- Risk Factors
- Signs & Symptoms
What is keratitis?
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 millimeters 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, abnormalities of the eyelids, injury, and a large variety of underlying medical diseases may all lead to keratitis. Some cases of keratitis result from unknown factors.
What are the risk factors for keratitis?
Major risk factors for the development of keratitis include any break or disruption of the surface layer (epithelium) of the cornea.
The use of contact lenses increases the risk of developing keratitis, especially if hygiene is poor, improper solutions are used to store and clean the lenses, or if contact lenses are worn improperly or in the presence of persistent irritation.
A decrease in the quality or quantity of tears predisposes the eye to the development of keratitis.
Disturbances of immune function through diseases such as AIDS or the use of medications such as corticosteroids, either in the form of eye drops or systemic, or chemotherapy also increase the risk of developing keratitis.
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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 both the cornea and the conjunctiva. Kerato-uveitis is inflammation of both 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. It typically produces dendritic keratitis, which is a defect in the surface of the cornea in a tree-branching configuration. 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 (VZV or varicella-zoster virus, the virus responsible for chickenpox and shingles) may also cause keratitis, particularly when shingles involve 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. In 2020, the COVID-19 virus was described as a cause of keratitis.
- 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. The 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. People with rheumatoid arthritis or other immune disorders may develop marginal corneal ulceration with thinning of the cornea.
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What are signs and symptoms keratitis?
The symptoms of keratitis usually include pain, tearing, redness, 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 and watery; and if the cornea has extensive keratitis, the normally clear cornea may look gray or have white to gray areas.
What types of doctors diagnose and treat keratitis?
If you develop keratitis, you should be examined promptly by a professional trained in the diagnosis and treatment of eye disease. These include ophthalmologists and optometrists.
How do healthcare professionals diagnose keratitis?
The diagnosis of keratitis is made by an ophthalmologist (a physician who specializes in diseases and surgery of the eye) through a history and a physical examination. The history consists of questions documenting a past medical and ocular history and the symptoms specific to the current visit. The eye examination will consist of checking your vision and careful inspection of the corneas using a slit lamp, which is a microscope with excellent illumination and magnification to view the entire ocular surface, including the cornea in detail. A special dye containing fluorescein in the form of eye drops may be placed in the eyes to assist with the examination.
In cases in which infection is suspected, a culture may be taken from the surface of the eye for specific identification of the bacteria, virus, fungus, or parasite causing the keratitis. Blood tests may also be done in certain patients with suspected underlying diseases.
What is the treatment for keratitis?
Treatment depends on the cause of the keratitis. Infectious keratitis generally requires antibacterial, antifungal, or antiviral therapy to treat the infection. This treatment can involve prescription eyedrops, pills, or even intravenous therapy. Any corneal or conjunctival foreign body should be removed. Wetting drops may be used if disturbance of the tears is suspected to be the cause of the keratitis. Steroid drops may be prescribed occasionally to reduce inflammation and limit scarring. This must be done carefully and judiciously since some infections can be worsened with their use.
Contact lens wearers are typically advised to discontinue contact lens wear, whether or not the lenses are related to the cause of the keratitis.
What is the prognosis of keratitis?
With proper diagnosis and appropriate treatment including follow-up care, keratitis can usually be managed without causing permanent visual disturbances.
What are the possible complications of keratitis?
Superficial keratitis involving only the superficial layers of the cornea most commonly does not lead to scarring. More extensive keratitis involves deeper layers of the cornea and may produce scarring. If a corneal ulcer develops, a scar may result. This will affect the vision if the central portion of the cornea is involved. With severe ulcerative keratitis, the cornea may perforate, which is an extremely serious situation.
Is it possible to prevent keratitis?
The risk of keratitis can be reduced through the use of precautions to avoid eye injury, careful contact-lens care including proper cleaning of contact lens cases, and the prompt treatment of early ocular symptoms.
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Guo, D., J. Xia, and Y. Wang, et al. "Relapsing viral keratoconjunctivitis in COVID-19: a case report." Virol J 17.97 (2020).
Lorenzo-Morales, J., N.A. Khan, and J. Walochnik. "An update on Acanthamoeba keratitis: diagnosis, pathogenesis and treatment." Parasite 22 (2015): 10.
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