WHY IS A 30-2 VISUAL FIELD CALLED A 30-2 ?

I came across this interesting article by Doug Rhett, OD, FAAO, where he discusses why a 30-2 visual field is called so. My perception of this was very basic, that 2 indicates the 2 hemispheres checked in a -2 field, similarly a -4 field compared 4 quadrants. However, it seems like things are more complicated than that. Take a look at Doug's article:

https://www.healio.com/optometry/technology/news/blogs/%7B5f21a6f6-b43c-4f92-993c-eac61463cc20%7D/doug-rett-od-faao/blog-why-is-it-called-30-2 

"Specifically, what’s the “2” signify? When I was asked this question, I felt like I had learned this answer at one point in my career, but since forgot it. Since visual fields are a test most of us order daily, it’s probably worth reviewing some details about which tests are best to order and how they differ from each other.

As most of us know, the “30” part of the name refers to the fact that this threshold exam tests the visual field extending out to 30 degrees from fixation. Importantly, it’s 30 degrees in all four directions, thus it tests the central 60 degrees of the patient’s visual field.

This contrasts with the 24-2 and the 10-2, which obviously test the central 48 degrees and 20 degrees, respectively. Recall that the optic nerve is approximately 15 degrees nasal to the fovea, which is why the physiological blind spot does not appear on the 10-2, but does appear on the 24-2 and 30-2.

The second number of the test, the “‘2”, came about because it was the second pattern for the threshold test that was developed. The first pattern is known as 30-1 (and 24-1, etc.) and differs primarily by where the stimuli are presented. The 30-1 test pattern presents stimuli directly on the x- and y-axes and then every 6 degrees extending from there (see the 30-1 figure). 

The 30-2 test pattern presents stimuli 3 degrees away from the x- and y-axes (see the 30-2 figure), and also every 6 degrees extending from there – essentially leap-frogging the 30-1 stimuli so that if you were to combine the tests, there would be stimuli every 3 degrees.

In fact, on Humphrey’s old HFA-1 visual field analyzer, you could combine the two tests. The clinician would perform a 30-1 and a 30-2, then combine the data to yield a test with much better effect resolution. You can also merge a 30-2 with a 30/60-2 (which tests points only between 30 degrees and 60 degrees from fixation) to get a very wide threshold field. Or merge a 30-1, 30-2, 30/60-1 and 30/60-2 to get the mother of all visual fields (which would probably take an hour and at least two office visits!). 

Alas, modern Humphrey machines don’t ship with the “1” fields as options. I spoke with a representative from Zeiss (which owns the Humphrey brand), who told me “These test patterns were only available on the HFA-1 instrument, which was replaced a long time ago. These patterns were replaced by similar patterns, 30-2 and 24-2 in HFA II-i and now by the HFA3. The difference is that the 30-1 and 24-1 had test points on the vertical and horizontal midline. The 30-2 and 24-2 moved these test points just off the midlines so you can tell if the patient is responding (or not responding) to one hemisphere or the other. The midline points did not show which side was affected.”

Thus, 30-2 (or 24-2, depending on your preference for detail versus reliability) became the standard over its “1” counterpart for neurological reasons. If a patient has a defect going right up to the axis (for example, a quadrantopsia or an altitudinal defect) and the visual field is tested with a 30-1 test, the stimulus presented on the axis of a scotoma might be detected by the “seeing” quadrant. This would effectively display a wider visual field than the patient truly has and also would not look as clean as a defect extending right up to the axis.

So, which field should you use: 24-2 or 30-2? I would answer that both have value. The 30-2 obviously tests more points, but if your patient has a problem with giving reliable fields, then you may be trading an extra 6 degrees of edge information for data reliability. I tend to order 24-2 fields because my patient population is older and has trouble with field testing. But many doctors order 30-2 fields with reliable results. Humphrey knows this and does not recommend one versus the other, but does add two additional points to the nasal field of the 24-2 (which is why it isn’t a perfect circle on the print out), extending the nasal field to 30 degrees from fixation to help catch any nasal steps.

The normal human visual field is widest horizontally, extending over 90 degrees temporally, 60 degrees nasally and superiorly, and 70 degrees inferiorly. In my state of Massachusetts (and in many states), the Division of Motor Vehicles requires 120 degrees of horizontal field in order to drive. And if the total normal horizontal field is just over 150 degrees, that means if you lose only 20% of your horizontal field, you could be in danger of failing the test. Relatedly, when we want to test a patient’s full field, we can order a binocular Esterman test, which tests the central 150 degrees in a horizontal pattern – perfect for a DMV screening.

But other times we want to know the patient’s full visual field, not just the horizontal aspect of it. Here we typically choose a screening (as opposed to a threshold) field, like the Full Field 81 or the Full Field 120. Both of these fields test 55 degrees from fixation, meaning the central 110 degrees. The only difference between the two are the amount of points in each tests. The Full Field 81 point will test 81 points and will be quicker but not as detailed as the longer Full Field 120. Again, the doctor should choose based on the patient. Keep in mind that 55 degrees from fixation is almost the entire visual field in every direction but the temporal field.

One last thing about visual fields: remember to keep the lights off while it boots up! No matter if you’re using an old machine or the newest model, they all work the same way. During boot-up it measures the ambient light and then adjusts the background illumination as a factor relative to the ambient light. And the stimulus intensity is a factor relative to the background illumination. So, to achieve accurate, repeatable results, it’s important that the room light during boot-up be the same as the room light during testing. Humphrey recommends testing “in a dimly lit room” and recommends to have the lights “dimmed or off” during boot-up. Incidentally, the reason Humphrey (and the older Goldmann perimeter) uses background illumination during the field test is so a patient who is coming from a lighted room will need less time to adapt to the testing illumination before perimetry can begin.

So, there you have it. All the questions you were afraid to ask about perimetry, plus a couple more. Hopefully this will serve as a nicer refresher to a complex test that we (myself in particular) have a tendency to take for granted.