Types of blindness and their causes

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Healthcare professionals divide types of blindness into the following categories:

• Total blindness: This is when a person cannot see anything, including light. Only 15% of individuals with eye disorders have total blindness.
• Low vision: Low vision describes visual impairments that healthcare professionals cannot treat using conventional methods, such as glasses, medication, or surgery.
• Legal blindness: “Legal blindness” is a term the United States government uses to determine who is eligible for certain types of aid. To qualify, a person must have 20/200 vision or less in their better-seeing eye, even with the best correction.
• Visual impairment: Visual impairment is a general term that describes people with any vision loss that interferes with daily activities, such as reading and watching TV.

People can also categorize types of blindness by their cause. Below are just some of the potential conditions that can result in vision loss.

Visual impairments are most common in older adults. Potential causes include:

• Age-related macular degeneration (AMD): AMD is an eye disorder that occurs when aging damages the macula, which controls sharp, straight-ahead vision.
• Cataract: A cataract is a cloudy area that develops over the eye’s lens. These can cause clouding or blurring of vision, color fading, and reduced night vision.
• Diabetic retinopathy: This diabetes complication is more likely to develop in older adults. It occurs when high blood sugar levels damage the tiny blood vessels supplying the retina.
• Glaucoma: Glaucoma is an eye disease that damages the optic nerve at the back of the eye. It is more common in people aged 60 years and over and can cause vision loss.

Eye injuries can lead to vision loss or blindness in some cases. They may occur due to:

• playing sports
• physical violence
• motor vehicle crashes
• chemical burns
• exposure to toxins
• falls or collisions
• objects such as darts, bullets, explosives, or fireworks

Many infectious diseases can result in vision loss. Some examples include:

• Trachoma: This is a bacterial infection that can cause eye itching, irritation, discharge, and eyelid swelling. Trachoma is the cause of more vision loss and blindness than any other infection worldwide, but it is rare in the U.S.
• Shingles: The same virus that causes chickenpox also causes shingles. When shingles affect the eyes, it may lead to corneal scarring, ulceration, and perforation, resulting in severely decreased vision.
• Cytomegalovirus (CMV): CMV is a herpes virus that can cause CMV retinitis, a severe viral infection that destroys the retina and damages the optic nerve. This is more common in people with compromised immune systems.
• Histoplasmosis: Histoplasmosis is a fungal lung infection. The infection can move from the lungs to the eyes, causing ocular histoplasmosis syndrome and vision loss.
• Keratitis: This is inflammation of the cornea that typically occurs due to a bacterial, viral, or fungal infection. Without treatment, it can result in severe complications that permanently damage vision.
• Syphilis: Syphilis is a sexually transmitted infection (STI) that can affect the eyes, leading to vision loss and permanent blindness in some cases.
• Toxoplasmosis: This occurs due to a parasitic infection. Some people are at risk of ocular toxoplasmosis, which causes retinal inflammation. Without treatment, it may lead to progressive vision loss.
• Uveitis: Uveitis is inflammation of the middle layer of the eye, known as the uvea, which can lead to loss of peripheral vision and complications that cause blindness.

Several noninfectious conditions can also lead to blindness. These include:

• Amblyopia: Amblyopia or “lazy eye” is an impairment that occurs in one or both eyes due to a breakdown in how the eye and brain work together. Without treatment, amblyopia may cause vision loss.
• Stroke: If a stroke damages part of the brain responsible for sight or eye movement, people can experience vision changes or loss.
• Retinopathy of prematurity (ROP): ROP is an eye disease that occurs in premature infants when atypical blood vessels develop in the retina. Advanced ROP may cause retinal detachment and blindness.
• Cancer: Certain types of cancer, such as orbital tumors, may cause complications, including retinal detachment and blindness.

Nutritional blindness occurs due to a vitamin A deficiency. This may lead to:

• xerophthalmia, which causes dry eyes and inflammation
• keratomalacia, which is drying and clouding of the cornea
• corneal necrosis, or the death of cells and tissue in the cornea

According to the World Health Organization (WHO), 250,000–500,000 children with vitamin A deficiency lose their sight each year.

Snow blindness occurs when the sun’s UV rays reflect off ice or snow, damaging the cornea and the conjunctiva. Snow blindness can also refer to the freezing of the cornea or severe drying of the cornea’s surface due to dry air.

It typically occurs in people who ski, climb mountains, or go snowmobiling. Although snow blindness is temporary, repeated exposure to UV rays without sunglasses may result in other eye disorders that cause permanent vision loss.

Night blindness, or nyctalopia, is when people experience poor vision in dimly lit environments or at night. Despite the name, people with night blindness are not blind but may have difficulty seeing in places such as movie theatres, restaurants, or driving at night.

People can develop night blindness due to eye conditions, such as nearsightedness, cataracts, and retinitis pigmentosa. Vitamin A deficiency may also cause it.

Several genetic conditions can cause blindness.

The blindness may be present from birth, in which case, it is known as congenital blindness. In some cases, though, blindness develops later on. Examples include:

• Leber hereditary optic neuropathy (LHON): LHON is an inherited disease that transmits from birth parent to child. It causes clouding and blurring of vision, progressing to blindness.
• Oculocutaneous albinism: Albinism is when a person does not have melanin in their hair, skin, or eyes. This can sometimes affect vision.
• Retinoblastoma: This type of eye cancer can be hereditary, occurring due to changes in certain genes.
• Inherited retinal diseases (IRDs): IRDs are rare conditions that can lead to vision loss and blindness. Genetic mutations in more than 260 genes may cause IRDs. IRDs may include conditions such as:
  • retinitis pigmentosa
  • choroideremia
  • Stargardt disease
  • cone-rod dystrophy
  • X-linked retinoschisis

Blindness can also occur due to congenital abnormalities, such as:

• anophthalmia, which is when a fetus does not develop one or both eyes
• microphthalmia, which is when one or both eyes are very small
• coloboma, which is when part of the eye does not develop in the typical way
• infantile glaucoma

Other causes of congenital blindness include:

• congenital cloudy cornea
• congenital cataracts
• optic nerve lesions
• cerebral visual impairment, which occurs due to brain damage
• ophthalmia neonatorum, which is an eye infection newborns can get if their birth parent has an STI

Color blindness does not cause blindness or vision loss. However, people with the condition see colors differently from others. It tends to run in families.

Color blindness can make it difficult for people to distinguish between certain colors, such as red and green or blue and yellow. Some people are completely color blind and cannot see any color. However, this is uncommon.

Age-related eye diseases, such as AMD and cataracts, are the leading causes of low vision and blindness in the U.S.

According to the Centers for Disease Control and Prevention (CDC), about 12 million people in the U.S. have some form of vision impairment, and 1 million are blind.

There are several ways to categorize blindness. Certain types describe how much vision loss a person has, ranging from some impairment to complete blindness. People may also categorize blindness based on its causes, which vary widely from congenital conditions to those that occur later in life.

The most common causes of blindness are age-related, including conditions such as AMD and glaucoma. Blindness can also result from injuries, infections, vitamin A deficiency, UV damage, congenital abnormalities, and many other conditions.