Nearsightedness (Myopia) - Surgery

earsightedness (Myopia) - Surgery
Several types of surgery for nearsightedness (myopia) can change the shape of the cornea and refocus light directly on the retina . But surgery cannot correct pathological myopia.

The goal of surgery is to allow people who are nearsighted to see clearly without corrective lenses or to be less dependent on corrective lenses. Most doctors consider 20/40 vision or better after surgery a satisfactory result. People with 20/40 vision or better are allowed to drive a car without corrective lenses. Overall, most people who have surgery achieve 20/40 vision or better after surgery.

Laser surgeries include:

• LASIK (laser in-situ keratomileusis), which is currently the most commonly used corrective surgery for nearsightedness. LASIK has a high success rate and low complication rate for low to moderate nearsightedness. It may also be used to correct more severe nearsightedness. In general, it requires less healing time, is less painful, and offers a faster visual recovery than photorefractive keratectomy (PRK).
• PRK (photorefractive keratectomy), LASEK (laser epithelial keratomileusis), and epi-LASIK (epithelial laser in-situ keratomileusis), which are similar surgeries. They are another type of corrective surgery for nearsightedness that is used frequently. Like LASIK, these surgeries have high success rates and low complication rates for low to moderate nearsightedness. They may be used instead of radial keratotomy (RK) to correct severe myopia.

RK (radial keratotomy)

Radial keratotomy (RK) is very successful and safe for people who have mild to moderate nearsightedness (less than 3 diopters). Although it is still used in some cases, it has been replaced in most cases by LASIK and PRK.

Corneal ring implants

This type of surgery to correct mild nearsightedness was approved by the U.S. Food and Drug Administration (FDA) in 2004. It uses corneal ring implants to change the shape of the cornea.

Intraocular lens implants

Intraocular lens implants (IOLs) may be used to correct severe myopia. IOLs, also called implantable contact lenses (ICLs), have been shown to be a safe and effective means of correcting moderate to severe myopia.⁴

Surgery Choices

Laser surgery for nearsightedness changes the shape of the cornea to refocus light directly on the retina. There are three main types of laser surgery used to treat nearsightedness.
    LASIK makes a small flap in the cornea and removes some of the tissue exposed by the flap.

    PRK, LASEK, and epi-LASIK use a laser to reshape the cornea. The doctor does not need to make a flap in the cornea first.

    RK uses several incisions on the cornea to change the curve of the cornea over the pupil.
Other types of surgery for nearsightedness involve the surgical placement of implants to either change the shape of the cornea or replace or assist the eye's natural lens.
    Corneal ring implants are clear pieces of plastic that can be surgically implanted to flatten the cornea and reduce nearsightedness.

    Intraocular lens implants replace the eye's natural lens with a clear plastic implant.

    Of the three main types of refractive surgery, RK has been in use the longest time. But PRK and LASIK are now more commonly used. Studies suggest that laser surgery such as PRK and LASIK is at least as accurate as radial keratotomy (RK) and has less risk of overcorrection or troublesome side effects. But some people may choose to wait for more information about the long-term outcomes of laser surgery before they decide to have it. If you find that glasses or contact lenses are inconvenient, surgery to correct nearsightedness may be appropriate. But nearsightedness is not a disease, and a nearsighted eye is otherwise normal and healthy. Weigh your desire to have clear vision without glasses or contacts against the risks of surgery. Ask your eye doctor any questions about surgery that you have (for example, what are the risks, benefits, possible outcomes, and alternatives to surgery) so that you have a good understanding of your options and can make the best decision. To find out more about each surgery, use this surgery information form.(What is a PDF document?) Some people may still need to wear glasses or contact lenses after surgery.

    Should I have laser surgery for nearsightedness?

     

Cure Myopia Naturally

Cure Myopia Naturally – Is It REALLY Possible?

Got myopia? Think glasses are your only hope? You can cure myopia naturally, just nobody wants to tell you. We are brainwashed to believe that it’s all “genetic”, but let’s have a look at the facts.

We have unnatural and incorrect diets. Prehistoric people didn’t have supermarkets or McDonalds. They lived off from the land. None of our ancestors had supermarkets and fast food restaurants either.

They grew their own vegetables, baked their own bread and killed their own meat. We’re not really adapted to doing all that today, but we could still eat a natural diet.

 The other factor which our ancestors did not have was TV and computers. The kids didn’t spend all day in front of the boob tube or playing computer games. They were out working and getting exercise.

Myopia – The Worldwide Phenomenon

Myopia is becoming a worldwide condition. No one race seems more susceptible than another.  For many years it was thought that myopia was caused by reading. Because the myopia rate was lower in the population with no formal education and didn’t read books. So, wearing glasses was a sign of intelligence.

They didn’t even realize that the people in the two groups ate different diets.  The people with no formal education were the poor people and they ate food they grew and hunted.  The formal education group could afford to eat more processed foods.

So we are led to believe that we have to wear glasses or butcher our eyes rather than cure myopia naturally…

Cure Myopia Naturally – The Environmental Factors

Studies have shown that when groups of people left primal living behind and moved on to a more urbanized existence, myopia rates increased within a single generation. This happened too quickly to be genetic.  Certain hunter-gatherer tribes have very low rates of myopia. But when given a western diet of highly refined carbs, their myopic rate was equal to those who had never eaten a natural diet.

It has been argued that the different lifestyle between our long ago ancestors and the people of today could also be based on the fact that we do a lot more reading, TV watching and computer work.  So, the diet cannot be the deciding factor.

However, the diets that are high in proteins and fats and low in carbohydrates don’t produce a spike in blood glucose and insulin levels in our bloodstream.   We know that high glucose levels leads to diabetes and diabetes raises havoc with our eyesight.  When societies that previously had low myopic rates, changed their lifestyles and introduced a high carbohydrate diet, they rapidly developed myopia rates that equaled or exceeded those in western societies.

So changing the life-style back, in the frame-work of today’s society, will cure myopia naturally.

Myopia, Genetics & The Twin Research

Christopher Hammond conducted a study in the Twin Research and Genetic Epidemiology Unit of St. Thomas’ Hospital in London (which was published in the American Journal of Human Genetics). He compared 506 pairs of unselected twins.  This study inferred the heritability of refractive error.

Refractive error is a common complex trait measured on a continuous scale, with myopia affecting  25-61% of the population. This is taking environment out of the equation and just using genes as a measurement.  The study has identified several genes that may be involved including PAX6. This one is known to be important in the development of the eye.

Even with this study, overwhelming evidence points to the fact that our unhealthy lifestyle is the leading cause of myopia.  An astounding thing came to light recently that confirmed this.  Dogs don’t read, watch TV or use the computer.  Wild dogs do not have myopia but some domesticated dogs do.

Why, because wild dogs eat meat and domesticated dogs are fed high carb based dog food and table scraps.  Exactly the same pattern found between our hunter-gatherer ancestors and our own couch-potato, high carb lifestyle.  Maybe when we have to start buying glasses for our pets, we will change our not so healthy habits, and that’s cure myopia naturally.

Pathological Myopia

Understanding Vision Loss from
Pathological Myopia

 

Pathological or Degenerative Myopia is quite different from the simple refractive myopia or nearsightedness that affects so many people around the world. Pathological myopia is an extremely high amount of nearsightedness that causes a major alteration of the shape or globe of the eye, which may lead to profound vision loss. In the United States, it occurs in about 2% of the population and is the seventh leading cause of legal blindness. It occurs most often in those of Chinese, Japanese, Middle Eastern, or Jewish descent. 

Thirty percent of all cases of pathological myopia occur at birth.  Sixty percent of patients are diagnosed with the condition between the ages 6 and 13, and it continues to progress throughout life.  Early examinations of children at risk are crucial, for the failure to detect high myopia in young children may lead to further vision loss from amblyopia.  

Associations with Other Conditions

  Pathological myopia has been associated with other ocular and systemic diseases. These conditions include Down’s Syndrome, Ocular Albinism, Infantile Glaucoma, Marfan’s Syndrome, Retinopathy of Prematurity, Ehler’s-Danlos Syndrome, low birth weight, and maternal alcoholism.  Patients with these diseases or conditions should be considered “at risk” for pathological myopia and carefully monitored.

Staphylomas

Pathological myopia causes the eye to elongate, which in turn stretches and thins the retina and the sclera of the eye. This leads to a bulging of the posterior portion of the eyeball. This condition is called a staphyloma.  By the age of sixty, 50% of patients with staphylomas in both eyes will be legally blind. 

Myopic Macular Degeneration

As the eyeball grows and stretches, it may also cause an area of atrophy and/or cracks in the layers under the retina. These cracks can serve as conduits for abnormal blood vessels to grow under the retina. These vessels can hemorrhage and scar which is similar to what happens in wet macular degeneration. The onset of the nets of abnormal blood vessels, called Fuch’s spots, often occur in the 4^th to 6^th decades of life.  Approximately 5% of pathological myopia patients develop Fuch’s spots, which lead to damage in the macular region of the eye and a subsequent loss of central vision.       

Retinal Detachment

The thinning of the layers of the eye may lead to degenerative changes in the peripheral retina including retinal holes and lattice degeneration which puts one at high risk for a retinal detachment.  If retinal breaks develop, fluid may leak behind the retina causing it to detach. The risk of a retinal detachment increases significantly as the amount of myopia increases. It is crucial for all highly myopic patients to undergo dilated retinal examinations every year.

Glaucoma

The incidence of glaucoma may increase due to alterations of the drainage angle of the eye causing which causes an increase in the fluid pressure of the eye. Detection of glaucoma may be more difficult since highly myopic patients have thicker, more rigid corneas, which can be interpreted as higher eye pressures even when the pressure is not really elevated. The thickness of the patient’s eyeglass lenses may impair visual fields testing, which is imperative in monitoring glaucoma. If the patient wears contact lenses, more accurate visual fields can be obtained.

High Refractive Error

Pathological myopia patients may present with prescriptions from –6 diopters to in excess of -40 diopters. This creates problems in correcting the patient’s refractive error. The lenses may be extremely thick and heavy. Fortunately, we are able to use small eyeglass frames combined with high index lenses to make the lenses thinner, lighter and more cosmetic. 

Contact Lenses

Contact lenses have been a good option for high myopes for many years. It eliminates the weight and thickness of the lenses in eyewear. It also eliminates the side vision difficulties inherent in these thick lenses. Myopic patients usually report having better vision when wearing contact lenses, because of eliminating the problems that the eyewear causes.

Refractive Surgery / LASIK

LASIK or laser refractive surgery has not been as effective in the highly myopic corrections, as it has in the lower ranges of myopia. An alternative for the profoundly myopic patients is a “Bear Claw” intraocular lens implant. It is affixed in the anterior chamber through a simple incision and can correct extreme amounts of myopia.  In some cases, a clear lens extraction may be performed. The procedure is identical to a cataract extraction. The patient’s lens is removed, but an intraocular lens in not inserted. By removing the lens, about fifteen diopters of myopia is automatically corrected.

Low Vision Considerations

Pathological myopia patients who wear eyeglasses may remove them to read. By removing their eyeglasses, it makes them automatically in focus at a few inches from their face, creating a significant amount of magnification. Most patients respond well to low vision care and low vision aids including magnifiers and CCTVs. These patients often present with mild photophobia and benefit from sunglasses. Patients with retinal detachments, myopic macular degeneration and staphylomas may have a poorer prognosis depending upon the location of the problem.

   

Safety Issues – Physical Education

Patients with pathological myopia have thinned retinas that can easily develop tears, holes and retinal detachments.  Patients should not participate in any physical activities that can lead to jostling or trauma to the eye. Contact sports and ball sports are too dangerous. Even activities like jumping rope or volleyball can lead to a retinal detachment. Children should not participate in physical education or competitive sports without the prior review and approval of their eye doctor.

Knowing the Signs of Retinal Detachment

Every patient with pathological myopia must know the signs of a retinal detachment. These include the sudden appearance of flashes of lights, like lighting flashes. Also floaters, little shadowy dots, and/or cobwebs, shadowy strands could be the first sign of an impending retinal detachment or a tear of the retina. If you experience the sudden onset of both flashes and floaters at the same time, the risk of detachment is extremely high.  In a retinal detachment, you may notice impairment in one area of your vision described like a curtain coming down. This can occur from any direction including the right or left side or from the top or bottom of the vision. You should seek an immediate retinal examination if any of these symptoms occur.  

Regular Retinal Monitoring

The best way to reduce the risk of vision loss is to have regular dilated retinal examinations, and to seek immediate eye care if you have any flashes, floaters or decrease in your vision.

Eye Anatomy

Eye Anatomy Introduction

Even though the eye is small, only about 1 inch in diameter, it serves a very important function - your sense of sight. Vision is arguably the most used of the 5 senses and is one of the primary means that we use to gather information from our surroundings.

The eye is often compared to a camera. Each gathers light and then transforms that light into a “picture.” Both also have lenses to focus the incoming light. A camera uses the film to create a picture, whereas the eye uses a specialized layer of cells, called the retina, to produce an image.

Orbit

The orbit is the eye socket, which is formed by the cheekbone, the forehead, the temple, and the side of the nose. The eye is cushioned within the orbit by pads of fat.


The orbit also contains the lacrimal gland that is located underneath the outer portion of the upper eyelid. The lacrimal gland produces tears that help lubricate and moisten the eye, as well as flush away any foreign matter that may enter the eye. The tears drain away from the eye through the nasolacrimal duct, which is located at the inner corner of the eye.

Eyelids and Eyelashes

The eyelids serve to protect the eye from foreign matter, such as dust, dirt, and other debris, as well as bright light that might damage the eye. When you blink, the eyelids also help spread tears over the surface of your eye, keeping the eye moist and comfortable.

The eyelashes help filter out foreign matter, including dust and debris, and prevent it from getting into the eye.
The conjunctiva is a thin, clear layer of skin covering the front of the eye, including the sclera and the inside of the eyelids. The conjunctiva keeps bacteria and foreign material from getting behind the eye.

Sclera

The white part of your eye that you see when you look at yourself in the mirror is the front part of the sclera. However, the sclera, a tough, leather-like tissue, also extends around the eye. Just like an eggshell surrounds an egg and gives an egg its shape, the sclera surrounds the eye and gives the eye its shape.

The sclera is also attached to the extraocular muscles, which, in turn, move the eye left and right, up and down, and diagonally.

Cornea

The cornea is a clear layer at the front and center of the eye. In fact, the cornea is so clear that you may not even realize it is there. The cornea is located just in front of the iris, which is the colored part of your eye. The main purpose of the cornea is to help focus light as it enters the eye. If you wear contact lenses, the contact lens rests on your cornea.

Anterior Chamber

The anterior chamber is the fluid-filled space immediately behind the cornea and in front of the iris. The fluid that fills this chamber is called the aqueous humor. The aqueous humor helps to nourish the cornea and the lens.

Iris and Pupil

The iris, which is the colored part of your eye, controls the amount of light that enters the eye. The iris is a ring shaped tissue with a central opening, which is called the pupil.

The iris has a ring of muscle fibers around the pupil, which, when contracted, causes the pupil to constrict (become smaller) in bright light. Another set of muscle fibers radiate outward from the pupil, which causes the pupil to dilate (become larger) in dim light or darkness.

Anterior Chamber Angle/Trabecular Meshwork

The anterior chamber angle and the trabecular meshwork are located where the cornea meets the iris. The trabecular meshwork is important because it is the site where the aqueous humor drains out of the eye. If the aqueous humor cannot properly drain out of the eye, the pressure can build up inside the eye, causing optic nerve damage and eventually vision loss, a condition known as glaucoma.

Posterior Chamber

The posterior chamber is the fluid-filled space immediately behind the iris but in front of the lens. The fluid that fills this chamber is called the aqueous humor. The aqueous humor helps to nourish the cornea and the lens.

Anterior Chamber

The anterior chamber is the fluid-filled space immediately behind the cornea and in front of the iris. The fluid that fills this chamber is called the aqueous humor. The aqueous humor helps to nourish the cornea and the lens.

Lens

The lens is a clear, flexible structure that is located just behind the iris and the pupil. A ring of muscular tissue, called the ciliary body, surrounds the lens. Together, the lens and the ciliary body help control fine focusing of light as it passes through the eye.

Vitreous Cavity

The vitreous cavity is located behind the lens and in front of the retina. It is filled with a gel-like fluid, called the vitreous humor. The vitreous humor helps maintain the shape of the eye.

Retina/Macula/Choroid

The retina acts like the film in a camera to create an image. To do this, the retina, a specialized layer of cells, converts light signals into nerve signals. After light signals are converted into nerve signals, the retina sends these signals to the optic nerve, which carries the signals to the brain. There, the brain helps process the image.

The retina is primarily made up of 2 distinct types of cells: rods and cones. Rods are more sensitive to light; therefore, they allow you to see in low light situations but do not allow you to see color. Cones, on the other hand, allow you to see color but require more light.
The macula is located in the central part of the retina. It is the area of the retina that is responsible for giving you sharp central vision.

The choroid is a layer of tissue that separates the retina and the sclera. It is mostly made up of blood vessels. The choroid helps nourish the retina.

Optic Nerve

The optic nerve, a bundle of over 1 million nerve fibers, is responsible for transmitting nerve signals from the eye to the brain. These nerve signals contain information on an image for processing by the brain. The front surface of the optic nerve, which is visible on the retina, is called the optic disk.

Extraocular Muscles

Six extraocular muscles are attached to each eye to help move the eye left and right, up and down, and diagonally.

Eye Pain

Eye Pain: Causes, Symptoms, Diagnosis, and Treatment

Nearly everyone suffers from eye pain or sore eyes at some point in life. Eye pain may be something that resolves on its own. It also can be a sign of something more serious. WebMD helps sort out the different causes of eye pain, their symptoms, and treatments.

What Causes Eye Pain?

A feeling of discomfort or pain in the eye can be caused by a problem in the eye itself. It can also be caused by a problem involving any of the structures around the eye. The pain might stem from problems with any of the following:

• Cornea -- the clear window in the front of the eye that does most of the focusing of incoming light
• Sclera - the white outside wall of the eye
• Conjunctiva -- the ultrathin covering of the sclera and the inside lining of the eyelid
• Iris -- the colored part of the eye, with the pupil in the middle
• Orbit -- the bony cave in which the eye and eye muscles are located
• Muscles of the eye (extraocular muscles) -- which rotate the eye
• Nerves -- which carry visual information from the eyes to the brain
• Eyelids -- which protect and continually spread moisture over the eyes

Problems can include:

• Corneal abrasions. The cornea is the clear disc covering the colored part of the eye, known as the iris. The cornea is vulnerable to injuries from children's flying fingers, errant tree branches, or tennis balls. A scratch on the cornea is called an abrasion. It can be very painful.
• Corneal infections. The cornea can also become inflamed or infected, a condition called keratitis. Herpes zoster, or shingles, which is caused by the same virus that causes chickenpox, can involve the cornea.
• Conjunctivitis is inflammation of the conjunctiva, the ultrathin lining over the front of the eye and on the inner side of the eyelids. Allergies or infections (viral or bacterial) can cause conjunctivitis, commonly called pinkeye. Blood vessels in the conjunctiva become engorged, and the normally white part of the eye looks red. Other symptoms usually include itchiness and discharge.
• Blepharitis occurs when there is inflammation or infection of the eyelid. A sensation of grit in the eyes, and sometimes pain, result.
• A sty or stye -- also known as hordeolum -- is an infection or inflammation of the eyelid margin that can come from the hair follicles of eyelashes or from oil glands. A sty is often painful and most often caused by infection.
• Foreign bodies. Sometimes there really is something in the eye -- a bit of dirt, plant debris, or a fragment of a contact lens. Foreign bodies are usually just irritating, and tears or a water rinse clears them out. If not removed, foreign bodies can cause corneal abrasions.
• Glaucoma is an eye condition that usually has no early symptoms. In some cases of glaucoma, though, pressure inside the eye rise suddenly. The condition is called acute angle closure glaucoma, and the symptoms include severe eye pain, nausea and vomiting, headache, and decreased vision. Acute angle closure glaucoma is an emergency and needs immediate treatment to prevent blindness.
• Iritis or uveitis is inflammation inside the eye. Iritis is uncommon, but can be due to trauma, infections, or autoimmune conditions. Symptoms include pain, red eye, and, often, decreased vision.
• Optic neuritis. The nerve traveling from the back of the eyeball into the brain can become inflamed. Multiple sclerosis or other autoimmune conditions or infections are often to blame. Symptoms include loss of vision and sometimes pain.
• Sinusitis can create pressure behind the eyes, causing eye pain on one or both sides.

What Symtoms Are Associated With Eye Pain?

Eye pain can occur by itself, or there may be various other symptoms present:

• Red eye or pinkeye
• Tearing
• Light sensitivity (photophobia)
• Discharge, which can be clear, or thick and colored (pus)
• The eye being crusted shut after sleep due to discharge
• Foreign body sensation -- the feeling that something is in the eye, whether or not anything actually is
• Nausea or vomiting
• Decreased vision
• Headache

Other symptoms accompanying sore eyes can be a clue to what is causing the eye pain.

How Is Eye Pain Diagnosed?

Severe eye pain needs medical attention, especially if decreased vision, headache, or nausea and vomiting are present.

A primary care doctor can diagnose some common eye conditions, such as conjunctivitis. For any potentially serious cause of sore eyes, your doctor will refer you to an ophthalmologist.

Ophthalmologists use a variety of tools to diagnose eye pain, all generally used in an office setting:

• A slit-lamp exam uses bright light to look into all the structures of the eye.
• Dilating drops expand the pupil to allow the doctor to see deep into the eye.
• A pressure-gauging instrument (tonometer or Tono-Pen) can detect high pressures from glaucoma.

How Is Eye Pain Treated?

The causes of sore eyes vary widely, and so do the treatments for eye pain. Treatments are tailored to the specific cause of eye pain:

• Conjunctivitis. Antibacterial eye drops can cure bacterial conjunctivitis. Antihistamines -- eye drops or in an oral form -- can often improve allergic conjunctivitis.
• Corneal abrasions heal on their own with time and are treated with pain medication and antibiotic ointments.
• Glaucoma severe enough to cause eye pain is treated urgently with eye drops to reduce eye pressure. If these don't work, surgery may be needed.
• Styes are usually cured by applying regular warm compresses at home for a few days.
• Infections of the cornea (keratitis) may require antiviral or antibacterial eye drops.
• Optic neuritis is generally treated with intravenous corticosteroids. Usually, this requires admission to the hospital.
• Iritis may be treated with antibiotic or antiviral eye drops, if an infection is the cause. For autoimmune causes, corticosteroids -- eye drops or in an oral form -- are used.

The only way to sort out the various potential causes of eye pain and to get appropriate treatment is to see a doctor. Your vision is precious -- protect it by taking eye pain seriously.

Refractive Error

Click here to overview diseases of eye
 
Refractive errors often are the main reason a person seeks the services of an optometrist or ophthalmologist. But what does it really mean when we're told that our vision is blurry because we have a refractive error?

We see the world around us because of the way our eyes bend (refract) light. Refractive errors are optical imperfections that prevent the eye from properly focusing light, causing blurred vision. The primary refractive errors are nearsightedness, farsightedness and astigmatism.

Refractive errors usually can be "corrected" with eyeglasses or contact lenses, or they can be permanently treated with LASIK and other vision correction surgery (also called refractive surgery).

How Light Travels Through the Eye

In order to see, we must have light. While we don't fully understand all the different properties of light, we do have an idea of how light travels. A light ray can be deflected, reflected, bent or absorbed, depending on the different substances it encounters.

When light travels through water or a lens, for example, its path is bent or refracted. Certain eye structures have refractive properties similar to water or lenses and can bend light rays into a precise point of focus essential for sharp vision.

Most refraction in the eye occurs when light rays travel through the curved, clear front surface of the eye (cornea). The eye's natural (crystalline) lens also bends light rays. Even the eye's tear film and internal fluids (aqueous humor and vitreous) have refractive abilities.

How the Eye Sees

The process of vision begins when light rays that reflect off objects and travel through the eye's optical system are refracted and focused into a point of sharp focus.

For good vision, this focus point must be on the retina. The retina is the tissue that lines the inside of the back of the eye, where light-sensitive cells (photoreceptors) capture images in much the same way that film in a camera does when exposed to light. These images then are transmitted through the eye's optic nerve to the brain for interpretation.

Just as a camera's aperture (called the diaphragm) is used to adjust the amount of light needed to expose film in just the right way, the eye's pupil widens or constricts to control the amount of light that reaches the retina.

In dark conditions, the pupil widens. In bright conditions, the pupil constricts.

Causes of Refractive Errors

The eye's ability to refract or focus light sharply on the retina primarily is based on three eye anatomy features: 1) the overall length of the eye, 2) the curvature of the cornea and 3) the curvature of the lens inside the eye.

• Eye Length: If the eye is too long, light is focused before it reaches the retina, causing nearsightedness. If the eye is too short, light is not focused by the time it reaches the retina. This causes farsightedness or hyperopia.
• Curvature of the Cornea: If the cornea is not perfectly spherical, then the image is refracted or focused irregularly to create a condition called astigmatism. A person can be nearsighted or farsighted with or without astigmatism.
• Curvature of the Lens: If the lens is too steeply curved in relation to the length of the eye and the curvature of the cornea, this causes nearsightedness. If the lens is too flat, the result is farsightedness.

More obscure vision errors, known as higher-order aberrations, also are related to flaws in the way light rays are refracted as they travel through the eye's optical system.

These types of vision errors, which can create problems such as poor contrast sensitivity, are detected through new technology known as wavefront analysis.

Detection and Treatment of
Refractive Errors

Your eye doctor determines the type and degree of refractive error you have by performing a test called a refraction.

This can be be done with a computerized instrument (automated refraction) or with a mechanical instrument called a phoropter that allows your eye doctor to show you one lens at a time (manual refraction).

Often, an automated refraction will be performed by a member of the doctor's staff, and then the eye care practitioner will refine and verify the results with a manual refraction.

An eye care practitioner performs a manual refraction. (Image: National Eye Institute)

Your refraction may reveal that you have more than one type of refractive error. For example, your blurred vision may be due to both nearsighted and astigmatism.

Your eye doctor will use the results of your refraction to determine your eyeglasses prescription. A refraction, however, does not provide sufficient information to write a contact lens prescription, which requires a contact lens fitting.

Eyeglass lenses and contact lenses are fabricated with precise curves to refract light to the degree necessary to compensate for refractive errors and bring light to a sharp focus on the retina.

Vision correction surgeries such as LASIK aim to correct refractive errors by changing the shape of the cornea, so that light rays are bent into a more accurate point of focus.

Strabismus

Click here to overview diseases of eye

Crossed eyes; Esotropia; Exotropia; Hypotropia; Hypertropia; Squint; Walleye; Misalignment of the eyes; Comitant strabismus; Noncomitant strabismus

Strabismus is a disorder in which the two eyes do not line up in the same direction, and therefore do not look at the same object at the same time. The condition is more commonly known as "crossed eyes."

Causes, incidence, and risk factors

Six different muscles surround the eyes and work "as a team" so that both eyes can focus on the same object.
In someone with strabismus, these muscles do not work together. As a result, one eye looks at one object, while the other eye turns in a different direction and is focused on another object.
When this occurs, two different images are sent to the brain -- one from each eye. This confuses the brain, and the brain may learn to ignore the image from the weaker eye.
If the strabismus is not treated, the eye that the brain ignores will never see well. This loss of vision is called amblyopia. Another name for amblyopia is "lazy eye." Sometimes amblyopia is present first, and it causes strabismus.
In most children with strabismus, the cause is unknown. In more than half of these cases, the problem is present at or shortly after birth (congenital strabismus).
Most of the time, the problem has to do with muscle control, and not with muscle strength.
Less often, problems with one of the nerves or muscles, or Graves' disease restriction may cause strabismus.
Other disorders associated with strabismus include:

• Brain and nerve disorders, such as traumatic brain injury, stroke, cerebral palsy, or Guillain-Barre syndrome
• Diabetes (causes a condition known as acquired paralytic strabismus)
• Damage to the retina in children who are born premature
• Hemangioma near the eye during infancy
• Injuries to the eye
• Tumor in the brain or eye
• Vision loss from any eye disease or injury

A family history of strabismus is a risk factor. Farsightedness may be a contributing factor, especially in children. Any other disease that causes vision loss may also cause strabismus.

Symptoms

Symptoms of strabismus may be present all the time, or only when you are tired or sick.
The eyes do not move together and may appear crossed at times.
The other eye will appear turned out, up, or down from wherever the first eye is focused.
Someone with strabismus may also have:

• Depth perception loss
• Double vision
• Vision loss

Signs and tests

A physical examination will include a detailed examination of the eyes. Tests will be done to determine how much the eyes are out of alignment.
Eye tests include:

• Corneal light reflex
• Cover/uncover test
• Retinal exam
• Standard ophthalmic exam
• Visual acuity

A brain and nervous system (neurological) examination will also be performed.

Treatment

The first step in treating strabismus is to prescribe glasses, if needed.
Amblyopia or lazy eye must be treated first. A patch is placed over the better eye. This forces the weaker eye to work harder.
Your child may not like wearing a patch or eyeglasses. A patch forces the child to see through the weaker eye at first. However, it is very important to use the patch or eyeglasses as directed.
If the eyes still do not move correctly, eye muscle surgery may be needed. Different muscles in the eye will be made stronger or weaker.
Eye muscle repair surgery does not fix the poor vision of a lazy eye. A child may have to wear glasses after surgery. In general, the younger a child is when the surgery is done, the better the result.
Adults with mild strabismus that comes and goes may do well with glasses and eye muscle exercises to help keep the eyes straight. More severe forms of adult strabismus will need surgery to straighten the eyes. If strabismus has occurred because of vision loss, the vision loss will need to be corrected before strabismus surgery can be successful.

Expectations (prognosis)

After surgery, the eyes may look straight but vision problems can remain.
The child may still have reading problems in school, and for adults driving may be more difficult. Vision may affect the ability to play sports.
With early diagnosis and treatment, the problem can usually be corrected. Delayed treatment may lead to permanent vision loss in one eye. About one-third of children with strabismus will develop amblyopia.
Because many children will get strabismus or amblyopia again, they need to be monitored closely.

Calling your health care provider

Strabismus requires prompt medical evaluation. Call for an appointment with your health care provider or eye doctor if your child:

• Appears to be cross-eyed
• Complains of double vision
• Has difficulty seeing

Note: Learning difficulties or problems at school can sometimes be due to a child's inability to see the blackboard or reading material.