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.
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.
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.
What are the causes of keratitis?
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
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
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
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
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.
The symptoms of keratitis usually include pain, tearing, and blurring of vision. The pain may be mild to severe, depending on the cause and extent of the inflammation. Sensitivity to light may also be present. To the observer, the eye may appear red, watery, and if the cornea has extensive keratitis, the normally clear cornea may look gray or have white to gray areas.