Overview of Evaluation of Visual Acuity
Checking your eyesight is an important part of an eye exam. This complete eye test includes various steps which test how well you see, how your pupils react, how well your eyes move and line up with each other, the pressure inside your eyes, your visual field, an examination of the outside of your eyes, a detailed look at the front parts of your eyes, and examining the inside back of your eyes (ocular fundus).
Your visual acuity, or how clearly you see, is often recorded by doctors using certain abbreviations like OU, OS, and OD. OU stands for Oculus uterque which means both eyes. OS refers to Oculus sinister, or the left eye. OD is for Oculus dexter, the right eye.
There are other terms like DVA (distance visual acuity), IVA (intermediate visual acuity), and NVA (near visual acuity) to describe your vision at different distances. These vision metrics could refer to uncorrected (as in, without glasses or contact lenses), corrected (with glasses or contact lenses), or two-eyed (binocular) vision.
The numbers from a visual acuity test are compared to what a person with typical vision can see from 20 feet (or 6 meters) away. So, if your vision is recorded as 20/20 or 6/6, it means you see as well as a typical person does from 20 feet away. But if it is 20/40 or 6/12, it means what you can see clearly from 20 feet (or 6 meters), a person with typical vision can see from 40 feet (or 12 meters) away.
Visual acuity measures the smallest object you can see at a certain distance. It depends on recognizing the different parts of an object. This small size of the clearest object you can see from a distance is called the minimum angle of resolution (MAR).
We measure four types of vision clarity:
- Minimum visible acuity: This measures detection of an object’s presence or absence.
- Minimum resolvable acuity: This checks the smallest separation between two parts of an object or two separate objects that you can tell apart. In a person with 20/20 vision, they can tell apart 2 objects that are very, very close together (1/60th of a degree apart).
- Minimum recognizable acuity: This is the smallest detail that you can identify or recognize, like identifying a letter. Although 20/20 is the standard, the average person can actually see a little better than 20/20.
- Minimum discriminable acuity: This refers to the smallest change in size, orientation, or position of an object that you can notice.
The smallest misalignment we can notice is Vernier acuity, named after Pierre Vernier who invented a scale used on ships. Humans are very good at detecting whether lines are aligned, so this is a widely-used test. In this test, humans can detect misalignments as tiny as 2 to 5/3600th of a degree!
Anatomy and Physiology of Evaluation of Visual Acuity
The minimum detectable visual acuity is a measurement that determines how well you can distinguish between the brightness of an object and its background, or in simpler terms, how well you can identify subtle changes in contrast.
The ability to clearly see details (referred to as minimum resolvable acuity and minimum recognizable acuity) depends on factors like how close together the photoreceptors (cells in your eyes that capture light) are, distortions in the eyes, and the size of the pupils. As an example, let’s consider the visual test using black and white stripes. There’s a limit to the frequency of these stripes, beyond which our eyes either see a grey field or misinterpret the size or orientation of the stripes, a phenomenon known as aliasing. At this limit, the center of the white stripes and black stripes each align to different cone photoreceptors in the eyes. Generally, such a maximum frequency visual pattern creates a visual angle of one minute or 60 seconds of arc.
An ability to see exceptionally fine details is referred to as ‘hyperacuity’. This ability exceeds what we would expect given the spacing between photoreceptors in our eyes. This hyperacuity is more sophisticated than merely the distance between the foveal cones, the specific photoreceptors located at the center of the retina (around 2.5 microns or 30 seconds of arc), which typically set the limit for our visual acuity. Experts have suggested multiple theories about how this exceptional detail-recognition works, which may involve changes in the photoreceptors or the optical properties of the eyes. However, it is generally believed that our brain’s processing of visual information, especially in relation to Vernier acuity (the ability to discern slight misalignments), may be key to understanding hyperacuity.
Why do People Need Evaluation of Visual Acuity
When a patient visits an emergency or outpatient department with any issues related to the eye, a properly recorded eye examination is indispensable. It’s a routine check every patient is subjected to, once they arrive at the eye department. The purpose of this examination is to assess the sharpness of the patient’s vision.
When a Person Should Avoid Evaluation of Visual Acuity
There are no specific reasons why someone can’t have their visual acuity (sharpness of vision) tested. However, there are various factors that can make the testing more difficult and potentially impact the results. These factors include:
Eye diseases: Conditions affecting the eyes may interfere with the test.
Lighting: The brightness or dimness of the room, including the chart used for testing, can affect the results.
Contrast: The differences in colors and shades in the testing environment can also play a part.
Pupil size: The size of the black part in the center of your eyes, called the pupil, can influence the accuracy of the test.
Type of optotype used: The particular types of letters, numbers, or symbols used for testing can complicate the test.
Refractive error: Problems with the focusing power of your eyes (like nearsightedness or farsightedness) can also interfere with the test.
Retinal eccentricity: This refers to how the light falls on different parts of the retina (the light-sensitive tissue at the back of the eye). If the light does not fall directly on the center of the retina, this can affect the results.
Duration of exposure: The length of time your eyes are exposed to the test can also have an impact.
Crowding due to interaction from adjacent contours: When lines or shapes on the test are too close together, it can be more difficult to read and might affect your score.
Light exposure before the vision testing: If your eyes are exposed to intense light before the test, it might affect your ability to see correctly during the test.
Patient cooperation or functional disease: The success of the test also depends on the patient’s ability to cooperate and any chronic diseases that might affect eye function.
Equipment used for Evaluation of Visual Acuity
Tests for Checking Eye Sharpness
There are several tests doctors use to check how well you can see. These tests can be grouped into three categories, with some of them needing special equipment.
The first group of tests is known as ‘detection acuity tests.’ These measure your ability to see the smallest things. Some examples of these tests are:
- Boeck candy bead test
- Catford drum test
- Dot visual acuity test
- Schwarting metronome test
- STYCAR graded balls test
The second group is ‘recognition acuity tests’, which measure your ability to recognize what you’re looking at. Some examples of these tests are:
- Bailey-Hall cereal test
- Bailey-Lovie chart
- Beale Collin picture charts
- Early Treatment of Diabetic Retinopathy Study (ETDRS) chart
- Landolt C test
- Lea symbols chart
- Lighthouse test
- Lippman’s HOTV test
- Sheridan letter test
- Sjogren hand test
- Snellen charts
- Snellen E test
The third group, ‘resolution acuity tests’, includes the Optokinetic drum and the Preferential looking test.
Eye Testing Options for Different Ages
Different tests may be used based on how old you are. Here’s how that breaks down:
For Infants:
- Cardiff acuity cards
- Catford drum test
- Fixation test
- OKNOVIS
- Preferential looking test – Teller acuity cards test
- Reflex response
- Visual evoked responses
For Children Aged 1 to 2 years:
- Boeck candy test
- Sheridan ball test
- Worth Ivory-ball test
For Children Aged 2 to 3 years:
- Coin test
- Dot visual acuity test
- Miniature toys test
For Children Aged 3 to 5 years:
- Landolt C
- Lea symbols chart
- Lippman’s HOTV test
- Sheridan letter test
- Tumbling E
Other Eye Tests
The Brückner test, using a direct ophthalmoscope, helps find ‘strabismus’, a condition where the eyes don’t move together. A patient with strabismus will show an increased light reflex in the eye that does not point in the right direction. This test can also find significant differences or errors in vision.
Relative afferent pupillary defect testing, done with a flashlight, helps find damage to or disease in the optic nerve or retina. The test involves shining a light into each eye one at a time and seeing how the pupil reacts. If a patient’s eye reacts differently than expected, it could indicate that there’s a problem.
The cover test can find ‘amblyopia’ in young children who can’t yet talk. If a child resists or cries when you cover the better eye, but shows little to no resistance when the weaker eye is covered, it’s an indication of this condition.
Who is needed to perform Evaluation of Visual Acuity?
Many different types of healthcare professionals are involved in checking your eyesight. This includes hospital staff, technical workers, eye doctors known as optometrists and ophthalmologists, and other medical professionals like doctors and nurse practitioners. The level of knowledge and training these professionals have can greatly influence how accurately your eyesight is measured.
Preparing for Evaluation of Visual Acuity
It’s important for doctors to explain any eye tests to patients in detail. This helps patients understand what to do, ensuring the best results, especially as eyesight tests rely on the patient’s experiences. Doctors might use special tests if they think a patient is pretending to have vision problems or if they’re seeing poor vision that doesn’t seem to be linked to a physical cause.
How is Evaluation of Visual Acuity performed
People often go through vision tests to understand how well they can see. One standard method to test how clearly you can see at a distance is by using a special eye chart known as the Snellen chart. To get an accurate reading, you need to stand a fixed distance from the chart. Your vision test will cover how well both your eyes work together as well as how each eye performs on its own. During the test, the doctor will assess your vision with glasses or contact lenses (corrected visual acuity), and without them (uncorrected visual acuity).
Your vision test results may have a few abbreviations. ‘V’, ‘VA’, or ‘Va’ represent visual acuity. Your uncorrected visual acuity is referred to as ‘UCVA’, while the best vision you can achieve with or without glasses or contact lenses is known as ‘BCVA’ or best-corrected visual acuity. ‘NV’ stands for near vision. Usually, the test starts with the assessment of how clearly you can see at a distance.
There are various types of eye charts used for vision testing. These charts may be printed, displayed on a computer or other screen, or projected. They can even use a mirror for the test in smaller rooms. Some of the popular charts include the Küchler chart, known to be the oldest eye chart, and the Snellen chart, which is commonly used nowadays. The Snellen chart has different rows of letters that decrease in size as you move down the chart. The letters in these charts are in high contrast against a white background, making it easier to read.
The size change from one line to another on the Snellen chart is not uniform, which means the letters on each line do not decrease in the same proportion. The letters used in these charts may also have different legibility. Some letters may be easier to identify than others. For example, letters like ‘A’, ‘L’, ‘T’, ‘Z’ could be easier to read compared to ‘S’, ‘F’, ‘C’, ‘B’.
To overcome these issues with the Snellen chart, the LogMAR chart was introduced, which provides a more accurate and reliable measurement of visual acuity. It uses a mathematical formula to measure how well you can see. This chart has equal letter sizes and spacing, making it easier to read and more accurate in assessing your visual acuity.
Regardless of which chart is used, eye tests are critical for checking your vision and detecting any potential vision problems.
What Else Should I Know About Evaluation of Visual Acuity?
An eye test, known as visual acuity testing, can give hints about several important health issues, both related and unrelated to the eyes. These can range from conditions affecting the eyes themselves, to more widespread diseases involving the whole body or even the brain. That’s why it’s usually used in combination with other eye tests.
Healthy eyesight can vary depending on a person’s age. In a study of 200 people, researchers found that until the age of 64, most people had vision better than 20/20. From age 25 to 54, people’s eyesight was usually even better than that. But starting at around ages 55 to 59, people’s eyesight started to get worse.
When it comes to testing the eyesight of babies too young to talk, experts use a few different methods. For example, they might use a test where they pass black and white stripes in front of the baby to get a reaction. In another test, babies are shown two pictures: one with stripes and one plain. A baby with normal eyesight will look at the striped picture for a longer time. They may also show the baby a pattern of grids or bars and monitor the brain’s response to this visual stimulation, noting whether the pattern is visible enough for the baby to react.
A very common method to evaluate eyesight in older children and adults is the Snellen chart, which displays rows of progressively smaller letters or numbers. A person’s score, known as their Snellen score, can be converted into another form called a logMAR score. However, this conversion might make the eyesight seem worse than it really is, especially in people with poor vision. Therefore, some experts caution against this method and suggest using logMAR charts directly.
The Snellen chart might not be able to pick up subtle differences in eyesight, especially in young children. This limitation means it may not be as effective in detecting a condition called amblyopia, or ‘lazy eye’. Some experts even suggest that it’s time to stop using the Snellen chart entirely due to these shortcomings.
Visual acuity measurements done with logMAR charts have been found to be more accurate than those done with Snellen charts. That’s why many professionals prefer using LogMAR charts to get reliable results. However, logMAR charts are not without their own limitations and challenges.