial keratitis. EyeWiki, AAO; 2023. Available from: https://eyewiki.aao.org/Bacterial_Keratitis
3. Microbial Keratitis. Clinical Management Guidelines. The College of Optometrists; 2022.
4. American Academy of Ophthalmology. 2019-2020 Basic and Clinical Science Course, Section 02: Fundamentals and Principles of Ophthalmology; 2019.

Written by Dr Weihan Ong (FY1) & reviewed by Dr Sarah Campbell (ST3 Ophthalmology)