Published in Glaucoma

Back to the Basics: Accurately Interpreting OCT and Visual Fields

This is editorially independent content
8 min read
In this session from Eyes on Glaucoma 2022, Mark T. Dunbar, OD, FAAO, and Danica J. Marrelli, OD, FAAO, cover the basics for the early detection and diagnosis of glaucoma.
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On June 10-11, eyecare practitioners from all over the world gathered online for Eyes On Glaucoma 2022, a two-day educational event all about glaucoma disease diagnosis, treatment, and management.

Enjoy this presentation from Mark T. Dunbar, OD, FAAO, and Danica J. Marrelli, OD, FAAO, and don't forget to check out our list of future events!

Please note these videos are provided for review only.

When it comes to glaucoma, we all know the standard routine: diagnosis, consultation, treatment, surgery, and so on. The worst of these glaucoma cases lead to blindness, with around 40% of all glaucoma patients reporting moderate-to-severe sight loss, according to a study reported in Glaucoma Stats and Facts™.
Maybe it’s time we take a step back to the glaucoma basics. Before we even get to treatment, there is of course, a diagnosis. Let’s take some time to explore these diagnoses, and take another look at how to accurately identify what is really going on with a patient. We have already seen why early detection is so important; it’s time we come up with better solutions, and provide overall better education on the subject.

What’s the issue?

Underdiagnoses, or misdiagnoses, is the root cause of the rapid growth of many glaucoma cases. If we aren’t looking for the right indicators due to lack of knowledge, how will we ever improve? In two different studies, it was suggested that over 50% of glaucoma patients have not been diagnosed, though having the disease.
Now, this of course includes the cases that simply don’t come in to get their eyes checked. However, we can be more effective to the ones that do annual eye checks.

How to properly diagnose glaucoma

Your first step is typically measuring the IOP to make sure it’s within a normal range, which for a traditional patient would be between 10 – 21 mmHg. However, we can’t always go by this reading, as we’ve found that between 25-35% of all patients with confirmed glaucoma have a normal IOP measurement.
We then look at the optic nerve to see if it appears abnormal, if there is evidence of “cupping”, or asymmetry between the two eyes? Well, when suspicions of glaucoma arise, we take it a step further. We capture a fundus photograph, and measure the visual field, which of course can be difficult to interpret alone, but combined may give a more definitive answer.

Testing the visual field

Most physicians aren’t excited to examine and test the visual field. Unfortunately, it doesn’t seem like it is going anywhere, so instead, let’s try and make it more manageable.

There are some very basic ways to interpret the results, by reviewing these five things:

  1. Reliability
  2. Grayscale
  3. Deviation plots
  4. Global indices
  5. Glaucoma Hemifield Test (GHT)

Reliability

An unreliable result is as good as no result at all. So how can we check for inaccuracies? The first would be any report showing 15 or more false positives within the metrics. As time goes on and technology advances, the process gets faster, and metrics unfortunately provide more false negatives than we used to see.
The next way to make sure the report is reliable is by looking at the grayscale, looking out for white scotomas. This is an indicator that those values are higher than they should be. Lastly, it comes down to the administrator to ensure the patient is staying still, relaxed, and is staying focused on the fixation spot. The fixation losses must be read and monitored to confirm the report is going to come out clear, accurate, and reliable.

Grayscale

With this method, we learn where the problem is. As extremely important as that is, know that this grayscale is just a factor, and we don’t ever want to rely on solely these results to come to any official result.

Deviation plots

This is one of the most important. It shows the response and compares it to their norm for their age range. It measures the sensitivity, as well as projects a probability plot after any adjustments had been made. We should be looking for asymmetry; not only between the two eyes, but to each eye itself.
Be sure the midline points back at the nerve, noting any common abnormalities such as nasal or arcuate bundles.

Global indices

There are three types of visual field indices: Mean Deviation (MD), Pattern Standard Deviation (PSD), and Visual Field Index (VFI).
  • The MD shows any asymmetries, showing which the stronger or weaker side is.
  • The PSD reflects any irregularities that are caused by localized defects, making sure it peaks at the sign of loss.
  • The VFI is the newest metric, which is an overall percent of total vision. This is overall an improved version of the MD, but from the pattern deviation plot.

Glaucoma Hemifield Test (GHT)

This test compares pattern deviation, scoring the eye in five different zones. Within these zones can be an array of messages that come up within the metrics, the most important being any readings outside of normal limits, such as depressions or high sensitivity

A visual field defect has been found–now what?

After the patient has been diagnosed, we need to stage the disease. A couple of years ago, Simpler Staging was made known, based solely on Mean Deviation numbers.

Staging: Modified Hadapp-Anderson-Parish Criteria

Mild

  • MD no worse than-5dB.
  • Less than 25% of points depressed below 5% level and 15% of points depressed below the 1% level.
  • No point within 5° of fixation with sensitivity <15dB.

Moderate

  • MD between -6 and -12dB.
  • Fewer than 50% of points depressed below 5% level and 25% of points depressed below the 1% level.
  • No point in central 5° of fixation with sensitivity 0dB.
  • Only 1 hemifield containing a point with sensitivity 15dB wit
When your 24-2 field test is showing mainly black on the greyscale, the next field test would be the 10-2 VF. As is the same case here, if we just look at things a little differently, specifically focusing on the ganglion cells.

What about the 10-2 VF?

  1. Central 8° from the center of the foveal contains more than 30% of retinal ganglion cells.
  2. 24-2 and 30-2 test strategies use a 6° test grid pattern: these points fall outside of the densest region of ganglion cells.
  3. 10-2 test strategy uses a 2° test grid.
  4. Recent research has shown that in some patients with small regions of macular ganglion cell loss, 10-2 testing may be better able to detect VF loss.

The missing piece for glaucoma diagnosis

When OCT came along, it seemed to fit as a piece of the puzzle. While it in no way undermined the other elements, it certainly explained a little more on how the congruity was functioning; how it all worked together.
When we see something like a suspicious nerve or a borderline visual field, the OCT can certainly provide a little more information to understand the symptoms and red flags as a whole, with an overall better ability to come to a proper diagnosis. Having the most success in detecting early stages of glaucoma, we need to know what to look for.
One thing we can focus on is OCT monitoring. Watching the progression, observing any changes, and looking for increased sensitivity loss. How much is too much? Anything more than 10 microns of a change should be a red flag that the glaucoma, or possibility of, has progressed.
It is highly recommended to take 2 to 3 RNFL scans at a time-it’s quick and simple, and is stronger data to use as a baseline for any future progression analysis that may be needed down the road.

Questions to ask yourself

When you’re reviewing any results, you always have to keep an open mind, constantly asking internal questions. One question you should always ask when looking at results is, can the optic nerve be applied to a normative database? Sometimes the answer can be yes, if you’re seeing enough red flags elsewhere. Well, how do you know when is the time? Let’s take a test case:
A patient has had glaucoma for the past 10 years, coming in as a new patient for a second opinion. She’s been on medications, has normal pressure readings, and a slightly abnormal visual field in the left eye, as well as a small depression. In this case, we cannot apply this to the normative data base, as these issues do not correlate directly with the optic nerve being abnormal. Thusly, the answer may be different if the red flags were raised elsewhere.

What we’ve learned about glaucoma diagnosis

We know that a combination of the visual field tests as well as the OCT are absolutely critical in a diagnosis. It’s always important to know your baseline, watching for changes more than 10 microns, so your information is the most reliable, along with an accurate staging level. Now, next time you have a potential glaucoma patient, you are equipped to produce a highly-accurate diagnosis.
Mark T. Dunbar, OD, FAAO
About Mark T. Dunbar, OD, FAAO

Dr. Dunbar is currently a Director of Optometry at the University of Miami. He has served as adjunct faculty at Nova Southeastern University, College of Optometry (Clinical Assistant Professor) and New England College of Optometry. He received his Doctor of Optometry (OD) from Ferris State University and went on to complete a residency with Omni Eye Services. His interests are comprehensive primary optometry, ocular diseases, and retinal disorders.

Mark T. Dunbar, OD, FAAO
Danica J. Marrelli, OD, FAAO, AAO Diplomate
About Danica J. Marrelli, OD, FAAO, AAO Diplomate

Danica Marrelli received her doctor of optometry degree from the University of Houston College of Optometry (UHCO), and completed a residency in hospital-based optometry at the Baltimore VA Medical Center in Baltimore, MD. She is a clinical professor at UHCO, where she serves as Assistant Dean of Clinical Education and Service Director of the Medical Eye Service. In the classroom, Dr. Marrelli teaches in the ocular pharmacology, glaucoma and case-based learning courses. Dr. Marrelli became a diplomate in the ocular disease (glaucoma) section of the American Academy of Optometry in November 2009 and is the vice president of the Optometric Glaucoma Society.

Danica J. Marrelli, OD, FAAO, AAO Diplomate
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