Blindness and Visual Impairment

Context

Worldwide, about 217 million people have low vision, and about 36 million are blind:

By age 60, around 1 in 9 people will be either blind or have moderate to severe visual impairment (MSVI). By age 80, the ratio increases considerably: around 1 in 3 people will be either blind or have MSVI. (Source)
About 90% of the world's visually impaired live in low-income settings.
81% of people who are blind or have moderate or severe vision impairment are aged 50 years and above.
80% of visual impairment can be prevented or cured.
China, India, and Africa account for approximately 8M, 8M, and 6M cases of blindness, respectively.

(WHO, Fact Sheet #282, 2017)

Causes

Age-Related Macular Degeneration (ARMD)

Incidence: perhaps 1/40 worldwide.
Blurs central vision needed for activities like driving or reading.

Bardet-Biedl syndrome (BBS)

Incidence: perhaps 1/150K worldwide.
A genetic, multisystem disorder that causes severe visual impairment. This condition is characterized by retinal dystrophy, obesity, digit anomalies, renal disease, and hypogonadism. (NIH)

Cataract

Incidence: perhaps 1/300 worldwide. Unoperated cataract remains the leading cause of blindness in low- and middle-income countries. (WHO, 2017)
Risk factors include:

Age: More than one-third of those over 65 have cataract.
Female: Females are 1.35 times more likely to have cataract.
Sunlight exposure: Those exposed to more than 11 hours of daily sunlight are nearly three times as likely to develop cataract as those exposed to less than 8 hours of sunlight.

Corneal blindness

Incidence: About 1.9 million people have corneal blindness, which accounts for about 5% of the total patients who have blindness. (Source)
Includes Xerophthalmia (nutritional blindness), Trachoma, corneal ulcer, and injuries (accidental, iatrogenic). (Dahal, 2018)

Cortical Visual Impairment (CVI, sometimes called Cerebral Visual Impairment)

Incidence: ~1/600 (Source)
A temporary or permanent visual impairment caused by brain damage or impairment rather than an eye problem. This is the most common cause of permanent visual impairment in children. (Perkins video). Usually diagnosed when poor visual performance cannot be explained by an eye examination.

Diabetic Retinopathy (DR)

Incidence: perhaps 1/300 worldwide. Diabetic retinopathy is the primary cause of vision loss in adults aged 20-74 years (Retina International).
"Chronically high blood sugar from diabetes is associated with damage to the tiny blood vessels in the retina, leading to diabetic retinopathy. The retina detects light and converts it to signals sent through the optic nerve to the brain. Diabetic retinopathy can cause blood vessels in the retina to leak fluid or hemorrhage (bleed), distorting vision." (National Eye Institute, NIH)

Glaucoma

Incidence: perhaps 1/100 worldwide.
A group of eye conditions that damage the optic nerve; often caused by abnormally high pressure in the eye. Glaucoma can occur at any age but is more common in older adults; it is one of the leading causes of blindness for people over the age of 60. (Mayo Clinic)

Leber Hereditary Optic Neuropathy (LHON)

Incidence: perhaps 1/40K worldwide.
Causes loss of central vision; predominately affects young adult males. Typically, vision loss in one eye is followed, after a few weeks or months, by vision loss in the other eye.

Leber’s Congenital Amaurosis (LCA)

Incidence: perhaps 1/40K worldwide.
Inherited, appears at birth or in infants. Causes severe vision loss or blindness. Some promising gene therapies.

Onchocerciasis (River blindness)

Incidence: The Pan American Health Organization estimates that there are 18 million infected people worldwide, including 270,000 who've lost their sight.
Caused by a roundworm infection spread by black flies.
The Farmer and Fisherman Who Lost His Sight to River Blindness

Retinitis Pigmentosa (RP)

Incidence: perhaps 1/4K worldwide.
Like other genetic eye diseases (Stargardt, LCA, LHON, etc.) caused by 260+ retinal disease genes, RP may be addressable by genetic methods like CRISPR.

Retinopathy Of Prematurity (ROP)

Incidence: About half of the estimated 28,000 premature babies born each year in the United States have some degree of retinopathy of prematurity. (Source)
Occurs in premature infants (<1250 gms, < 31 weeks) when abnormal blood vessels spread throughout the retina. "These abnormal blood vessels are fragile and can leak, scarring the retina and pulling it out of position. This causes a retinal detachment. Retinal detachment is the main cause of visual impairment and blindness in ROP." (National Eye Institute, NIH)

Stargardt Disease

Incidence: perhaps 1/10K worldwide.
"An inherited disorder of the retina – the tissue at the back of the eye that senses light. The disease typically causes vision loss during childhood or adolescence, although in some forms, vision loss may not be noticed until later in adulthood. It is rare for people with the disease to become completely blind. For most people, vision loss progresses slowly over time to 20/200 or worse." (National Eye Institute, NIH)

Trachoma

Incidence: perhaps 1/4K worldwide blinded, 1/150 affected.
A chronic, contagious infection that eventually causes blindness; manifests gradually. Largely found in poor, rural communities in developing countries, particularly in sub-Saharan Africa (NIH). Tends to occur in clusters, infecting families or communities. WHO uses a SAFE (Surgery, Antibiotics, Face-washing, Environmental improvement) strategy against Trachoma. (Dahal, 2018)

Xerophthalmia/Vitamin A deficiency/Night Blindness

Incidence: According to the World Health Organization, there are about 2.8 million children with xerophthalmia.
Caused by severe vitamin A deficiency, and described by pathologic dryness of the conjunctiva and cornea.

Multiple impairment

Deaf-Blind

Incidence: 1/33K births.
"Deaf-blindness refers to the combination of hearing and visual loss that severely impedes communication, education, employment, and independent living. While some deaf-blind individuals are totally deaf and blind, most deaf-blind people have different levels of vision and hearing loss." (NLS)
Play-by-play soccer for a deaf-blind person
Striped canes signify deaf-blindness
Library of Congress resources