Overview of Visual Acuity
A visual acuity test is just one part of a complete eye exam. This test checks how clear your vision is. It measures how well you can recognize different letters or symbols from a certain distance. This test relies on various parts of your visual system working properly like the light entering your eye, the health of your retina (the back part of your eye that’s sensitive to light), and your brain’s ability to understand what you’re seeing.
In the 1800s, doctors started creating charts to standardize how they test visual acuity. The most common chart used today is the Snellen chart. Other charts that were created around the same time include the Tumbling E chart, Landolt C chart, and Allen chart. A newer chart, the LogMAR (also known as the ETDRS chart), is liked because it can provide more accurate results and is easier to use in vision studies.
The results of a visual acuity test are often reported as 20/20 which describes standard vision. The first number, 20, is how far away you are from the chart (usually 20 feet). The second number tells you the distance that a person with normal vision can read the same line. So, if someone has 20/60 vision, they can read at 20 feet what a person with normal vision can read at 60 feet. On the logMAR chart, the results are reported as a single number where 0.0 means normal vision. As this number increases, vision gets worse; as the number decreases, vision gets better. Even though 20/20 vision is often called “perfect vision,” it’s important to know that it’s only one part of total vision – it does not account for other things like depth perception, side vision, and colorblindness.
There are many reasons why someone might not have perfect visual acuity. Common problems can be nearsightedness (you can’t see far away) or farsightedness (you can’t see things close up). Other issues could be astigmatism (blurred vision), amblyopia (lazy eye), retinal detachment, macular degradation, ischemia, cataracts, glaucoma, corneal abrasion, or eye injuries. Vision tests are important because catching these problems early can often help improve them. Knowing a person’s best possible corrected vision is also essential. According to the World Health Organisation (WHO), a person with low vision has a best possible corrected vision of 20/60 or worse, while being blind means having a best-corrected vision worse than 20/400. In the United States, legal blindness is defined as 20/200. These official definitions can affect things like getting special help (accommodations) or being able to drive a car.
Anatomy and Physiology of Visual Acuity
Seeing clearly involves a lot of parts of your eye working together perfectly. First, light enters your eye through the cornea, the clear front surface of your eye. This can be affected by injuries like corneal abrasion, or scrapes and scratches on the cornea; corneal ulceration, which is an open sore on the cornea; or UV keratitis, which is inflammation caused by UV rays.
Next, the light travels through your pupil, a small opening in the eye. The size of this opening is controlled by your iris (the colored part of your eye). An example of how this system works is the fact that you may have blurry vision after an eye exam, as your pupils have been dilated.
The light then moves through the lens of your eye, which changes shape to focus light onto your retina, the part of the back of your eye that captures images. The lens adjusts using the ciliary bodies and suspensory ligaments. These readjustments help to focus light or images onto the retina. When this fails, it may cause eye conditions like myopia (nearsightedness), hyperopia (farsightedness), or presbyopia (difficulty focusing on close objects as you get older). Cataracts can also affect vision, as the lens in the eye becomes cloudy.
Once the light reaches the retina, it’s turned into nerve signals that are sent to the brain to be interpreted as images. Problems at the retina level, like retina separation (when the retina detaches from the back wall of the eye) or macular degeneration (a condition that damages the center of your retina), can affect this process. In fact, issues at any level from the retina to the brain (the visual cortex) can affect your field of vision. Therefore, in order to have good vision, all parts of the eye need to be working properly.
Why do People Need Visual Acuity
Getting your eyes checked is a vital part of maintaining your overall health. These eye tests are called visual acuity testing and they are part of a thorough eye examination. There are several reasons why you might need to get this test done:
1. To check for any problems with your vision, this is known as screening for visual impairments.
2. To assist in diagnosing eye conditions that are difficult to treat, also known as refractory eye diseases.
3. To evaluate sudden changes in your vision, whether they are caused by an accident or not.
4. To reassess your vision after a procedure or treatment has been done to correct any issues with your eyesight.
5. To keep track of your vision for certifications where good vision is important, such as renewing your driver’s license.
When a Person Should Avoid Visual Acuity
If you get a chemical in your eye, it’s important to wash it out straight away. You shouldn’t wait. After properly washing your eye, your doctor can check your eyesight. Apart from this, there are no reasons why you shouldn’t get your eye washed out if a chemical gets in it.
Equipment used for Visual Acuity
Depending on how well the patient can see, and their other abilities, they might need one or several of the following tools during their eye examination:
– An optotype chart: This is a chart with letters or symbols in different sizes that’s used to test vision. Examples are Snellen, tumbling E, Landolt C, Allen or LogMAR charts.
– An occluder card or patch: This is used to cover one eye at a time while the other is being tested.
– A pinhole occluder: This tool has small holes and is used to help focus light into the eye which can sometimes help people see clearer. It’s especially useful for individuals who may have issues with blurred or distorted vision.
– Penlight/flashlight: This is a small, portable light used to examine the eye more closely.
How is Visual Acuity performed
The Snellen chart is the most commonly used tool to test eyesight. The process involves the person sitting a standardized distance (usually 20 feet or 6 meters) from the chart in a well-lit area. If the space is small, mirrors can help get the required distance, and if a person needs glasses to read, they might use a near Snellen chart at 14 inches. There are also smartphone apps that can display the Snellen chart at a distance of about 4.5 feet, or the length to the end of a hospital bed. These apps are handy but might not be as accurate as the traditional method. However, they do seem to be reliable for tracking changes in eyesight over time.
During the test, you have to cover one eye with your hand or a card. Some people like to test the same eye first every time, or you might start with the eye with the poorer vision to prevent remembering letters from the chart. The test is often repeated with and without glasses or contact lenses, and a pinhole can be used to help focus light better if your vision remains blurry, even with corrective lenses. The goal is to measure your best possible vision.
You’ll start by reading the top line of the chart from left to right, continuing to the smallest line you can read. If you miss one or two letters on the smallest line you read, your vision is still considered equivalent to that line. If you can’t read the top line, you’ll move closer until you can, and your vision will be written as a fraction of the distance from the chart (for example, 10/200 if you can only read it at 10 feet).
If you can’t read the chart at all, you’ll start by counting fingers at different distances, identifying hand motions, or recognizing light. If you can’t recognize light, that’s recorded as having no light perception.
The test then repeats for your other eye, and it might be repeated with corrective lenses or a pinhole if necessary. The results are noted using specific terms or abbreviations, such as CC for with correction, SC for without correction, OD for the right eye, OS for the left eye, OU for both eyes, PH for pinhole, CF for counting fingers, HM for hand motion, LP for light perception, and NLP for no light perception.
For example, if your vision in your right eye with glasses is 20/30, it would be recorded as ccOD 20/30.
Testing vision in children over three years old who can talk requires a chart suitable for their understanding level. Providing a quiet, calm environment can help them feel comfortable, and they might sit on a parent’s lap or have a parent help cover their eye. For children under three, doctors test vision by checking for normal eye-tracking and movements.
What Else Should I Know About Visual Acuity?
Testing how well you can see, known as visual acuity testing, is important as both a routine check and during emergency situations. If you injure your eye or suddenly lose your vision, one of the first things doctors do is measure your visual acuity. This initial measurement helps them understand the extent of the injury or disease and they can compare it to any previous tests if available. This also gives them a starting point for further tests after treatment. If your eye is injured, it’s important not to wear contact lenses, but you can still use glasses or a small tool called a pinhole occluder to test your vision. A perfect score (20/20 vision) doesn’t guarantee that your eyes are healthy, so it’s always important to take follow-up tests.
Regular eye tests for children, which include checking visual acuity, can catch eye problems early on. These problems could include ‘lazy eye’ (amblyopia) or an eye cancer called retinoblastoma. However, visual acuity tests are most efficient at finding issues in the way the eye focuses light (refractive defects), which is a common reason for vision problems in children.
Teaching children to use glasses when needed can help stop the progress of vision problems and can also provide developmental, educational, and social benefits. In the United States, kids usually start having their eyesight tested as soon as they turn 3. There are standard levels of vision that children of specific ages should have. If a child cannot read the line on an eye chart that we expect from their age group, they should be referred to an eye specialist (ophthalmologist). Generally, children with normal vision should have their sight checked once every 1-2 years.
Getting eyes checked frequently is crucial when children are growing up, but it’s not necessary or cost-effective for adults to have frequent eye tests. This is why the recommendation in the United States is that the frequency of tests should be based on risk factors. People who don’t have symptoms or higher risk of eye disease should get a comprehensive eye exam every 5-10 years if they are less than 40 years old, every 2-4 years between 40 and 54, every 1-3 years between 55 and 64, and every 1-2 years if they are over 65.
The frequency of check-ups should increase as we age to check for diseases that are common in older people. For instance, people at higher risk of developing an eye condition called glaucoma should have their eyes examined frequently, even if they don’t have symptoms. People diagnosed with type 1 diabetes should get their eyes checked, including testing their visual acuity, five years after their diagnosis and then every year after that, while people diagnosed with type 2 diabetes should get yearly eye tests starting at the time of diagnosis. Additionally, women with diabetes should get a complete eye test, including visual acuity, during the first trimester of their pregnancy. The aim of these screening tests is to catch and treat any problems early and to improve quality of life.
