Microbial keratitis

• Summary
• Bacterial Keratitis
• Fungal Keratitis
• Herpes Simplex Keratitis
• Acanthamoeba Keratitis
• Radiology in Focus
• Optical coherence tomography (OCT)
• Summary tables
• References
• Author(s)

Summary

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.

Bacterial Keratitis

Aetiology

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:

• Pseudomonas aeruginosa
• Staphylococcus aureus

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”

• Corynebacterium
• Haemophilus
• Neisseria species (gonorrhoea and meningitis
• Listeria

Bacterial keratitis seldom occurs in healthy, normal eyes. Risk factors include:

• Contact lens wear
• Trauma
• Ocular surface disease: e.g. dry eye, chronic blepharitis, trichiasis, entropion, exposure, severe allergic eye disease, corneal anaesthesia
• Vitamin A deficiency and immunosuppression

Clinical features

Pain, conjunctival injection, photophobia, blurred vision, mucopurulent discharge, hypopyon, anterior chamber flare

Diagnosis

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.

Management

• 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

Aetiology

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
• Contact lenses

Clinical features

• Symptoms often have a more gradual onset
• Stromal infiltrates with fluffy margins
• Satellite lesions
• 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

Diagnosis

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

Management

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

Aetiology

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.

Epithelial 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

Diagnosis for HSV keratitis is usually clinical, however corneal scrapings can be placed in viral culture media, with Giemsa stain.

Management

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.

Acanthamoeba Keratitis

Aetiology

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.

Clinical features

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.

Diagnosis

Corneal scrapings again can be cultured using periodic acid–Schiff or calcofluor white. Immunohistochemistry, PCR and in vivo confocal microscopy are also options.

Management

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.

Radiology in Focus

Microbial keratitis is a serious corneal infection that can lead to significant vision loss if not promptly diagnosed and treated. While the diagnosis is primarily clinical, imaging techniques can provide valuable insights into the extent of the infection, the depth of corneal involvement, and any associated complications.

Optical coherence tomography (OCT)

OCT provides high-resolution cross-sectional images of the cornea, allowing for detailed assessment of the corneal layers. It can help visualise the depth and extent of the infiltrates, assess the presence of corneal oedema, and evaluate any associated scarring or thinning.

Summary tables

Culture media for corneal scrapings

Stains for corneal scrapings

References

1. Salmon, John F., and Jack J. Kanski. Kanski’s Clinical Ophthalmology: A Systematic Approach. Ninth Edition, Elsevier, 2020.
2. Bouhenni, Rachida, et al. ‘Proteomics in the Study of Bacterial Keratitis’. Proteomes, vol. 3, no. 4, Dec. 2015, pp. 496–511. PubMed Central, https://doi.org/10.3390/proteomes3040496.
3. Leck, Astrid, and Matthew Burton. ‘Distinguishing Fungal and Bacterial Keratitis on Clinical Signs’. Community Eye Health, vol. 28, no. 89, 2015, pp. 6–7. PubMed Central, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4579991/.
4. Huang, Yi-Hsun, et al. ‘Early Diagnosis and Successful Treatment of Cryptococcus Albidus Keratitis: A Case Report and Literature Review’. Medicine, vol. 94, no. 19, May 2015, p. e885. PubMed, https://doi.org/10.1097/MD.0000000000000885
5. Gilani, Christopher J., et al. ‘Differentiating Urgent and Emergent Causes of Acute Red Eye for the Emergency Physician’. The Western Journal of Emergency Medicine, vol. 18, no. 3, Apr. 2017, pp. 509–17. PubMed, https://doi.org/10.5811/westjem.2016.12.31798
6. Chomicz, Lidia, et al. ‘Emerging Threats for Human Health in Poland: Pathogenic Isolates from Drug Resistant Acanthamoeba Keratitis Monitored in Terms of Their In Vitro Dynamics and Temperature Adaptability’. BioMed Research International, vol. 2015, 2015, p. 231285. PubMed, https://doi.org/10.1155/2015/231285

Author(s)

Dr Sara Memon

Sara is the Co-Founder of Ophtnotes. She is a doctor who graduated from UCL Medical School in London. She won the Allen Goldsmith Prize in Ophthalmology. Sara is also the co-founder of PAMSA: an organisation linking doctors and medical students of Pakistani origin. She’s especially passionate about teaching and education, having presented a workshop she designed herself at the 2019 Annual GMC Conference.

Dr Abhiyan Bhandari

Abhiyan is the Co-Founder and Radiology & Imaging Lead of Ophtnotes. He is a doctor who graduated from UCL Medical School in London. He scored in the top 10% of candidates who sat the Duke Elder examination and runs ophthalmology and Duke Elder revision sessions aimed at medical students. He also runs a YouTube channel aimed at medical students, covering topics ranging from study tips, productivity and vlogs of his journey through medical school.

Published: 2/10/22

Last updated: 29/1/25