|The Merck Manual of Medical Information--Home Edition
|Section 20. Eye Disorders
The cornea, the domed covering in the front of the eye that protects the iris and lens and helps focus light on the retina, consists of cells and fluid and is normally clear. (see illustration, page 1026) Corneal disease or damage can cause pain and a loss of vision.
Superficial Punctate Keratitis
Superficial punctate keratitis is a condition in which cells on the surface of the cornea die.
The cause may be a viral infection, a bacterial infection, dry eyes, exposure to ultraviolet light (sunlight, sunlamps, or welding arcs), irritation from prolonged use of contact lenses, or irritation from or an allergy to eyedrops. The condition can also be a side effect of certain drugs taken internally, such as vidarabine.
Symptoms and Treatment
The eyes are generally painful, watery, sensitive to light, and bloodshot, and vision may be slightly blurry. When ultraviolet light causes the condition, symptoms usually don't occur for several hours, and they last for 1 to 2 days. When a virus causes the condition, a lymph node in front of the ear may be swollen and tender.
Almost everyone who has this condition recovers completely. When the cause is a virus, no treatment is needed, and recovery usually occurs in 3 weeks. When the cause is a bacterial infection, antibiotics are used. When the cause is dry eyes, treatments such as ointments and artificial tears (eyedrops prepared with substances that simulate real tears) are effective. When the cause is exposure to ultraviolet light or contact lens irritation, an antibiotic ointment, an eyedrop that dilates the pupil, (see illustration, page 1036) and an eye patch may provide relief. And when the cause is a drug reaction, the drug must be stopped.
A corneal ulcer is a pitting of the cornea, generally from an infection by bacteria, fungi, viruses, or the protozoan Acanthamoeba, and sometimes from an injury.
Bacteria (often staphylococci, pseudomonades, or pneumococci) can infect and ulcerate the cornea after the eye is injured, a foreign object lodges in the eye, or the eye is irritated by a contact lens. Other bacteria, such as gonococci, and viruses, such as herpes, can also cause corneal ulcers. Fungi may cause slowly growing ulcers. Rarely, vitamin A or protein deficiency may lead to corneal ulceration.
When the eyelids don't close properly to protect and moisten the cornea, corneal ulcers may develop from dryness and irritation, even without an infection.
Symptoms and Treatment
Corneal ulcers cause pain, sensitivity to light, and increased tear production, all of which may be mild. A whitish yellow spot of pus may appear in the cornea. Sometimes, ulcers develop over the entire cornea and may penetrate deeply. Additional pus may accumulate behind the cornea. The deeper the ulcer, the more severe the symptoms and complications.
Corneal ulcers may heal with treatment, but they may leave a cloudy, fibrous material that causes scarring and impairs vision. Other complications include deep-seated infection, perforation of the cornea, displacement of the iris, and destruction of the eye.
A corneal ulcer is an emergency that should be treated immediately by an eye doctor (ophthalmologist). To see an ulceration clearly, a doctor may apply eyedrops that contain a dye called fluorescein. Antibiotic therapy and surgery may be required.
Herpes Simplex Infection
When a corneal herpes simplex infection (see page 916 in Chapter 186, Viral Infections) (herpes simplex keratoconjunctivitis, keratitis) begins, it may resemble a mild bacterial infection because the eyes are slightly painful, watery, red, and sensitive to light. Corneal swelling makes vision hazy. However, the herpes infection doesn't respond to antibiotics, as a bacterial infection would, and often it continues to worsen.
Most often, the infection produces only mild changes in the cornea and goes away without treatment. Rarely, the virus deeply penetrates the cornea, destroying its surface. The infection may recur, further damaging the surface of the cornea. Several recurrences may result in ulceration, permanent scarring, and a loss of feeling when the eye is touched. The herpes simplex virus can also cause an increased growth of blood vessels, visual impairment, or total loss of vision.
A doctor may prescribe an antiviral drug such as trifluridine, vidarabine, or idoxuridine. These drugs are usually prescribed as an ointment or a solution to be applied to the eye several times a day. However, they're not always effective; sometimes, other drugs must be taken by mouth. Sometimes, to help speed healing, an ophthalmologist may have to gently swab the cornea with a soft cotton-tipped applicator to remove dead and damaged cells.
Herpes Zoster Infection
Herpes zoster is a virus that grows in nerves and may spread to the skin, causing shingles. (see page 918 in Chapter 186, Viral Infections) The condition doesn't necessarily threaten the eye, even when it appears on the face and forehead. But if the ophthalmic division of the fifth cranial nerve (trigeminal nerve) becomes infected, the infection is likely to spread to the eye. The infection produces pain, redness, and eyelid swelling. An infected cornea can become swollen and severely damaged and scarred. The structures behind the cornea can become inflamed, a condition called uveitis, and the pressure in the eye can increase, a condition called glaucoma. Common complications of a corneal infection include a lack of feeling when the cornea is touched and permanent glaucoma.
When herpes zoster infects the face and threatens the eye, early treatment with acyclovir taken by mouth for 7 days reduces the risk of eye complications. Corticosteroids, usually in eyedrops, may also help. Atropine drops are often used to keep the pupil dilated and help keep pressure in the eye from increasing. People over age 60 who are in generally good health may find that taking corticosteroids for 2 weeks helps prevent the pain that may occur after the herpes sores disappear; this pain is called postherpetic neuralgia.
Peripheral Ulcerative Keratitis
Peripheral ulcerative keratitis is an inflammation and ulceration of the cornea that often occurs in people who have connective tissue diseases such as rheumatoid arthritis.
The condition impairs vision, increases sensitivity to light, and produces a sensation of a foreign object trapped in the eye. The condition is probably caused by an autoimmune reaction. (see page 816 in Chapter 168, Immunodeficiency Disorders)
Of the people who have rheumatoid arthritis and peripheral ulcerative keratitis, about 40 percent die within 10 years of developing peripheral ulcerative keratitis unless they are treated. Treatment with drugs that suppress the immune system reduces the death rate to about 8 percent in 10 years.
Keratomalacia (xerophthalmia, xerotic keratitis) is a condition in which the cornea becomes dry and clouds over because of deficiencies in vitamin A, protein, and dietary calories.
The surface of the cornea dies, and corneal ulcers and bacterial infections may follow. The tear glands and conjunctiva are also affected, resulting in inadequate tear production and dry eyes. Night blindness (poor vision in the dark) may develop because of vitamin A deficiency. Antibiotic eyedrops or ointments can help cure infections, but correcting the vitamin A deficiency with oral vitamin supplements or malnutrition with an improved diet or supplements is more important.
Keratoconus is a gradual change in the shape of the cornea, causing it to resemble a cone.
The condition begins between ages 10 and 20. One or both eyes may be affected, producing major changes in vision and requiring frequent changes in the prescription for eyeglasses or contact lenses. Contact lenses often correct the vision problems better than glasses, but sometimes the change in corneal shape is so severe that contact lenses either can't be worn or can't correct vision. In extreme cases, corneal transplantation may be needed. (see box, page 833)
Bullous keratopathy is a swelling of the cornea that's most common in older people.
Occasionally, bullous keratopathy occurs after eye surgery, such as cataract surgery. The swelling leads to fluid-filled blisters on the surface of the cornea that can rupture, causing pain and impairing vision.
Bullous keratopathy is treated by reducing the amount of fluid in the cornea with salty solutions or soft contact lenses. Rarely, corneal transplantation is needed.