Astigmatism: Causes, Symptoms, and Treatments - Medical News Today Astigmatism: Causes, Symptoms, and Treatments Written by Christian NordqvistReviewed by University of Illinois-Chicago, School of Medicine 4 23 In astigmatism, the front surface of the eye (cornea) is not curved in the normal way. The primary symptom of astigmatism is blurred vision. Because of the abnormal curve of the cornea, when light rays enter the eye, they do not focus correctly on the retina, resulting in a blurred image. Astigmatism may also be caused by an irregularly shaped lens, which is located behind the cornea. Astigmatism can occur in children and adults; it is fairly common and not contagious. It is usually congenital - present at birth - but can develop after an eye operation or an injury to the eye. This condition belongs to a group of eye conditions called refractive errors; these include myopia (short-sightedness), hyperopia (long-sightedness), and presbyopia (aging of the lens in the eye). These conditions are common, around half of all adults in the United States aged 20 and older have refractive errors in their eyes.

Contents of this article: Fast facts on astigmatism Here are some key points about astigmatism. More detail and supporting information is in the main article. Astigmatism is a type of refractive error Astigmatism is a common condition in the population It is caused by an abnormal curve of the cornea or lens Laser surgery can often correct astigmatism Causes of astigmatism Astigmatism is very common. Regular eye tests are important for diagnosis. Astigmatism can be due to an abnormal curvature of the cornea (corneal) or the lens (lenticular): Corneal astigmatism The cornea is a clear (transparent) layer of tissue that covers the front of the eye. As well as transmitting and focusing light into the eye, it protects the eye from infection and damage. The cornea needs to have a perfect curve in order to bend (refract) light properly as it enters the eye. If the cornea does not curve perfectly, the light that hits it will not refract properly and the retina at the back of the eye will receive an imperfect image. The person will have blurred vision from that eye - astigmatism.

People with astigmatism commonly have an oblong-shaped (oval-shaped) cornea rather than a perfect sphere. A ping-pong ball is a perfect sphere, while an American football or a rugby ball has an oblong shape. If the cornea's curve is like an oblong, the light rays will focus on two points in the retina, rather than just one. Experts are not sure why some people are born with a cornea that does not curve properly. Twin studies indicate that there may be a genetic component to astigmatism. A higher percentage of babies born prematurely have astigmatism, compared with babies that are born closer to their due date. Certain types of surgery or eye injuries that cause scarring of the cornea may cause astigmatism. Keratoconus is a degenerative disorder of the eye where the cornea gradually thins and changes to a more conical shape; this can also cause astigmatism. Lenticular astigmatism This is similar to corneal astigmatism but exists in the lens rather than the cornea. Instead of a perfect curve, the lens has variations that cause images to reach the back of the eye (retina) imperfectly.

Most patients with lenticular astigmatism have a normally shaped cornea. Symptoms of astigmatism The following are common symptoms of astigmatism : Blurred or distorted vision at all distances Headaches Excessive squinting Eye strain - occurs more often when the eye has to focus for long periods, as in reading from paper or a computer monitor Diagnosis of astigmatism A visual acuity test is commonplace when having an eye exam. An eye specialist may use the following tools to examine the eyes: Visual acuity test - this involves reading letters on a chart. The letters become progressively smaller on each line. Astigmatic dial - a chart showing a series of lines which make up a semi-circle. People with perfect vision will see the lines clearly, while those with astigmatism will see some more clearly than others. Keratometer (ophthalmometer) - this device measures the reflected light from the surface of the cornea. It measures the radius of the curvature of the cornea and can assess the degree of abnormal curvature. Corneal topography - this process gives more information about the shape and curve of the cornea. The importance of regular eye tests Astigmatism is very common.

Many children who are born with astigmatism will not realize they have it until they have an eye test. Reading and concentrating at school may be affected if a child has undiagnosed astigmatism. Therefore, regular eye tests are important. The American Optometric Association recommend that children have an eye exam at age 6 months, age 3 years, before first grade, and every 2 years after that. For high-risk children, an eye exam is recommended every year. Adults should have an eye test every couple of years, and more often if they have chronic conditions, like diabetes . Treatments for astigmatism If the astigmatism is very mild, the doctor will commonly suggest no treatment at all. Corrective lenses for astigmatism Corrective lenses bend the incoming light rays in a way that compensates for the error caused by faulty refraction so that images are properly received onto the retina. Whether the corrective lenses are in the form of glasses or contact lenses is up to the patient - they are equally effective.

Experts say that children can wear contact lenses as long as they are careful about using them properly - this includes not wearing them for too long and cleaning them correctly, which might be difficult to achieve if the child is under the age of 12. People of any age who use contact lenses need to be aware of good lens hygiene. Otherwise, there is a significant risk of eye infection. People with astigmatism may be prescribed rigid gas permeable lenses, which are made from plastics, silicone and/or fluoropolymers. These lenses are hard and hold their shape, but allow oxygen to pass freely to the eye. Individuals with more severe astigmatism may prefer these. Soft contact lenses are commonly used for more minor astigmatism. Laser eye surgery Some people with astigmatism can be treated with laser eye surgery, the most common of which is LASIK. Photorefractive keratectomy (PRK) - some of the outer protective layer of the cornea is removed. An excimer laser changes the shape of the cornea by removing tissue. When the cornea heals, it usually has a more even and spherical curve. This procedure can be moderately to very painful.

LASEK (Laser epithelial keratomileusis) - alcohol is used to loosen the surface of the cornea before it is operated on; a much thinner layer is affected, making the eye less vulnerable to damage or injury, compared with PRK. A laser is then used to change the shape of the cornea. LASEK may be a better option for a patient whose cornea is thin. This procedure is usually less painful than PRK, but slightly more painful than LASIK. Laser in situ keratectomy (LASIK) - the doctor uses a device called a keratome to make a thin, round hinged cut into the cornea. The flap is then lifted, and an excimer laser sculpts the shape of the cornea under the flap. LASIK causes less pain than the other procedures, and the patient will recover their vision within a few days; because of this, LASIK is usually the preferred laser treatment option. LASEK and PRK are better procedures than LASIK in cases where the cornea is thin. Laser eye surgery may not suitable if: The patient is under the age of 18.

Vision is still changing - the vision of some older people may still be changing. Experts say that a person's vision should be stable for at least 1 year before undergoing laser surgery. Patients with diabetes - in some cases, laser surgery may worsen abnormalities in the eye caused by diabetes. Pregnant or breast-feeding mothers - during pregnancy and breast-feeding, hormone fluctuations still exist within the eye, making it more difficult to carry out surgery accurately. People with some immune conditions - people with rheumatoid arthritis , lupus, or HIV, for example, may find it harder to recover after surgery. People with other existing eye conditions - they will need to have other conditions treated first before becoming eligible candidates for laser eye surgery. Examples include cataracts and glaucoma. People taking certain medications - if the patient is taking medications, such as Accutane or oral prednisone, they should not undergo laser eye surgery. Risks of laser eye surgery Correction error - the surgeon may have taken out the wrong amount of tissue and the patient's vision worsens.

Regression - vision defects can recur after surgery Visual loss - some people's vision might become slightly worse after surgery Dry eyes - dry eye is very common after laser eye surgery In most countries, the risk of complications from laser eye surgery is very small. It depends on how experienced the surgeon is, and how accurately the patient was assessed beforehand. Written by Christian Nordqvist Click the stars to rate this article Astigmatism: Causes, Symptoms, and Treatments Public / Patient Astigmatism: Definition and Tests Medical Author: Andrew A. Dahl, MD, FACS Andrew A. Dahl, MD, FACS Andrew A. Dahl, MD, is a board-certified ophthalmologist. Dr. Dahl's educational background includes a BA with Honors and Distinction from Wesleyan University, Middletown, CT, and an MD from Cornell University, where he was selected for Alpha Omega Alpha, the national medical honor society. He had an internal medical internship at the New York Hospital/Cornell Medical Center. Medical Editor: Melissa Conrad Stöppler, MD Melissa Conrad Stöppler, MD Melissa Conrad Stöppler, MD, is a U.S.

board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology. Quick GuideCommon Eye Problems and Infections Astigmatism facts Astigmatism is an eye disorder in which the cornea (the clear tissue covering the front of the eye) is abnormally curved, causing out-of-focus vision. It is commonly treated with glasses, contact lenses , or refractive surgery. What is the definition of astigmatism? In order to see clearly, the eye must be able to focus light into a single plane at the surface of the retina . The word astigmatism comes from the Greek "a" meaning "without" and "stigma" meaning "spot." In astigmatism, a point (or spot) of light is focused at two different planes, causing blurred vision . An optical system (or eye) without astigmatism is called "spherical" and has only one plane of focus for all rays of light.

An optical system with astigmatism is one in which rays that propagate in two perpendicular planes have different foci. For example, if an optical system with astigmatism is used to form an image of a plus sign, the vertical and horizontal lines will never be in focus at the same time, since they are in sharp focus at two distinctly different distances from the plus sign. In an eye without astigmatism, the surface of the cornea is shaped like a sphere the way a ping-pong ball is, where all the curves are the same. This is called a spherical surface. In an eye with astigmatism, the surface of the cornea is shaped more like the bottom of a spoon, where there are two different surface curves located 90 degrees apart. This is called an astigmatic or toric surface. What are the different types of astigmatism? Share Your Story There are various classification systems for astigmatism, based on the anatomical source of the astigmatism, the regularity/ irregularity of astigmatism, or the direction of astigmatism.

Most astigmatism in the human eye has its source within the cornea, although there are irregularities of the lens that can lead to astigmatism, known as lenticular astigmatism. Most corneal astigmatism is regular, signifying that the cornea is most curved (steepest) 90 degrees away from the surface of the cornea that is the least curved (flattest) and that the transition from most curved to least curved surface occurs in a regular manner. Regular astigmatism can be corrected with glasses, toric soft lenses, rigid lenses, or refractive surgery. Irregular astigmatism is defined as the focus resulting from any corneal surface that is neither spherical nor regularly astigmatic. Irregular astigmatism cannot be corrected with glasses or soft contact lenses. A historical classification of astigmatism differentiates “with the rule” astigmatism from "against the rule" astigmatism. In "with the rule" astigmatism the steepest curvature (most curved part of the corneal surface) lies in or close to the vertical meridian, similar to the surface of a spoon lying on it side.

In “against the rule” astigmatism, the steepest (most curved) part of the cornea is in or close to the horizontal meridian, similar to the surface of a football standing upright. Medically Reviewed by a Doctor on 11/20/2015 Astigmatism: Causes, Symptoms, Diagnosis, and Treatment How Is Astigmatism Treated? What Is Astigmatism? It’s a big word, but it simply means your eye isn’t completely round. Almost all of us have it to some degree. A normal eyeball is shaped like a perfectly round ball. Light comes into it and bends evenly, which gives you a clear view. But if your eye is shaped more like a football or the back of a spoon, light gets bent more in one direction than another. That means only part of an object is in focus. Objects at a distance may look blurry and wavy. It’s fairly easy for an eye doctor to fix with glasses , contacts , or surgery. What Causes It? It’s totally natural and most people are born with it. We don’t know the exact cause. You can also get it after an eye injury, eye disease, or surgery.

There’s a myth that you can get it if you read in low light or sit too close to the TV, but that isn’t true. What Are the Symptoms? Blurry vision . You might chalk it up to fatigue or eyestrain, but it’s the major sign of astigmatism . If you can’t see clearly, schedule an eye exam to find the source of your problems. How Is It Diagnosed? You’ll need a thorough eye exam . Your doctor may also find another problem -- you could be nearsighted or farsighted . Because astigmatism symptoms come on slowly, you should go to an eye doctor if you notice changes in your vision . How Is Astigmatism Treated? Almost all cases can be corrected with glasses or contacts . But if you only have a slight astigmatism -- your doctor may refer to it as a degree -- and you don’t have another vision problem, you may not need them. Irregular astigmatism is far less common and is linked to problems with your cornea , the front part of the eye. A common one is keratoconus , in which your normally round cornea becomes cone-shaped. There are two treatments for the common levels of astigmatism: Corrective lenses.

That means glasses or contacts. If you have astigmatism, your doctor will probably prescribe a special type of soft contact lenses called toric. They can be made to bend light more in one direction than the other. If your case is more severe, you might go with a gas-permeable rigid contact lens. Your eye doctor will figure out which one is best for you. Refractive surgery. This laser surgery changes the shape of your cornea. There’s more than one type, so your doctor will help you pick the one that’s right for you. You’ll need to have healthy eyes with no retina problems or corneal scars. WebMD Medical Reference Reviewed by Alan Kozarsky, MD on September 20, 2016 Sources Astigmatism Astigmatism Search our database of 28,332 Optometrists. Show only InfantSEE® Providers How is astigmatism treated? Astigmatism is a common vision condition that causes blurred vision. It occurs when the cornea (the clear front cover of the eye) is irregularly shaped or sometimes because of the curvature of the lens inside the eye.

An irregularly shaped cornea or lens prevents light from focusing properly on the retina, the light-sensitive surface at the back of the eye. As a result, vision becomes blurred at any distance. This can lead to eye discomfort and headaches. Most people have some degree of astigmatism. Slight astigmatism usually doesn't affect vision or require treatment. Astigmatism frequently occurs with other vision conditions like myopia (nearsightedness) and hyperopia (farsightedness) . Together these vision conditions are referred to as refractive errors because they affect how the eyes bend or "refract" light. The specific cause of astigmatism is unknown. It can be hereditary and is usually present from birth. It can decrease or increase over time. A comprehensive optometric examination will include testing for astigmatism. If necessary, your optometrist can provide eyeglasses or contact lenses that correct the astigmatism by altering the way light enters the eyes. Another option for treating astigmatism is a corneal procedure called orthokeratology (ortho-k) . In this painless, noninvasive procedure, the patient wears a series of specially designed rigid contact lenses to gradually reshape the curvature of the cornea.

Laser surgery can also treat some types of astigmatism. The laser changes the shape of the cornea by removing a small amount of eye tissue. [back to top] What causes astigmatism? The curvature of the cornea and lens bends the light entering the eye in order to focus it precisely on the retina at the back of the eye. In astigmatism, the surface of the cornea or lens has a somewhat different curvature. the surface of the cornea is shaped more like a football instead of round like a basketball, the eye is unable to focus light rays to a single point. Vision becomes out of focus at any distance. In addition, the curvature of the lens inside the eye can change, resulting in an increase or decrease in astigmatism. This change frequently occurs in adulthood and can precede the development of naturally occurring cataracts. Sometimes astigmatism may develop following an eye injury or eye surgery. Astigmatism also occurs due to a relatively rare condition called keratoconus in which the cornea becomes progressively thinner and cone-shaped. This results in a large amount of astigmatism, which causes poor vision that cannot be clearly corrected with eyeglasses.

People with keratoconus usually need contact lenses for clear vision and eventually may need a corneal transplant. [back to top] How is astigmatism diagnosed? An optometrist can diagnose an astigmatism through a comprehensive eye examination . Testing for astigmatism measures how the eyes focus light and determines the power of any optical lenses needed to improve vision. This examination may include: Visual acuity-When you read letters on a distance chart, you are measuring your visual acuity. Visual acuity is given as a fraction (for example, 20/40). The top number is the standardized testing distance (20 feet) and the bottom number is the smallest letter size read. A person with 20/40 visual acuity would have to get within 20 feet to read a letter that should be seen clearly at 40 feet. Normal distance visual acuity is 20/20. Keratometry/Topography-A keratometer is the primary instrument used to measure the curvature of the cornea. By focusing a circle of light on the cornea and measuring its reflection, it is possible to determine the exact curvature of that area of the cornea's surface.

This measurement is particularly critical in determining the proper fit for contact lenses. A corneal topographer, which is gaining use, generates a contour map of the cornea and provides even more detail of the cornea's shape. Refraction-Using an instrument called a phoropter, your optometrist places a series of lenses in front of your eyes and measures how they focus light. This is performed using a handheld lighted instrument called a retinoscope or an automated instrument that evaluates the approximate focusing power of the eye. Based on your responses, the power is then refined to determine the lenses that allow the clearest vision. Despite improved technology, patient input remains integral in determining vision needs. With the information from these tests, your optometrist can determine if you have astigmatism. Your optometrist will use these findings, combined with those of other tests performed, to determine the power of any lens correction you need to provide clear, comfortable vision. Once testing is complete, your optometrist can discuss treatment options. How is astigmatism treated? People with astigmatism have several options to regain clear vision. They include: Eyeglasses. People with astigmatism primarily choose eyeglasses to improve their vision.

The eyeglasses contain a special cylindrical lens prescription that compensates for the astigmatism. This provides additional power in specific parts of the lens. Generally, a single-vision lens is prescribed to provide clear vision at all distances. However, patients over age 40 who have presbyopia may need a bifocal or progressive addition lens. Contact lenses. Some people will have better vision with contact lenses rather than eyeglasses. Contact lenses may provide clearer vision and a wider field of view. However, since contact lenses are worn directly on the eyes, they require regular cleaning and care to safeguard eye health. Standard soft lenses may not be effective in correcting astigmatism. However, special toric soft contact lenses can correct for many types of astigmatism. Because rigid gas-permeable contact lenses maintain their regular shape while on the cornea, they can compensate for the cornea's irregular shape and improve vision for people with astigmatism. Orthokeratology. Orthokeratology (ortho-k) involves the fitting of a series of rigid contact lenses to reshape the cornea. The patient wears contact lenses for limited periods, such as overnight, and then removes them.

People with moderate astigmatism may be able to temporarily obtain clear vision without lenses for most of their daily activities. Orthokeratology does not permanently improve vision. If patients stop wearing the retainer lenses, their vision may return to its original condition. Laser and other refractive surgery procedures. Astigmatism can also be corrected by reshaping the cornea through LASIK (laser in situ keratomileusis) or PRK (photorefractive keratectomy). PRK removes tissue from the superficial and inner layers of the cornea. LASIK removes tissue only from the inner layer of the cornea. If you have an astigmatism, you have a wide range of options to correct your vision problem. In consultation with your optometrist, you can select the treatment that best meets your visual and lifestyle needs. Understanding Astigmatism -- the Basics Understanding Astigmatism -- the Basics What Causes Astigmatism? What Is Astigmatism? Most of the focusing power in the eye occurs along the front surface, called the cornea , which is the clear "window" in the front of the eyeball. The next structure involved in focussing is the lens, which sits behind the cornea inside the eye .

The ideal cornea has a round, symmetrically curved surface, like a basketball. Astigmatism is caused by an "out of round" cornea or a lens that is not symmetrical. As a result, people with significant astigmatism may have distorted or blurry vision . Astigmatism is measured in diopters. More than 1 diopter typically requires correction with glasses or soft contact lenses . Astigmatism can run in families and often occurs in combination with other refractive problems such as nearsightedness or farsightedness . It can also increase over time due to age. What Causes Astigmatism? The cause of astigmatism is almost always unknown and not associated with an external or genetic cause. One in 400 people has a higher level of astigmatism that comes from keratoconus (KEHR-a-toh-kohn-nus), a condition in which the cornea progressively becomes cone-shaped and thin. Keratoconus usually appears around puberty or early adulthood. The cornea is made of transparent collagen fibers that hold the cornea shape in place much like a building is held up by steel beams. Eyes with keratoconus have weaker collagen fibers.

This causes the cornea to bulge out and distort vision. Keratoconus may progress to the point where corneal rigid contact lenses no longer solve the vision problems and transplantation of the cornea is necessary. Corneal transplantation for keratoconus is used when other measures are no longer sufficient to produce high quality, comfortable vision and has a high success rate. Corneal cross linking is a very promising treatment to arrest the progression of keratoconus and help patients avoid a cornea transplant. Another form of astigmatism is lenticular astigmatism. It is caused by abnormalities in another curved structure inside the eye -- the natural internal crystalline lens that also focuses incoming rays. Blurred vision from lenticular astigmatism can be treated with eyeglasses and contact lenses . WebMD Medical Reference Reviewed by Alan Kozarsky, MD on March 07, 2015 Sources The Human Balance System | Vestibular Disorders Association Vestibular Disorders Association Information on this website is not intended to be used for medical diagnosis or treatment. VEDA does not specifically endorse any product or service advertised on this site. VEDA is a Top Rated Nonprofit!

The Human Balance System Good balance is often taken for granted. Good balance is often taken for granted. Most people don’t find it difficult to walk across a gravel driveway, transition from walking on a sidewalk to grass, or get out of bed in the middle of the night without stumbling. However, with impaired balance such activities can be extremely fatiguing and sometimes dangerous. Symptoms that accompany the unsteadiness can include dizziness, vertigo, hearing and vision problems, and difficulty with concentration and memory. What is balance? Balance is the ability to maintain the body’s center of mass over its base of support.1 A properly functioning balance system allows humans to see clearly while moving, identify orientation with respect to gravity, determine direction and speed of movement, and make automatic postural adjustments to maintain posture and stability in various conditions and activities. Balance is achieved and maintained by a complex set of sensorimotor control systems that include sensory input from vision (sight), proprioception (touch), and the vestibular system (motion, equilibrium, spatial orientation); integration of that sensory input; and motor output to the eye and body muscles. Injury, disease, certain drugs, or the aging process can affect one or more of these components.

In addition to the contribution of sensory information, there may also be psychological factors that impair our sense of balance. Sensory input Maintaining balance depends on information received by the brain from three peripheral sources: eyes, muscles and joints, and vestibular organs (Figure 1). All three of these information sources send signals to the brain in the form of nerve impulses from special nerve endings called sensory receptors. Figure 1. Balance is achieved and maintained by a complex set of sensorimotor control systems. Input from the eyes Sensory receptors in the retina are called rods and cones. Rods are believed to be tuned better for vision in low light situations (e.g. at night time). Cones help with color vision, and the finer details of our world. When light strikes the rods and cones, they send impulses to the brain that provide visual cues identifying how a person is oriented relative to other objects. For example, as a pedestrian takes a walk along a city street, the surrounding buildings appear vertically aligned, and each storefront passed first moves into and then beyond the range of peripheral vision. Input from the muscles and joints Proprioceptive information from the skin, muscles, and joints involves sensory receptors that are sensitive to stretch or pressure in the surrounding tissues.

For example, increased pressure is felt in the front part of the soles of the feet when a standing person leans forward. With any movement of the legs, arms, and other body parts, sensory receptors respond by sending impulses to the brain. Along with other information, these stretch and pressure cues help our brain determine where our body is in space. The sensory impulses originating in the neck and ankles are especially important. Proprioceptive cues from the neck indicate the direction in which the head is turned. Cues from the ankles indicate the body’s movement or sway relative to both the standing surface (floor or ground) and the quality of that surface (for example, hard, soft, slippery, or uneven). Input from the vestibular system Sensory information about motion, equilibrium, and spatial orientation is provided by the vestibular apparatus, which in each ear includes the utricle, saccule, and three semicircular canals. The utricle and saccule detect gravity (information in a vertical orientation) and linear movement. The semicircular canals, which detect rotational movement, are located at right angles to each other and are filled with a fluid called endolymph.

When the head rotates in the direction sensed by a particular canal, the endolymphatic fluid within it lags behind because of inertia, and exerts pressure against the canal’s sensory receptor. The receptor then sends impulses to the brain about movement from the specific canal that is stimulated. When the vestibular organs on both sides of the head are functioning properly, they send symmetrical impulses to the brain. (Impulses originating from the right side are consistent with impulses originating from the left side.) Integration of sensory input Balance information provided by the peripheral sensory organs—eyes, muscles and joints, and the two sides of the vestibular system—is sent to the brain stem. There, it is sorted out and integrated with learned information contributed by the cerebellum (the coordination center of the brain) and the cerebral cortex (the thinking and memory center). The cerebellum provides information about automatic movements that have been learned through repeated exposure to certain motions. For example, by repeatedly practicing serving a ball, a tennis player learns to optimize balance control during that movement. Contributions from the cerebral cortex include previously learned information; for example, because icy sidewalks are slippery, one is required to use a different pattern of movement in order to safely navigate them.

Processing of conflicting sensory input A person can become disoriented if the sensory input received from his or her eyes, muscles and joints, or vestibular organs sources conflicts with one another. For example, this may occur when a person is standing next to a bus that is pulling away from the curb. The visual image of the large rolling bus may create an illusion for the pedestrian that he or she—rather than the bus—is moving. However, at the same time the proprioceptive information from his muscles and joints indicates that he is not actually moving. Sensory information provided by the vestibular organs may help override this sensory conflict. In addition, higher level thinking and memory might compel the person to glance away from the moving bus to look down in order to seek visual confirmation that his body is not moving relative to the pavement. Motor output As sensory integration takes place, the brain stem transmits impulses to the muscles that control movements of the eyes, head and neck, trunk, and legs, thus allowing a person to both maintain balance and have clear vision while moving. Motor output to the muscles and joints A baby learns to balance through practice and repetition as impulses sent from the sensory receptors to the brain stem and then out to the muscles form a new pathway.

With repetition, it becomes easier for these impulses to travel along that nerve pathway—a process called facilitation—and the baby is able to maintain balance during any activity. Strong evidence exists suggesting that such synaptic reorganization occurs throughout a person’s lifetime of adjusting to changing motion environs. This pathway facilitation is the reason dancers and athletes practice so arduously. Even very complex movements become nearly automatic over a period of time. This also means that if a problem with one sensory information input were to develop, the process of facilitation can help the balance system reset and adapt to achieve a sense of balance again. For example, when a person is turning cartwheels in a park, impulses transmitted from the brain stem inform the cerebral cortex that this particular activity is appropriately accompanied by the sight of the park whirling in circles. With more practice, the brain learns to interpret a whirling visual field as normal during this type of body rotation. Alternatively, dancers learn that in order to maintain balance while performing a series of pirouettes, they must keep their eyes fixed on one spot in the distance as long as possible while rotating their body.

Motor output to the eyes The vestibular system sends motor control signals via the nervous system to the muscles of the eyes with an automatic function called the vestibulo-ocular reflex (VOR). When the head is not moving, the number of impulses from the vestibular organs on the right side is equal to the number of impulses coming from the left side. When the head turns toward the right, the number of impulses from the right ear increases and the number from the left ear decreases. The difference in impulses sent from each side controls eye movements and stabilizes the gaze during active head movements (e.g., while running or watching a hockey game) and passive head movements (e.g., while sitting in a car that is accelerating or decelerating). The coordinated balance system The human balance system involves a complex set of sensorimotor-control systems. Its interlacing feedback mechanisms can be disrupted by damage to one or more components through injury, disease, or the aging process. Impaired balance can be accompanied by other symptoms such as dizziness, vertigo, vision problems, nausea, fatigue, and concentration difficulties. The complexity of the human balance system creates challenges in diagnosing and treating the underlying cause of imbalance.

The crucial integration of information obtained through the vestibular, visual, and proprioceptive systems means that disorders affecting an individual system can markedly disrupt a person’s normal sense of balance. Vestibular dysfunction as a cause of imbalance offers a particularly intricate challenge because of the vestibular system’s interaction with cognitive functioning,2 and the degree of influence it has on the control of eye movements and posture. Authors: the Vestibular Disorders Association, with contributions by Mary Ann Watson, MA, and F. Owen Black, MD, FACS, and Matthew Crowson, MD DID THIS INFORMATION HELP YOU? Thanks to VEDA, vestibular disorders are becoming recognized for their impacts on people's lives and our economy. We see new diagnostic tools and research studies, more accessible treatments, and a growing respect for how life-changing vestibular disorders can be. Your support of VEDA matters. Please help us continue this important work by becoming a member or donor . References Shumway-Cook A, Woollacott MH. Motor Control: Theory and Practical Applications. Philadelphia: Lippincott, Williams & Wilkins; 2001. Hanes DA, McCollum G.

Journal of Vestibular Research 2006;16(3):75–91. Eyes Eyes Los ojos Eyes: Small But Mighty We depend on sight more than any other of our senses to maneuver through the space around us. In a single glance, lasting a fraction of a second, our eyes work with our brains to tell us the size, shape, color, and texture of an object. They let us know how close it is, whether it's standing still or coming toward us, and how quickly it's moving. Every day, our eyes give us messages that help us understand the world around us. Although the eyes are small compared with most of the body's other organs, their structure is incredibly complex. The eyes work together to perceive depth, enabling us to judge distance and the size of objects to help us move around them. Not only do the two eyes work together, they also work with the brain, muscles, and nerves to produce complicated visual images and messages. And our eyes constantly adapt to the changing environment — for example, they are able to adjust so that we can easily move around in a nearly dark room or bright sunlight.

To understand more about how the eyes work, it's important to know about the structures that make up the eye and about conditions and diseases that can interfere with vision. continue The Basics on Eyes Only part of the eye is visible in a person's face. The whole eye — the eyeball — is about the size and shape of a ping-pong ball. The eye — both the parts that are visible and those that aren't — is extremely delicate. The body has several ways of protecting this vulnerable organ. The eyeball sits in the eye socket (also called the orbit) in a person's skull, where it is surrounded by bone. The visible part of the eye is protected by the eyelids and the eyelashes, which keep dirt, dust, and even harmful bright light out of the eye. Our eyes are also protected by our tears, which moisten the eyes and clean out dirt, dust, and other irritants that get past the defenses of our eyelashes and eyelids. Tears also help protect the eyes against infection. Every time we blink, our eyelids spread a layer of moisture over the cornea, which covers the front of the eye.

The lacrimal (pronounced: LAK-ruh-mul) glands in the upper outer corner of each eye socket produce tears. After they've done their job moistening the eyes, the tears flow into canals in the eyelids, which drain into the lacrimal sac, a pouch in the lower inner corner of each eye socket. Tears then exit through a passage that leads to the nose. For you to see, the eye has to move. Six muscles, called extraocular (pronounced: eks-truh-OK-yuh-ler) muscles, surround the eyeball in the skull. These muscles act like the strings on a puppet, moving the eye in different directions. The muscles of each eye normally move together at the same time, allowing the two eyes to remain aligned. The wall of a person's eyeball is made up of three layers, rather like the layers on an onion: The sclera (pronounced: SLEER-uh) is the outermost protective layer. This tough, fibrous tissue surrounds the eyeball and attaches to the cornea, which is the clear front surface of the eye. What we see as the white of the eye is the sclera.

The conjunctiva (pronounced: kon-junk-TY-vuh) a clear skin layer that protects the eye from becoming dry, sits over the sclera and also covers the inner surface of the eyelid. The choroid (pronounced: KOR-oyd) is the middle layer that contains blood vessels that deliver oxygen and nutrients to the inside parts of the eye. The retina (pronounced: RET-nuh), the innermost of the three layers, lines the inside of the eyeball. The retina is a soft, light-sensitive layer of nervous system tissue. The optic nerve carries signals from the retina to the brain, which interprets them as visual images. The space in the center of the eyeball is filled with a clear jelly-like material called the vitreous (pronounced: VIH-tree-us) humor. This material allows light to pass through to the retina. It also helps the eye keep its round shape. previous continue How We See Vision is the process by which images captured by the eye are interpreted by the brain, and the visible part of the eye is where the process of sight begins. On the front surface of the eye is the see-through, circle-shaped cornea.

You can't see a person's cornea the way you can see the colored part of the eye behind it — the cornea is like a clear window that focuses light into the eye. Behind the cornea is a watery fluid called the aqueous humor. The cornea and aqueous humor form an outer lens that refracts (bends) light on its way into the eye. This is where most of the eye's focusing work is done. The colored circular membrane in the eye just behind the cornea is called the iris. The iris controls the amount of light entering the eye through the pupil, which is the opening in the center of the iris that looks like a tiny black circle. Like a camera shutter, which controls the amount of light coming in to prevent both over- and under-exposure, the iris becomes wider and narrower, changing the size of the pupil to control the amount of light entering the eye. The pupil gets bigger when more light is needed to see better and smaller when there's plenty of light. The eye's lens sits just behind the iris. Just like a camera lens, the eye's lens focuses light to form sharp, clear images.

Light that has been focused through the cornea and aqueous humor hits the lens, which then focuses it further, sending the light rays through the vitreous humor and onto the retina. To focus on objects clearly at varying distances, the eye's lens needs to change shape. The ciliary (pronounced: SIL-ee-air-ee) body contains the muscular structure in the eye that changes the shape of the eye's lens. In people who have normal vision, the ciliary body flattens the lens enough to bring objects into focus at a distance of 20 feet (6 meters) or more. To see closer objects, this muscle contracts to thicken the lens. Young children can see objects at very close range; many people over 45 have to hold objects farther and farther away to see them clearly. This is because the lens becomes less elastic as we age. The retina (the soft, light-sensitive layer of tissue that lines the back of the eyeball wall) is made up of millions of light receptors. These are called rods and cones. Rods are much more sensitive to light than cones. Each eye has about 120 million rods that help us see in dim light and detect shades of gray, but they cannot distinguish colors.

In comparison, the 6 million cones in each eye allow us to see in bright light, and they also sense color and detail. The macula (pronounced: MAK-yuh-luh) is a small, specialized area on the retina. The macula helps our eyes see fine details when we look directly at an object. It contains mainly cones and few rods. When focused light is projected onto the retina, it stimulates the rods and cones. The retina then sends nerve signals through the back of the eye to the optic nerve. The optic nerve carries these signals to the brain, which interprets them as visual images. The portion of the brain that processes visual input and interprets the messages that the eye sends is called the visual cortex. Most people use both eyes to see an object. This is called binocular vision. Through binocular vision, images are formed on the retina of each eye. These images are slightly different, because the object is being viewed from slightly different angles. Nerve signals representing each image are sent to the brain, where they are interpreted as two views of the same object. Some of the nerve fibers from each eye cross, so each side of the brain receives messages from both eyes.

Through experience, the brain learns to judge the distance of an object by the degree of difference in the images it receives from the two eyes. This ability to sense distance is called depth perception. Our vision is a fine-tuned process. All the parts of the eye — and the brain — need to work together so a person can see correctly. Because the eye's structure is so complex, though, a lot of things can go wrong. previous continue Things That Can Go Wrong With the Eyes and Vision Some of the most common eye problems are called refractive errors. These are the problems that eye doctors check for routinely in your vision test. Refraction means bending of light rays to focus the light coming from an image. Refractive errors are problems with the focusing of the eye, which causes the image you see to be blurred. Refractive errors include: Astigmatism (pronounced: uh-STIG-muh-tih-zum). In astigmatism, there's a problem with the curve of the cornea. This causes part of the eye's image to be blurry. Corrective lenses such as contact lenses or glasses can usually correct vision in people with astigmatism. Myopia (pronounced: my-OP-ee-uh).

Also called nearsightedness or shortsightedness, myopia occurs when the eye focuses the image of an object in front of the retina instead of directly on it. In most cases, people can't see well far away, but can see objects clearly close up. The condition tends to get somewhat worse through childhood and adolescence, but stabilizes in adulthood. People with this condition may need to wear glasses or contacts to correct their vision. Laser eye surgery is now commonly used in adults to correct nearsightedness permanently by changing the shape of the cornea. Laser surgery is not used for teens because the eye may still be growing and the refractive error changing. Hyperopia (pronounced: hy-per-OP-ee-uh). Also called farsightedness or longsightedness, hyperopia occurs when the incoming image is not focused on the retina, but behind it. This may make it difficult to see close objects clearly, with far-off objects seen more easily. Many younger children are hyperopic, but because of the ability of the eye to focus itself, may not need glasses to correct this. Glasses or contact lenses can correct this problem in kids and teens when needed.

Some of the other eye problems that can affect teens include: Blindness. This is the loss of useful sight. Blindness can be temporary or permanent and it has many causes. Birth defects or damage to any portion of the eye, the optic nerve, or the area of the brain responsible for vision can lead to blindness. The visual impairment cannot be corrected with surgery or corrective lenses, and the condition can make it difficult to perform everyday activities. Diabetes and eye changes because of aging account for most blindness in adults in the United States. But eye injuries or eye trauma are the most common causes for blindness in kids and teens. Color blindness. Color blindness is usually caused by problems in the pigments of the cones in the retina. Most people who are color-blind can see some colors. A person who is color-blind often confuses some colors with others, such as red and green. People can be born color-blind, or they may develop the condition over time. The most common form of color blindness is an inherited condition that affects boys much more often than girls. Color blindness cannot be corrected. Conjunctivitis (pronounced: kon-junk-tuh-VY-tus).

Conjunctivitis refers to inflammation (redness, pain, and swelling) of the conjunctiva. One type of conjunctivitis, pinkeye, is a common contagious infection in which the eyes become pinkish red and watery, and pus may form. Pinkeye is usually treated with eyedrops. Conjunctivitis also can be due to allergies or from something that irritates the eye. Eye injuries . Injuries to the eye are one of the most common preventable causes of blindness. Eye injuries may be caused by irritants such as sand, dirt, or other foreign bodies on the eye's surface. Chemicals or foreign bodies that become embedded in the eye can cause pain and loss of vision. Forceful blows to the eye can cause bleeding inside the eye and damage to the cornea, retina, and other important eye structures. Stye . This red, painful bump on the eyelid is caused by a backed-up oil gland. Styes are treated by applying warm compresses to allow them to drain. Sometimes antibiotic drops or ointments are necessary if it gets infected. Sometimes, a doctor may need to make a tiny cut in the eyelid to let out the clogged-up material.

Wondering how to keep your eyes in excellent shape? One of the best things you can do for your eyes is to have them checked by your doctor whenever you have a physical examination. If you're having trouble seeing or you've been getting frequent headaches at the end of the day, make arrangements to have your eyes examined by an eye specialist. Weightlessness and Its Effect on Astronauts Weightlessness and Its Effect on Astronauts By Elizabeth Howell, Space.com Contributor | September 30, 2013 09:07pm ET MORE "Avatar" Director/Producer James Cameron floats weightless in Zero-G along with X PRIZE Trustees in a flight to raise funds for the X PRIZE Foundation. Left to right: Rob McEwen (Chairman, US Gold), James Cameron, Peter H. Diamandis (Chairman/CEO, X PRIZE), Elon Musk (Chairman/CEO, SpaceX), Jim Gianopulos (Chairman/CEO, Fox Filmed Entertainment). Full story . Credit: Bryan Rapoza The sensation of weightlessness, or zero gravity, happens when the effects of gravity are not felt.

Technically speaking, gravity does exist everywhere in the universe because it is defined as the force that attracts two bodies to each other. But astronauts in space usually do not feel its effects. The International Space Station , for example, is in perpetual freefall above the Earth. Its forward motion, however, just about equals the speed of its "fall" toward the planet. This means that the astronauts inside are not pulled in any particular direction. So they float. Not having to bear weight on your feet sounds relaxing, but in the long term there are many health problems associated with it. Bones and muscles weaken, and other changes also take place within the body. One of the functions of the ISS is to study how astronaut health is affected by long periods in weightlessness. This will also be a huge focus of the first one-year ISS mission in 2015. Experiencing weightlessness You don't have to leave Earth to (briefly) escape the bonds of gravity . Anyone who crested the top of the hill in a fast roller coaster, or who sat in a small plane pushed down suddenly by the wind, briefly experienced weightlessness. More sustained periods are possible in planes that fly a parabola.

NASA's reduced gravity flight program , for example, flies planes in a series of about 30 to 40 parabolas for researchers to conduct experiments on board. Each climb produces a force about twice the force of gravity for 30 seconds. Then, when the plane reaches the top of the parabola and descends, passengers feel microgravity for about 25 seconds. The film crew and actors on the movie "Apollo 13" (1995) spent hours aboard a plane that flew parabolic flights over and over again. This allowed the actors to really "float" during their time in the movie spacecraft rather than relying on cumbersome wires. Astronauts, however, experience weightlessness for much longer periods. The longest sustained time spent in space took place in 1994-95, when Valeri Polyakov spent almost 438 days in space. Even a few days in space can present temporary health problems, as Heidemarie Stefanyshyn-Piper discovered after spending two weeks in space during STS-115 in 2006. During a press conference after the landing, Piper collapsed as she was not quite readjusted to gravity.

Temporary health effects Weightlessness causes several key systems of the body to relax, as it is no longer fighting the pull of gravity. Astronauts' sense of up and down gets confused, NASA said, because the vestibular system no longer can figure out where the ground and the ceiling are. Spacecraft designers take this into account; the ISS, for example, has all of its writing on the walls pointing in the same direction . Crewmembers also experience a disruption in their proprioceptive system, which tells where arms, legs and other parts of the body are oriented relative to each other. "The first night in space when I was drifting off to sleep," one Apollo astronaut said in a NASA interview, "I suddenly realized that I had lost track of ... my arms and legs. For all my mind could tell, my limbs were not there." This disorientation can cause astronauts to become queasy for a few days. One famous example took place during Apollo 9 in 1969. Rusty Schweickart had to change a planned spacewalk because he was feeling ill.

The concern was that if he vomited while in his spacesuit, the fluid could spread through his helmet (making it hard to see) or interfere with the breathing apparatus and cause him to potentially choke to death. Spacecraft also must be designed to take microgravity into account. During spacewalks, for example, astronauts require extra handholds and footholds on the exterior of their spacecraft so that they can anchor themselves and not float away. (Astronauts also attach to them in tethers in case they lose their grip.) Long-term health effects Astronauts in space for weeks to months can run into trouble. Calcium in bones secretes out through urine. As the bones weaken, astronauts are more susceptible to breaking them if they slip and fall, just like people with osteoporosis. Muscles also lose mass. Astronauts typically exercise two hours a day in space to counteract these effects, but it still takes months of rehabilitation to adjust on Earth after a typical six-month space mission. More recently, doctors have discovered eye pressure changes in orbit. NASA has tracked vision changes in astronauts that were on the space station , but nothing so serious as to cause concern.

The first one-year mission to the ISS will take place in 2015. One of the astronauts, Scott Kelly, has a twin, Mark, who will remain on Earth. The brothers have volunteered to be guinea pigs to look at effects on Scott Kelly's body in weightlessness in comparison to Mark Kelly's body on Earth. "They will provide insight into future genetic investigations that can build on this study, but with a larger study population of unrelated astronauts," NASA said in 2013. Author Bio Elizabeth Howell, Space.com Contributor Elizabeth Howell is a contributing writer for Space.com who is one of the few Canadian journalists to report regularly on space exploration. She is pursuing a Ph.D. part-time in aerospace sciences (University of North Dakota) after completing an M.Sc. (space studies) at the same institution. She also holds a bachelor of journalism degree from Carleton University. Besides writing, Elizabeth teaches communications at the university and community college level. To see her latest projects, follow Elizabeth on Twitter at @HowellSpace . Elizabeth Howell, Space.com Contributor on Latest on Weightlessness and Its Effect on Astronauts

Question: Astigmatism affects which part of the human body?
Answer: Eye