Visual impairment , or vision impairment , usually means that someone’s eyesight is reduced (impaired) to the extent that it can’t be corrected to a normal level. This means full correction is not even possible with the help of glasses, contact lenses, medication or vision surgery.

The definition of visual impairment can vary depending on who is using it. Different medical groups, organizations and doctors may use the term in slightly different ways. It can even vary among people who are visually impaired themselves.

Some only use the term to describe visual acuity , the sharpness of one’s vision.

One common method uses these measurements:

• Moderate visual impairment

• Visual acuity: 20/70 to 20/160

• Severe visual impairment

• Visual acuity: 20/200 to 20/400 and/or

• Visual field: 20 degrees or less

• Profound visual impairment

• Visual acuity: 20/500 to 20/1000 and/or

• Visual field: 10 degrees or less

For reference, the standard measurement for normal visual sharpness (after correction) is 20/20 . Someone with 20/20 vision could read small letters on a Snellen eye chart that would look very blurry to someone with impaired visual acuity.

READ MORE: Low vision eye exams

Exact measurements and classifications vary. The term “visual impairment” implies that someone’s vision problems affect their ability to perform certain everyday activities, even with the aid of glasses or contact lenses.

The impact of vision impairment depends on how much — and in what way — someone’s vision is impaired.

People with severe or profound visual impairment may not be able to read any of the letters on a standard eye chart.

Impaired vision can be confused with low vision , which is classified by the level of visual impairment. Low vision is often defined by two major aspects of vision, visual acuity and visual field:

• Decreased visual acuity – Visual acuity that is 20/70 or worse in the better eye with best correction.

• Visual field loss – If the visual field loss is large enough, a person may be classified as having low vision.

The WHO defines a person with low vision as having a central visual field of less than 20 degrees. In the United States, this degree of visual field loss is classified as legally blind.

Total blindness is the absence of all light.

Most people use “low vision” and “visual impairment” with separate definitions, but some medical professionals use them interchangeably. When this happens, an eye doctor might use either term to describe any uncorrectable vision problem that affects someone’s daily life.

Legal blindness , on the other hand, is considered a severe to profound visual impairment.

In the United States, people are considered legally blind when they have 20/200 vision or worse (with best correction) or a visual field less than 20 degrees.

SEE ALSO: How to help someone who is visually impaired

Many different issues can lead to impaired vision. Some cases may develop slowly and worsen over time, while others happen instantly. Some people with impaired vision are born with it or develop it at a young age.

Eye disease

Certain eye conditions can lead to different degrees of permanent vision loss.

Macular degeneration gradually creates a blind spot in the center of someone's vision. This is a type of visual impairment.

In the United States, the most common vision-threatening eye diseases are:

• Macular degeneration

• Diabetic retinopathy

• Glaucoma

• Cataracts

• Serious eye infections

Some of these conditions are treatable, and the odds of successful treatment are usually higher with an early diagnosis.

For example, when eye-teaming problems like amblyopia and strabismus are cared for early in life, a child usually has a much lower chance of their vision becoming impaired later on.

Other conditions cannot be cured. In these cases, an eye doctor may be able to help someone manage the disease and slow the rate of vision loss. Many support groups and foundations are available to help the affected person cope with permanent vision loss and adapt to a new way of life.

Eye problems can also happen before, during or shortly after childbirth. Some cases of retinopathy of prematurity and congenital glaucoma , for example, can lead to impaired vision.

Genetics and family history can also play a role, depending on the circumstances.

Illness

Diseases in other parts of the body can lead to vision impairment too. These problems can lead to vision loss in different ways.

Some diseases affect vision through the central nervous system, while others directly affect the anatomy of the eye(s) . Some illnesses increase the risk of developing a sight-threatening eye infection.

These conditions include:

• Diabetes

• Stroke

• Thyroid disease

• Myasthenia gravis

• Lyme disease

• AIDS

• Tumors located in or around the eye

Injury

Head injuries can have long-lasting or permanent effects on someone’s eyesight.

In order to see, the eyes need to transmit information along a delicate pathway to the brain . If any part of this pathway gets disrupted by an injury, it can cause vision loss.

One analysis called vision loss after head trauma “common.” It notes that injuries can cause eyesight problems when they affect any of the following areas:

• The eye itself

• Optic nerve

• Orbit (area around each eye)

• The vision pathways inside the head

These injuries can be hard to detect, especially when a patient is unconscious and can’t talk to the doctor about their symptoms.

READ MORE: Anchor Center for Blind Children

Vision impairment can come with everyday difficulties, but people with impaired vision can still live a fulfilling, independent life.

Together with doctors and other medical professionals, humanitarian groups and organizations such as the National Federation of the Blind , the American Council for the Blind and Hadley can offer a helping hand to those affected by vision loss.

To learn more about adapting to life with impaired vision, visit All About Vision’s comprehensive, step-by-step guides:

• Adapting your home for a visual impairment

• How to make the internet easier to navigate with impaired vision

• Tips for traveling with impaired vision

• How to get your visually impaired child ready to start school

• Guide to disability benefits for the visually impaired

• How to Get a Guide Dog

SEE RELATED: DeafBlind Awareness Week