The eye is a slightly asymmetrical globe, about an inch in diameter. The front part of the eye (the part you see in the mirror) includes:
- » The iris (the pigmented part)
- » The cornea (a clear dome over the iris)
- » The pupil (the black circular opening in the iris that lets light in)
- » The sclera (the white part)
- » The conjunctiva (a thin layer of tissue covering the front of the eye, except the cornea)
Just behind the iris and pupil lies the lens, which helps to focus light on the back of the eye. Most of the eye is filled with a clear gel called the vitreous. Light projects through the pupil and the lens to the back of the eye. The inside lining of the eye is covered by special light-sensing cells that are collectively called the retina. The retina converts light into electrical impulses. Behind the eye, the optic nerve carries these impulses to the brain. The macula is a small extra-sensitive area within the retina that gives central vision. It is located in the center of the retina and contains the fovea, a small depression or pit at the center of the macula that gives the clearest vision.
Eye color is created by the amount and type of pigment in the iris. Multiple genes inherited from each parent determine a person’s eye color.
What Is Myopia ?
Myopia, or nearsightedness, is a refractive error, which means that the eye does not bend or refract light properly to a single focus to see images clearly. In myopia, close objects look clear but distant objects appear blurred. Myopia is a common condition that affects an estimated 25 percent of Americans. It is an eye focusing disorder, not an eye disease.
Myopia in children: Myopia is inherited and is often discovered in children when they are between ages eight and 12 years old. During the teenage years, when the body grows rapidly, myopia may become worse. Between the ages of 20 and 40, there is usually little change. Myopia can also occur in adults.
High myopia: If the myopia is mild, it is called low myopia. Severe myopia is known as high myopia. High myopia will usually stabilize between the ages of 20-30 years old. With high myopia, you can usually correct vision easily with glasses, contact lenses or sometimes with refractive surgery.
Patients with myopia have a higher risk of developing a detached retina. Ask your Eye Specialist to discuss the warning signs of retinal detachment with you if you are in this risk category. If the retina does detach and it is discovered early enough, a surgical procedure can usually repair it. It is important to have regular eye examinations by an ophthalmologist to watch for changes in the retina that might lead to retinal detachment.
What Is Hyperopia ?
Hyperopia (farsightedness), is a refractive error, which means that the eye does not bend or refract light properly to a single focus to see images clearly. In hyperopia, distant objects look somewhat clear, but close objects appear more blurred.
People experience hyperopia differently. Some people may not notice any problems with their vision, especially when they are young. For people with significant hyperopia, vision can be blurry for objects at any distance, near or far. It is an eye focusing disorder, not an eye disease.
What Is Astigmatism ?
Astigmatism is an imperfection in the curvature of your cornea — the clear, round dome covering the eye's iris and pupil — or in the shape of the eye's lens. Normally, the cornea and lens are smooth and curved equally in all directions, helping to focus light rays sharply onto the retina at the back of your eye. However, if your cornea or lens isn't smooth and evenly curved, light rays aren't refracted properly. This is called a refractive error.
When the cornea has an irregular shape, it is called corneal astigmatism. When the shape of the lens is distorted, you have lenticular astigmatism. As a result of either type of astigmatism, your vision for both near and far objects appears blurry or distorted. It's almost like looking into a fun house mirror in which you appear too tall, too wide or too thin.
In a normal eye, the cornea and lens focus light rays on the retina.
In astigmatism, images focus in front of and beyond the retina, causing both close and distant objects to appear blurry.
People can be born with astigmatism — in fact, most people probably are born with some degree of astigmatism — and they may have it along with other refractive errors: nearsightedness (myopia) orfarsightedness (hyperopia).
While adults with a higher degree of astigmatism may realize their vision isn't as good as it should be, children who have astigmatism symptomsmay not be aware they have this condition, and are unlikely to complain about blurred or distorted vision. But uncorrected astigmatism can seriously impact a child's ability to achieve in school and sports. And some forms of astigmatism can be a sign of keratoconus, a cone-like bulge of the cornea, which can distort vision. That's why it is crucial that children have regular eye exams to detect astigmatism or other vision problems as early as possible.
Astigmatism symptoms may include blurry vision or areas of distorted vision, eyestrain, headaches, squinting to try to see clearly or eye discomfort.
Having these symptoms may not necessarily mean that you have astigmatism, but they do indicate the need for a visit to your eye specialist for a complete eye exam.
What Is Keratoconus ?
Keratoconus is an uncommon condition in which the normally round, dome-like cornea (the clear front window of the eye) becomes thin and develops a cone-like bulge. Keratoconus literally means “cone-shaped cornea.”
The cornea is a very important part of your eye. Light enters the eye through the cornea, which refracts, or focuses, the light rays so that you can see clearly.
Left: normal cornea; right: cornea with keratoconus.
With keratoconus, the shape of the cornea is altered, distorting your vision. Keratoconus can make some activities difficult, such as driving, typing on a computer, watching television or reading.
What Is Lazy Eye ?
Amblyopia is poor vision in an eye that did not develop normal sight during early childhood. It is sometimes called "lazy eye."
When one eye develops good vision while the other does not, the eye with the poorer vision is called amblyopic. Usually, only one eye is affected by amblyopia, but it is possible for both eyes to be "lazy." This condition is called bilateral amblyopia.
The condition is common; approximately two or three out of every 100 people has amblyopia. The best time to correct amblyopia is during infancy or early childhood.
Amblyopia in children and adults :Newborn infants are able to see, but as they use their eyes during the first months of life, their vision improves. During early childhood years, their visual system changes quickly and their sight continues to develop.
In order to have normal vision, it is important that both eyes develop equal vision. If a child has amblyopia and cannot use his or her eyes normally, vision does not develop properly and may even decrease. After the first nine years of life, the visual system is normally fully developed and usually cannot be changed.
If amblyopia treatment is not begun as early as possible, several problems can develop that can seriously affect vision from childhood into adulthood:
- » the amblyopic eye may develop a serious and permanent visual defect
- » depth perception (seeing in three dimensions) may be lost, because good vision in both eyes is needed
- » if the stronger eye becomes diseased or injured, it can mean a lifetime of poor vision.
People with amblyopia in one eye are more than twice as likely to lose vision in the healthy eye from trauma. If the vision in one eye should be lost later in life from an accident or illness, it is essential that the other eye have normal vision.
Another important reason to make sure amblyopia is detected and treated as early as possible in childhood: people who have good vision in only one eye may find they are limited in the kinds of jobs they can perform.
Your ophthalmologist can teach you how amblyopia can be treated, and can help you and your child successfully carry out this treatment.
What is refractive (or anisometropic) amblyopia ?
Refraction is when the eye focuses light onto the retina to form a visual image. A refractive error occurs when the light is not properly focused in the eye and vision is blurry. When a child has refractive, or anisometropic, amblyopia, it means he or she has a different amount of refractive error in each eye. When this is the case, the brain will use the better-seeing eye and essentially "turn off" vision from the weaker eye. At first, eyeglasses may help by correcting the refractive error in both eyes, allowing them to work equally together. Then the amblyopia may be further treated to help improve vision and depth perception.
What Are Cataracts ?
Cataract is a clouding of the eye's lens. When we look at something, light rays travel into our eye through the pupil and are focused through the lens onto the retina, a layer of light-sensitive cells at the back of the eye. The lens must be clear in order to focus light properly onto the retina. If the lens has become cloudy, this is called a cataract.
Vision problems with cataracts : If your vision has become blurry, cloudy or dim, or things you see are not as bright or colorful as they used to be, a cataract may have developed in one or both of your eyes. Many people say that their vision with cataracts is similar to the effect of looking through a dirty car windshield.
As a cataract slowly begins to develop, you may not notice any changes in your vision at first. But as the cataract progresses, you may begin to find that it interferes with your daily activities. Performing a complete eye exam, your Eye Specialist can tell you whether cataract or another problem is the cause of your vision loss.
While cataracts are one of the most common causes of vision loss, especially as we age, they are treatable with cataract surgery. Since most cataracts are part of the normal aging process, they cannot be reversed. There are no medications or eye drops that will make cataracts go away—surgery is the only treatment.
A cataract may not need to be removed right away if your lifestyle isn't significantly affected. In some cases, simply changing your eyeglass prescription may help to improve your vision. Contrary to popular belief, a cataract does not have to be "ripe" to be removed. However, once you are diagnosed with a cataract, your ophthalmologist needs to monitor your vision regularly for any changes.
Cataract surgery for clearer vision : When a cataract causes bothersome vision problems that interfere with your daily activities, your ophthalmologist may recommend surgery to remove the cataract. With cataract surgery, your eye's cloudy natural lens is removed and replaced with a clear artificial lens implant (called an intraocular lens or IOL).
You and your ophthalmologist can discuss the cataract surgery procedure, preparation for andrecovery after surgery, benefits and possible complications of cataract surgery, cataract surgery costs and other important information. Together, you can decide if cataract surgery is appropriate for you.
What Is Glaucoma ?
Glaucoma is a disease that damages your eye's optic nerve. It usually happens when fluid builds up in the front part of your eye. That extra fluid increases the pressure in your eye, damaging the optic nerve.
It is estimated that three million Americans have glaucoma, but only about half of them know that they have glaucoma. Glaucoma is a leading cause of blindness for people over 60 years old. But blindness from glaucoma can often be prevented with early treatment. When glaucoma develops, usually you don’t have any early symptoms and the disease progresses slowly. In this way, glaucoma can steal your sight very gradually. Fortunately, early detection and treatment (with glaucoma eyedrops, glaucoma surgery or both) can help preserve your vision.
The optic nerve is connected to the retina — a layer of light-sensitive tissue lining the inside of the eye — and is made up of many nerve fibers, like an electric cable is made up of many wires. The optic nerve sends signals from your retina to your brain, where these signals are interpreted as the images you see.
In the healthy eye, a clear fluid called aqueous (pronounced AY-kwee-us) humor circulates inside the front portion of your eye. To maintain a constant healthy eye pressure, your eye continually produces a small amount of aqueous humor while an equal amount of this fluid flows out of your eye. If you have glaucoma, the aqueous humor does not flow out of the eye properly. Fluid pressure in the eye builds up and, over time, causes damage to the optic nerve fibers.
Open-angle glaucoma : The most common form of glaucoma is called primary open-angle glaucoma. It occurs when the trabecular meshwork of the eye gradually becomes less efficient at draining fluid. As this happens, your eye pressure, called intraocular pressure (IOP), rises. Raised eye pressure leads to damage of the optic nerve. Damage to the optic nerve can occur at different eye pressures in different patients. There is not one ‘right’ eye pressure that is the same for everyone. Your ophthalmologist (Eye M.D.) establishes a target eye pressure for you that he or she predicts will protect your optic nerve from further damage. Different patients have different target pressures.
Typically, open-angle glaucoma has no symptoms in its early stages and your vision remains normal. As the optic nerve becomes more damaged, blank spots begin to appear in your field of vision. You usually won’t notice these blank spots in your day-to-day activities until the optic nerve is significantly damaged and these spots become large. If all of the optic nerve fibers die, you will be blind.
Half of patients with glaucoma do not have high eye pressure when first examined. Eye pressure is not always the same – it rises and falls from day to day and hour to hour. So a single eye pressure test will miss many people who have glaucoma. In addition to routine eye pressure testing, it is essential that the optic nerve be examined by an ophthalmologist for proper diagnosis.
Normal-tension glaucoma : Eye pressure is expressed in millimeters of mercury (mmHg), the same unit of measurement used in weather barometers.
Although "normal" eye pressure is considered a measurement less than 21 mmHg, this can be misleading. Some people have a type of glaucoma called normal-tension, or low-tension glaucoma. Their eye pressure is consistently below 21 mmHg, but optic nerve damage and loss of vision still occur. People with normal-tension glaucoma are usually treated in the same way as people who have open-angle glaucoma.
Angle-closure glaucoma (also called "closed-angle glaucoma" or "narrow-angle glaucoma")
This type happens when someone’s iris is very close to the drainage angle in their eye. The iris can end up blocking the drainage angle. You can think of it like a piece of paper sliding over a sink drain. When the drainage angle gets completely blocked, eye pressure rises very quickly. This is called an acute attack. It is a true eye emergency, and you should call your ophthalmologist right away or you might go blind. People of Asian descent and those with hyperopia (farsightedness) tend to be more at risk for developing this form of glaucoma.
Symptoms of an acute attack include :
» Your vision is suddenly blurry
» You have severe eye pain
» You have a headache
» You feel sick to your stomach (nausea)
» You throw up (vomit)
» You see rainbow-colored rings or halos around lights
A closed-angle glaucoma attack is a medical emergency and must be treated immediately. Unfortunately, people at risk for developing closed-angle glaucoma often have few or no symptoms before the attack.
People at risk for closed-angle glaucoma should avoid over-the-counter decongestants and other medications where the packaging states not to use these products if you have glaucoma. These products are usually safe to use once your narrow angle has been treated with laser iridotomy. Always ask your ophthalmologist if it is safe for you to use products with this warning.
Congenital glaucoma :Congenital glaucoma is a rare type of glaucoma that develops in infants and young children and can be inherited. While less common than the other types of glaucoma, this condition can be devastating, often resulting in blindness if not diagnosed and treated early.
Secondary glaucoma :
Secondary glaucoma is glaucoma that results from another eye condition or disease. For example, someone who has had an eye injury, someone who is on long-term steroid therapy or someone who has a tumor may develop secondary glaucoma. The most common forms of secondary glaucoma are: pseudoexfoliative glaucoma, pigmentary glaucoma, and neovascular glaucoma.
Glaucoma suspect :
Some people have normal eye pressure but their optic nerve or visual field looks suspicious for glaucoma. These people must be watched carefully because some eventually develop definite glaucoma and need treatment.
Other people have an eye pressure that is higher than normal, but they do not have other signs of glaucoma, such as optic nerve damage or blank spots that show up in their peripheral (side) vision when tested. This condition is called ocular hypertension. Individuals with ocular hypertension are at higher risk for developing glaucoma compared to people with lower, or average, eye pressure. Just like people with glaucoma, people with ocular hypertension need to be closely monitored by an ophthalmologist to ensure they receive appropriate treatment.
What Is Diabetic Retinopathy?
Diabetic retinopathy, the most common diabetic eye disease, occurs when blood vessels in the retina change. Sometimes these vessels swell and leak fluid or even close off completely. In other cases, abnormal new blood vessels grow on the surface of the retina.
The retina is a thin layer of light-sensitive tissue that lines the back of the eye. Light rays are focused onto the retina, where they are transmitted to the brain and interpreted as the images you see. The macula is a very small area at the center of the retina. It is the macula that is responsible for your pinpoint vision, allowing you to read, sew or recognize a face. The surrounding part of the retina, called the peripheral retina, is responsible for your side or peripheral vision.
Diabetic retinopathy usually affects both eyes. People who have diabetic retinopathy often don't notice changes in their vision in the disease's early stages. But as it progresses, diabetic retinopathy usually causes vision loss that in many cases cannot be reversed.
Diabetic eye problems
Background or nonproliferative diabetic retinopathy (NPDR)
Nonproliferative diabetic retinopathy (NPDR) is the earliest stage of diabetic retinopathy. With this condition, damaged blood vessels in the retina begin to leak extra fluid and small amounts of blood into the eye. Sometimes, deposits of cholesterol or other fats from the blood may leak into the retina.
NPDR can cause changes in the eye, including:» Microaneurysms: small bulges in blood vessels of the retina that often leak fluid.
» Retinal hemorrhages: tiny spots of blood that leak into the retina.
» Hard exudates: deposits of cholesterol or other fats from the blood that have leaked into the retina.
» Macular edema: swelling or thickening of the macula caused by fluid leaking from the retina's blood vessels. The macula doesn't function properly when it is swollen. Macular edema is the most common cause of vision loss in diabetes.
» Macular ischemia: small blood vessels (capillaries) close. Your vision blurs because the macula no longer receives enough blood to work properly.
Many people with diabetes have mild NPDR, which usually does not affect their vision. However, if their vision is affected, it is the result of macular edema and macular ischemia. Watch how macular edema and macular ischemia affect your eyes.
Proliferative diabetic retinopathy (PDR) :
Proliferative diabetic retinopathy (PDR) mainly occurs when many of the blood vessels in the retina close, preventing enough blood flow. In an attempt to supply blood to the area where the original vessels closed, the retina responds by growing new blood vessels. This is called neovascularization. However, these new blood vessels are abnormal and do not supply the retina with proper blood flow. The new vessels are also often accompanied by scar tissue that may cause the retina to wrinkle or detach.
PDR may cause more severe vision loss than NPDR because it can affect both central and peripheral vision. PDR affects vision in the following ways:
Vitreous hemorrhage: delicate new blood vessels bleed into the vitreous — the gel in the center of the eye — preventing light rays from reaching the retina. If the vitreous hemorrhage is small, you may see a few new, dark floaters. A very large hemorrhage might block out all vision, allowing you to perceive only light and dark. Vitreous hemorrhage alone does not cause permanent vision loss. When the blood clears, your vision may return to its former level unless the macula has been damaged.
Neovascular glaucoma: if a number of retinal vessels are closed, neovascularization can occur in the iris (the colored part of the eye). In this condition, the new blood vessels may block the normal flow of fluid out of the eye. Pressure builds up in the eye, a particularly severe condition that causes damage to the optic nerve.
Watch how vitreous hemorrhage affects your eyes.
What Are Eye Allergies?
Eye allergies, called allergic conjunctivitis, are a common condition that occurs when the eyes react to something that irritates them (called an allergen). The eyes produce a substance called histamine to fight off the allergen. As a result, the eyelids and conjunctiva — the thin, filmy membrane that covers the inside of your eyelids and the white part of your eye (sclera) — become red, swollen and itchy, with tearing and burning. Unlike bacterial or viral conjunctivitis, allergic conjunctivitis is not spread from person to person.
People who suffer from eye allergies usually (though not always) have nasal allergies as well, with an itchy, stuffy nose and sneezing. It is usually a temporary (acute) condition associated with seasonal allergies. However, in other cases, eye allergies can develop from exposure to other environmental triggers, such as pet dander, dust, pollen, smoke, perfumes, or even foods. If the exposure is ongoing, the allergies can be more severe, with significant burning and itching and even sensitivity to light.
What Is Age-Related Macular Degeneration ?
Age-related macular degeneration (AMD) is a deterioration or breakdown of the eye's macula. The macula is a small area in the retina — the light-sensitive tissue lining the back of the eye. The macula is the part of the retina that is responsible for your central vision, allowing you to see fine details clearly.
The macula makes up only a small part of the retina, yet it is much more sensitive to detail than the rest of the retina (called the peripheral retina). The macula is what allows you to thread a needle, read small print, and read street signs. The peripheral retina gives you side (or peripheral) vision. If someone is standing off to one side of your vision, your peripheral retina helps you know that person is there by allowing you to see their general shape.
Many older people develop macular degeneration as part of the body's natural aging process. There are different kinds of macular problems, but the most common is age-related macular degeneration.
With macular degeneration, you may have symptoms such as blurriness, dark areas or distortion in your central vision, and perhaps permanent loss of your central vision. It usually does not affect your side, or peripheral vision. For example, with advanced macular degeneration, you could see the outline of a clock, yet may not be able to see the hands of the clock to tell what time it is.
Causes of macular degeneration include the formation of deposits called drusen under the retina, and in some cases, the growth of abnormal blood vessels under the retina. With or withouttreatment, macular degeneration alone almost never causes total blindness. People with more advanced cases of macular degeneration continue to have useful vision using their side, or peripheral vision. In many cases, macular degeneration's impact on your vision can be minimal.
When macular degeneration does lead to loss of vision, it usually begins in just one eye, though it may affect the other eye later.
Many people are not aware that they have macular degeneration until they have a noticeable vision problem or until it is detected during an eye examination.
Types of macular degeneration: dry macular degeneration and wet macular degeneration
There are two types of macular degeneration:
Dry, or atrophic, macular degeneration (also called non-neovascular macular degeneration) with drusen
Most people who have macular degeneration have the dry form. This condition is caused by aging and thinning of the tissues of the macula. Macular degeneration usually begins when tiny yellow or white pieces of fatty protein called drusen form under the retina. Eventually, the macula may become thinner and stop working properly.
With dry macular degeneration, vision loss is usually gradual. People who develop dry macular degeneration must carefully and constantly monitor their central vision. If you notice any changes in your vision, you should tell your ophthalmologist right away, as the dry form can change into the more damaging form of macular degeneration called wet (exudative) macular degeneration. While there is no medication or treatment for dry macular degeneration, some people may benefit from a vitamin therapy regimen for dry macular degeneration.
Using an Amsler grid to test for macular degeneration :If you have been diagnosed with dry macular degeneration, you should use a chart called an Amsler grid every day to monitor your vision, as dry macular degeneration can change into the more damaging wet form.
To use the Amsler grid, wear your reading glasses and hold the grid 12 to 15 inches away from your face in good light.
» Cover one eye.
» Look directly at the center dot with the uncovered eye and keep your eye focused on it.
» While looking directly at the center dot, note whether all lines of the grid are straight or if any areas are distorted, blurry or dark.
» Repeat this procedure with the other eye.
» If any area of the grid looks wavy, blurred or dark, contact your ophthalmologist.
» If you detect any changes when looking at the grid, you should notify your ophthalmologist immediately.
Wet, or exudative, macular degeneration (also called neovascular macular degeneration)
About 10 percent of people who have macular degeneration have the wet form, but it can cause more damage to your central or detail vision than the dry form.
Wet macular degeneration occurs when abnormal blood vessels begin to grow underneath the retina. This blood vessel growth is called choroidal neovascularization (CNV) because these vessels grow from the layer under the retina called the choroid. These new blood vessels may leak fluid or blood, blurring or distorting central vision. Vision loss from this form of macular degeneration may be faster and more noticeable than that from dry macular degeneration.
The longer these abnormal vessels leak or grow, the more risk you have of losing more of your detailed vision. Also, if abnormal blood vessel growth happens in one eye, there is a risk that it will occur in the other eye. The earlier that wet macular degeneration is diagnosed and treated, the better chance you have of preserving some or much of your central vision. That is why it is so important that you and your ophthalmologist monitor your vision in each eye carefully.
What Is Dry Eye?
When you blink, a film of tears spreads over the eye, making the surface of the eye smooth and clear. Without this tear film, good vision would not be possible.
Sometimes people don't produce enough tears or the right quality of tears to keep their eyes healthy and comfortable. This condition is known as dry eye.
The tear film consists of three layers:
» An oily layer
» A watery layer
» A layer of mucus
Each layer has its own purpose. The oily layer, produced by the meibomian glands, forms the outermost surface of the tear film. Its main purpose is to smooth the tear surface and reduce evaporation of tears.
The middle watery layer makes up most of what we ordinarily think of as tears. This layer, produced by the lacrimal glands in the eyelids, cleanses the eye and washes away foreign particles or irritants.
The inner layer consists of mucus produced by the conjunctiva. Mucus allows the watery layer to spread evenly over the surface of the eye and helps the eye remain moist. Without mucus, tears would not stick to the eye.
Normally, the eye constantly bathes itself in tears. By producing tears at a slow and steady rate, the eye stays moist and comfortable.
The eye uses two different methods to produce tears. It can make tears at a slow, steady rate to maintain normal eye lubrication. It can also produce a lot of tears in response to eye irritation or emotion. When a foreign body or dryness irritates the eye, or when a person cries, excessive tearing occurs.
It may not sound logical that dry eye would cause excess tearing, but think of it as the eye's response to discomfort. If the tears responsible for maintaining lubrication do not keep the eye wet enough, the eye becomes irritated. Eye irritation prompts the gland that makes tears (called the lacrimal gland) to release a large volume of tears, overwhelming the tear drainage system. These excess tears then overflow from your eye.
Conjunctivitis: What Is Pink Eye?
Conjunctivitis is the term used to describe swelling (inflammation) of the conjunctiva — the thin, filmy membrane that covers the inside of your eyelids and the white part of your eye (known as the sclera). Often this condition is called "pink eye."
The conjunctiva, which contains tiny blood vessels, produces mucus to keep the surface of your eye moist and protected. When the conjunctiva becomes irritated or swollen, the blood vessels become larger and more prominent, making your eye appear red. Signs of pink eye may occur in one or both eyes.
There are three types of conjunctivitis:
Bacterial conjunctivitis :
This is a highly contagious form of pink eye caused by bacterial infections. This type of conjunctivitis usually causes a red eye with a lot of pus.
Viral conjunctivitis :
The most common cause of pink eye is the same virus that causes the common cold, and is also very contagious.
Allergic conjunctivitis :
This form of conjunctivitis is caused by the body's reaction to an allergen or irritant. It is not contagious.