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 simply not enough time to discuss in the exam room. We’ve been talking about Visual Field Testing, and today I’d like to go over some of the other tests that are used to complement visual fields and why they are necessary.
So, let’s go!
Now, visual field test—as we’ve talked about, is critically important in both the detection and monitoring of progression of glaucoma. However, we’ve also talked about many of the issues with visual field testing. And there’s two in particular that I’d like to discuss today. One is sensitivity, and the other is reliability.
So, sensitivity is essentially the likelihood that a test will pick up something that’s there. So, for example, a test that is a hundred percent sensitive would detect a condition 100 percent of the time that it is there, alright. Whereas something that is less sensitive won’t pick it up every time, and you could potentially miss a condition that you’re looking for.
Visual field testing has pretty poor sensitivity with regard to detecting glaucoma. So, for example, it’s possible to lose about a third of the retinal nerve fiber layer, which is what’s lost with glaucoma before any visual field defects are picked up on standard automated perimetry or a visual field testing (that’s commonly used in the in the office). So, we need something that is more sensitive to glaucoma detection. Otherwise, we risk not, actually being able to find these conditions until damage has already been done.
Now, the other issue is reliability. Visual field testing is not very reliable (with regard to detecting glaucoma). And we’ve discussed some of the things that impact the reliability. And frankly, if a visual field testing was a car, it’d probably be a Yugo. For those who may remember that car, it was not a very reliable car. But if your only other option is a horse and buggy, you’ll take the Yugo. And visual field testing is essentially the only option that we have to assess the function of the optic nerve. So, we need it, but it’s not sufficient on its own.
So, what are some of the other things that we can use to detect and monitor glaucoma? Well one of those is the Dilated Fundus Exam, which is almost as disliked as visual field testing …dilating the eyes takes a while so it’s a long exam in the office; eyes maintain dilation usually for hours, which means that the vision is blurred for hours after the exam; there’s light sensitivity, so if you do it on a bright day it’s uncomfortable. But there are things that can be picked up with a dilated fundus exam or dilated eye exam, which just can’t be picked up with anything else other than photography of the optic nerve.
So, a skilled physician can actually detect glaucoma just by looking at the optic nerve. And there are things that a physician can look, for such as hemorrhages or small little bleeds on the optic nerve, that indicate that someone is at risk for progression. And that’s something that also can’t be picked up without photography. And most of the photography also requires dilation to be done really well.
So, what else? Well, fortunately we now have very sophisticated methods of scanning the optic nerve. So, the typical method of scanning is called Optical Coherence Tomography or OCT, for short. And what this does is it actually creates scans of the optic nerve that can give us information about the shape of the optic nerve, the thickness of the fiber that goes into the optic nerve, which is lost with glaucoma. So, this now provides us an objective measurement of the problem that we’re looking for with glaucoma.
So, one of the questions that that I get from my patients is well if this is objective–meaning it’s a measurement and it’s pretty reliable in terms of measurement to measurement the variation is very small so it’s about four micrometers are four millionths of a meter so pretty-pretty impressive there. Now, why can’t we just use OCT? Why do we have to do a visual field test?
It’s a good question and the way it I generally explain it is it’s a lot like taking your car to a mechanic. So, if you have an issue with your car and you go to the mechanic, the mechanic is going to both open the hood, take a look at the engine and the connections, as well as take it for a test drive. And both of those inspections are important. The same is pretty much true with trying to detect or monitor glaucoma. So, the OCT is the measurement of the optic nerve, is kind of like looking at the engine, seeing what the anatomy is, but you don’t know how it’s functioning, just by looking. And the visual field test is very much the test drive. So, the two also complement each other in terms of what they do best. So, for example, we talked about the fact the visual field testing can miss early glaucoma, whereas an OCT testing of the optic nerve head can detect it up to five-six seven maybe even eight years before loss of a visual field. So, OCT testing can be very sensitive for early loss—so, it’s great for early detection, but on the flip side scanning the retinal nerve fiber layer…the optic nerve head, is not very helpful in more advanced glaucoma, where it really can’t pick up changes in the nerve fiber layer very well. So, in the more moderate to severe glaucoma, visual field testing really is a better method of monitoring for glaucoma changed. So, the two dovetail and have a little bit of overlap there.
Now there are also considerations in terms of times when visual field testing may just not be reasonable. And, in those cases were pretty much forced to use these other methods of monitoring for glaucoma. So, for example, if you’ve got somebody who just can’t sit still for five minutes— somebody who has a tremor, someone who has cervical disease (making it difficult to sit upright at the visual field)—a severe dry eye, severe arthritis of the hands, dementia, or other cognitive disorders that keep someone from being able to pay attention for a period of time, vision that’s just too poor to see the stimuli (the visual field). So, all of these things can make visual field testing less reliable or just not possible to perform, which is why we really do need these other methods of testing.
So, in any case, monitoring for glaucoma requires checking a lot of things: the pressure, the visual fields…the optic nerve head itself (ideally both by OCT testing as well as a direct examination periodically by the physician). So, there is a lot to this, and you know, we’re fortunate we have these methods. And that needs to be balanced in its view to the annoyance of having to take multiple tests, coming to the office multiple times— potentially in a year. But fortunately, if these tests are used in the right way, at the right time in terms of the diagnosis and treatment of glaucoma, they can be very effective at detecting things before the individual loses a significant amount of vision. And that’s ultimately what we’re trying to achieve in the treatment of glaucoma.
So, I hope you found this helpful, and we’re almost done with my series on glaucoma visual field testing. I think I’ve got one or two more topics that I’d like to discuss, but I hope again that you’re finding this informative and I look forward to my next commute.
- Kerrigan-Baumrind LA, Quigley HA, Pease ME, et al. Number of ganglion cells in glaucoma eyes compared with threshold visual field tests in the same persons. Invest Ophthalmol Vis Sci. 2000;41(3):741–748
- Airaksinen PJ, Heijl A. Visual field and retinal nerve fibre layer in early glaucoma after optic disc haemorrhage. Acta Ophthalmol. 1983;61(2):186–194
- Bowd C, Zangwill LM, Berry CC, et al. Detecting early glaucoma by assessment of retinal nerve fiber layer thickness and visual function. Invest Ophthalmol Vis Sci. 2001;42(9):1993–2003.
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David Richardson, MD
Medical Director, San Marino Eye
David Richardson, M.D. is recognized as one of the top cataract and glaucoma surgeons in the US and is among an elite group of glaucoma surgeons in the country performing the highly specialized canaloplasty procedure. Morever, Dr. Richardson is one of only a few surgeons in the greater Los Angeles area that performs MicroPulse P3™ "Cyclophotocoagulation" (MP3) glaucoma laser surgery. Dr. Richardson graduated Magna Cum Laude from the University of Southern California and earned his Medical Degree from Harvard Medical School. He completed his ophthalmology residency at the LAC+USC Medical Center/ Doheny Eye Institute. Dr. Richardson is also an Ambassador of Glaucoma Research Foundation.