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Why the Snellen Chart Fails with Certain Vision Problems

Why the Snellen Chart Fails with Certain Vision Problems

You’re probably familiar with the eye chart used by most schools for vision screenings.  It’s called the Snellen chart, and despite its widespread usage, it tends to miss significant vision problems.The Snellen Chart has given us the term “20/20” vision.  This refers to acuity, which the American Optometric Association defines as the “clarity or sharpness of vision.”  Unfortunately, acuity does not include a wide range of other vision issues that can result in a learning problem.

What a Snellen Chart does well:  Visual acuity

Many vision screening tests performed in schools are confined to the Snellen chart.  Developed in 1862 by the Dutch ophthalmologist Hermann Snellen, the chart is used to measure visual acuity, defined as the visual ability to resolve fine detail.

On the chart, the first line consists of very large letters. Subsequent rows have an increasing number of letters that decrease in size.  The smallest row that can be read accurately indicates the visual acuity of the eye.

The most familiar acuity test places the test at a standard distance of twenty feet. At the bottom of the chart is the 20/20 line. A person with 20/20 vision can accurately read this line while standing at the test distance of 20 feet.

A person with 20/30 vision would not be able to see the 20/20 line at 20 feet and instead would read the 20/30 line, which is two lines above the 20/20 line, and has larger print.

A person with 20/20 vision could read the 20/30 line at a distance of 30 feet, while the person with 20/30 vision would have to move up to 20 feet in order to see it.

This is a time-tested way to determine acuity.  But can it detect other types of vision skills - ones that, if problematic, can cause a learning problem?

Vision Problems missed by the Snellen Chart

According to developmental optometrist Dr. Kellye Knueppel of The Vision Therapy Center, the Snellen chart is very useful for detecting near-sightedness.  However, it’s profoundly lacking in other areas.

Besides not being very useful for detecting farsightedness, the Snellen chart often does not detect astigmatism, eye teaming difficulties and/or a ‘lazy eye’.  All of these factors can cause a learning problem.

“If a child’s eyes don’t work together well it can cause great difficulty with tracking the lines of text across the page of a book or a computer screen,” Dr. Knueppel explained.  “The Snellen chart doesn’t test for this.”

Even worse, Dr. Knueppel notes that the eye chart is susceptible to cheating.  In the cases of adult patients worried about losing driving privileges, or children worried about wearing glasses, patients can often remember the sequence of letters, and then repeat them back when an alternate eye is tested.

“I’ve even had patients peek without knowing they are doing so,” Dr. Knueppel recalls.

The takeaway?  If your child exhibits any symptoms of undetected vision problems, consider scheduling a Functional Vision Test. A Functional Vision Test is beneficial because so many vision problems are hard to detect, and often children don’t realize they have any issues. 

Expanding the search beyond visual acuity could help you find these problems, and potentially discover the root cause of a child’s learning problem.


 The Vision and Learning Guide from The Vision Therapy Center. Learn how undetected vision problems can impact a child's ability to learn.  Download your free Vision and Learning Guide.

Posted by   Greg Mischio