Visual Acuity

Anatomy and Physiology

Visual acuity requires each level of a complex process to be functioning correctly. As light initially passes through the cornea, it can be affected by injuries such as corneal abrasion, corneal ulceration, or UV keratitis. Light next travels through the pupil, which is an opening with the aperture controlled by the iris. A typical example of visual changes arising from this system is seen with visual disturbances after a dilated eye exam. Light will then pass through the lens.

The lens changes shape and refraction using a system that includes the ciliary bodies and suspensory ligaments, adjusting the focal length of the eye and allowing the image to be focused on the retina. Failure of proper focusing can lead to myopia and hyperopia, while age-related changes can result in presbyopia. Additionally, the lens can opacify and obstruct vision in cataracts.

Once the light reaches the retina, light signals are converted to neuronal signals that fire through a system of neurons and ganglia to the visual cortex where the signal is interpreted. Visual changes can occur at the level of the retina due to issues such as retinal detachment or macular degeneration. There can also be issues at any level of the signal transfer to the visual cortex that will impact visual fields based upon the area of insult. In order to have visual acuity intact, all of these systems must be functioning properly.[1][3][5]

Indications

Visual acuity testing should be performed as part of a comprehensive eye exam, such instances include:

• Screening for visual impairments
• Aid in diagnosis of refractory eye disease
• Evaluation of acute changes in vision, traumatic or nontraumatic
• Reevaluate vision after corrective procedure or intervention
• Monitor vision for certification where it is necessary such as driver’s license renewal[1][6]

Contraindications

In the event of acute chemical exposure to the eye, there should be no delay in irrigation. Visual acuity testing can then be performed after thorough irrigation. Besides that, there are no contraindications.[7]

Equipment

Depending on the patient's vision and ability, one or more of the following may be required:

• Optotype chart (e.g., Snellen, tumbling E, Landolt C, Allen, or LogMAR chart)
• Occluder card or patch
• Pinhole occluder
• Penlight/flashlight[6]

Technique or Treatment

Select the preferred chart. The most widely used is the Snellen chart. 

Position the patient in a well-lit area so that they are a standard distance from the chart. The testing distance is typically 20 feet (6 m), but this may vary. In smaller spaces, mirrors can be used to achieve the required distance. Additionally, a near Snellen chart may be used at 14 inches in some cases, which would require reading glasses if applicable. There are also many phone applications available that use 4.5 feet or approximately the length to the bottom of a hospital bed. While phone applications are becoming more frequently used for convenience, it is worth noting that they may not be as accurate at measuring visual acuity in a more classic manner. However, phone applications do appear to have reproducible results for tracking changes in visual acuity.[8][9]

Cover the patient's eye with their hand or an occluder card. Some testers prefer to test the eyes in the same order on all patients. An alternative is to test the eye with worse vision first to reduce remembered letters. The second eye can also read the letters backward to reduce remembered letters. Depending upon the indication, visual acuity testing can be performed with corrective lenses or with and without corrective lenses. Pinhole testing can be used to reduce refractive error further if visual acuity remains impaired with corrective lens use. Typically, the goal is to determine the best-corrected vision.

Instruct the patient to read the top line of the chart left to right. Continue to the smallest line that the patient can read. If a patient misses one or two letters on the lowest line, they can still be considered to have vision equivalent to that line. To save time, some testers will have the patient start at the lowest line they can easily read.

Move the patient closer to the chart if they are unable to read to the top line, the new distance from the chart becomes the numerator in a fraction reporting system. For example, if able to read the top line at 10 feet, the patient's vision would be represented as 10/200.

Test the patient with finger counting if unable to read the chart at any distance. Visual acuity can be documented based upon the distance where they can accurately finger count.

Test the patient with hand motion if unable to finger count. This can again be documented based upon the distance the patient is able to recognize hand motion.

Test the patient with light perception if unable to recognize hand motion. This can then be documented as positive light perception or no light perception.

Switch eyes after the visual acuity is documented for the first eye and repeat the above steps to test the second eye. This can then be used to test both eyes.

Repeat with corrective lenses or pinhole occluder if necessary. Once completed, the visual acuity should be documented properly. If using a fraction to report the results, these can be made more precise by indicating errors. The ability to read a partial line can be marked by mistakes on the lowest line read, such as 20/40-1, for one mistake is made on the 20/40 line. Correct letters without completing the lower line are indicated by addition. For example, 20/40+3 for three letters correct on the line below 20/40 without qualifying for the lower line. Documentation will also occasionally use a shorthand from Latin derivatives when describing visual acuity in the eye. Commonly used abbreviations are listed below:

• CC-With corrections
• SC-Without corrections
• OD-Right eye
• OS-Left eye
• OU-Both eyes
• PH-Pinhole
• CF-Counting fingers
• HM-Hand motion
• LP-Light perception
• NLP-No light perception

For example, ccOD 20/30 indicates a 20/30 vision in the right eye using corrective lenses.

Testing a special population, such as children, may require some adjustments to the testing procedure. Verbal children older than three years old can have visual acuity testing with a chart attempted. Ensure to select an optotype chart to their level of comprehension. It is best to limit distractions and make the patient as comfortable as possible. This may include sitting on a parent's lap, having a parent cover the eye, minimizing distractions in the room, showing the chart one line at a time, and showing patience. Children under this age typically have vision indirectly tested with age-appropriate eye-tracking and movements.[1][6][10]

Clinical Significance

Visual acuity testing plays a role in both the acute and screening settings. In the acute setting, it is important to document an initial visual acuity score. This is true for both mechanical injury of the eye as well as sudden vision loss, for example, from glaucoma, retinal detachment, or ischemia. This visual acuity can be compared to previous visual acuity testing if available and will serve as a baseline for reevaluation after intervention in the acute and subacute setting. If there is a mechanical injury to the eye, contacts should not be used to correct vision; however, pinhole occluders or glasses are still acceptable. It is important to note that 20/20 vision on initial evaluation does not rule out significant pathology that could result in long term visual impairment.[3][7]

Visual screening tests, including visual acuity, have numerous benefits to children. Some tests encompassed by a comprehensive visual screening exam can lead to early detection of pathologies such as amblyopia or retinoblastoma. However, visual acuity is best at discovering the refractory error, which is the leading cause of visual impairment and reduced vision in children.

With early intervention, the progression of visual impairment can be limited. Using corrective lenses such as glasses in refractive error can have long-term developmental, educational, and social benefits. In the United States, visual acuity screening will typically begin as early as age 3. There is a critical line that the child should be able to complete on a visual acuity chart by age group. The critical line for children between the ages of three to four is 20/50, four to five is 20/40, and five or older is 20/30. Failure to read this line should prompt referral for a comprehensive examination by an ophthalmologist. The current recommendations in the United States are for children with age-appropriate visual acuity to have repeat visual screening performed every 1 to 2 years.[10][11][12]

During the initial development of children, it is important to have more frequent visual examinations. However, it has not been found to be of benefit or be cost-effective to have frequent visual screening exams on all adults. This has led to recommendations for visual screening exams in the United States, including visual acuity testing to be based upon risk factors. In asymptomatic adults without significant risk factors for visual disease, a comprehensive eye exam may be performed every 5 to 10 years when under age 40, 2 to 4 years between age 40 and 54, 1 to 3 years between age 55 and 64, and 1 to 2 years over age 65.

The change in frequency is to better assess for age-related ocular disease. Populations at higher risk for glaucoma should have comprehensive eye examinations performed more frequently, even when asymptomatic. Individuals diagnosed with type 1 diabetes should have an eye exam, including visual acuity, five years after diagnosis followed by yearly exams, while individuals diagnosed with type 2 diabetes should have yearly testing starting at the time of diagnosis. Additionally, women with diabetes should have a comprehensive eye exam, including visual acuity during the first trimester of pregnancy. The goal of these screening exams is to allow for early intervention to prevent long-term morbidity and enhance the quality of life.[3][4][13]

Enhancing Healthcare Team Outcomes

Identifying individuals and children with visual impairment or low vision is a critical intervention that can occur at an earlier stage if completed and followed appropriately. The goal of these interventions is to preserve the remaining vision and improve the overall quality of life. While it is easy to think that this is the responsibility of eye care specialists, many of these patients will see long-term benefits from a multifaceted approach to caring for their low vision. For children, the team supporting their vision is not limited to healthcare specialists.

While pediatricians, ophthalmologists, optometrists, nurses, social workers, and rehabilitation specialists can be involved in specialty teams targeted at intervention, other individuals in a child’s life, such as teachers and parents, maybe the first to notice a child has visual issues. Once a child has been identified, and intervention has started, these same individuals can continue to monitor the progress and continue to evaluate social interactions and academic performance. Additionally, in children with neurological disabilities, addressing a visual component as part of their interprofessional care team can have developmental and emotional benefits.

There is also evidence that using specialist teams from multiple disciplines, including ophthalmology, nursing, social work, and rehabilitation specialists, can benefit adults by improving their ability to perform activities of daily living in adults diagnosed with low vision. Interprofessional teams and support systems can improve the quality of life of patients found to have low vision with ongoing intervention and adjustment.[14][15][16][17][18] [Level 4]