Bacterial Keratitis

Epidemiology: Over 90% of all corneal inflammations are caused by bacteria.

Etiology: The pathogens listed in Table 5.1 are among the most frequent causes of bacterial keratitis in the urban population in temperate climates.

Table 5.1 The most common bacterial pathogens that cause keratitis

Bacterium

Typical characteristics of infection

Staphylococcus aureus

Infection progresses slowly with little pain.

Staphylococcus epidermidis

As in Staphylococcus aureus infection.

Streptococcus pneumoniae

Typical serpiginous corneal ulcer: the cornea is rapidly perforated with early intraocular involvement; very painful.

Pseudomonas aeruginosa

Bluish green mucoid exudate, occasionally with a ring-shaped corneal abscess. Progression is rapid with a tendency toward melting of the cornea over a wide area; painful.

Moraxella

Painless oval ulcer in the inferior cornea that progresses slowly with slight irritation of the anterior chamber.

Most bacteria are unable to penetrate the cornea as long as the epithelium remains intact. Only gonococci and diphtheria bacteria can penetrate an intact corneal epithelium.

Symptoms: Patients report moderate to severe pain (except in Moraxella infections; see Table 5.1), photophobia, impaired vision, tearing, and purulent discharge. Purulent discharge is typical of bacterial forms of keratitis; viral forms produce a watery discharge.

Diagnostic considerations: Positive identification of the pathogens is crucial. Serpiginous corneal ulcers are frequently associated with severe reaction of the anterior chamber including accumulation of cells and pus in the inferior anterior chamber (hypopyon, Fig. 5.7 a) and posterior adhesions of the iris and lens (posterior synechia).

Differential diagnosis: Fungi (positive identification of the pathogen is required to exclude a fungus infection).

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