One of the questions that you run in to a lot when you have a very little one in glasses is how an eye doctor can determine the prescription of kids who can’t read letters yet — and in many cases, aren’t verbal yet. It was one of the things I wondered about when we took Zoe in at 9 months old. It turns out that eye doctors (optometrists and ophthalmologists) have a number of tools to help them do this. At Zoe’s early appointments, they used Teller Cards (grey cards with black and white lines on them) as well as dilating her eyes and using the retinoscope to look at the shape of her eye. The Teller cards are an example of a subjective measurement of her acuity – it requires some response from her, in this case, it was whether or not she looked at the black and white squares. Other examples of subjective measurements include eye charts, which can use letters or symbols. The retinoscopy was an objective refraction. That is, it looked at the shape of her eye to see how well she could focus without requiring a response from her.
Teller Acuity Cards
The way these work is based on the idea of preferential looking: that a child will look at the most visually interesting part of an object. In the case of the cards, the square of alternating black and white lines is more interesting than the grey background. If your child can see the lines clearly enough, they’ll prefer to look at that square. However, if a child’s vision is blurry, the lines blur together at some point and the square will be the same shade of grey as the rest of the card and they won’t look at the square. The eye doctor shows the child the cards with thinner and thinner lines until the child stops looking at the square. That point at which the lines blur together too much to see a separate square tells you about the child’s visual acuity.
Much of how clearly a person can see is related to the shape of the eye and where the lens of the eye focuses the light. If you’re hyperopic (farsighted or longsighted), light focuses behind the retina. If you’re myopic (nearsighted or shortsighted), light is focused before it reaches the retina. So understanding how the eye is shaped and where light is focused tells the eye doctor what shape corrective lenses need to be to get the light focusing in the right spot. The video below gives the best description I’ve seen of how retinoscopy works.
Putting it all together
Subjective measures like the Teller cards require some cooperation and response from the patient. The retinoscopy will tell you how someone’s eye is shaped, but there are other things that go in to how clearly a person can see. So both of those methods are used by eye doctors to come up with the final corrective lens prescription.
For a more complete explanation of how it all comes together, I turned to Dr. Dominick Maino, OD, MEd, FAAO, FCOVD-A. He is a Professor of Pediatrics/Binocular Vision at the Illinois Eye Institute/IllinoisCollege of Optometry and is in private practice in Chicago, Il. He also writes about latest research in vision and vision care of children at MainosMemos.
An objective examination of refractive error (myopia, also known as nearsightedness or shortsightedness; hyperopia, also known as farsightedness or longsightedness; and astigmatism) can be completed several ways. You can use an auto-refractor (though most do not work well for little ones) where the child looks at this computerized device and it tells you the refractive error. Eye doctors can also use a retinoscope. This is a small handheld flashlight that directs a light into the eye. When it is reflected back out, depending upon the type of refractive error, it will move in a certain way. We then neutralize this movement by placing lenses in front of the eye. Once we see no movement, we know the refractive error. In terms of accuracy, they are all accurate depending upon several factors.
The same pretty much applies to the subjective measurement of visual acuity. Teller Cards, Snellen Chart and the Lea Symbols all measure visual acuity but slightly differently. Once again all are accurate. I am not quite as concerned about the number generated by the visual acuity test at this age. I am much more concerned with both eyes having similar numbers. You want both eyes to see equally as well. [emphasis mine – Ann Z]
Now on to prescribing for little ones: Giving glasses is still as much art as it is science (something about humans being so…well human!). For example, most people want to be fully corrected if they have myopia (nearsightedness / shortsightedneess) so they can see clearly at all distances. Hyperopia (farsightedness / longsightedness) is even more tricky. If you are farsighted and a child, you can compensate for the farsightedness by kicking in your focusing ability. Unfortunately this can lead to major problems such as accommodative esotropia: when you compensate for the high amount of hyperopia by using your focusing system, an eye turn inwards results (from MainosMemos, “What is Accomodative Esotropia?“). To complicate matters even more, ophthalmologists and optometrists have different philosophies when it comes to prescribing glasses. Ophthalmologists tend to prescribe for higher amounts of farsightedness, while Optometrists lower. The good news is that for infants and toddlers we probably prescribe in a similar way.
The important thing is to find a doctor you trust and follow his/her advice. Don’t be afraid to seek out a second opinion and don’t be surprised if that second opinion if different from what you’ve been hearing. Ask lots of questions. Ask why the doctor is recommending what they are recommending. If the doc doesn’t have the time to fully explain what he/she is doing, find a new doc. I hope this helps!!!”
— Ann here again. Many, many thanks again to Dr. Maino for the explanations.