Keratitis and Corneal Inflammation

Keratitis is inflammation of the cornea, the transparent dome at the front of the eye. It may be infectious, driven by bacteria, fungi, viruses, or the protozoan Acanthamoeba, or non-infectious, arising from injury, dryness, immune disease, or loss of corneal nerve function. Because the cornea is the eye's principal refractive surface, keratitis is a leading cause of corneal opacity and avoidable monocular blindness.

Definition

Keratitis is inflammation of the cornea, characterised clinically by corneal infiltrate, epithelial defect, or stromal involvement, and classified by cause (infectious versus non-infectious) and by the layer of cornea affected.

Scope

This entry covers the concept and classification of keratitis, the main infectious and non-infectious mechanisms, the role of risk factors such as contact-lens wear and ocular trauma, and the global burden of microbial keratitis. It is a reference overview of the disease entity and does not provide diagnostic protocols or treatment instructions.

Key concepts

• Infectious versus non-infectious keratitis
• Microbial keratitis (bacterial, fungal, viral, Acanthamoeba)
• Contact-lens-associated keratitis
• Corneal infiltrate and epithelial defect
• Stromal melt and corneal perforation
• Neurotrophic keratopathy
• Antimicrobial resistance

Mechanisms

Keratitis begins when the corneal epithelial barrier is breached, by contact-lens wear, trauma, surface disease, or reduced corneal sensation, allowing micro-organisms to invade the stroma or allowing sterile inflammation to develop. Invading pathogens and the host inflammatory response release proteases and recruit neutrophils, producing an infiltrate, epithelial defect, and, in severe cases, stromal thinning (melt) and perforation. Healing by scarring leaves a corneal opacity. In neurotrophic keratopathy the mechanism is loss of trigeminal corneal innervation, which impairs epithelial maintenance and predisposes to persistent defects independent of infection.

Clinical relevance

Keratitis is one of the commonest sight-threatening anterior segment emergencies and a major contributor to corneal blindness worldwide; distinguishing infectious from non-infectious causes, and identifying the responsible organism, is central to ophthalmic evaluation. This entry describes the disease for reference and study and is not a substitute for clinical assessment or treatment.

Epidemiology

Microbial keratitis carries a substantial global burden, with markedly higher incidence in low- and middle-income countries and a large fungal component in tropical and agricultural regions; one synthesis estimated more than a million cases of fungal keratitis annually. In high-income settings contact-lens wear is the dominant risk factor, while in lower-income settings ocular trauma, often agricultural, predominates.

Debates

Rising antimicrobial resistance in bacterial keratitis

Increasing resistance among common corneal pathogens complicates empirical therapy and is a recognised threat to keratitis outcomes, prompting calls for better surveillance and stewardship.

Underdiagnosis of fungal and Acanthamoeba keratitis

Fungal and Acanthamoeba keratitis are frequently misdiagnosed or diagnosed late because of limited laboratory access and overlapping clinical features, contributing to worse outcomes especially where diagnostic capacity is scarce.

Seminal works

• ung-2019
• brown-2021-fungal
• lorenzo-morales-2015

Frequently asked questions

Is keratitis the same as a corneal ulcer?

They overlap but are not identical: keratitis means corneal inflammation, while a corneal ulcer is an epithelial defect with underlying stromal inflammation or infection. Microbial keratitis often progresses to a corneal ulcer, but keratitis can also be superficial or non-ulcerative.

Why is contact-lens wear linked to keratitis?

Contact lenses can disrupt the corneal surface, reduce oxygen supply, and harbour micro-organisms in biofilm, which together raise the risk of microbial keratitis, including bacterial and Acanthamoeba infection.