The term blindness is a general term that can include those with low vision and legal blindness.
Low vision is a term used to describe those whose vision cannot be fully corrected by glasses, contact lenses, refractive surgery, or other surgery. Another term commonly used is a visual impairment to describe those whose decreased visual function interferes with the ability for one to perform their activities of daily living. Visual impairment is defined based on function, instead of using visual acuity or visual field cutoff values.
Legal blindness is a term defined by the United States Social Security Administration (SSA) to determine those who are eligible to receive disability benefits, tax exemption programs, and low vision training. SSA uses visual acuity or visual field results to determine eligibility. A person is considered legally blind if he/she has central visual acuity of 20/200 or worse in the better-seeing eye with best correction (using glasses or contact lenses) at a distance, or if he/she has visual field restriction where the widest diameter is 20 degrees or less in the better-seeing eye. The tests used to measure visual acuity or visual fields have been outlined as follows:
Visual acuity testing for distance is carried out using Snellen visual acuity chart or another test that is comparable to the Snellen methodology. In 2007, SSA updated the criteria for measuring the visual acuity by allowing newer low vision test charts to be used instead of Snellen acuity charts alone. Under this update, if a person cannot read at least one letter on the 20/100 line, he or she will be classified as legally blind.
Acceptable tests for visual field testing include automated static perimetry such as Humphrey Field Analyzer (HFA) 30-2, HFA 24-2, and Octopus 32, kinetic perimetry such as Goldmann perimetry or HFA "SSA Test Kinetic." Screening tests including confrontation tests, tangent screen tests, and static screening tests are not accepted forms of testing to determine legal blindness.
There can be many different causes of blindness. The leading causes of blindness worldwide are cataracts, age-related macular degeneration, glaucoma, diabetic retinopathy, and trachoma.
The number of persons with reduced vision due to uncorrected refractive error exceeds that of those with the conditions mentioned above, however, uncorrected refractive errors can be easily treated or "cured" with proper vision correction.
It is estimated that approximately 90% of the visually impaired population live in developing countries or low-income circumstances, and about 80% of all visual impairment worldwide can be prevented, treated, or cured with proper access to eye care.
The most common causes of visual impairment globally are as follows:
The most common causes of blindness are as follows:
According to World Health Organization (WHO), there are approximately 285 million people with a visual impairment. Of those, 39 million are blind, and 246 million have low vision. Of those who are blind, 90% live in developing countries. For each blind person worldwide, an average of 3.4 people has low vision, with the country and regional variation ranging from 2.4 to 5.5 people.
More than 82% of all people who are blind are 50 years of age and older. Due to the expected number of years lived in blindness, childhood blindness is a significant problem, with an estimated 1.4 million blind children below age 15. Females have a significantly higher risk of being visually impaired than males.
Conditions causing visual impairment vary widely. It can be a genetic, congenital, or acquired condition. Visual impairment and can occur gradually or suddenly and can include central vision loss, peripheral vision loss, overall blur, a decrease in contrast sensitivity, color vision, glare, or light sensitivity. The cause of the vision loss determines whether it involves one type alone or a combination of presentations.
Performing a thorough case history is important. Case history should include:
After a thorough case history, an ocular examination is necessary to assess the patient's current visual status and function.
In addition to taking a good history, it is important to perform a thorough evaluation. Ocular health exam should typically include visual acuity, visual field, extraocular muscles, pupil, binocular vision testing, intraocular pressure, anterior segment and posterior segment evaluation with dilated fundus exams. Supplemental testing such as formalized visual field testing, visual evoked potential, electroretinography, electrooculography, optical coherence tomography, or fluorescein angiography may be necessary to help determine the etiology, monitor and properly manage the condition.
Comprehensive eye examinations are necessary to prevent, detect, treat, and manage the ocular conditions that can lead to blindness. Eye exams should include ocular health exams and not just vision and sight checks for glasses or contact lens prescription.
Many world, national, and local organizations are working to bring more awareness about vision and eye health, provide access to health care, and fight blindness. Research is being done worldwide to study the etiologies of many eye conditions causing permanent vision loss, and to develop a proper treatment to control or cure these conditions.The World Health Organization (WHO) is the leader in working to monitor trends, raise awareness, and coordinate efforts to fight blindness.
Low vision rehabilitation services are available to help patients maximize their remaining vision and maintain their independence and quality of life. Low vision rehabilitation is provided by a team of low vision optometrists, ophthalmologists, low vision therapists, occupational therapists, orientation and mobility specialists, social workers, and other rehabilitation low vision professionals. Please refer to the review article titled "Low Vision Rehabilitation" for additional information.