Common Errors while checking vision

Common Errors while checking vision



Have you ever pondered over the accuracy of your vision check? “What if I didn’t push my patient enough? What if they were squinting and I never caught it? Or maybe the patient’s eyes were too dry. Should I have put a few drops of artificial tears in?” These questions have been posed amongst many optometric personal over the years, yet there seems to be no well-defined rule answering these questions in a viable way. Isn’t It odd how you can get an astonishing 20/20 on the chart, only to be told 10 seconds later that the image is no longer clear? Well don’t worry my friends. I’m here to answer your question. The answer IS……………… THERE IS NO ANSWER! Allow me to explain.

As we have learned from our studies, the eye (much like the rest of the human body) is a fluid system that is constantly in a state of change. This ranges from the adjusting for the interocular lens to the stability of the tear film on the surface of the eye. In a perfect world, all components would seamlessly align to produce the optimal refractive medium. But let’s face it, the odds are forever against us with this. Most often things are in tune just long enough to give us a narrow ballpark, but hitting a targeted pinpoint is just unlikely. Still, we want to maximize our chances of getting the best possible answer. Here are 3 ways to improve accuracy.


  1. Skip lines in chart – This trick of the trade can help in many ways. For starters, it helps speed up the work up and decrease the chair time. This isn’t to take away from the thoroughness of the exam, but it is well proven that the longer the human eye concentrates on an object, the more time the lens in the eye has a chance to accommodate in or even out of focus. So, by simply reducing the number of letters you show the patient, you in turn allow for less stimuli to interfere with the natural lens. How can you apply this? Well its simple. If a patient can read the initial 20/60 line with ease, why not drop them down to 20/30 or 20/25 immediately after. Though this is a big jump, you can often gauge how close the patient is to seeing 20/20 by how they preform on the larger print. It would be both time consuming and visually taxing to slowly walk the patient down line by line in this scenario. On the other hand, it is always wise to go line for line in the case that the patient is not preforming well or struggling to read the bigger lines.


  1. Blinking to help improve vision – As mentioned, the eye is not a static system in which everything stays consistent. One of the most under diagnosed ocular complications is dry eye. This can be linked to things such as excessive television, computer and cell phone usage. By any rate, it is not uncommon for your patient to have fluctuating vision while gazing at the Snellen chart. Most often a poor tear film is the results of this. Think of a car windshield while raining. Virtually impossible to drive like this without using the windshield wipers. Now when you think of it, windshield wipers are not designed to “dry “the windshield per say. They are mainly used to displace the droplets in order to form a smooth/even layer on the surface. This allows the light to refract through the shield much easier and in turn allows the driver to see better. This is no different then when a person blinks. If said dry eye is a result of displaced mucous, oil and water on the cornea, it is happily evened out by a simple…. Blink! The upper lid acts as the windshield wiper giving the patient the small gap of time in order to read at their best. But be quick. Depending on how short the tear break up time will let you know how many seconds your patient has before things get blurred again.


  1. Screen contrast – Third, but certainly not least, what type of chart are we using? Back in the early 50’s, everyone was excited about the latest invention. A Color TV! Though heavily pixelated with poor resolution quality, for its time it was the top of the line. Hard to envision this old box being such a game changer when compared to the current televisions we own in our homes today. You see, things are fine to use when you are none the wiser. But when things change around you, you can become complacent very quickly if you don’t also adapt. Sure, you could turn on the 1980’s projector with the half burnt out bulb and have the patient try to guess the letters as best as they can. But the reality is, this method is vastly inferior and has a direct impact on the performance of your patient. Modernized digital charts have made it so that the contrast and pixels accurately reflect what is seen in the real world today. By upgrading to a new eye chart not only will you get a more accurate acuity measure, but you will also attract more clientele. After all, patients love to see modern tools in healthcare that enable the user better outcomes with higher reliability.

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