Visual Field Testing – What It Is | Driving with Dr. David Richardson – Series 2 Ep 02

VIDEO TRANSCRIPT: 

Hello I’m Dr. David Richardson. I’m a cataract and glaucoma surgeon in Southern California and on my morning commute, I like to discuss those topics that there’s just not enough time to discuss in the exam room. So today, I’d like to continue the series of discussions about visual field testing.

Today I’m going to discuss what visual field testing is as well as the patient experience in the hopes that knowing this will make your experience less frustrating when you’re in the doctor’s office. So let’s go!

So our visual field—what is it? Well it’s the area in which we can see. So, in front—up above, below…to side…and in humans, the normal visual field spans about 160 degrees. So one, six, zero. Now, testing the visual field—we’ve talked about why this is important—because it tends to be reduced in those with glaucoma as well as other conditions.

Visual field testing has been done for about a hundred years. Now when it was first done, it was pretty simple it was essentially done on the back of a clinic door and testing this way it was pretty impossible to detect subtle changes. The only thing that could be detected at that time was what we call, “scotoma”—essentially an absolute loss of an area vision. Now with glaucoma, we or anything that involves the visual field, we’d really like to detect it before it becomes an absolute scotoma.

And over the decades, the testing has become far more sophisticated and sensitive to earlier field loss. Modern perimeters in the modern visual field devices are called, “automated perimeters”. It uses a computer to essentially determine how the test will be performed, rather than an individual which used to be the way we do it. And it generally involves a white ball or screen upon which dots of light our little circles of light are projected. And this is done in a seemingly random pattern although there’s actually a computer algorithm that’s pretty sophisticated in determining where the spots of light should be shown at what rates as well as at what intensity. And the entire point is to find what’s called, “the threshold of vision”. So essentially, how low an intensity of light can one see.

Now, the patient experience tends to be somewhat frustrating. Well, I’ll tell you why that is… there’s a number of reasons. First of all, in terms of positioning, you do get to sit so there’s that but it’s not so relaxing because you have to lean forward with your chin up against the chin-rest, your forehead up against a forehead rest, and if you have some issues with your neck or back, it’s not necessarily a very comfortable test. You’ll be asked to look straight ahead, at a point in the white screen or wall, which is called the fixation point. And it’s critical that you maintain fixation, keep looking at that point because if you’re looking around, it’ll end up being a poor quality test. And you want a high quality test. If you’re going to sit through this, both you and your physician want it to be high quality. So you must maintain that fixation, which of course is challenging to do because when we see something in our visual fields, the natural reaction is to look toward it, right? So you have to overcome that kind of a natural tendency. When you do see a point of light, you will depress a button so you will have been given a handheld controller on which there’s a button that you should be instructed to press the button whenever you see a point of light.

So I mentioned that it could be frustrating. And the reason that it can be frustrating—well there are numerous reasons actually—one is, it’s incredibly boring. So you’re sitting there in front of light screen and dots of light appear and you press a button. So it’s essentially the most boring video game you’ll ever be asked to play.  No colorful graphics, no interesting sounds, no points, no bonus rounds (although you probably wouldn’t want to bonus round in this case). And so given that it’s so boring even though the field only lasts about 4 to 8 minutes on average, it seems like it lasts a really long time as anything does that’s boring, right? And you have to do one eye at a time. So double at four to eight minutes it becomes, you know, an eight to sixteen minute a test, which doing anything that you find boring for you know a quarter of an hour can be quite challenging. Even for us even for us non-millennials. So, there’s the boring aspect…there’s the aspect that it seems to take a long time because it’s so boring. But there are some other things that are actually somewhat interesting to me that still make the test a bit challenging for those who are taking it.

So one interesting thing that can happen during the testing (I just parked in my office lot here) is that at times, after you’ve been taking the test for a while, the entire field can just kind of “white out”. It can be disconcerting the first time that it happens to you. Everything just looks like you’re caught in a blizzard or your entire field has turned into an old-style cathode ray TV with no broadcast. And what’s important to know is that this is this is somewhat normal. Essentially, when there’s no input coming into our visual system, the brain, which is used to seeing something,  just put something there. And once you actually have something going on in your visual field then that  will disappear. And so, the thing to do if this happens to you is to generally depress the button and hold it down, which in most systems indicates that you want to pause the test, sit back, look around and your vision will come back. Blink a couple times, put your chin and forehead back where they need to be and then go on with the testing.

Now that’s interesting, but there’s something else that I find even more interesting but unfortunately for almost everybody who takes the test is something that can be quite confusing and that’s something related to the actual threshold of vision. Now, before I discuss that I want to back up a little bit and discuss something related to the threshold which is that the field test itself can make you feel like you’re failing the test and the reason for that is we are testing the threshold of vision so that is right where you are just able to detect something. And in order to do so, the algorithm tests points in your field with light that you can see but then it’s also going to test points with illuminated spots that are just a little bit below that until it finds that point where you can see spot and then a little bit less illumination you can’t. So it’s essentially “straddling your threshold”.

Well if you’ve got a test a point that you can and a point that you can’t that means you’re going to miss about 50% of the points of a field. Well you know in our culture where we’re all trying to achieve missing 50% of anything it’s gonna be frustrating. But it helps to know that that’s completely expected. So don’t feel bad that you’re missing half of the points you’re supposed to.

Now the confusing aspect of this straddling the field is that something very very interesting happens right at that razor’s edge of your threshold, which occurs when photons of light from that little spot of light hit the photo-receptors in your eye. So they’re actually detected by these cells whose job is to detect light. These cells send a signal back through the optic nerve into the brain. Now once the signal enters the brain is split into two pathways. One pathway goes into your subconscious awareness system. So it indicates hey something’s there. The other signal and this is right at the threshold of your vision. The other signal goes back to your visual cortex which is in the back of your head here. And on the way to the visual cortex, the processing algorithms that are our brain perform to essentially get rid of noise in the visual system decide ah there was nothing there actually it was just noise. So what you end up with is this awareness something was there but you didn’t actually see anything. Your retina photo-receptors detected something but you didn’t actually see anything. So it’s actually really fascinating but can be really confusing and frustrating while you’re taking the test.

So what do you do when you’ve sensed that something was there but you didn’t actually see it. What I generally instruct my patients to do is “only press the button when they know they saw something” and that helps reduce some of the frustration but hopefully for you the next time you take a visual field when you experience that sense, that something was there but you didn’t actually see it rather than be confused or frustrated by it you’ll have an extra appreciation of the complex system of vision that we have.

Anyway, so hopefully, this will make your next visual field testing experience a bit less frustrating. Just knowing about you know some of these details of the testing and I look forward to continuing my discussion about visual field testing on my next video, all right.

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1. Visual Field Testing – Why We Do It | Driving with Dr. David Richardson – Series 2 Ep 01
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3. Holistic Approach to Glaucoma | Driving with Dr. David Richardson Ep 01