Blindness

Blindness is a term that refers to both legal blindness and visual impairment.

It is estimated that 2.2 billion people worldwide suffer from some degree of blindness, and approximately half of these are due to preventable or treatable causes.

The definition of blindness is vision that is so poor in both eyes to interfere with activities of daily living despite attempted correction with glasses, contacts, or refractive surgery. Poor vision can result from damage to any part of the visual pathway, from the eyes to the nerves that transmit the images to the occipital lobe of the brain where vision is perceived and processed.

The term blindness encompasses both visual impairment and legal blindness. Visual impairment means the best corrected vision (vision with glasses, contacts, or refractive surgery) is poor and results in difficulty functioning. Legal blindness is a specific term defined by the United States Social Security Administration to determine disability benefit eligibility. A person is deemed legally blind if the best corrected central visual acuity in the better-seeing eye is 20/200 or worse, or if the visual field is restricted to 20 degrees or less in the better-seeing eye.

Worldwide, the most common causes of visual impairment and blindness include unaddressed correction of refractive errors, cataracts, glaucoma, and corneal opacities from trachoma infection and injuries. In the United States and most other developed nations, the four most common causes of blindness are age-related macular degeneration, cataract, diabetic retinopathy, and glaucoma. Glaucoma is the leading cause of irreversible blindness in African Americans. Additional causes of blindness include inflammatory conditions like optic neuritis, inherited degenerations such as retinitis pigmentosa, retinopathy of prematurity (formerly called retrolental fibroplasia) seen in premature babies, vascular diseases, eye neoplasias such as retinoblastoma or optic glioma, and blocked blood vessels to the visual pathway due to strokes.

Worldwide, the leading risk factor for blindness is lack of access to health care resources to correct refractive errors and cataracts. Poor hygiene and lack of access to clean water plays a major role in the spread of trachoma.

The rising rate of obesity worldwide has led to marked increases in diabetes and diabetic retinopathy. Increasing life span has led to increased numbers of people with eye conditions that are naturally more prevalent with age, such as cataracts, glaucoma, and age-related macular degeneration.

A family history of eye disease is a risk factor. However, lifestyle choices and socioeconomic factors affect the incidence and severity of these inherited diseases. Smoking, obesity, and lack of access to health care can increase the risk of eye diseases progressing to blindness.

Symptoms of poor visual acuity may range from trouble focusing to glare to difficulty perceiving light.

Markedly constricted visual fields are seen in conditions such as advanced glaucoma and pigmentary retinopathies. In these cases, despite a small central area of relatively clear vision, the surrounding severe peripheral vision loss results in tunnel vision that can be disabling.

People born with sight who later lose vision can still form images in their mind. However, in people who lose sight due to damage to the portions of the brain that process visual input, there can be a loss of ability to form images in the mind. This is the case with trauma or strokes affecting the occipital cortex of the brain.

Most blindness is unassociated with physical pain. However, angle-closure glaucoma and various infectious and inflammatory eye disease can cause both pain and severe visual loss.

In most cases of blindness, there are no outward signs evident to others. In some cases, however, a rhythmic back and forth motion of the eyes (nystagmus) occurs in some forms of congenital blindness (blindness from birth). When one eye has poor sight relative to the other, it may wander and be misaligned relative to the eye with good sight. This misalignment is called strabismus. The most common cause of visual loss in one eye in children is amblyopia associated with strabismus or an uncorrected high refractive error. Eyes with inflammation may appear reddened. Corneal scars and swelling can appear as a grayish patch. Eyes with a dense cataract may have a visible white color in their pupil. Blind eyes with long-standing severe inflammation can also shrink in size (phthisis).

Reading an eye chart one eye at a time is the standard way to measure visual acuity, and visual field testing measures the size and location of blind spots in the vision. Both tests are subjective and require patient cooperation and a minimum amount of vision remaining to be able to take the tests. There are also objective tests such as pupillary exam, optokinetic testing, electroretinography, electrooculography, and others that can pick up defects in certain parts of the visual pathway from the eye to the visual cortex of the brain.

Optometrists and ophthalmologists assess vision loss by performing eye examinations and testing, prescribing glasses or contact lenses and providing treatment for certain eye diseases. Ophthalmologists are medical doctors who can also offer surgical treatment when necessary. Often the patient's primary care provider is the first to discover the problem. Pediatricians are may find a vision problem in an otherwise healthy child during a routine well-child check. Geriatricians who care for seniors will often refer patients to eye care professionals for assessment of decreased vision that may impair a patient's ability to drive safely.

Treatment is dependent on the cause. Some conditions are entirely curable, such as simple refractive errors and cataracts. Others are not entirely curable but often manageable, and with proper treatment, a patient may be able to retain good vision. Glaucoma is an example of a chronic condition that can often be well controlled.

Surgery treats cataracts. Medications, laser, and/or surgery treats diabetic retinopathy, depending on the type of retinopathy. Macular degeneration is treated with medications that slow down the progression of the wet (hemorrhagic) form. Lowering eye pressure with medication, laser, and/or surgery controls glaucoma. Corneal transplants can correct many types of corneal opacities due to scarring or swelling.

A patient's primary care doctor and other specialists are often involved in treatment when the eye condition is part of a systemic disease and requires oral or intravenous medications.

For patients with poor vision, a low vision specialist can offer visual aids such as magnifiers that can help with daily activities.

Research is under way for new drugs and gene therapies such as gene transfer or CRISPR gene editing to prevent and treat a variety of eye conditions.

The prognosis depends on the cause. Some conditions are treatable and have an excellent chance of full visual recovery. Other conditions result in irreversible vision loss, however, treatments may significantly slow down or delay the progression of vision loss.

For children, timing of treatment is particularly important, and generally, the earlier the condition is diagnosed and treated, the better the outcome. This is because in children, the visual pathways from the eye to the brain may weaken or not develop fully when vision to one or both eyes is blurred or blocked. This type of vision impairment is called amblyopia, or lazy eye. Once the underlying eye condition is treated, the child should be examined for signs of amblyopia. Amblyopia is often treatable with aggressive vision rehabilitation such as weeks or months of patching therapy of the good eye to improve the vision of the amblyopic eye.

Improved access to health care, clean water, and better hygiene are key factors for preventing blindness worldwide. Early treatment of eyelid scarring in trachoma can significantly reduce the later chances of blindness.

Routine eye examinations are important to detect eye problems early, particularly for treatable conditions such as glaucoma.

A particular set of vitamins was found to be effective in slowing down the progression of vision loss in patients with moderate to severe macular degeneration of the dry form in the Age Related Eye Disease Study 2, and these vitamins are sold as the AREDS-2 formula. An eye doctor can assess the stage of macular degeneration to determine if a patient should take these supplements.

Proper eye protection can prevent injuries from accidents, chemical burns, and sports injury.

Health habits also play a significant role in preventing blindness. Smoking, poor cardiac health, sedentary lifestyle, poor nutrition, and obesity are linked to more vision loss in diabetic retinopathy, glaucoma, and macular degeneration, so while these conditions may not be entirely preventable, the severity of vision loss can certainly be affected by lifestyle choices.