Updated: Nov 15, 2022
Microbial Keratitis is an inflammation of the cornea secondary to infection. In this article, we will discuss the various forms of microbial keratitis, including their presenting features and management.
In most cases, bacterial keratitis only occurs where the ocular defences have been compromised, for example following an abrasion. It is typically common for this reason in contact lens wearers, particularly those who use soft lenses. The most common pathogens include:
Some bacteria however are able to penetrate through the intact corneal epithelium, without there being a prior abrasion or defect. A nice way to remember this is the organisms that can create a “CNHL”
Neisseria species (gonorrhoea and meningitis
Bacterial keratitis seldom occurs in healthy, normal eyes. Risk factors include:
Contact lens wear
Ocular surface disease: e.g. dry eye, chronic blepharitis, trichiasis, entropion, exposure, severe allergic eye disease, corneal anaesthesia
Vitamin A deficiency and immunosuppression
⭐ Pseudomonas aeruginosa is responsible for >60% of contact-lens associated keratitis!
Pain, conjunctival injection, photophobia, blurred vision, mucopurulent discharge, hypopyon, anterior chamber flare
Corneal scraping may be useful, particularly in cases of epithelial defects. Samples can be placed in various culture media in order to highlight the various possible organisms, a summarised table can be found below.
Discontinuation of contact lenses
Empirical broad spectrum antibiotics to be initiated before culture sensitivities are back, usually a fluoroquinolone
Most cases may be managed with a low intensity antibiotic and/or topical steroid, with a pause in contact lens use.
Prevention: correct contact lens use education, and use of protective eye wear during sports and other dangerous activities
Fungal keratitis is a major cause of visual loss in developing and tropical countries. There are two main types of fungus that can cause keratitis:
Yeasts (e.g, Candida), more common in temperate climates
Filamentous fungi (e.g. Fusarium and Aspergillus), more common in tropical climates
Risk factors include:
Long term topical steroids (often associated in corneal transplant patients)
Systemic immunosuppression, diabetes
Trauma, particularly involving organic/agricultural material
Symptoms often have a more gradual onset
Stromal infiltrates with fluffy margins
Feathery branch-like or ring shaped extensions
Candida infection typically is characterised by a small ulcer with an expanding infiltrate in a ‘collar stud’ formation
Corneal scrapes (see table below) using Sabourand dextrose agar will grow fungal organisms. Polymerase chain reaction (PCR) is rapid and highly sensitive. Confocal microscopy may allow in vivo visualisation of the organisms
General education measures on contact lens hygiene are important, as for bacterial keratitis. Topical antifungal agents: first line for candida infection is amphotericin B, and first line for filamentous species is natamycin. In some cases, a broad spectrum antibiotic may be considered to prevent bacterial co-infection.
Herpes Simplex Keratitis
💡 Herpes eye disease is the most common cause of infectious blindness in the developed world
Herpes simplex virus (HSV) is a double-stranded DNA virus encased by a cuboidal capsule. After primary infection by the virus, it is carried to the sensory root ganglion for the dermatome where the virus is incorporated into host DNA and remains latent. Stressors such as fever, stress, infection or trauma may result in reactivation where the virus is shed to the periphery via the sensory axons, as it uses host DNA to replicate. Symptoms depend on the pattern of reactivation, which can be far away from the initial infection site.
We will cover two types of of corneal inflammation due to HSV: epithelial keratitis and disciform (endothelial) keratitis.
This form of keratitis is associated with HSV reactivation and active virus replication. It is characterised by the classical “linear-branching” dendritic (tree-like) ulcer, most frequently located centrally and staining well with fluoroscein. Virus laden cells at the margins of the ulcer stain well with rose bengal. Reduced corneal sensation is characteristic of a viral keratitis.
Disciform (endothelial) keratitis
The aetiology of this type of keratitis is more complex: thought to result from a hypersensitivity reaction of the cornea to the HSV antigen as opposed to direct infection. Discomfort tends to be of a milder and more gradual onset than in epithelial disease. Characteristic signs include central circular stromal oedema, keratic precipitates, and a Wessely ring: this is an antigen/antibody complex within the stroma.
Diagnosis for HSV keratitis is usually clinical, however corneal scrapings can be placed in viral culture media, with Giemsa stain.
Epithelial keratitis: Topical aciclovir. Topical steroids are avoided: this is because they increase the risk of corneal perforation.
Disciform keratitis: Topical/oral aciclovir, with topical steroids, ensuring that the epithelium is intact before commencing.
💡 Review corneal anatomy to understand the different layers targeted by these two forms of HSV keratitis, to help better remember the symptoms and features!
Acanthaemoeba are a protozoa species found in soil, , dust, fresh water sources), brackish water (such as a marsh), and sea water. It is a feared complication in contact lens wearers, resulting from swimming or showering in contacts, or rinsing them in tap water.
Patients usually report pain which is out of proportion to the clinical picture. Early signs are not too specific and misdiagnosis with herpetic or fungal keratitis can be made. The pathognomonic sign is perineural infiltrates, which may coalesce to form ring abscess.
Corneal scrapings again can be cultured using periodic acid–Schiff or calcofluor white. Immunohistochemistry, PCR and in vivo confocal microscopy are also options.
Acanthamoeba cysts are resistant to most antimicrobial agents. Polyhexamethylene biguanide (PHMB) or chlorhexidine have been proven to be amoebicidal. Topical steroids should be avoided during active infection. Pain control with NSAIDs may be of symptomatic benefit.
Contact lens wear, trauma, or ocular surface disease
Conjunctival injection, mucopurulent discharge, anterior chamber flare, hypopyon if severe. Epithelial defect may be present
Trauma involving agricultural material, immunosuppression, contact lens wear
Feathery branch like extensions, satellite lesions
Primary infection is normally in childhood with a mild systemic infection. Reactivation results in ocular disease
Punctate/stellate pattern, linear branching with reduced corneal sensation
Contact lens wear, history of swimming/showering
Pain out of proportion with clinical findings, characteristic perineural infiltrates, which may coalesce to form ring abscess
Culture media for corneal scrapings
Most bacteria and fungi (except Haemophilus, Neisseria, Moraxella)
Fastidious bacteria (e.g. Haemophilus, Neisseria, Moraxella, the ones not picked up on blood agar)
Non-nutrient agar seeded with E.coli
Cooked meat broth
Stains for corneal scrapings
Bacteria, fungi, acanthamoeba
Grocott-Gomori methenamine silver
Periodic-acid Schiff (PAS)
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