Dry eye is a syndrome in which eyes become itchy, painful, and irritated because the eye either produces too few tears or the tears evaporate from the surface of the eye too quickly. Dry eye is different from an eye allergy, which may cause similar symptoms but is the result of the eye having an allergic reaction to something in the environment.
Tears are composed of a film made up of three layers produced by three different types of cells. The outermost layer that interfaces with the air is produced by the meibomian glands. This layer contains lipids (oils). Its main function is to prevent tears from evaporating too rapidly. The middle layer is the aqueous layer. It is mainly water, a few salts, and some proteins. This layer is produced by the lachrymal glands and makes up about 90% of the volume of tears. The main function of the aqueous layer is to keep the eyes clean and wash away any foreign particles that get on the surface of the eye. The innermost layer is called the mucin layer. It is produced by the goblet cells of the conjunctiva (the white part of the eye). The function of the mucin layer is to help tears spread evenly across the eye. Dry eye can develop if there are defects in the production of any of these three layers. Generally both eyes are affected.
Dry eye is a common condition, especially in people over age 40. It occurs more frequently in women than in men. It is especially common in postmenopausal women who are taking estrogen hormones. Dry eye occurs with approximately equal frequency worldwide, affecting between 10% and 30% of the population. In 2005, it was estimated that between 10 and 14 million Americans experienced moderate to severe dry eye with perhaps 20 million more having mild dry eye.
Causes and symptoms
As people age, they naturally produce fewer tears. Dry eye can develop because a normal volume of tears forms but they evaporate too quickly, because the eye makes an inadequate volume of tears, or because a normal volume of tears spreads unevenly over the eye leaving dry patches.
Evaporative dry eye
Evaporative dry eye occurs most often because of meibomian gland dysfunction (MGD). In MGD, the meibomian gland does not make enough material to protect the aqueous layer from evaporation. Ocular rosacea and blepharitis can also interrupt the flow of material from the meibomian glands to the surface of the eye. Abnormalities of the eyelid that prevent tears from being spread adequately across the eye can cause evaporative dry eye. Abnormalities can be structural or the result of disorders such as Bell's palsy (paralysis of the face muscles). If the mucin layer is defective, tears will not spread evenly, and dry patches may develop.
Tear-deficient dry eye
Dry eye caused by inadequate tear production can be the result of connective tissue diseases such as Sjögren's syndrome, rheumatoid arthritis , or systemic lupus erythematosus (SLE). Other causes of inadequate tear production may include vitamin A deficiency, previous herpes eye infection, excessive contact lens wear, diabetes, trauma to the eye such as a chemical burn, laser-assisted in-situ keratomileusis (LASIK) surgery, and natural aging. Medications that can decrease tear production and cause dry eye include diuretics, antihistamines, beta blockers , oral contraceptives, tricyclic antidepressants , and opiate painkillers such as morphine and codeine.
Environmental factors that aggravate dry eye include:
- forced hot air heat
- long-term exposure to windy environments
- low humidity environments (the desert, an airplane cabin)
- air pollution (cigarette smoke, industrial pollution)
- activities that reduce blinking (reading, prolonged computer use, driving, watching television or anything that interferes with a normal blinking rate of about 13 blinks per minute)
Symptoms of dry eye tend to be similar regardless of the cause. These include:
- stinging or brining eyes
- itchy eyes
- a feeling that foreign material is in the eye
- mucus or crust around the eyes, especially when waking
- eye strain after short periods of reading or computer work
- difficulty wearing contact lenses comfortably
- excessive sensitivity to light
Oddly, people who have dry eye may have excessive tearing. This occurs because there are two control mechanisms for tear production. One produces tears in a slow, steady, automatic way people are not conscious of. The other produces tears in response to irritation in the eye or emotions. These are called reflex tears. When the eye is irritated because it is dry, the production of reflex tears kicks in to try to remove the irritation. If enough reflex tears are produced, they overflow the eyes. Reflex tears are mostly water, and for this reason they do not help alleviate dry eye.
Diagnosis of dry eye can be made by an eye care professional. The doctor begins with a complete eye examination . In addition, the patient may be asked to respond to a questionnaire that helps to quantify dry eye symptoms. Two frequently used diagnostic questionnaires are the Ocular Surface Disease Index, which asks patients to rate 12 different symptoms on a scale of one to four, and the McMonnies & Ho Dry Eye Questionnaire.
Based on the results of the examination and questionnaire, two other tests may be performed. The Schirmer's I test measures the quantity of tears produced. A narrow strip of filter paper is placed at the lower eyelid and the distance moisture wicks along the filter paper in a specific period of time (usually about 5 minutes) is measured. This gives an indication of the quantity of tears produced.
The fluorescein break up time (FBUT) measures how fast tears evaporate from the surface of the eye. A bit of fluorescein dye is put in the eye. The patient blinks a few times and then stares straight ahead without blinking. Under a special light, the dye makes the tear layer visible. The doctor then times with a stopwatch how long it takes the tear layer to break up and evaporate. A variation on this test is the noninvasive break up time, in which no dye is used, but with special equipment, tear thinning and tear break up can be observed and timed.
There are four aspects to treating dry eye syndrome. First, environmental factors that aggravate dry eye should be eliminated or minimized. This may mean using a humidifier in the home or at work, wearing glasses instead of contact lenses , avoiding exposure to cigarette smoke, and positioning computer monitors below eye level. This monitor placement causes the eyelid to cover part of the eye and helps slow evaporation.
The second aspect of treatment involves replenishing tears. There are many types of sterile artificial tears, but they break down into a two main categories: artificial tears with preservatives and artificial tears without preservatives. Artificial tears with preservatives are less expensive, but can be used less often (usually up to four times daily) than artificial tears without preservatives. Some preservative-free tears can be used as often as once an hour. Other artificial tears contain oils that help slow evaporation. Artificial tears are also available in gel form. No single type of artificial tear is best for every person with dry eye. The selection of an artificial tear should be made in consultation with an eye care professional.
QUESTIONS TO ASK YOUR DOCTOR
- What type of artificial tears are best for me?
- Do I have underlying health problems that are aggravating my dry eye?
- With the treatment you recommend, how long should it take to feel an improvement in symptoms?
- If symptoms do not improve, what is the next step in treatment?
- Am I at risk for vision loss from dry eye?
- What lifestyle changes should I make to help relieve my symptoms?
- Do I have any problems that require referral to a specialist?
- Should I schedule a follow-up visit, and if so, when?
The third aspect of treatment involves preserving tears the eye makes naturally. Tears normally drain into the nasal cavity. A surgical procedure can insert tiny silicon plugs, called punctal plugs, into the drainage channel so that the tears stay in the eye. One drawback is that these plugs can shift position and lose their effectiveness. A non-surgical way of preserving tears is to sleep in swimming goggles or use windless sunglasses during the day, both of which create a moisture chamber.
The fourth aspect of treatment is to correct anatomical problems with the eyelids, treat eye infections, and treat underlying diseases or disorders that may cause or aggravate dry eye. Adjustments to medication may also improve dry eye.
Drinking at least six to eight glasses of water daily helps the body stay hydrated so that fluid is available for tear production. Diuretics such as coffee and alcohol may worsen dry eye.
Beta blocker —An anti-hypertensive drug that limits the activity of epinephrine, a hormone that increases blood pressure.
Blepharitis —An inflammation of the eyelids that causes crusting, swelling, and burning or irritation.
Fluorescein dye —An orange dye used to make visible various structures in the eye.
Ocular rosacea —Rosacea is a disorder that causes the face to flush and red rash-like breakouts on the skin. Ocular rosacea causes the eyes to become red, irritated, and sometimes painful.
Sjögren's syndrome —An autoimmune disease in which the immune system of the body destroys tear glands and salivary glands. Symptoms include dry eye and dry mouth.
Systemic lupus erythematosus (SLE) —A chronic, inflammatory, autoimmune disorder in which the individual's immune system attacks, injures, and destroys the body's own organs and tissues. It may affect many organ systems including the skin, joints, lungs, heart, and kidneys.
Research has shown that omega-6 fatty acids and omega-3 fatty acids are important in preventing dry eye. Most Americans get enough omega-6 fatty acid in their regular diet , but most do not get enough omega-3 fatty acid. Omega-3 fatty acid is found mainly in fish and flaxseed oil. Eating more fish or taking an omega-3 dietary supplement may help to prevent dry eye.
Most people who seek treatment for symptoms of dry eye find relief in treatment. Dry eye may lead to some loss of visual acuity (sharpness), but rarely leads to blindness. Severe, prolonged, untreated dry eye can cause ulcers on the cornea (the clear covering of the eye), but this complication is rare.
Many people develop dry eye as a natural part of aging. Controlling environmental factors that aggravate dry eye is the best method of prevention. Seeking prompt treatment can control uncomfortable symptoms.
Caregivers should be alert to any signs of mucus or crust around the eyes of the people in their care or to any complaints of itchy, uncomfortable eyes. All people age 65 and older should have a complete eye examination once a year. At that time, the eye care professional can check for dry eye.
Gordon, Sandra. The Aging Eye. New York: Fireside, 2001.
Latkany, Robert. The Dry Eye Remedy: The Complete Guide to Restoring the Health and Beauty of Your Eyes. New York: Hatherleigh Press, 2007.
“Dry Eye Information Center.” Allergan, Inc. [cited February 18, 2008]. http://www.dryeye.com.
“Dry Eyes.” Mayo Clinic. June 14, 2006 [cited February 18, 2008]. http://www.mayoclinic.com/health/dry-eyes/DS00463.
“Dry Eyes.” The Eye Digest. June 17, 2007 [cited February 18, 2008]. University of Illinois Eye & Ear Infirmary. http://www.agingeye.net/dryeyes/dryeyesinformation.php.
Haddrill, Marilyn. “Ten Warning Signs of Age-Related Eye Problems.” All About Vision. September 2007 [cited April 6, 2008]. http://www.allaboutvision.com/over60/warning-signs.htm.
Heiting, Gary. “How Your Vision Changes as You Age.”All About Vision. February 2008 [cited April 6, 2008]. http://www.allaboutvision.com/over60/vision-changes.htm.
American Optometric Association, 243 N. Lindbergh Blvd., St. Louis, MO, 63141, (800) 365-2219, http://www.aao.org.
EyeCare America Foundation of the American Academy of Ophthalmology, P.O. Box 429098, San Francisco, CA, 94142-9098, (877) 887-6327, (800) 324-EYES (3937), (415) 561-8567, [email protected], http://www.eyecareamerica.org.
Tish Davidson AM