Occasionally I send patients to another clinic to get a laser scan of the back of their eye. What is this test, and why do we do it?
The official name of this procedure is called an “optical coherence tomography” scan or “OCT” for short. The machine emits a very short wavelength (~100 nm) laser beam that hits certain structures in the retina. The onboard processor is able to translate the information that the laser records into a topographical profile of whatever structure the doctor wants to see. This affords him the ability to notice any disruptions below the retinal surface.
I take retinal pictures of every single patient in order to examine key anatomical and functional components of the eye. Retinal photos are a brilliant form of technology that allows me to see very detailed things. Occasionally I also utilize special lenses to determine if a problem is present. However, these two techniques are sometimes not detailed enough to examine key hidden structures.
Instead of thinking about the retina, let’s use an apple analogy for a moment. If you take a picture of the side of an apple, you could see that it is red. You could see the outline and perhaps infer that it curves outward towards you. You wouldn’t be able to tell how deep the apple was or what the insides look like. Taking an OCT of the apple would be like cutting it in half and observing a cross section. You would now be able to see that red color is actually a very thin skin. You could also measure the depth of the apple, see that the majority of it’s volume is composed of a fibrous network, and notice a small cavity in the center that’s filled with seeds. If the seeds were displaced to the side, a traditional camera picture wouldn’t notice anything out of the ordinary since the external appearance is untouched. However, an OCT would be able to tell that they are not in their proper position. For almost every patient, a retinal photo is way above the standard of care that’s required to examine their eyes. However, some patients’ eyes either present with abnormalities or have a suspicious appearance. Those patients require the detailed level of imaging and analysis that an OCT can provide.
It’s important to clarify a few misconceptions about getting an OCT:
-Does it hurt?
It doesn’t hurt at all. It’s actually slightly more gentle than a camera flash
-Will it fix my problem.
No. There are many types of lasers that doctors use on the eye. An OCT does diagnostic imaging only. That means that it shows the doctor what is the issue and does not fix it.
-Why didn’t my doctor see what the laser saw?
A doctor only orders an OCT when a known or suspected problem lies in an area of the retina that’s not visible to human eyes and traditional machines. Unless your doctor has x-ray vision, he won’t be able to see the deep hidden retinal layers.
-Come on doc. Is this really important?
Absolutely! With many retinal pathologies the OCT reading is how the doctor determines what treatment or monitoring protocols he will use. Additionally, he now has a snapshot of the retina at a given time and can track it’s evolution in the future.
-Why doesn’t my doctor have an OCT?
Most patients don’t require such a scan. Typically I will see only 1 patient who needs an OCT each week. Although I can perform the scan and interpret the results with ease, I prefer my patients to be scanned by a doctor who does 10 a day and specializes in that type of care.
-You’re sending me to a disease specialist? I must have something really bad.
Needing an OCT does not mean that you have a serious disease. It does mean that your eyes present slightly atypically and an extra scan is the prudent thing to do. Frequently the doctor who does my scanning will say, “It looks okay. We’ll just monitor it. Go see Dr. Ritenour in a year.” Some patients I know are healthy, but have an elevated risk for developing a problem in the future. In theses cases, the OCT scan is a baseline “snapshot” that we can always compare back to in the future.
Leave a comment or email me if you have any questions or concerns.
Spencer Ritenour, O.D.
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Optometrists love to toss around the term 20/20: “You have 20/20 vison.” or “Gosh, you’re seeing better than 20/20.” Perhaps, you use it at home: “How does this dress look?” “Sorry honey – I don’t see 20/20.” The news program “20/20” was actually named after the visual measurement. Everyone knows 20/20 is ideal, but few actually know what it means.
To know how well someone sees, we must use a standardized system. This affords doctors the ability to know if someone needs more or less correction and to compare the vision between members of a population. For example, it’s means very little to accurately check the vision of an eight month old girl and then record it as “pretty good” or “seems normal.” It’s far superior to be able to record her vision as 20/60 or 20/80. Then visual changes may be tracked with tighter accuracy, her visual acuity may be compared against what’s normal for 8 month olds, and her vision may be corrected or observed against a quantifiable system.
The 20/xx system is also known as the Snellen visual acuity system. The top number refers to the testing distance in feet. Twenty feet is considered to be “optical infinity” due to how our eyes focus and the behavior of light waves at this distance. However, most offices do not have patients sit 20 feet from a chart due to space constraints. Through the use of mirrors and chart calibrations, the shorter distance is compensated for remarkably well.
The bottom number is a bit more complex. It’s the distance at which each element of a letter has an angular height of 1 minute of arc (or 1/60th of a degree). The letter “E” on an eye chart has 5 elements. There is the top bar, a space, the middle bar, another space, and the bottom bar. When one stands 400 feet from the big E at the top of the chart, each of these elements subtends 1 minute of arc. Therefore the size of the letter is called 20/400. For a letter on the 20/20 line, one must stand 20 feet away for each element to subtend 1 minute of arc.
The neurological processing in the human eye and key areas of the brain allows people to easily discriminate between letters with 1 minute of arc (or smaller) features. With a (compensated or actual) testing distance of 20 feet, optometrists therefore embrace the 20/20 line as a perfect endpoint. Please don’t fret if you can’t see 20/20 without glasses or contacts. That will be the subject of a blog post in the near future. Seeing 20/20 with or without visual correction means that at 20 feet away, your eyes see as well as they are expected see. If you can’t read 20/20, please schedule an appointment, and I’d love to help boost your vision.
Spencer Ritenour, O.D.