Physical Structures of the Eye Important for Vision Cornea-The outermost layer of the eye, it refracts light into the lens, which further focuses it onto the retina. Iris-The colored part of the eye, which helps to regulate light intake by regulating pupil size. Pupil-The opening at the center of the iris which expands and contracts to regulate light intake. Macula-Small centrally located area on retina that gives “central vision.” (NEI. 2011) Optic Nerve-Carries electrical stimuli from the retina to the brain. (NEI. 2011)
Lens Lens- A transparent structure that bends in order to focus light onto the retina. It consists of many layers “like an onion.” (National Eye Institute,2011). The lens thickens as you like at something close and thins as you look in the distance Green-Shows the layers of the lens. Blue-Nuclei of other cells in the eye. National Eye Institute (2011).
Physical Structures of Eye Retina-A layer at the back of the eye where light hits photoreceptor cells. There are two types of these nerve cells. Rods and cones, which receive light and convert this into electrical signals delivered to the brain. Rods allow for night vision. Cones allow for color vision and vision in a brightly lit environment. There are three types: 1.S cones which are sensitive to shorter wavelengths in the VLS (blue), 2.M cones which are sensitive to medium wavelengths (green). 3.L cones which are sensitive to longer wavelengths (red).
National Eye Institute (2011). Retina: Night Vision Shows the interactions of nerve cell networks operating during night vision (rod use.) Blue-Nerve endings of rod bipolar cells which receive info from the rods. Green- Helps facilitate communication through neurotransmitters.
Cone Cells-Color and Bright Light Vision Cells in blue are S cones, sensitive to short wavelength blue light. Other cones are M cones sensitive to medium wavelength green light. National Eye Institute92011).
How does the Eye Work? Light first enters the eye through the cornea which helps refract light into the lens. The amount of light arriving at the lens is controlled by the iris expanding or contracting the pupil, which functions much like a camera aperture. As light passes through the lens it is focused on the back of the eye at the retina. Millions of photoreceptor cells in the retina convert this light energy into electrical signals. These signals travel through the optic nerve and reach the brain, where the brain interprets them.
Common Measures of Visual Acuity Snellan Chart Snellan Chart Developed in the mid 19 th Century by the Dutch ophthalmologist Snellan, this chart measures vision by determining the smallest line a patient can read from 20 feet.
Common Measures of Visual Acuity ETDRS Chart ETDRS Chart Developed in 1970’s Backlit by a lightbox to ensure equal illumination. Has the same number of letters on all lines. Lines diminish in size based on a mathematical formula. Measures the smallest line read from 20 feet.
Isn’t 20/20 a show on CBS? How are vision results reported and what do they mean? The first number refers to the patient’s vision at 20 feet. The second number refers to how many feet away a person with average vision can read the letters. 20/20 is average vision. 20/15 is above average vision. What a visually average person can see at 15 feet, the patient can see from 20 feet. 20/50 is below average vision. What a person with average vision can see from 50 feet, the patient must be at 20 feet to see.
Visual Impairments “Low vision is defined as a visual impairment not corrected by standard eyeglasses, contact lenses, medication, or surgery that interferes with the ability to perform everyday activities.” Defined by the WHO as only correctable to 20/60 or less “(Central Association for the Blind and Visually Impaired, 2012). Partially Sighted- Refers to a loss of vision that results in a student needing Special Education Services.
Blind Total Blindness- Is marked by a complete lack of light or form. Legal Blindness- Defined in North America as 20/200 visual acuity or less or as having a field of vision of 20 degrees or less which are not correctable.
What are the major causes of blindness and visual impairment?
Globally The major causes of visual impairment are: –Uncorrected refractive errors (myopia, hyperopia or astigmatism): 43% –Cataract: 33% –Glaucoma: 2% 80% of all visual impairment can be prevented or cured.
United States The major causes of visual impairment in the U.S are: –Macular degeneration –Cataract –Glaucoma –Diabetic retinopathy
Macular Degeneration Macular degeneration affects the macula. Associated with aging that gradually destroys sharp, central vision. –Central vision is needed for seeing objects clearly and for common daily activities like reading and driving.
Two forms of macular degeneration Wet: occurs when abnormal blood vessels behind the retina start to grow under the macula. These new blood vessels tend to be very fragile and often leak blood and fluid. The blood and fluid raise the macula from its normal place at the back of the eye. Damage to the macula occurs rapidly. Dry (more common): occurs when the light-sensitive cells in the macula slowly break down, gradually blurring central vision in the affected eye. As dry AMD gets worse, you may see a blurred spot in the center of your vision. Over time, as less of the macula functions, central vision is gradually lost in the affected eye.
Cataract A clouding of the lens in the eye that affects vision. Most cataracts are related to aging. –The lens is a clear part of the eye that helps to focus light, or an image, on the retina. –The lens must be clear for the retina to receive a sharp image. –If the lens is cloudy from a cataract, the image you see will be blurred.
Types of cataracts Secondary cataract. Cataracts can form after surgery for other eye problems, such as glaucoma. Cataracts also can develop in people who have other health problems, such as diabetes. Cataracts are sometimes linked to steroid use. Traumatic cataract. Cataracts can develop after an eye injury, sometimes years later. Congenital cataract. Some babies are born with cataracts or develop them in childhood, often in both eyes. These cataracts may be so small that they do not affect vision. If they do, the lenses may need to be removed. Radiation cataract. Cataracts can develop after exposure to some types of radiation.
Glaucoma Glaucoma is a group of diseases that damage the eye’s optic nerve –The optic nerve is a bundle of more than 1 million nerve fibers. It connects the retina to the brain. –A healthy optic nerve is necessary for good vision. Congenital glaucoma in infants is a rare condition that may be inherited. It is the result of incorrect or incomplete development of the eye drainage canals before birth. –Can be treated with medication and surgery.
Diabetic retinopathy Most common diabetic eye disease Caused by changes in the blood vessels of the retina. –In some people with diabetic retinopathy, blood vessels may swell and leak fluid. –In other people, abnormal new blood vessels grow on the surface of the retina.
Diabetic retinopathy has four stages Mild Nonproliferative Retinopathy. At this earliest stage, microaneurysms occur. They are small areas of balloon-like swelling in the retina's tiny blood vessels. Moderate Nonproliferative Retinopathy. As the disease progresses, some blood vessels that nourish the retina are blocked. Severe Nonproliferative Retinopathy. Many more blood vessels are blocked, depriving several areas of the retina with their blood supply. These areas of the retina send signals to the body to grow new blood vessels for nourishment. Proliferative Retinopathy. At this advanced stage, the signals sent by the retina for nourishment trigger the growth of new blood vessels. This condition is called proliferative retinopathy. These new blood vessels are abnormal and fragile. They grow along the retina and along the surface of the clear, vitreous gel that fills the inside of the eye. By themselves, these blood vessels do not cause symptoms or vision loss. However, they have thin, fragile walls. If they leak blood, severe vision loss and even blindness can result.
Retinal Disorders Retinoblastoma Cancer that develops in the cells of the retina Mutation of chromosome 13, the RBI Gene Portions of the code are missing or altered Can develop in the early stages of fetal development Symptoms -Enlarged or dilated pupil -Deterioration of vision -Red and irritated eye -Inflammation of tissue around eye -Crossed eyes -Faltering grown or delayed development
Retinal Disorders Retinitis Pigmentosa Deformation in the receptors of the retina Retina has difficulty perceiving light Progressive dysfunction for the photoreceptors (rods and cones) Leads to cell loss and eventual deteriorate of several retina layers Symptoms - Defective dark adaptation (night blindness) -Loss of central vision -Reduction of the peripheral visual field (tunnel vision)
Retinal Disorders Retinal Detachments Occur when the retina peels away from the underlying layer of the support tissue Occur when the retina peels away from the underlying layer of the support tissue Without rapid treatment the entire retina may detach and lead to vision loss and blindness Without rapid treatment the entire retina may detach and lead to vision loss and blindness Usually occurs in one eye but has a 15% chance of developing in the other eye Usually occurs in one eye but has a 15% chance of developing in the other eye
Retinal Disorders Retinal Detachments 1. Tear, break, or hole in the retina allowing fluid from the middle of the eye to collect under the retina 1. Tear, break, or hole in the retina allowing fluid from the middle of the eye to collect under the retina 2. Inflammation, injury or vascular abnormalities that results in fluid accumulating underneath the retina. Fluid buildup causes the layers of the retina to separate 2. Inflammation, injury or vascular abnormalities that results in fluid accumulating underneath the retina. Fluid buildup causes the layers of the retina to separate 3.Traction pulls the retina away from the layers beneath it 3.Traction pulls the retina away from the layers beneath itSymptoms -Flashes of light at the edge of the field of vision -Increase in the number of floaters (strands of vitreous gel) -See a shadow across part of the visual field that does not go away -New or sudden vision loss
Retinal Disorders Retinopathy of Prematurity Affects babies who are born premature/low birth weight Scaring or retinal detachment Five Stages I: Mildly abnormal blood vessel growth II: Moderately abnormal blood vessel growth III: Blood vessel growth is severely abnormal. IV: Severely abnormal growth and partially detached retina V: There is a total retinal detachment Symptoms - Abnormal eye movements -Crossed eyes -Severe nearsightedness -White-looking pupils
Optic Nerve Disorders Optic Nerve Atrophy Tissue death of the nerve that carries information from the eye to the brain Caused by: -Poor blood flow -Damage due to shock, toxic substances, radiation, or trauma -Various eye, brain, and central nervous system diseases Symptoms - Dimming of vision -Loss of the ability to see fine detail -Color seems faded -The pupils reaction to light may diminish and may eventually be lost
Optic Nerve Disorders Hemianopia Common after stroke and/or brain injury Symptoms decreased vision or blindness in one quarter or half of the visual field of one or both eyes -Blind on the same side in both eyes. This allows only objects to the right or to the left sides to be seen.
Optic Nerve Gliomas Neurofibromatosis Genetic disorders that cause tumors to grow in the nervous system Genetic disorders that cause tumors to grow in the nervous system tumors grow on nerves and produce other abnormalities such as skin changes, bone deformities, and/or damage to the eyes tumors grow on nerves and produce other abnormalities such as skin changes, bone deformities, and/or damage to the eyesSymptoms - Cataracts at an early age or changes in the retina that may affect vision -Two or more growths on the iris or the optic nerve
Disorders of the brain that affect vision Cortical Blindness Abnormal visual responses due to brain dysfunction that cannot be attributed to the eyes themselves Due to: - infections like Meningitis and Encephalitis -Lack of oxygen -Drugs during pregnancy Symptoms - Abnormal light response -Avoidant social gaze -Brief fixations and intermittent gaze -Poor visual perception -Visual field loss
Congenital Cataracts What is it? –Congenital cataract is the clouding of the lens of the eye which is present at birth. Causes –Congenital cataracts are rare and in most patients no cause has been found. They may occur in association with other medical problems such as: Down Syndrome Trisomy 13 RubellaSymptoms –Grey or white cloudiness of the pupil (which is normally black) –Infant does not seem to be able to see –“Red eye” glow of the pupil is missing in photos, or is different between the two eyes. –Unusual rapid eye movements
Congenital Glaucoma What is it? –Children with congenital glaucoma are born with a drainage defect which slows the normal drainage of fluid and leads to a higher eye pressure than normal. This occurs in about one out of 10, 000 infants. –It affects males twice as often as females and has no racial preference. –In 2/3rds of the cases it affects both eyes. Causes –Heritable and is caused by abnormal eye development. Symptoms –Symptoms are usually noticed when the child is a few months old. These include the following: –Cloudy eyes –Sensitivity to light –Excessive tearing –Enlargement of one eye or both eyes
Ocular Albinism What is it? –Ocular albinism in an inherited condition in which the eyes lack melanin pigment which is needed for the growth processes that occur before birth. Therefore, the fovea does not fully develop. Causes –Ocular albinism is caused by mutations in the GPR143 gene. This gene provides instructions for making protein that plays a role in pigmentation of the eyes and skin. Symptoms –Reduced visual acuity from 20/60 to 20/400 and sometimes as good as 20/25 in African-Americans, –Reduced visual acuity from 20/60 to 20/400 and sometimes as good as 20/25 in African-Americans, –Nystagmus (involuntary back-and-forth movement of the eyes) –Strabismus (crossed eyes or "lazy" eye) –Sensitivity to bright light and glare. –Sensitivity to bright light and glare.
Achromatopsia What is it? –It is a rare hereditary vision disorder that affects 1 in 33, 000 in the U.S. –Those who have achromatopsia do not have normal “cone vision.” The eyes of achromats, lacking normal cone vision and having only rod vision, are not able to adapt normally to higher levels of illumination. Rods do not provide color vision or good detail vision. Therefore, persons with achromatopsia are either totally colorblind or almost totally colorblind, and they have poor visual acuity. –Those who have achromatopsia do not have normal “cone vision.” The eyes of achromats, lacking normal cone vision and having only rod vision, are not able to adapt normally to higher levels of illumination. Rods do not provide color vision or good detail vision. Therefore, persons with achromatopsia are either totally colorblind or almost totally colorblind, and they have poor visual acuity. Cause –Achromatopsia is hereditary. Symptoms –Photophobia (sensitivity to light) –Poor visual acuity –Color blindness –Nystagmus
Aniridia What is it? –It is a condition in which the individual is born without the iris. The iris is the colored part of the eye that controls the amount of light that goes into the eye. Cause -2/3 rd of the time it is hereditary. Symptoms –Decreased visual acuity –Photophobia (sensitivity to light) –Dry eye
What is it? –Individuals with hyperopia have difficulty viewing objects up close. Farsightedness occurs if your eyeball is too short or the cornea has too little curvature, so light entering your eye is not focused correctly. Cause –This vision problem occurs when light rays entering the eye focus behind the retina, rather than directly on it. The eyeball of a farsighted person is shorter than normal. Symptoms –Common signs of farsightedness include difficulty in concentrating and maintaining a clear focus on near objects, eye strain, fatigue and/or headaches after close work, aching or burning eyes, irritability or nervousness after sustained concentration. Hyperopia (farsightedness) Hyperopia (farsightedness)
Myopia (Nearsightedness) What is it? –Nearsightedness is when light entering the eye is focused incorrectly, making distant objects appear blurred. Nearsightedness is a type of refractive error of the eye. Cause –Occurs equally in males and females. –People with a family history of Myopia than they are more likely to develop it. Symptoms –Difficulty seeing objects at a distance. –Eyestrain
Amblyopia (Lazy Eye) What is it? –Amblyopia, or "lazy eye," is the loss of one eye's ability to see details. It is the most common cause of vision problems in children. Causes –Amblyopia occurs when the nerve pathway from one eye to the brain does not develop during childhood. This occurs because the abnormal eye sends a blurred image or the wrong image to the brain. –Strabismus is the most common cause of amblyopia. There is often a family history of this condition. –Childhood cataracts –Farsightedness, nearsightedness, or astigmatism, especially if it is greater in one eye. Symptoms –Eyes that turn in or out –Eyes that do not appear to work together –Inability to judge depth correctly –Poor vision in one eye
Astigmatism What is it? –Astigmatism is a condition in which the uneven curvature of the cornea blurs and distorts both distant and near objects. The cornea is abnormally curved like the back of a spoon which causes vision to be out of focus. Causes –The specific cause is unknown. It is usually present from birth, and often occurs together with nearsightedness or farsightedness. Symptoms Symptoms –Astigmatism makes it difficult to see fine details, either close up or from a distance.
Educational Implications Student may need optical aids like: - sunglasses or tinted contact lenses to cope with bright light in the classroom -eye patch -thick glasses -protective eyewear during sports and other hazardous activities. Student may need magnifiers to help with reading Increased illumination Some are recommended to avoid contact sports and strenuous activity
Educational Implications –Some students may have difficulties reading and playing ball sports due to the low visual acuity. –Some potential modifications include: Moving the student’s seat to the front row so the student can see the chalkboard, marker board, overhead projector, or video Changing where the student sits so they can avoid glare from overhead lights or windows. Provide the student with a hard copy of the material that is presented on the white board or via the overhead project. Allow the student to copy another student’s notes if they can’t see the board. Enlarge tests and handouts.
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