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Infectious Disease and Microbiology - Keratitis
Keratitis refers to inflammation of the cornea that may be caused by infectious or noninfectious processes. Infectious keratitis can result from bacteria, viruses, fungi, or parasites, and bacterial keratitis is often referred to as a corneal ulcer. A major global cause of keratitis-related blindness is trachoma due to Chlamydia trachomatis, which leads to chronic inflammation, eyelid deformities, and corneal scarring.
The condition occurs worldwide, with bacterial keratitis being more common among contact lens users. Risk factors include disruption of the corneal epithelial surface, poor contact lens hygiene, overnight lens use, and ocular trauma. In critically ill patients, factors such as inability to close the eyes, reduced blinking, and dry eye contribute to risk. Fungal keratitis is more common in warm climates and often follows trauma with plant material, while trachoma is associated with poor hygiene and limited access to water.
The pathophysiology involves a break in the corneal epithelium that allows microorganisms to enter and proliferate in the corneal stroma. Some organisms possess adhesins that facilitate attachment to corneal cells. Viral causes such as Herpes simplex virus type 1 can remain latent in the trigeminal ganglion and reactivate, leading to recurrent disease. Similarly, Varicella zoster virus may reactivate later in life, causing herpes zoster ophthalmicus when the trigeminal nerve is involved.
A wide range of organisms can cause keratitis. Bacterial causes include Pseudomonas, Staphylococcus, and Streptococcus species, with Pseudomonas being particularly aggressive in contact lens users. Viral causes include HSV and VZV, with HSV producing characteristic dendritic lesions. Fungal organisms such as Aspergillus and Fusarium cause more indolent infections, while parasites such as Acanthamoeba can produce severe pain and characteristic ring-shaped infiltrates.
Patients typically present with eye pain, redness, foreign body sensation, photophobia, tearing, discharge, and reduced vision. Severe pain that is disproportionate to clinical findings is suggestive of Acanthamoeba infection. Viral keratitis may lead to reduced corneal sensation. Herpes zoster ophthalmicus may be preceded by a flu-like illness and is associated with a dermatomal rash, especially involving the tip of the nose.
On examination, slit-lamp findings include epithelial defects, stromal infiltrates, and anterior chamber inflammation. Fluorescein staining highlights epithelial defects. Severe cases may show hypopyon. Bacterial ulcers may appear as rapidly spreading infiltrates, while fungal keratitis often shows feathery borders and satellite lesions. HSV infection classically produces branching dendritic ulcers, while late Acanthamoeba infection may show a ring infiltrate.
Diagnosis is primarily clinical but supported by laboratory studies. Corneal scrapings are obtained for staining and culture to identify the causative organism. Special stains and media are used depending on suspected pathogens. PCR and viral testing may be used for herpetic infections, while confocal microscopy can help identify Acanthamoeba cysts.
Management depends on the underlying cause but must be initiated promptly. Suspected bacterial keratitis requires immediate empiric broad-spectrum topical antibiotics, often administered hourly. Severe cases may require fortified antibiotic drops. Fungal keratitis is treated with topical antifungals such as natamycin or amphotericin B, often for prolonged periods. Acanthamoeba infections require specific agents such as polyhexamethylene biguanide and are difficult to eradicate. Viral keratitis due to HSV is treated with topical antivirals, while deeper stromal disease may require cautious use of topical steroids along with antiviral coverage. Herpes zoster ophthalmicus is treated with systemic antiviral therapy.
Prognosis depends on the severity, location, and timeliness of treatment. Early treatment generally leads to good outcomes, but delayed therapy can result in significant visual impairment. Complications include corneal scarring, thinning, perforation, and secondary infections such as endophthalmitis. Chronic or recurrent infections, particularly with HSV, may lead to permanent visual loss and require surgical intervention such as corneal transplantation.
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