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What are the different types of keratitis?
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. It may involve one eye (unilateral) or both eyes (bilateral).
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, social history, and a review of all symptoms are 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. Worldwide, the incidence of HSV keratitis is about 1.5 million, including 40,000 new cases of related blindness each year. The herpes zoster virus (the virus responsible for chickenpox and shingles) may also cause keratitis, particularly when shingles involves the forehead. The U.S. Centers for Disease Control and Prevention (CDC) has recently described adult patients with conjunctivitis and keratitis resulting from the Zika virus.
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 noninfected. 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 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 corneal drying, a condition termed exposure keratitis. This can occur in Bell's palsy, which is a facial nerve weakness sometimes associated with Lyme disease.
Disorders of the eyelids or eyelashes may also cause keratitis. If the lower eyelid is turned inward, a condition known as entropion, eyelashes will rub against the cornea. Lashes growing in the wrong direction may also cause surface damage to the cornea.
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.