Summary

Marginal keratitis refers to inflammation of the peripheral cornea, thought to represent a type III hypersensitivity reaction to Staphylococcus aureus, classically associated with acne rosacea and blepharitis.

Background

<aside> ⭐ Marginal keratitis is thought to result from a hypersensitivity reaction to Staphylococcus aureus antigens, as opposed to being caused directly by the bacteria, differentiating it from other causes of microbial keratitis

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A type III reaction is an immune complex-mediated reaction where the formation of antigen-antibody aggregates may precipitate in various tissues, activating the classical complement pathway. This triggers the recruitment of inflammatory cells, which release lysosomal enzymes, triggering tissue damage. In the case of marginal keratitis, this response leads to stromal infiltrates, which can later progress to marginal ulcers.

Although staphylococcus aureus blepharoconjunctivitis is classically linked to this, Haemophilus, Moraxella and Streptococcus species have also been known to trigger the inflammatory response.

Clinical features

• Stromal infiltrates in the peripheral cornea, where the eyelid margin most likely comes into contact with the limbus (at 2, 4, 8 and 10 o clock positions). These infiltrates are usually located 1-2mm away from the limbus, with healthy cornea in between.
• Prolonged inflammation results in the stromal infiltrated developing into an epithelial infiltrate and later a marginal ulcer
• Features of blepharitis: redness and swelling of the eyelid margin, telangectasia, hard scales in the eyelashes
• Features of acne rosacea

Marginal keratitis in the peripheral cornea in a 2 o clock position, showing a clear margin of around 1-2mm away from the limbus with healthy cornea in between. Image courtesy of Babu and Maralihalli.

Diagnosis

The diagnosis of marginal keratitis is made clinically. However if there are epithelial defects, corneal scrape cultures would be useful to differentiate between other forms of ulceration.

Radiology in Focus

Although the diagnosis of marginal keratitis is mainly clinical, imaging techniques can provide valuable insights into the degree of corneal involvement.

Optical Coherence Tomography (OCT)

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

OCT of Fuch’s superficial marginal keratitis, showing hyper-reflective infiltration along Bowman’s layer and mild peripheral stromal thinning. [Image source](https://eyewiki.org/Fuchs'_Superficial_Marginal_Keratitis).