Visual Field Findings
Chat Highlights
October 7, 2009

On Wednesday, October 7, 2009, Dr. Michael Pro, a glaucoma specialist at Wills, and the glaucoma chat group discussed "Visual Field Findings".

Moderator: Hello Dr. Pro. Our topic this evening is Visual Field Findings. As patients, we often request copies of our visual fields. What is the one aspect we can look at to assess the condition and progress of our eyes?

Dr. Pro: Well, let me back track for a moment, and excuse me if my typing is slow as I cut my finger this morning. Visual fields are one of the tools we use to either diagnose glaucoma or determine if it is getting worse. The idea is testing peripheral vision, which is preferentially lost in glaucoma. It is one of the most important exams we do and it gives very useful data, but it has limitations, such as patient performance on tests and normal performance fluctuation from one test to another. In fact, some glaucoma specialists feel that to really accurately determine definitive progression, you need to do seven tests!

For the patient, the test can be frustrating, exhausting, and also confusing as there is more than one brand or test type. What is important is for you as a patient is to ask your doctor to explain the visual field findings with you and explain whether the test looks stable or worsening.
So to answer the question, there is no one test that will tell you that the field is worse or not. I suggest that interested patients keep copies of fields especially if they are transferring their care to another doctor or getting a second opinion.

Moderator: To follow up on one of your comments--how many tests to do *you* need to determine progression, doctor?

Dr. Pro: Well, I mentioned the theoretical need for seven tests, but that is not practical. Most of us consider at least three tests to be sufficient, but each test must be on the same type of machine, with the same testing protocol (and there are several different ones), and the tests need to be fairly reliable.

P: What is pattern deviation and what does it tell a physician? What is total deviation?

Dr. Pro: Pattern deviation is a calculation of how tested points are different from what is expected from age matched controls (people without glaucoma). This value is determined from the total deviation, which plots out how each point is different from normal controls, but the total deviation can be influenced by the presence of a cataract. The pattern deviation "cleans up" the noise from a cataract and highlights the visual field lost due to glaucoma.

P: Can you explain the false positives?

Dr. Pro: A false positive means that you clicked the button (meaning you saw a light) when there wasn't a light displayed by the machine, so you are "trigger happy," in this case the field may look a bit better than it truly is and is less reliable to interpret.

P: If the patient is over 85 years old and his coordination is not good, would this give a false reading in the visual field test?

Dr. Pro: Absolutely, and I see this every day. Some patients also have a hard time positioning in the machine, or they get sleepy, all this makes the visual field test less accurate.

P: What is the GHT, and its relevance in normal tension glaucoma?

Dr. Pro: Glaucoma Hemifield Test compares the response of nerve bundles above and below the horizontal midline. This is based on the fact that the nerve fibers emerge from the nerve and cover the entire retina, the fibers are strictly separated by development in the horizontal midline of the retina, and a difference from test response from one part of the eye to another is a sensitive test to diagnose glaucoma. Normal tension glaucoma can be tough to detect early on and an abnormal GHT can help to diagnose it.

P: Do the numbers listed on the visual field test printout reflect percentages of loss?

Dr. Pro: There are lots of normals. The top plot is a raw value and the other plots diagram decibel loss (a unit of light) from what would be expected in age matched controls.

P: Why do some patients experience loss in the center rather than the peripheral visual field?

Dr. Pro: That is not understood. Central visual field loss can indicate disease other than glaucoma, but is also seen in glaucoma patients. For whatever reason, the optic nerve at a particular location (and the axons present there) are damaged and the vision is impaired in the region of the retina where those axons serve.

P: Is the definition of peripheral sometimes misunderstood? How is peripheral the visual filed tested if the angle of the visual field test is normally limited by the machine to 24-30 degrees?

Dr. Pro: Great question! Firstly, let's understand that the majority of axons from the optic nerve serve your central vision. Typical glaucoma patterns of visual loss are seen in that central 24 degrees. Yes your vision extends out to 120 degrees and glaucoma damage can extend out there, but it is not practical or even useful to test out there.

P: So if the VF test shows only 10 percent loss and the patient can still see perfectly at 180 degrees, does this mean he has glaucoma? Should he or she be treated with eye drops or until some peripheral loss is visible?

Dr. Pro: Well, there are several issues that you raise. First, there is the issue of velocity of visual field loss. Have you had a very slow progression? What is your life expectancy? If your have 10 percent loss in a corner of your vision and you have other illnesses that may shorten your life, then maybe that 10 percent is not so important. But understand that to have a demonstrated visual field defect some studies have found that you have to have lost 50 percent of your axons which together make up the optic nerve. So in a person with a long life expectancy, that 10 percent is important as it portends ongoing lost unless something is done to stabilize the glaucoma.

P: When would you request a visual field test be performed in a glaucoma suspect; especially a child? Of the different types of tests and machines, which do you think is easier for a young patient to perform?

Dr. Pro: Every child is different. I have had some seven year old children perform admirably, while others can't sit through the exam. Today’s kids are often good because of their skill at video games! That being said, the best test is probably the shortest, and there are newer testing strategies and machines (such as the new Octopus) which may be better for kids.

P: Can you tell from a visual field test what percentage of your central or peripheral vision is lost?

Dr. Pro: Grossly, yes. You can look at the printout and make a rough estimate, but it doesn't always tell how that field loss affects you. I have some patients who have a central white spot in a black field who barely notice a problem, while others have a peripheral black spot and they complain about it.

P: If a certain percent of your visual field has been lost in say a three year period, and progression is continuing, can you project the loss in the next three years?

Dr. Pro: Yes, but not with total accuracy. Programmers are working on software that may also be able to do that.

P: Does visual field damage point to specific types of glaucoma?

Dr. Pro: Well, I have noticed that normal tension patients tend to have central loss more frequently.

P: Based on a visual field test, does a physician know what a patient can and cannot see?

Dr. Pro: We know what points of light you can or can't see in a dark room with your head in a visual field machine, but we have no idea what you can or can't "see" in the real world.

P: If md and psd refer to the pattern deviation and total deviation you talked about earlier, should those numbers be increasing or decreasing over time, and how quickly?

Dr. Pro: They should be holding steady, if the MD is decreasing, or the PSD is increasing then the glaucoma may be getting worse.

Moderator: Thank you Dr. Pro; great answers on a topic that is always of interest.

Dr. Pro: You are welcome!

On November 9, Dr. Pro discussed "Refractive Concerns" in the Chat room. Click here for highlights of that meeting.

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