Perhaps the most iconic piece of the optometric examination is the visual acuity-traditionally measured by reading letters on a chart 20 feet away. This is a very important piece of information because it tells the doctor how well the macula is working. The macula is the area of the retina that gives us visual details and color vision. This is all well and good until the patient in the chair cannot read letters. What happens now?!?! Rest assured, there is more than one way to do this.
- Resistance to Occlusion (RTO): This is perhaps the most basic test of vision–determining if the patient has a preference for one eye over another. If the patient fights me when I try to cover one of their eyes, it is an indicator that the patient is favoring one of the eyes. I have had patients try and bat my hand away, turn away from my hand, yell, scream and cry when I attempt to cover their favorite eye.
- Fix and Follow (F&F): Fix and follow is often paired with RTO–allowing me to assess both tracking and vision at the same time. Infants learn how to fixate on an object of interest and track it (like mom’s face) fairly soon. The presence of a high prescription or damage to the macula can impede vision development and prevent a strong fix and follow response.
What about the patient who doesn’t know their letters? There are a lot of different options for getting acuity with picture charts. My system has Allen figures (click here for example) and they work well. If the patient will sit in my chair , this is a great tool. I am always impressed that the kids identify the old rotary telephone as a phone without any prompting from me. The last time I saw a rotary phone was in my great-grandmother’s house.
There are lea figures, which come on cards. These are my favorite, as I can play matching games with the kids and thus eliminate the need for the kids to be able to reliably identify pictures such as birds and horses. We are only dealing with a square, circle, heart and house, all of which are designed to blur to a circle once I reach the patient’s vision threshold. They come on cards, which means they are portable! This is fantastic because I can take acuities at 5 feet, (not so far away that the kids quit paying attention, but not so close that I am all up in their personal space) and easily covert to 20 feet distance acuities. Simple and versatile is better.
The tumbling E’s are the bane of my existence. I hate these. When I was in college, I worked for a pediatric ophthalmologist who would have the kids indicate which way the E was tumbling with their hands. Fingers up, down, left or right. This doc must have had magical hand sign interpreting powers. What do you do when the kid displays fingers on the diagonal??!?!? Needless to say, I do not use these.
Once we have gotten past the age where I have to entice the kids to play games with me, they can usually start reading numbers and then letters, which brings us back to the traditional chart.
Why go through all of this trouble to make sure we can quantify visual acuity? As long as I have a consistent way to measure how well a patient can see, I can track progress in vision therapy, notice changes in refractive error, and catch subtle pathological conditions that need to be addressed sooner rather than later.
Stay tuned for….the COVER TEST! As a professor of mine put it, the KING OF ALL TESTS.