Visual impairment,Types and Causes and many more...

The human eye is like a camera that collects, focuses, and transmits light through a lens to create an image of its surroundings. In a camera, the image is created on film or an image sensor. In the eye, the image is created on the retina, a thin layer of light-sensitive tissue at the back of the eye.

Like a camera, the human eye controls the amount of light that enters the eye. The iris (the colored circular part of the eye) controls the amount of light passing through the pupil. It closes up the pupil in bright light and opens it wider in dim light. Thecornea is the transparent, protective surface of the eye. It helps focus light, as does the lens, which sits just behind the iris.

When light enters the eye, the retina changes the light into nerve signals. The retina then sends these signals along the optic nerve (a cable of more than 1,000,000 nerve fibers) to the brain. Without a retina or optic nerve, the eye can't communicate with the brain, making vision impossible.

Julian’s Story

When Julian was almost two years old, he developed this adorable habit of closing one eye when he looked at you. It almost seemed as if he were winking. The possibility that Julian had a visual impairment didn’t initially occur to his parents, but when Julian’s right eye started crossing inward toward his nose…

Off they went to the eye doctor, who confirmed that, yes, Julian had a visual impairment—amblyopia, often called “lazy eye.” As the most common cause of vision problems in children, amblyopia is the medical term used when vision in one eye is reduced because that eye and the brain are not working together properly. (1)  Julian was also very farsighted, especially in the eye he’d taken to closing.

Soon Julian had a brand-new pair of durable glasses suited to his active two-year-old self. The eye doctor also put an eyepatch over Julian’s better eye, so that he would have to use the weaker eye and strengthen its communication with the brain. Otherwise, the eye doctor said, the brain would begin to ignore the images sent by the weaker eye, resulting in permanent vision problems in that eye.

Julian took good care of his glasses, but he didn’t take well to the patch, unfortunately. He ripped it off every time his parents put it on…and back on… and back on again. So today his eye still turns inward if he doesn’t wear his glasses.

What Is Visual Impairment?

Visual impairment is when a person has sight loss that cannot be fully corrected using glasses or contact lenses.

Many people have some type of visual problem at some point in their lives. Some can no longer see objects far away. Others have problems reading small print. These types of conditions are often easily treated with eyeglasses or contact lenses.

But when one or more parts of the eye or brain that are needed to process images become diseased or damaged, severe or total loss of vision can occur. In these cases, vision can't be fully restored with medical treatment, surgery, or corrective lenses like glasses or contacts.

The American Foundation for the Blind estimates that 10 million people in the United States are visually impaired. Visual impairment is a term experts use to describe any kind of vision loss, whether it's someone who cannot see at all or someone who has partial vision loss.

Some people are completely blind, but many others have what's called legal blindness. They haven't lost their sight completely but have lost enough vision that they'd have to stand 20 feet from an object to see it as well as someone with perfect vision could from 200 feet away.

Visual Impairments in Children

Vision is one of our five senses. Being able to see gives us tremendous access to learning about the world around us—people’s faces and the subtleties of expression, what different things look like and how big they are, and the physical environments where we live and move, including approaching hazards.

When a child has a visual impairment, it is cause for immediate attention. That’s because so much learning typically occurs visually. When vision loss goes undetected, children are delayed in developing a wide range of skills. While they can do virtually all the activities and tasks that sighted children take for granted, children who are visually impaired often need to learn to do them in a different way or using different tools or materials. (2) Central to their learning will be touching, listening, smelling, tasting, moving, and using whatever vision they have. (3) The assistance of parents, family members, friends, caregivers, and educators can be indispensable in that process. More will be said about this in a moment.

Types of Visual Impairment

Not all visual impairments are the same, although the umbrella term “visual impairment” may be used to describe generally the consequence of an eye condition or disorder.

The eye has different parts that work together to create our ability to see. When a part of the eye doesn’t work right or communicate well with the brain, vision is impaired.

To understand the particular visual impairment a child has, it’s helpful to understand the anatomy of the eye and the functions of its different parts. Rather than go into those details here, in this general fact sheet, we’re pleased to refer you to the experts for easy-to-understand explanations and diagrams of the visual system.

Most of us are familiar with visual impairments such as near-sightedness and far-sightedness. Less familiar visual impairments include:

• strabismus, where the eyes look in different directions and do not focus simultaneously on a single point;
• congenital cataracts, where the lens of the eye is cloudy;
• retinopathy of prematurity, which may occur in premature babies when the light-sensitive retina hasn’t developed sufficiently before birth;
• retinitis pigmentosa, a rare inherited disease that slowly destroys the retina;
• coloboma, where a portion of the structure of the eye is missing;
• optic nerve hypoplasia, which is caused by underdeveloped fibers in the optic nerve and which affects depth perception, sensitivity to light, and acuity of vision; and
• cortical visual impairment (CVI), which is caused by damage to the part of the brain related to vision, not to the eyes themselves.

There are also numerous other eye conditions that can cause visual impairment. For a more comprehensive glossary of conditions, here are two resource pages you’ll find helpful:

What Causes Visual Impairment?

People rarely lose their eyesight during their teen years. When they do, it's usually caused by an injury like getting hit in the eye or head with a baseball or having an automobile or motorcycle accident.

Some babies have congenital blindness, which means they are visually impaired at birth. Congenital blindness can be caused by a number of things — it can be inherited, for instance, or caused by an infection (like German measles) that's transmitted from the mother to the developing fetus during pregnancy.

Conditions that may cause vision loss after birth include:

• Amblyopia (pronounced: am-blee-OH-pee-uh) is reduced vision in an eye caused by lack of use of that eye in early childhood. Some conditions cause a child's eyes to send different messages to the brain (for example, one eye might focus better than the other). The brain may then turn off or suppress images from the weaker eye and vision from that eye then stops developing normally. This is also known as a "lazy eye." Strabismus (misaligned or crossed eyes) is a common cause of amblyopia, since the brain will start to ignore messages sent by one of the misaligned eyes.
• Cataracts are cloudy areas in part or all of the lens of the eye. In people without cataracts, the lens is crystal clear and allows light to pass through and focus on the retina. Cataracts prevent light from easily passing through the lens, and this causes loss of vision. Cataracts often form slowly and usually affect people in their 60s and 70s, but sometimes babies are born with congenital cataracts. Symptoms include double vision, cloudy or blurry vision, difficulty seeing in poorly lit spaces, and colors that seem faded.
• Diabetic retinopathy (pronounced: reh-ton-AH-pa-thee) occurs when the tiny blood vessels in the retina are damaged due to diabetes. People with retinopathy may not have any problems seeing at first. But if the condition gets worse, they can become blind. Teens who have diabetes should be sure to get regular eye exams because there are no early warning signs for this condition. To help prevent retinopathy, people with diabetes should also avoid smoking, keep their blood pressure under control, and keep their blood sugar at an even level.
• Glaucoma is an increase in pressure inside the eye. The increased pressure impairs vision by damaging the optic nerve. Glaucoma is mostly seen in older adults, although babies may be born with the condition and children and teens can sometimes develop it as well.
• Macular (pronounced: MAH-kyoo-lur) degeneration is a gradual and progressive deterioration of the macula, the most sensitive region of the retina. The condition leads to progressive loss of central vision (the ability to see fine details directly in front). Macular degeneration is often age related (it occurs in older people, especially older than 60), but sometimes it can occur in younger people. Excessive exposure to sunlight and smoking can increase the risk for age-related macular degeneration. Symptoms may include increased difficulty reading or watching TV, or distorted vision in which straight lines appear wavy or objects look larger or smaller than normal.
• Trachoma (pronounced: truh-KO-muh) occurs when a very contagious microorganism called Chlamydia trachomatis causes inflammation in the eye. It's often found in poor rural countries that have overcrowded living conditions and limited access to water and sanitation. Blindness due to trachoma has been virtually eliminated from the USA.
• • age-related macular degeneration (AMD) – where the central part of the back of the eye (the macula, which plays an important role in central vision) stops working properly
  • cataracts – where cloudy patches can form within the lenses of the eyes
  • glaucoma – where fluid builds up inside the eye, damaging the optic nerve (which relays information from the eye to the brain)
  • diabetic retinopathy – where blood vessels that supply the eye become damaged from a build-up of glucose

What Do Doctors Do?

If you, your parent, or your doctor suspects a visual problem, you'll probably pay a visit to an ophthalmologist (pronounced: af-thal-MAH-luh-jist), a medical doctor who specializes in examining, diagnosing, and treating eyes and eye diseases. When someone goes for an examination, the ophthalmologist will look at the structure of that person's eye.

Other simple tests an ophthalmologist may perform include:

• Visual acuity test. A person reads an eye chart to measure how well he or she sees at various distances.
• Visual field test. Ophthalmologists use this test to measure side, or peripheral, vision.
• Tonometry test. This test determines the fluid pressure inside the eye to evaluate for glaucoma.

If your doctor determines that you have an eye condition that is likely to cause visual impairment, many treatments are available. Options may include eyeglasses, contact lenses, and eye drops or other medicines.

In some cases, surgery may be required. For instance, cataracts are often treated by removing the clouded lens and replacing it with an intraocular lens (an artificial plastic lens that requires no special care and restores vision).

Other methods can compensate for vision loss. Guide dogs can help people get from place to place independently. Braille allows those with visual impairment to read and write. Special equipment such as microscopic and telescopic glasses and voice-recognition software can make school and homework easier.

What's It Like to Be Visually Impaired?

Just as you don't think about your eye color every day, people with visual impairment don't always think about their condition every day either.

Someone with sight problems can become isolated from others more easily, though. If a visually impaired person asks for assistance, don't hesitate to help. But someone who uses a cane or a guide dog is probably self-sufficient and may not need help.

By Dr Ananya Mandal, MD

The definition of vision impairment by the Centers for Disease Control and Prevention (CDC) says a visually impaired person’s eyesight cannot be corrected to a “normal level”.

It may be said that visual impairment is the functional limitation of the eye or eyes or the vision system. This leads to (1-5) –

• Loss of visual acuity and inability of the person to see objects as clearly as a healthy person
• Loss of visual field meaning inability of an individual to see as wide an area as the average person without moving the eyes or turning the head.
• Photophobia – inability to look at light
• Diplopia – double vision
• Visual distortion or distortion of images
• Visual perceptual difficulties or difficulties of perception
• Or any combination of the above features

Low visual acuity

The CDC and the World Health Organization suggest that low visual acuity means vision between 20/70 and 20/400 with the best possible correction, or a visual field of 20 degrees or less.

Blindness

Blindness is defined as a visual acuity worse than 20/400 with the best possible correction, or a visual field of 10 degrees or less.

Legal blindness in the US means visual acuity of 20/200 or worse with the best possible correction, or a visual field of 20 degrees or less. (1)

Measuring visual acuity

Visual acuity is measured by use of a Snellen’s chart.

The chart consists of random letters and numbers of various sizes set at a distance of 6 feet.

The best possible vision if 6/6. Visual field is the width of the vision without moving or turning the head. It is measured in degrees.

Causes of vision impairment

Vision may be impaired due to multiple reasons. These could be due to eye damage, failure of the brain to receive and read the visual cues sent by the eyes etc.

Underlying diseases may also cause visual impairment. The commonest cause is diabetic retinopathy, age-related macular degeneration, formation of cataracts and raised pressure within the eyes leading to glaucoma.

Who does visual impairment affect?

Although impairment of vision can occur at any point in life, it is more common among the elderly.

Sometimes visual impairment may be inherited. It manifests at birth or in childhood in such cases.

Common conditions are retinitis pigmentosa, genetic or developmental abnormalities etc.

These children with partial or complete impairment are often developmentally delayed especially in gross and fine motor skills. Visually impaired adults struggle with gainful employment and day to day activities. (1-5)

How many people are visually impaired?

Some estimates suggest the approximately 10 million people in the United States are blind or visually impaired.

Some sources suggest that one million adults over the age of 40 are blind, and 2.4 million are visually impaired.

It is estimated that as the elderly population swells over the years, the number of adults with vision impairments is expected to double.

Figures also show that only 46% of working-age adults have vision impairments and 32% of legally blind adults of working age are gainfully employed. (1-5)

Types of visual impairment

According to the CDC and the World Health Organization the classification of visual acuity and impairment includes (1, 2) –

• Low visual acuity means vision between 20/70 and 20/400 with the best possible correction, or a visual field of 20 degrees or less
• Blindness is defined as a visual acuity worse than 20/400 with the best possible correction, or a visual field of 10 degrees or less
• Legal blindness in the US means visual acuity of 20/200 or worse with the best possible correction, or a visual field of 20 degrees or less.
• Visual acuity of 20/70 to 20/400 (inclusive) is considered moderate visual impairment or low vision.

Diagnosis of visual impairment in children

An ophthalmologist, optometrist, paediatrician or other specialist may examine a child between birth and 3 months, between 6 months to 1 year, around 3 years or 5 years of age.

This is especially important if there is someone in the family with vision problems. (1-4)

Signs of vision problems in children

Signs of vision problems in children include (3) –

• The child closes or covers one eye
• The child squints the eyes or frowns
• He or she complains that things are blurry or difficult to see
• He or she has trouble reading or doing other close-focussed work, or holds objects close to eyes
• The child blinks more than usual or seems frustrated when doing close-up work (such as looking at books)
• Eyes could be watery, and eyelids could look red-rimmed, swollen or crusted with secretions

Process of diagnosis

Diagnosis begins with complete history and family history of visual impairment in the patient.

External examination of the eye including lids, conjunctiva, cornea, iris, lens, etc. are the next step.

Tests for visual impairment

The tests that are used to diagnose visual acuity and visual impairment include the Snellen test, visual field test and so forth. (1-4)

Snellen test

The Snellen test is also known as the visual acuity test.

Usually a chart called the Snellen’s chart is used. It contains progressively shortening random letters and numbers and is placed 6 meters away from the patient.

The patient is asked to read the letters with each eye separately and both together. Ability to read the letters at each size determines the visual acuity.

After the test a score is obtained. It is made up of two numbers.

The first number represent how far away from the chart the patient was when he or she was able to successfully read the letters on the chart.

The second number represents how far away a person with healthy vision should be able to read the chart.

Healthy vision scores 6/6. If the score is 6/60, it means that the patient can only read something 6 metres away what a person with healthy eyesight can read 60 metres away.

Being partially sighted, or sight impaired means if level of sight loss is moderate and blindness, or severe sight impairment means when level of vision loss is so severe that a person is unable to complete any activities that require eyesight.

Partial sight or sight impairment is defined as 3/60 to 6/60 vision or having a combination of moderate visual acuity (up to 6/24) and a reduced field of vision.

Blindness is defined as having poor visual acuity (less than 3/60) but having a full field of vision or having poor visual acuity (between 3/60 and 6/60) and a severe reduction in the vision field or having average visual acuity (6/60 or above) and an severely reduced field of vision.

Visual field test

Visual field is the range of vision that a person can see without tilting or turning one’s head. This measures the peripheral vision of the eyes.

The test uses a device strapped over the patient’s eyes. Lights are flashed on and off in the patient’s peripheral vision.

He or she is then asked to press a button every time they see a light. Any gap of field of vision is detected.

Tonometry test

This test uses specialized instruments to determine fluid pressure inside the eye to evaluate for glaucoma.

Ocular Motility Assessment

This tests if there is squint of other problems in the movement of the eyeballs.

Other tests

Other tests like Visually evoked potential (VEP), Electroretinogram (ERG), Electro-oculogram (EOG) are sometimes prescribed to test if the signals from the eye are travelling adequately to the brain.

These may help if the patient is very young and clinical examination is difficult or if there are multiple handicaps that make diagnosis difficult.

Considerations for treatment

For management for patients some considerations are important (3) –

• The degree of impairment, disability, or handicap
• Cause of visual impairment and possible outcome
• Age of the patient and level of development
• General health of the patient
• Patient’s other impairment and adjustment to the vision loss and patient’s expectations from therapy

Treatments for visual impairment

Treatment and management of visual impairment include (1-6):

• Control of diabetes – This prevents progression of diabetic retinopathy and resulting visual impairment to some extent. However in advanced cases this may be of little help.
• Cataract surgery – In cases of visual impairment due to cataracts, surgery may be opted. Usually an artificial lens is placed within the eye as a replacement of the damaged and clouded lens. This, more often than not restores vision.
• Magnification systems – This is achieved using appropriate lens, reading telescopes or surgical systems etc. Hand magnifiers may also be offered. For improvement of visual fields prisms and mirror systems may be prescribed.
• Glaucoma – medications in eye drops are prescribed to control glaucoma. These may contain drugs like Latanoprost, tafluprost, travoprost which are prostaglandin analogues.The drugs may include Beta-blockers like betaxolol hydrochloride or carbonic anhydrase inhibitors dorzolamide and Sympathomimetics like brimonidine tartrate.
  Glaucoma may also be managed surgically using open microscopic surgery, laser corrections, etc.
• At present there are no cures for age-related macular degeneration (AMD) especially of the dry variety.For the wet AMD there are therapies that help to stop the progression. These include Photodynamic therapy. This uses a light-sensitive medicine called verteporfin injected into a vein of the arm.
  The drug identifies the abnormal blood vessels in the eyes and attaches itself to the proteins in these blood vessels. This is followed by a low-powered laser therapy.
  Some new types of medications called Anti-VEGF medication may also be used. One example is Ranibizumab that has recently been approved by the National Institute for Health and Clinical Excellence (NICE) for use on the NHS.

The Help Available under IDEA

If you suspect (or know) that your child has a visual impairment, you’ll be pleased to know there’s a lot of help available under IDEA—beginning with a free evaluation of your child. IDEA requires that all children suspected of having a disability be evaluated without cost to their parents to determine if they do have a disability and, because of the disability, need special services under IDEA. Those special services are:

Early intervention | A system of services to support infants and toddlers with disabilities (before their 3rd birthday) and their families.

Special education and related services | Services available through the public school system for school-aged children, including preschoolers (ages 3-21).

Visual impairment, including blindness, is one of the disabilities specifically mentioned and defined in IDEA. If a child meets the definition of visual impairment in IDEA as well as the State’s criteria (if any), then he or she is eligible to receive early intervention services or special education and related services under IDEA (depending on his or her age).

Accessing early intervention (EI) 
To identify the EI program in your neighborhood, consult NICHCY’s State Organizations page for your state, online at:

Developing a written plan of services

In both cases—in early intervention for a baby or toddler with a visual impairment and in special education for a school-aged child, parents work together with program professionals to develop a plan of services the child will receive based on his or her needs. In early intervention, that plan is called the IFSP (individualized family service plan). In special education, the plan is called the IEP (individualized education program). Parents are part of the team that develops their child’s IFSP or IEP.

There’s a lot to know about early intervention for infants and toddlers with disabilities and about special education and related services for school-aged children. Visit NICHCY’s website and find out more about these crucial services for eligible children with visual impairments, beginning at: