Vision Impairment

What is visual impairment?

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 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)

Edited by April Cashin-Garbutt, BA Hons (Cantab)

Types of visual impairment

The World Health Organization (WHO) International Classification of Impairment, Disabilities, and Handicaps (ICIDH) system is used to classify the types of visual impairment.

This system, as the name suggests, is used to classify disorders, impairments, disabilities, and handicaps.

Definition of impairment

Impairment is defined as “any loss or abnormality in an anatomical structure or a physiological or psychological function.”

Similarly a disability is “any restriction or lack (resulting from an impairment) of ability to perform an activity in the manner or within the range considered normal for a human being.”

This places an individual in a handicap that is a person’s disadvantaged position in society due to an impairment or disability.

Definition of visual impairment

Visual impairment is defined as the limitation of actions and functions of the visual system.

The National Eye Institute defines low vision as a visual impairment not correctable by standard glasses, contact lenses, medication or surgery that interferes with the ability to perform activities of daily living.

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.

Causes of visual impairment

Types of impairment are different for different causes of visual impairment. In total vision loss for example there may be total darkness of the visual fields. Other types include visual impairment in glaucoma, age-related macular degeneration and so forth. (1-5)

Visual impairment in glaucoma

This condition is due to the rise of normal fluid pressure inside the eyes. The type of vision is usually like a tunnel.

The intact vision remains in the center while progressively the peripheries start decreasing. The center of the tunnel reduces in size progressively till total vision is lost if left uncorrected.

Age-Related Macular Degeneration

A central area of woolly or cottony opacity obscures the central part of the vision.

The peripheries may be normally seen. AMD usually blurs the sharp, central vision that is needed for closely viewed activities like reading, sewing, and driving. This is a painless condition.


There is general clouding of the vision. As the whole eye lens is affected the blurring of vision may be diffuse until it is totally lost.

There may be other symptoms like photophobia – inability to see the light; diplopia – double vision etc. Cataracts are very common in older people.

Diabetic Retinopathy

Diabetes leads to damage of the smaller arteries and blood vessels at the back of the eyes over the retina.

Diabetic retinopathy is the most common diabetic eye disease and a leading cause of blindness in adults.

Usually vision impairment in diabetics begins as black spots or floating shapes that appear in the field of vision. Slowly complete vision may be lost if left unchecked.

Near sightedness

Myopia or near sightedness or short sightedness means a person can see nearby objects clearly but distant objects appear blurred. High myopia may lead to vision impairment.

Retinitis Pigmentosa

This is a genetic or inherited condition. Initially it manifests as night blindness.

As the disease progresses there may be a tunnelling of vision with loss of peripheral vision followed by complete blindness.

Edited by April Cashin-Garbutt, BA Hons (Cantab)

Causes of visual impairment

There are many causes that may lead to loss of vision or lead to impairment of vision.

Common causes that lead to vision loss or visual impairment include injury to the eye, inherited conditions, infections and so forth. (1-4)

Injury to the eyes

Injury to the eyes while playing or at work or due to accidents may result in vision loss and impairment.

Particularly injuries to the cornea are the commonest cause of vision loss.

Inherited conditions of blindness and vision impairment

Retinitis pigmentosa is the most common cause of inherited blindness.

Infections of the eyes

Sometimes if the mother has had a viral infection like German measles that is transmitted from the mother to the developing fetus during pregnancy the baby may be born with blindness or visual impairment.

Trachoma of the eyes caused by contagious microorganism called Chlamydia trachomatis may also damge eye sight. This is seen in the developing and underdeveloped countries with poor water and sanitation facilities.


This is basically impaired vision in one eye due to lack of its use in early childhood.

This is seen in squint or “lazy eye” since both the eyes project differently and send in different messages to the brain the brain may then turn off or suppress images from the weaker eye.

This stops development of the weaker eye leading to amblyopia in that eye.


Clouding of part or the entire lens of the eye.

Normally, the lens is clear to let in the light that focuses on the retina. Cataracts prevent light from easily passing through the lens, and this causes loss of vision.

This condition usually affects the elderly. Symptoms include cloudy or blurry vision, difficulty in seeing in dimly lit areas and bright lights, colors appear faded, double vision etc.

Cataract affects 20.5 million (1 in 6) Americans age 40 and older. By 80 years of age, more than one half of Americans have cataract.

Cataract is the leading cause of blindness in the world (47.8%) compared to other eye disorders.

Diabetic retinopathy

Diabetes affects the small blood vessels in the retina. When damaged this leads to impairment of vision.

This is the commonest cause of blindness and visual impairment in the United States.

An estimated 23.6 million people in the U.S. have diabetes.

Of those, 5.7 million are undiagnosed. Currently, 1 in 10 individuals has diabetes. Between 40% to 45% of all people with diabetes have diabetic retinopathy.


This condition results due to raised pressure within the eyes. The increased pressure impairs vision by damaging the optic nerve.

This may be seen in older adults and in some babies as well who are born with the condition.

Approximately 2.3 million Americans (1.9%) age 40 and older, have glaucoma.

African-Americans (age 40 and over) are 4 to 5 times more likely than any other ethnic group to have glaucoma

Age related Macular degeneration

Age related Macular degeneration or AMD that is a progressive loss of the visual acuity due to damage to the macula that is the most sensitive part of the retina.

AMD affects more than 1.75 million individuals in the U.S. This number is expected to increase to almost 3 million by 2020 due to the rapid rise in the aging of the U.S. population.

The center of the visual field appears blurry or opaque. The patient is unable to focus clearly. This mainly occurs in the elderly.

The risk of AMD rises in those exposed to excess sunlight and those who smoke excessively.

AIDS related visual impairment

This is usually caused by viral infections of the eyes called Cytomegalovirus or CMV retinitis.

The estimated proportion of persons with AIDS who will develop CMV retinitis ranges from 20 to 40%.

Cancer of the eyes

Retinoblastoma is the most common eye cancer of children. There are between 300 and 400 new cases diagnosed annually.

Edited by April Cashin-Garbutt, BA Hons (Cantab)

Diagnosis of visual impairment

If visual problems are suspected an ophthalmologist usually examines the eyes.

An ophthalmologists specializes in diagnosing, and treating eyes and their diseases.

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.

Edited by April Cashin-Garbutt, BA Hons (Cantab)