Medically Reviewed on 5/4/2023

What is blindness?

Color blindness is the inability to perceive differences in various shades of colors, particularly green and red, that others can distinguish.

Blindness is defined as the state of being sightless. A blind individual is unable to see. In a strict sense, the word "blindness" denotes the inability of a person to distinguish darkness from bright light in either eye. The terms blind and blindness have been modified in our society to include a wide range of visual impairments.

Blindness is frequently used today to describe a severe visual decline in one or both eyes with the maintenance of some residual vision.

Vision impairment, or low vision, means that even with eyeglasses, contact lenses, medicine, or surgery, someone doesn't see well. Vision impairment can range from mild to severe. Worldwide, between 300 million-400 million people are visually impaired due to various causes. Of this group, approximately 50 million people are totally blind. Approximately 80% of blindness occurs in people over 50 years of age.

When is one considered legally blind?

Legal blindness is not a medical term. It is defined by lawmakers in nations or states to either limit allowable activities, such as driving, by individuals who are "legally blind" or to provide preferential governmental benefits to those people in the form of educational services or monetary assistance.

Under the Aid to the Blind program in the Social Security Act passed in 1935, the United States Congress defined legal blindness as either central visual acuity of 20/200 or less in the better eye with corrective glasses or central visual acuity of more than 20/200 if there is a visual field defect in which the peripheral field is contracted to such an extent that the widest diameter of the visual field subtends an angular distance no greater than 20 degrees in the better eye. Blindness in one eye is never defined as legal blindness if the other eye is normal or near-normal.

It is estimated that more than 1 million people in the United States meet the legal definition of blindness.

What are the three types of blindness?

There are three main types of blindness, which include the following:

  • Color blindness is the inability to perceive differences in various shades of colors, particularly green and red, that others can distinguish. It is most often inherited (genetic) and affects about 8% of males and under 1% of women. People who are color blind usually have normal vision otherwise and can function well visually. This is not true blindness.
  • Night blindness is difficulty in seeing under situations of decreased illumination. It can be genetic or acquired. The majority of people who have night vision difficulties function well under normal lighting conditions; this is not a state of sightlessness.
  • Snow blindness is a loss of vision after exposure of the eyes to large amounts of ultraviolet light. Snow blindness is usually temporary and is due to the swelling of cells on the corneal surface. Even in the most severe cases of snow blindness, the individual is still able to see shapes and movement.

People often say, "I am 'blind as a bat' without my glasses." All bat species have eyes, and most have excellent vision at night but not in daylight. More importantly, the term blindness means the inability to see despite wearing glasses. Anyone who has access to glasses and sees well with the glasses cannot be termed blind.


The colored part of the eye that helps regulate the amount of light that enters is called the: See Answer

What are the causes of blindness?

The many causes of blindness differ according to the socio-economic condition of the nation being studied. In developed nations, the leading causes of blindness include:

In third-world nations where 90% of the world's visually impaired population lives, the principal causes are infections, cataracts, glaucoma, injury, and inability to obtain any glasses. In developed nations, the term blindness is not used to describe those people whose vision is correctable with glasses.

Infectious causes in underdeveloped areas of the world include:

The most common infectious cause of blindness in developed nations is herpes simplex.

Other causes of blindness include:

  • Vitamin A deficiency
  • Retinopathy of prematurity
  • Blood vessel diseases involving the retina or optic nerve include:
    • Stroke, infectious diseases of the cornea or retina
    • Ocular inflammatory disease
    • Retinitis pigmentosa
    • Primary or secondary malignancies of the eye, congenital abnormalities, hereditary diseases of the eye
    • Chemical poisoning from toxic agents such as methanol

What are risk factors for blindness?

A principal risk factor for blindness is living in a third-world nation without ready access to modern medical care.

Other risk factors for blindness include:

What are signs and symptoms of blindness?

All people who are blind or have visual impairment have the common symptom of difficulty seeing. People with similar levels of visual loss may have very different responses to that symptom.

  • If one is born blind, there is much less adjustment to a non-seeing world than there is for people who lose their vision late in life, where there may be limited ability to cope with that visual loss.
  • Support systems available to individuals and their psychological makeup will also modify the symptom of lack of sight.
  • People who lose their vision suddenly, rather than over the years, also can have more difficulty adjusting to their visual loss.

Associated symptoms of blindness may be present or absent, depending on the underlying cause of the condition, and may include:

  • Discomfort in the eyes
  • Awareness of the eyes
  • Foreign body sensation
  • Pain in the eyes or discharge from the eyes

A blind person may have no visible signs of any abnormalities when sitting in a chair and resting. However, when blindness is a result of an infection of the cornea (the dome in front of the eye), the normally transparent cornea may become white or gray, making it difficult to view the colored part of the eye.

In blindness from cataracts, the normally black pupil may appear white. Depending on the degree of blindness, the affected individual will exhibit signs of visual loss when attempting to ambulate. Some blind people have learned to look directly at the person they are speaking with, so it is not obvious they are blind.

What specialists diagnose and treat blindness?

Ophthalmology is the specialty of medicine that deals with the diagnosis and medical and surgical treatment of eye disease. Therefore, ophthalmologists are specialists who have the knowledge and tools to diagnose the cause of blindness and to provide treatment, if possible.

How do healthcare professionals diagnose blindness?

Blindness is diagnosed by testing each eye individually and by measuring the visual acuity and the visual field, or peripheral vision. People may have blindness in one (unilateral blindness) or both eyes (bilateral blindness). Historical information regarding blindness can help diagnose the cause of blindness. Poor vision that is sudden in onset differs in potential causes from blindness that is progressive or chronic. Temporary blindness differs in cause from permanent blindness. The cause of blindness is made by a thorough examination by an ophthalmologist.

What is the treatment for blindness?

The treatment of visual impairment or blindness depends on the cause.

  • In third-world nations where many people have poor vision as a result of a refractive error, merely prescribing and giving glasses will alleviate the problem.
  • Nutritional causes of blindness can be addressed by dietary changes.
  • There are millions of people in the world who are blind from cataracts. In these patients, cataract surgery would, in most cases, restore their sight.
  • Inflammatory and infectious causes of blindness can be treated with medication in the form of drops or pills.
  • Corneal transplantation may help people whose vision is absent as a result of corneal scarring.

What is the prognosis for blindness?

The prognosis for blindness is dependent on its cause. In patients with blindness due to optic nerve damage or a completed stroke, visual acuity can usually not be restored. Patients with long-standing retinal detachment, in general, cannot be improved with surgical repair of their detachment. Patients who have corneal scarring or cataract usually have a good prognosis if they can access surgical care for their condition.

How can you prevent going blind?

Blindness is preventable through a combination of education and access to good medical care. Most traumatic causes of blindness can be prevented through eye protection. Nutritional causes of blindness are preventable through proper diet. Most cases of blindness from glaucoma are preventable through early detection and appropriate treatment. Visual impairment and blindness caused by infectious diseases have been greatly reduced through international public-health measures.

The majority of blindness from diabetic retinopathy is preventable through careful control of blood-sugar levels, exercise, avoidance of obesity and smoking, and emphasis on eating foods that do not increase the sugar load (complex, rather than simple carbohydrates). There has been an increase in the number of people who are blind or visually impaired from conditions that are a result of living longer. As the world's population achieves greater longevity, there will also be more blindness from diseases such as macular degeneration. However, these diseases are so common that research and treatment are constantly evolving. Regular eye examinations may often uncover a potentially blinding illness that can then be treated before there is any visual loss.

There is ongoing research regarding gene therapy for certain patients with inheritable diseases such as Leber's congenital amaurosis (LCA) and retinitis pigmentosa. Improvements in the diagnosis and prevention of retinopathy of prematurity, a potentially blinding illness seen in premature babies, have made it an avoidable cause of blindness today.

Patients who have untreatable blindness need tools and help to reorganize their habits and how they perform their everyday tasks. Organizations, such as the Braille Institute, offer helpful resources and support for people with blindness and their families. Visual aids, text-reading software, and Braille books are available, together with many simple and complex technologies to assist people with severely compromised vision in functioning more effectively. In the United States and most other developed nations, financial assistance through various agencies can pay for the training and support necessary to allow a blind person to function.

John Milton and Helen Keller are well known for their accomplishments in life despite being blind. There are countless other unnamed individuals with blindness, however, who, despite significant visual handicaps, have had full lives and enriched the lives of those who have interacted with them.

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Medically Reviewed on 5/4/2023
American Academy of Ophthalmology. "Eye Health Statistics at a Glance." <>.

Switzerland. World Health Organization. "Visual Impairment and Blindness." Oct. 2017. <>.