Keratitis

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Medical Author:

Andrew A. Dahl, MD, FACS

Andrew A. Dahl, MD, FACS

Andrew A. Dahl, MD, is a board-certified ophthalmologist. Dr. Dahl's educational background includes a BA with Honors and Distinction from Wesleyan University, Middletown, CT, and an MD from Cornell University, where he was selected for Alpha Omega Alpha, the national medical honor society. He had an internal medical internship at the New York Hospital/Cornell Medical Center.

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Medical Editor:

Melissa Conrad Stöppler, MD

Melissa Conrad Stöppler, MD

Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.

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Keratitis is the medical term for inflammation of the cornea. The cornea is the dome-shaped window in the front of the eye. When looking at a person's eye, one can see the iris and pupil through the normally clear cornea. The cornea bends light rays as a result of its curved shape and accounts for approximately two-thirds of the eye's total optical power, with the lens of the eye contributing the remaining one-third.. Only the very thin tear film lies between the front of the cornea and our environment.

The cornea is about 0.5 millimeter thick. The back of the cornea is bathed in the aqueous fluid that fills the anterior chamber of the eye. The cornea has a diameter of about 13 millimeters (½ inch) and, together with the sclera (the white part of the eye) forms the entire outer coat of the eye.

Keratitis, the eye condition in which the cornea becomes inflamed, has many potential causes. Various types of infections, dry eyes, injury, and a large variety of underlying medical diseases may all lead to keratitis. Some cases of keratitis result from unknown factors.

Keratitis can be classified by its location, severity, and cause.

If keratitis only involves the surface (epithelial) layer of the cornea, it is called superficial keratitis. If it affects the deeper layers of the cornea (the corneal stroma), it is called stromal keratitis or interstitial keratitis . It may involve the center of the cornea or the peripheral part of the cornea (that portion closest to the sclera) or both. Keratitis may affect one eye or both eyes.

Keratitis may be mild, moderate, or severe and may be associated with inflammation of other parts of the eye. Keratoconjunctivitis is inflammation of the cornea and the conjunctiva. Kerato-uveitis is inflammation of the cornea and the uveal tract, which consists of the iris, ciliary body, and choroid.

Keratitis may be acute or chronic. It may occur only once or twice in an eye or be recurrent. It may be limited in its effects on the eye or be progressive in its damage.

The various causes of keratitis may result in different clinical presentations, so defining the location, severity, and frequency of the condition can often assist in pinpointing the exact cause. Other helpful facts in establishing the cause of keratitis can include demographic information such as the age, sex, and geographic location of the patient. A medical history is often useful as well in finding the cause of keratitis.

Infection is the most frequent cause of keratitis. Bacteria, viruses, fungi, and parasitic organisms may all infect the cornea, causing infectious or microbial keratitis.

• Bacteria most frequently responsible for keratitis include Staphylococci, Hemophilus, Streptococci, and Pseudomonas. If the front surface of the cornea has been damaged by a small scratch and the surface is not intact, almost any bacteria, including atypical mycobacteria, can invade the cornea and result in keratitis. Ulcerations of the cornea may occur, a condition known as ulcerative keratitis. Before the advent of antibiotics, syphilis was a frequent cause of keratitis.



• Viruses that infect the cornea include respiratory viruses, including the adenoviruses and others responsible for the common cold. The herpes simplex virus is another common cause of keratitis. There are about 20,000 new cases of ocular herpes in the United States annually, along with more than 28,000 reactivations of the infection. There are about 500,000 people in the U.S. with a history of herpes simplex eye disease. The herpes zoster virus (the virus responsible for chickenpox and shingles) may also cause keratitis if shingles involve the forehead.



• Fungi such as Candida, Aspergillus, and Nocardia are unusual causes of microbial keratitis, more frequently occurring in people who are immunocompromised because of underlying illnesses or medications. Fusarium keratitis, a type of fungal infection, occurs primarily in contact-lens wearers. Bacterial co-infection can complicate fungal keratitis.



• Contact-lens wearers are also susceptible to acanthamoeba keratitis caused by an amebic parasite. "River blindness," or onchocercal keratitis, is another parasitic infection of the cornea, rarely seen in developed countries but very common in the Third World.

Physical or chemical trauma is a frequent cause of keratitis. The injury may become secondarily infected or remain noninfectious. Retained corneal foreign bodies are frequent sources of keratitis. Ultraviolet light from sunlight (snow blindness), a tanning light or a welder's arc, contact-lens overwear, and chemical agents, either in liquid form splashed into the eye or in gases in the form of fumes can all result in noninfectious keratitis. Chemical injury or contact lens-related keratitis often causes superficial punctate keratitis, in which the examiner notices myriads of injured surface cells on the affected cornea.

Disturbances in the tear film may lead to changes in the corneal surface through drying of the corneal epithelium. This type of keratitis is usually superficial and most commonly is related to dry eyes and is known as keratitis sicca. If the eyes are extremely dry, the surface cells may die and form attached filaments on the corneal surface, a condition known as filamentary keratitis. Inability to close the eyelids properly can also lead to cornea drying, which is a condition termed exposure keratitis.

Allergies to airborne pollens or bacterial toxins in the tears may also cause a noninfectious type of keratitis. Autoimmune diseases create a similar appearance, often affecting the periphery of the cornea, termed marginal keratitis or limbic keratitis.