What is Visual Acuity?

Visual Acuity is a measurement of how well someone can see at standardized distance of 20 feet. The goal is that either with or without correction that a patient will be able to see at a distance of 20 feet what should normally be seen at a distance of 20 feet. This is known as 20/20 vision. If a patient can only see at a distance of 20 feet what should normally be seen at a distance of 40 feet, this would be 20/40 vision.

How to Measure Visual Acuity

At Professional Vision Care, to measure acuity, we use a digital acuity chart program called Acuity Pro that displays a Snellen Chart on a monitor fixed to the wall. The Snellen chart is a standardized chart of rows of letters of different sizes that demonstrate different levels of acuity. The smallest row corresponds to 20/20 vision. The largest letter at the top of the chart corresponds to 20/400 vision. Because our rooms are not 20 feet long, the chart is calibrated to adjust the size of the letters to simulate a testing distance of 20 feet (in some rooms, mirrors are incorporated into this process).

Because a Snellen Chart is larger than can fit on the monitor at one time, it is manipulated using a remote that can adjust the size of letters displayed, change the letters that are displayed, change the number of letters displayed, and also display alternative charts (such as pictures for children, numbers, tumbling E, etc). To issue commands using the remote it must be pointed at the receiver which is usually located nearby the room’s computer.

To test visual acuity, first ask the patient if they have glasses or contacts that they wear for distance activities. If so, have them put them on if they are not already wearing them.

Next, hand the patient an occluder (see below) and have them use it to cover their left eye.

Next, have them look at the chart on the monitor and read out the lowest line they are able to read.

If the patient reads . . .

. . . then you would record for their right eye that they were able to read 20/20 with correction.

Next, have them cover the other eye and again read the lowest line they are able to. If they can only read . . .

. . . then you would record for their left eye that they were able to read 20/200 with correction.

Sometimes, patients will not be able to read the entire line correctly. For example, if a patient who does not wear glasses covers their left eye and reads . . .

. . . the letters they missed are reflected by recording the line minus the number of missed letters: in this case they are able to read 20/20-2 in their right eye without correction.

If they switch to cover their right eye and read . . .

. . . you would not record 20/20-6. In order to qualify for a line, the patient must read at least half of the line correctly. In this case the patient would be asked to read the line above the 20/20 row. If they read that line correctly they would qualify for the 20/25 row; however, it should be reflected that when attempting the 20/20, the patient was able to read two of the letters. This is recorded as the line they qualified for + the number of letters they read correctly on the harder row: in this case, 20/25+2.

Some measure of acuity must be obtained for each eye in the distance (The only exception being if the patient has a prosthetic eye). If the patient is unable to read the 20/400 letter, you will stand 5 feet in front of the patient, still covering one eye, and hold up fingers for them to count. If they can count your fingers, then you record CF 5’ (counting fingers at five feet).

If they cannot count your fingers at 5 feet, then move closer to 3 feet. If they can count your finger, then you record CF 3’.

If they cannot count your fingers at 3 feet, then you remain at that distance, wave your hand back and forth, and ask them if they can see what you are doing. If they see your hand waving back and forth, then you record HM 3’ (hand motions at three feet).

If they cannot see your hand moving back and forth at 3 feet, then you will need to check if they have LP or light perception. To check for LP, find a transilluminator.

The transilluminator emits a very bright light from its end when turned on. You turn it on by depressing and rotating the button on its black collar. Once the light is on, you will stand in front of the patient and bring the light in and out of their direct line of sight (directly in front of their pupil). If the patient is able to appreciate the change in illumination when the light is directly in front of their eye, you would record LP (light perception).

If the patient is unable to appreciate any difference when the light is right in front of their eye, you record NLP (no light perception).

Pinhole Testing

If the patient with their most recent correction is unable to read better than 20/40 at distance with either eye, it is worthwhile to quickly check to see if this is likely due to a change in prescription or if there is something physically wrong with the eye, such as worsening cataracts. How we check this is through something called pinhole testing. The pinhole testing is completed using the pinhole occluder.

Have the patient hold up the pinhole occluder like opera glasses overtop of their most current glasses (or without if they do not have glasses). One side of the pinhole occluder is completely blocked off; the other side has tiny pinholes that the patient will look through using the eye that they were unable to see better than 20/40 with. These pinholes work in the same manner as squinting. If the patient’s visual acuity is better looking through the pinholes, this likely means that they have only had a change in their glasses prescription. If the acuity does not improve through the pinhole, there may be something physically wrong with the eye that needs to be addressed before the patient updates their glasses.

How to Record Visual Acuity in the Chart

Visual Acuity is recorded on the lower left corner of the workup tab.

The Visual Acuity section of the workup tab is made up of five columns and six rows. The columns and rows have several abbreviations which let you know where each acuity should be recorded.

OD: right eye

OS: left eye

OU: both eyes together

SC: without correction

CC: with correction

PH: with pinhole

Dist: tested at distance

Near: tested at near

There are two main divisions in the visual acuity section: testing with or without correction and testing at distance and near.

Once you know these, you can record each acuity in the appropriate section. For example if a patient who does not wear glasses reads 20/20 for each eye in the distance and 20/25 at near, the chart would look like this.

If the patient wore contact lenses and in the distance read 20/20 in the right eye, 20/20-3 in the left eye, and 20/60 at near it would be recorded like this:

If the patient wears glasses and in the distance for the right eye read 20/30, for the left eye read 20/40, pinholed in the left eye to 20/20, and removed the glasses for near and read 20/20, it would be recorded like this:

If the patient wears classes and in the distance for the right eye read 20/30, for the left eye was only able to count fingers at 5 feet, but pinholed in the left eye to 20/400, and read 20/25 at near with the glasses on, it would be recorded like this:

Here’s a video on how to enter acuity findings.

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