The lowest eye vision number refers to the poorest visual acuity or sharpness of vision that an eye can have. Visual acuity is measured during an eye exam using a Snellen chart which displays letters or symbols of gradually decreasing sizes. The results are expressed as a fraction, with the distance at which a person can read a certain line of letters over the distance at which a person with “normal” sharp vision can read the same line.
How is visual acuity measured?
Visual acuity is measured by having a person read letters or symbols on an eye chart (also called an eye test chart or vision chart) from a set distance, usually 20 feet. The most commonly used eye chart is the Snellen eye chart, developed by Dutch ophthalmologist Hermann Snellen in the 1860s.
Snellen Eye Chart
The Snellen eye chart contains 11 lines of block letters. The first line consists of one large letter, usually the letter “E”. Subsequent rows have increasing numbers of letters that decrease in size. A person taking the eye exam covers one eye and reads aloud the letters of each row, beginning at the top. The smallest row that can be read accurately indicates the visual acuity in that eye.
The results are written as a fraction. The numerator is the testing distance, usually 20 feet. The denominator is the distance at which a person with “normal” vision could read the same line. For example, if a person can read at 20 feet a line that people with normal vision can read at 40 feet, the visual acuity would be written as 20/40.
20/20 vision indicates normal sharpness of vision, meaning the person can see at 20 feet what should normally be seen at that distance. 20/40 vision means the person sees at 20 feet what a person with normal vision sees at 40 feet, indicating slightly impaired visual acuity. The lower the second number, the worse the visual acuity.
Other Eye Charts
There are other eye charts that may be used besides the Snellen chart. These include:
– Sloan letters chart – Uses letters that are equally legible, unlike the Snellen chart. This provides a more accurate assessment.
– Tumbling E chart – Tests not only visual acuity but also how well eyes work together for depth perception.
– Lea symbols chart – Used for testing children who may not know the alphabet. Consists of 4 simple shapes – circle, square, house, apple.
– Landolt C chart – Tests visual acuity by having the person identify the direction of the opening in a broken circle.
– LogMAR chart – Allows more refined measurement of visual acuity loss than the Snellen chart.
What is the lowest possible visual acuity?
The lowest visual acuity possible is 20/200 (or 6/60 in metric notation). This means the person sees at 20 feet what someone with normal vision can see at 200 feet.
20/200 vision is considered legally blind or significantly visually impaired. Below 20/200, vision loss is profound, with counting fingers, hand motion, light perception, or no light perception only.
Some key points about the lowest visual acuity:
– 20/200 is the cutoff for legal blindness in the United States and many other countries. At this level, vision is so impaired that it affects daily functioning and independence.
– 20/200 means an extremely blurry image. Details are difficult or impossible to see. Faces may be unrecognizable until very close. Reading regular print is not possible.
– In the metric scale, 6/60 vision equates to 20/200. 6 meters is roughly equivalent to 20 feet.
– Other visual impairments may also be classified as “legally blind” even if the visual acuity is better than 20/200. These include visual field loss.
– Visual acuity can fluctuate. Legal blindness is defined as 20/200 or worse in the better seeing eye with best correction (eyeglasses or contacts).
Causes of Extremely Low Vision
There are a number of eye diseases and conditions that can result in severely reduced visual acuity of 20/200 or worse. Common causes include:
Age-Related Macular Degeneration
Age-related macular degeneration (AMD) is the leading cause of legal blindness and low vision in people over 50. It affects the macula, which controls central vision needed for reading, recognizing faces, driving, and seeing details. AMD can progress from mild to profound vision loss.
Diabetic Retinopathy
Uncontrolled diabetes can cause damage to the blood vessels in the retina at the back of the eye. Proliferative diabetic retinopathy results in abnormal blood vessel growth which can lead to serious vision loss. Diabetes is the leading cause of blindness in American adults.
Glaucoma
Glaucoma is a group of eye diseases that damage the optic nerve, which transmits visual information from the eye to the brain. Untreated glaucoma initially causes peripheral vision loss but can progress to tunnel vision and eventual blindness without treatment.
Cataracts
A cataract causes the normally clear lens of the eye to become cloudy. As it progresses, vision blurs and dims, like looking through a dirty window. Untreated cataracts can cause complete vision loss. Cataract surgery is usually highly effective at restoring vision.
Retinitis Pigmentosa
Retinitis pigmentosa is an inherited, progressive condition affecting the retina’s ability to detect light. It causes night blindness and tunnel vision which can eventually progress to complete blindness.
Retinal Detachment
Retinal detachment occurs when the retina separates from its underlying supportive tissue. This causes severe, sudden vision loss and requires emergency surgery to prevent permanent blindness. Macula-off detachments can cause profound vision loss.
Eye Injuries or Trauma
Significant damage to the eye from an injury or trauma, such as a ruptured globe or detached retina, can result in very low visual acuity. Timely treatment is needed to try to restore or preserve vision.
Neurological Disease
Diseases affecting the visual centers of the brain, like stroke or brain tumor, can cause blindness or profound vision loss. Optic neuritis, an inflammation of the optic nerve, can also severely reduce visual acuity.
Improving Extremely Low Vision
While there are some cases of profoundly reduced visual acuity that cannot be improved, there are options to enhance functional vision for many people. These may include:
Regular Eye Exams
Comprehensive eye exams by an ophthalmologist are important to detect and monitor vision loss from eye diseases. Early detection and treatment can help slow progression.
Medical Treatment
Medications or injections can treat certain eye conditions if caught early. These include drugs that slow the progression of macular degeneration or laser or other treatments that improve retinal detachments or diabetic retinopathy.
Surgery
Surgeries like cataract removal, retinal repair, cornea transplant, or glaucoma surgery can significantly improve visual acuity in many people. Even people legally blind from cataracts can sometimes regain functional vision with cataract surgery.
Glasses and Low Vision Devices
Strong eyeglasses, contact lenses, or telescopic and microscopic lenses can maximize remaining vision. Other devices like video magnifiers can assist with reading or seeing faces and details.
Vision Rehabilitation
Vision rehabilitation teaches skills for performing daily tasks with low vision. People learn how to optimize remaining vision and use low vision aids and adaptive techniques to safely navigate and function independently.
Coping with Extremely Impaired Eyesight
Coping with vision loss, especially severe or progressive vision impairment, is challenging but there are many ways to enhance quality of life. These include:
Support and Counseling
Peer support groups and professional counseling help people deal with vision loss emotionally. Connecting with others facing similar challenges reduces isolation and encourages sharing practical solutions.
Orientation and Mobility Training
Special orientation and mobility training teaches safe navigation skills for people with limited vision. This includes using a white cane, service animal, or technology and building confidence in independent travel.
Assistive Technology
Technology like screen readers, screen magnification software, audio books, and voice assistants greatly improve access to information for people with reduced vision. Smart home tools also increase safety and independence.
Accessible Home Modifications
Simple home modifications like improved lighting, high-contrast markings, large print labels, color-coded systems, organized spaces, and smart home automation promote easier functioning for those with low vision.
Transportation Options
Accessible transportation resources include paratransit services, rideshares, community shuttles, public transit options with reduced fares, travel training, and supportive driving networks. These allow people with visual impairments to get around safely.
Support Services
Many communities have services to assist people experiencing vision loss including home care, meal delivery, assistive technology instruction, reading services, and help accessing benefits or workforce programs. These services aim to maximize independence for people with low vision.
Conclusion
The lowest visual acuity a person can have is 20/200 vision, considered legally blind. While there are many causes, the most common are age-related eye diseases. Some degree of functional vision can often be restored through regular eye exams, treatment, surgery, or adaptive devices. With proper support and training, most people can adjust and cope with extreme vision impairment to lead full lives. Advances in medical treatment and technology continue to expand possibilities for those with even the most reduced visual acuity.