How to Tackle Fungal Corneal Infections

When a patient presents with infiltrative keratitis, often our first inclination is to think of bacterial infection. Yet, it's important to remember that other infectious conditions can cause infiltrative keratitis, including fungal infections.

The human cornea is vulnerable to two types of fungal infections: filamentous fungi such as Aspergillus or Fusarium; and yeasts such as Candida. Fungal keratitis associated with filamentous fungi can occur following corneal trauma in otherwise immunocompetent patients. The traumatic break in the epithelium allows the fungus to adhere to the corneal surface and develop into an infection.

These presentations occur more commonly in farm workers and in those who suffer a corneal injury from organic material such as a tree branch. Filamentous fungal infections are much more prevalent in the southern United States and other warm climates. Other predisposing factors for fungal infections include contact lens wear and topical steroid use.

The clinical manifestations of fungal ulcers caused by filamentous fungi may be difficult to distinguish from bacterial infection. Patients often develop acute symptoms of pain, photophobia and decreased vision. Yet, the progression of symptoms often follows a more indolent course than that of bacterial ulcers.

In your slit lamp assessment, look for a dense, gray-white infiltrate and corneal ulceration. The infiltrate may display a feathery or indistinct border. There may be one or more satellite lesions either adjacent to the main infiltrate or several millimeters away. Many times the epithelium remains intact over a fungal infiltrate, resulting in a fungal corneal abscess. A ring infiltrate may surround the primary lesion, although such infiltrates can also occur in bacterial, viral and parasitic corneal infections.

The anterior chamber reaction can be rather severe, out of proportion to the patient's symptoms and corneal findings. Expect to see endothelial plaque formation and hypopyon. Lesions located near the limbus may develop deep neovascularization.

A fungal corneal ulcer may mimic the presentation of a bacterial infection.

Yeast infections present a different clinical picture. They develop more commonly in immunocompromised patients, those with preexisting corneal disease, or those on topical steroids. These infections produce an oval, elevated, plaque-like infiltrate. The surrounding cornea is often edematous, and the edges of the infiltrate tend to be less feathery than with filamentous fungal infections. Satellite lesions rarely develop in yeast infections.

Although the history and clinical appearance may strongly suggest fungal keratitis, cultures are necessary to confirm your diagnosis. Plate multiple deep scrapings of the lesion on blood agar and Sabouraud media. Also obtain samples for microscopic analysis; these smears will sometimes reveal the presence of fungi even when cultures show no growth. The culture results should be available in two to three days, although some fungi take up to three weeks to grow.

Treatment of fungal keratitis is often difficult because no single agent works against all types of infection. Treatment often requires six weeks or more of topical therapy. Natamycin (Natacyn) 5 percent, the topical drug of choice, is effective against various filamentous fungi and yeasts. A standard regimen is to administer natamycin every 30 minutes for the first three to four days, then continue six times a day until the infection resolves. The drug works best for superficial infections, yet can be toxic to the corneal epithelium.

Before we had natamycin, amphotericin B was the most widely used drug for fungal infections. It still may be effective in some cases today, but its use is limited by resistance, toxicity and the topical discomfort it can cause. Amphotericin B may be most useful in cases where the infection extends to the intraocular tissues. In addition, the imidazole compounds miconazole (Monistat) and ketoconazole (Nizoral) may be effective in treating fungal ulcers.

A word of caution: Never use topical steroids to treat a fungal infection. Suppressing the natural host response may worsen the flare-up.