Let’s just cut to the chase – visual fields can be confusing. There are a million options on a hundred instruments, and a poor choice could doom you and your patient to thirty minutes in a dark room with some dots and beeps. Interpreting the darn things is equally frustrating. Which of the machines should you choose? Is it pattern deviation or mean that matters more in your patient’s case? What exactly is a GHT and what does it mean when it isn’t within normal limits? Fear not, fellow eye-lovers, over the next few weeks we’re going to try to clear the fog around this subject. Buckle up, because we’re diving in as deep as the word count will let me go! I’ll be presenting this in three segments: obtaining, reading, and interpreting. Stick around for the whole trilogy! Let’s start with the basics:
What exactly am I measuring?
Remember the hill of vision from first year? It’s the most basic visual that describes what we’re measuring:
The high points correlate to high SENSITIVITY. We measure that in a visual field by finding THRESHOLDS, or the minimum brightness at which that location in your retina recognizes light half of the time. The sensitivity also correlates with the density of receptors packed in that area of retina, so this is a handy diagram to understand. Note the punched out hole as the blind spot/optic nerve. Note also how the threshold decreases as we get farther from the fovea/point of fixation. These are all basic things you probably already know, but it helps to point them out now so interpreting later is easier to grasp.
When do I need to run visual fields?
There are some obvious answers here, as well as some not-so-obvious ones. Of course, patients with glaucoma, strokes, macular degeneration, and suspected lesion/tumor patients need visual fields. So do patients with ptosis, dermatochalasis patients who may not be safe to drive anymore, patients who take Plaquenil and other high risk medications, and suspected temporal arteritis patients. In short, always do confrontation visual fields early in the exam. If it isn’t normal or you’re not sure what a patient’s vision really looks like, it’s a safe bet to get a visual field.
What kinds of visual fields are out there?
Automated vs Manual
Historically, fields were taken by a human perimetrist sitting opposite the patient behind a bowl or graph of some sort, manually drawing in locations where the patient could/couldn’t see. Eventually, technology developed to where a computer could run a comparable test with much less outside input. By far, we use automated visual fields more than manual fields. Perimetrists are hard to come by these days, but it’s still a useful tool. Perimetrists can use the kinetic or static Goldmann visual fields to determine defects outside the usual test zone of 30 degrees. However, manual perimetries take much longer to execute by comparison and may not be prudent or possible considering your patient’s particular defect or attention span.
Static vs Kinetic
Static is the format you likely know and love. Humphrey fields are static, meaning a stationary dot is flashed at different locations across the bowl. Patients click a button when they see the stimulus while looking at a central target. Usually the dot is a Goldmann III equivalent stimulus, about a 0.5 degree size.HFA, HFA II, and HFA III are all Humphrey instruments that offer a variety of static fields.
Kinetic fields, like what is available on the Octopus, slowly slide a stimulus from the perimeter towards the center along radial axes. The patient fixates on the central target and clicks the button when they see the stimulus in their periphery. These are helpful for qualifying patient’s visual fields for driver’s licenses.
There are other options out there such as instruments that utilize frequency doubling technology, as well. This is a static field type that alternates a series of white and black stripes (sine wave gratings). It may detect some disease processes earlier because it activates magnocellular pathways, but it isn’t used as often and is more useful for screening.
Threshold vs Suprathreshold
In a clinical setting, threshold tests are preferred. Threshold tests find exactly how much light it takes to stimulate a part of the retina. This gives us more valuable data than suprathreshold tests, which essentially show what is missing, or what is left that can be seen.
I chose an instrument, but which test do I Run?
So many choices on one little screen! Realistically, though, there are only a few tests we run on a regular basis.
A 30-2 is tested at 76 locations in the central 30 degrees of fixation.
Most commonly, we use a 24-2, which tests 54 locations in the central 24 degrees of vison. We tend towards this test because it is shorter and is still very comparable to the 30-2 while also limiting testing artifacts from the trial lens size. This test is good for general field testing and neurological testing.
A 10-2 tests the central 10 degrees of vision. Test points are packed together more tightly than the previous two tests. Because of this, a 10-2 shows subtle changes in field for end stage glaucoma when a 24-2 would just be black. It’s a good idea to occasionally run it before late stage as well; it can detect early changes in the field.
Some alternative test types:
Short Wavelength Automated Perimetry (SWAP)- This interesting test uses a Goldmann size V blue stimulus on a yellow background to try to isolate ganglion cells that deal with short wavelengths. In a perfect world with perfect patients, this test would detect glaucoma at an earlier stage. However, cataracts affect this test, and many glaucoma suspects may have cataracts. It is also a longer test, making it unpopular with patients and of limited service to doctors.
Chloroquin testing: It’s suggested that patients taking Plaquenil get 10-2 fields done every few years to watch for maculopathy. It’s a longer test that uses a red stimulus, so be sure to select the right setting!
What does SITA mean, and why is it faster?
SITA stands for Swedish Interactive Thresholding Algorithm. Standard testing requires a threshold to be missed a certain number of times, but SITA factors in probability of certainty. SITA Fast is even faster to finish than the SITA at 2-5 minutes, but it’s more difficult to complete for the elderly or patients who might struggle with testing because the starting stimulus is so close to threshold.
How do I run visual fields?
Visual field instruments are kept in dim spaces, and testing is run in dim illumination. Bright light exposure for too long can throw off the calibration of the field, so avoid it if you can. Bowl-based perimetries require the patient to be patched. It might be a good idea to use a folded tissue under the patch for comfort, especially for longer tests.
A common question is whether the patient needs to be dilated. For most cases, the answer is no, though testing for cataract patients may be made easier with dilation. Regardless, if a patient’s previous tests have been dilated, it’s best to be consistent and continue with a dilated test. To correct for the dilation, remember to always add 3 diopters to the patient’s distance prescription to the trial lens holder, and be sure those trial lenses are clean!
If your machine has a liquid lens (fancy!) be sure to put the right prescription into the appropriate box on the screen. A liquid lens only has a maximum cylinder correction of -2 diopters, so bear that in mind when choosing which instrument to use. Never use alcohol to clean a liquid lens!
Get the patient as comfortable as possible before starting the test. Four minutes per eye may not sound like a long time, but to your 80-year-old patient with arthritis, it can be excruciating. Remind the patient occasionally to keep looking at the central target, and give them updates estimating how far along they are. It’s important to balance being encouraging and distracting. Last but not least, don’t forget to transmit the test to whatever forum your school/office is using!
This just isn’t working – HELP!
So you’ve done everything right, but the patient won’t hold still, looks all over the place, talks constantly, and keeps pulling off the patch. Troubleshooting and visual fields go hand in hand, and it’s important to be flexible. Take a deep breath and assess the situation. Is the patient dilated? Inexperienced with the test? Elderly? Fidgeting patients may be tired, or may not understand the test. Stop the test, explain again, and restart. Begin with the good eye, especially on first timers, so they have a good idea of how to complete the task. If all else fails, the patient may have been in the office for hours, and can’t muster enough attention to complete the test. Scheduling a better, fresher time for testing is okay. A bad visual field doesn’t tell us anything, so it’s definitely worth the effort.
And that’s all for today! Thanks to Dr. Mary Hoang for helping to inspire and inform this article.