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Accommodation The lens in the eye is an elastic structure that changes shape in order to focus things near or far onto the retina. Optically, the greater the curvature of the lens, the greater its refractive power. To bring close objects into focus on the retina the lens must have a greater refractive power than in its resting state. This is accomplished via contraction of the ciliary muscles that hold the lens in place. When these muscle fibers contract, the lens, via its elasticity, springs into a more convex shape allowing a focused image of near objects. This is an energy requiring process since muscular effort is necessary, and therefore can be tiring.

Amblyopia or  “Lazy Eye” Amblyopia is a term used to describe an uncorrectable loss of vision in an eye that appears to be normal.  It’s commonly referred to as “lazy eye” and can occur for a variety of reasons. A child’s visual system is fully developed between approximately the ages of 9-11. Until then, children readily adapt to visual problems by suppressing or blocking out the image.  If caught early, the problem can often be corrected and the vision preserved.  However, after about age 11, it is difficult if not impossible to train the brain to use the eye normally.  Some causes of amblyopia include:  strabismus (crossed or turned eye), congenital cataracts, cloudy cornea, droopy eyelid, unequal vision and uncorrected nearsightedness, farsightedness or astigmatism.  Amblyopia may occur in various degrees depending on the severity of the underlying problem.  Some patients just experience a partial loss; others are only able to recognize motion. Patients with amblyopia lack binocular vision, or stereopsis – the ability to blend the images of both eyes together.  Stereopsis is what allows us to appreciate depth.  Without it, the ability to judge distance is impaired. (http://www.stlukeseye.com)

Astigmatism Astigmatism means that the cornea is oval like a football instead of spherical like a basketball.  Most astigmatic corneas have two curves – a steeper curve and a flatter curve.  This causes light to focus on more than one point in the eye, resulting in blurred vision at distance or near.  Astigmatism often occurs along with nearsightedness or farsightedness. (http://www.stlukeseye.com)

Blepharitis A chronic or long-term inflammation of the eyelids and eyelashes. It affects people of all ages. Among the most common causes of blepharitis are poor eyelid hygiene; excessive oil produced by the glands in the eyelid; a bacterial infection (often staphylococcal); or an allergic reaction. Seborrheic blepharitis is often associated with dandruff of the scalp or skin conditions like acne. It can appear as greasy flakes or scales around the base of the eyelashes and a mild redness of the eyelid. It may also result in a roughness of the normally smooth tissue that lines the inside of the eyelid. Ulcerative blepharitis is less common, but more serious. It is characterized by matted, hard crusts around the eyelashes, which when removed, leave small sores that ooze or bleed. There may also be a loss of eyelashes, distortion of the front edges of the eyelids and chronic tearing. In severe cases, the cornea, the transparent front covering of the eyeball, may also become inflamed. In many cases, good eyelid hygiene and a regular cleaning routine may control blepharitis. This includes frequent scalp and face washing; warm soaks of the eyelids; and eyelid scrubs. In cases where bacterial infection is a cause, eyelid hygiene may be combined with various antibiotics and other medications. Eyelid hygiene is especially important upon awakening because debris can build up during sleep. (http://www.aoanet.org/ed-blepharitis.html)

Cataracts  Cataracts are opacities of the lens and can be caused by aging, diabetes, trauma, radiation, drugs, or intraocular disease. They may also be seen from birth as a result of birth trauma, maternal infection (rubella), genetic, metabolic or chromosomal defects. When the cause is not severe or prolonged the cataract will regress when normal lens growth resumes. The degree of visual blurring will depend on the site of the cataract within the lens and its proximity to the visual axis. Mature or congenital cataracts can be removed but the refractive capabilities of the eye are compromised. This can be corrected by contact lenses, glasses or intraocular lens.

Color Blindness  Color blindness may be a hereditary condition or caused by disease of the optic nerve or retina.  Acquired color vision problems only affect the eye with the disease and may become progressively worse over time.  Patients with a color vision defect caused by disease usually have trouble discriminating blues and yellows.    Inherited color blindness is most common, affects both eyes, and does not worsen over time.  There are several types of color blindness based primarily on which of the three cone systems work in that individual. For instance a person my have all three color sensing cones but one is weak (trichromate); or maybe only 2 cone systems work as in dichromats. Monochromates have only one cone color sensing system. Abnormal color vision is more prevalent in males due to the fact that the genes for two of the cone pigments (green and red) are located on the X chromosome. Some 8% of Caucasian males and 0.4% of females have abnormal color vision. Because all the cells in a male's body, except sperm, have an X and Y chromosome in addition to the 44 other chromosomes, color blindness is found in males with the abnormal gene on the X chromosome. In order for women to be color blind, they must receive two of the recessive genes, one from both parents. Female children of a colorblind male have one recessive gene and pass the defect onto half of their sons. X-linked color blindness appears in males of every second generation. The most common tests for color blindness are yarn-matching tests and the Ishihara charts. In the yarn test the individual is asked to match one color of yarn to others close to it. Ishihara charts are multicolored plates that have numbers or letters made of colored spots on a background of different colored spots. The numbers or letters are printed in colors that would look the same as the background spots to someone with color blindness of the various types. ).  Complete color blindness is very rare and those who are completely color blind usually have other serious eye problems as well. (http://www.stlukeseye.com)

Color Vision  In the human, some 6 million cones are responsible for filling in the details and colors in a visual image. The cone photopigment is called iodeosin. There are three types of cones (red, green, and blue) based on their maximum sensitivity to the visible light spectrum, which is determined, by the particular variety of iodeosin produced by that cone. It is important to note that the sensation of any color can be derived from the proper mix of these three primary colors. Cones require a higher light intensity to illicit a response than the rods because they have a higher threshold for stimulation.

Conjunctivitis or “Pink Eye”  An infection of the conjunctiva (the outer-most layer of the eye that covers the sclera).  The three most common types of conjunctivitis are: viral, allergic, and bacterial.  Each requires different treatments.  With the exception of the allergic type, conjunctivitis is typically contagious. The viral type is often associated with an upper respiratory tract infection, cold, or sore throat.  The allergic type occurs more frequently among those with allergic conditions.  When related to allergies, the symptoms are often seasonal.  Allergic conjunctivitis may also be caused by intolerance to substances such as cosmetics, perfume, or drugs.  Bacteria such as staphylococcus and streptococcus often cause bacterial conjunctivitis.  The severity of the infection depends on the type of bacteria involved.(http://www.stlukeseye.com)

Dark Adaptation:  The human eye can see things in quite a remarkable range of light intensity, bright sunshine to a moonless sky. This is due to dark adaptation, which is due to a change in the visual threshold. For instance, when a person comes inside to a movie theater from a bright sunny day their eyes gradually adapt to the darker environment. It takes about 20 minutes for full adaptation. The reverse process, adaptation from dark to light, takes about 5 minutes. Pilots, radiologists and others who need maximal visual sensitivity in the dark can avoid the 20-minute dark adaptation period if they wear red glasses or goggles in the light. Red lighting is used in photographic print development as well because the photographer can see acutely without ruining the print. This is because the rods are only minimally simulated by red light and they are chiefly involved in dark adaptation. The time required for dark adaptation is due to the time required to build up Rhodesian stores within the rods.

Dry Eye The tears your eyes produce are necessary for overall eye health and clear vision. Dry eye means that your eyes do not produce enough tears or that you produce tears, which do not have the proper chemical composition. Often, dry eye is part of the natural aging process. It can also be caused by blinking or eyelid problems, medications like antihistamines, oral contraceptives and antidepressants, a dry climate, wind and dust, general health problems like arthritis or Sjogren's syndrome and chemical or thermal burns to your eyes. If you have dry eye, your symptoms may include irritated, scratchy, dry, uncomfortable or red eyes, a burning sensation or feeling of something foreign in your eyes and blurred vision. Excessive dry eyes may damage eye tissue, scar your cornea (the front covering of your eyes) and impair vision and make contact lens wear difficult. If you have symptoms of dry eye, see your optometrist for a comprehensive examination. Dry eye cannot be cured, but your optometrist can prescribe treatment so your eyes remain healthy and your vision is unaffected. Some treatments that your optometrist might prescribe include blinking more frequently, increasing humidity at home or work, using artificial tears and using a moisturizing ointment, especially at bedtime. In some cases, small plugs are inserted in the corner of the eyes to slow tear drainage. Sometimes, surgical closure of the drainage ducts may be recommended. (http://www.aoanet.org/ed-dry-eye.html)

Farsightedness Farsightedness or hyperopia, occurs when light entering the eye focuses behind the retina, instead of directly on it.  This is caused by a cornea that is flatter, or an eye that is shorter, than a normal eye.  Farsighted people usually have trouble seeing up close, but may also have difficulty seeing far away as well. (http://www.stlukeseye.com)

Glaucoma  Glaucoma is the name given to a group of conditions that cause intraocular pressure (IOP) to rise above normal (21 mmHg) and share characteristic changes to the visual field and optic disc. In primary glaucoma there is no known mechanism for the disease, in secondary glaucoma the eye symptoms are secondary to another ocular disease. Normal IOP is determined by the balance between the formation and removal of the aqueous humor. Normally the IOP in both eyes is roughly equivalent and varies throughout the day. The most common form of glaucoma, primary open angle glaucoma, affects about 0.5% of adults in western countries. It often goes undetected until visual acuity is lost. Therefore, screening of high-risk groups and early diagnosis are important. High-risk groups are those with a family history, high myopia, diabetes and the elderly.

Grave’s Disease Grave’s Disease or hyperthyroidism is a condition in which there is an excessive amount of thyroid hormones circulating in the blood because of an overactive thyroid gland. ("Hyper" means "over" in Greek). Thyrotoxicosis is a term that refers to a toxic condition that is caused by an excess of thyroid hormones from any cause. Thyrotoxicosis can be caused by an excessive intake of thyroid hormone and inflammation of the thyroid gland. Because both physicians and patients often use these words interchangeably, we will take some liberty by using the term “hyperthyroidism” throughout this article. Hyperthyroidism is a condition in which there is an excessive amount of thyroid hormones. Thyroid hormones regulate the metabolism of the cells. Normally, the rate of thyroid hormone production is controlled by the brain at the pituitary gland. There are many possible causes of hyperthyroidism. Common symptoms of hyperthyroidism include restlessness, tremors, weight loss despite an increased appetite, sweating, rapid heart rate, intolerance to heat, and frequent bowel movements. Treatments for hyperthyroidism include medications and surgery. (www.medterms.com)

Hyperopia  Farsightedness is caused by having an eyeball that is too short and/or a cornea that is too flat. Light rays focused by the lens are projected on the retina prior to the focal point. Convex lenses can correct for this by shortening the focal distance.

Keratitis or Ocular Herpes is a recurrent viral infection that is caused by the herpes simplex virus and is the most common infectious cause of corneal blindness in the U.S. Previous studies show that once people develop ocular herpes, they have up to a 50 percent chance of having a recurrence. This second flare-up could come weeks or even years after the initial occurrence. Ocular herpes can produce a painful sore on the eyelid or surface of the eye and cause inflammation of the cornea. Prompt treatment with anti-viral drugs helps to stop the herpes virus from multiplying and destroying epithelial cells. However, the infection may spread deeper into the cornea and develop into a more severe infection called stromal keratitis, which causes the body's immune system to attack and destroy stromal cells. Stromal keratitis is more difficult to treat than less severe ocular herpes infections. Recurrent episodes of stromal keratitis can cause scarring of the cornea, which can lead to loss of vision and possibly blindness. Some factors believed to be associated with recurrence include fever, stress, sunlight, and eye injury. (http://www.nei.nih.gov)

Legal Blindness  In the United States, (1) visual acuity of 20/200 or worse in the better eye with corrective lenses (20/200 means that a person must be at 20 feet from an eye chart to see what a person with normal vision can see at 200 feet), or (2) visual field restricted to 20 degrees diameter or less (tunnel vision) in the better eye. Legal blindness defines visual conditions that, when present, connote eligibility for government or other benefits and services. An individual who is legally blind has a visual acuity of 20/200 in the better eye with the best correction or a visual field of no more than 20 degrees. (http://www.sola.com/eyesight/glossary/glossary.html)

Low Vision Low vision means that even with regular glasses, contact lenses, medicine, or surgery, people find everyday tasks difficult to do. Reading the mail, shopping, cooking, seeing the TV, and writing can seem challenging. Millions of Americans lose some of their vision every year. Irreversible vision loss is most common among people over age 65. Low vision is a clinical diagnostic term used to describe impaired vision that cannot be improved by conventional eyeglasses, contact lenses, medications, or surgery in which some good usable vision remains. People with low vision can learn to make the best use of the vision available to them. (http://www.afb.org)

Macular Degeneration  Macular degeneration is the most common cause of blindness in developed countries. It is most common in the elderly Caucasian population. The highest incidence is in the 60-70 age group. Dietary and environmental factors may be important as well as age, heredity and race in the incidence of macular degeneration. The loss of vision is caused by accumulations of lipid and protein in a membrane at the back of the eye and the formation of new blood vessels that interfere with normal vision. Sometimes laser treatment is used to destroy the new blood vessels but the basic disease process remains and continues such that the long-term stability of visual acuity is doubtful. Oral zinc and antioxidants may have some beneficial effect on senile macular degeneration.

Myopia Nearsightedness or myopia, occurs when light entering the eye focuses in front of the retina instead of directly on it.  This is caused by a cornea that is steeper, or an eye that is longer, than a normal eye.  Nearsighted people typically see well up close, but have difficulty seeing far away. (http://www.stlukeseye.com)

Near Point  The nearest point to the eye that an object can be brought into clear focus is called the near point of vision. The near point gets farther away throughout life due to a loss of lens elasticity. For example, the near point at age 10 might be 9 cm and 83 cm at age 60. The loss of accommodation is often sufficient by age 40-45 to make close work and reading difficult. This can be corrected by wearing glasses with convex lenses.

Nyctalopia  Night blindness is the first sign of a deficiency of Vitamin A. Long-term deficiency results in irreversible degeneration of the rods and cones. Treatment with Vitamin A can restore vision if given before the photoreceptive cells are destroyed. (http://www.stlukeseye.com)

Presbyopia  This is a loss of visual accommodation caused by aging and a loss of elasticity of the lens. The nearest point to the eye that an object can brought into clear focus gets farther away with age. Usually by the age of 40-45 the loss in accommodation is such that additional (convex) lenses are helpful for reading and close work. (http://www.stlukeseye.com)

Retinal Detachment  The most common cause of the detachment of the photosensitive retina from the underlying pigmented epithelium is a break or tear that allows fluid for the vitreous humor to get behind the retinal epithelium. Tearing of the retina is often accompanied by bleeding that is experienced as "floaters" within the visual field. If the retina remains detached it will atrophy in just months. Sometimes, though rarely, the retina reattaches spontaneously. Treatment modes consist of laser or cryosurgery, vitrectomy (drainage of subretinal fluid and reattachment of the retina by microsurgery), or the placement of external devices to indent the sclera (so as to bring the retina in closer contact with the pigmented epithelium) in the region of the tear. Gas tamponade can be used to close breaks as well. In this procedure a bubble of air is injected intraocularly so as to apply pressure to bring the detached flap in closer proximity to the pigmented epithelium. Gas tamponade is often used along with the other methods for an improved outcome.

Retinitis Pigmentosa  Retinitis pigmentosa (or night blindness) is a group of inherited diseases that damage the light-sensitive rods and cones located in the retina, the back part of our eyes. Rods, which provide side (peripheral) and night vision are affected more than the cones, which provide color and clear central vision. Signs of RP usually appear during childhood or adolescence. The first sign is often night blindness followed by a slow loss of side vision. Over the years, the disease will cause further loss of side vision. As the disease develops, people with RP may often bump into chairs and other objects as side vision worsens and they only see in one direction – straight ahead. They see as if they are in a tunnel (thus the term tunnel vision). Fortunately, most cases of retinitis pigmentosa take a long time to develop and vision loss is gradual. It may take many years for loss of vision to be severe. Currently, there is no cure for RP, but there is research that indicates that vitamin A and lutein may slow the rate at which the disease progresses. Your doctor of optometry can give you more specific information on nutritional supplements that may help you. Also, there are many new low vision aids, including telescopic and magnifying lenses, night vision scopes as well as other adaptive devices, that are available that help people maximize the vision that they have remaining. An optometrist, experienced in low vision rehabilitation, can provide these devices as well as advice about other training and assistance to help people remain independent and productive. Since it is an inherited disease, research into genetics may one day provide a prevention or cure for those who have RP. (http://www.aoanet.org/ed-rp.html)

Retinopathy  Retinopathy is damage to the retina caused by microvascular changes.  Diabetic Retinopathy is a disease that affects vision.  It is a progressive disease that destroys capillaries (the smallest blood vessels linking arteries to veins) in the eye by depositing an abnormal material along the walls of the tiny blood vessels in the retina.  Blurred vision and often blindness follow. Upon eye examination mild retinal abnormalities are often seen about seven years after the onset of diabetes, but the damage that threatens vision usually does not occur until much later.  The condition can be treated and vision spared with laser photocoagulation, if it is detected early. (http://www.retinopathy.org /)

Sjogren’s Syndrome  Sjogren’s Syndrome is an autoimmune disease that classically combines dry eyes, dry mouth, and another disease of the connective tissues such as rheumatoid arthritis (most common), lupus, scleroderma or polymyositis. Sjogren's syndrome is an inflammatory disease of glands and other tissues of the body. Inflammation of the glands that produce tears (the lacrimal glands) leads to decreased tears and dry eyes. Inflammation of the glands that produce the saliva in the mouth (salivary glands, including the parotid glands) leads to dry mouth. Sjogren's syndrome can consequently be complicated by infections of the eyes, breathing passages, and mouth. (http://www.medterms.com)

Uvetis. Uveitis is a general term that refers to inflammation or swelling of the eye's structures responsible for its blood supply.  These structures are collectively known as the uveal tract, and include the iris, ciliary body, and choroid.  Uveitis is classified by the structures it affects, the underlying cause, and whether it is chronic (lasting more than 6 weeks), or acute in nature.  There are four main categories of uveitis.  Anterior uveitis (also known as iritis) involves the iris and ciliary body and is the most common type; intermediate uveitis affects the ciliary body, vitreous and retina; posterior uveitis involves the retina, choroid and optic nerve; and diffuse uveitis affects structures both in the front and back of the eye.   (http://www.stlukeseye.com)

Visual Impairment: Visual impairment describes vision that cannot be fully corrected by ordinary prescription lenses, medical treatment, or surgery. Used by the National Center for Health Statistics for studying visual impairment in the population, The term visually impaired describes visual impairment in people who have difficulty reading ordinary newsprint even with correction. Like the term severe visual impairment, visual impairment is used by researchers who study the population, and is not used in clinical references. Severe visual impairment is a term used by researchers at the National Center for Health Statistics (NCHS) to describe visual impairment in people who are unable to read ordinary newsprint even with correction. This term, used primarily for studying visual impairment in the population, is not used in clinical references by eye care professionals. People with a severe visual impairment may or may not be legally blind.

Photographs and Disease Simulations by:  www.NEI.NIH.GOV