VISUAL FIELD EVALUATION

So your eye doctor has told you that you need a visual field evaluation. What does this mean? Here you will find answers about what this is and why it is done

Visual field is measured by a perimetry test, sometimes also called campimetry. This short test of about 10-15 minutes is similar to playing a video game.

A perimetry test begins when you first are asked to focus on an image appearing on the screen. Then, other similar images will appear around it and you record the number of extra images that you see using something like a joystick. The original image then moves to a different position on the screen and the process is repeated.


PERIMETRY

A perimeter is a dedicated machine that is used to precisely map and quantify your visual field. There are different kinds of perimeters, some more complex than others.

Kinetic perimetry involves the presentation of a moving stimulus of a specific size and intensity. The stimulus is moved from a non-seeing area toward a fixation point (line of sight) until it is seen. Kinetic perimetry particularly is useful for mapping visual field boundaries.

Threshold static perimetry is generally done using automated equipment. It is a rapid-screening technique used for follow up of diseases involving deficits such as scotomas, loss of peripheral vision and more subtle forms of vision loss. A scotoma is an area or island of reduced visual acuity surrounded by a field of normal vision.

Visual field evaluation through perimetry testing is important in the screening, diagnosing, and monitoring of assorted eye, retinal, optic nerve and brain disorders, including glaucoma.

How should I prepare for a perimeter test?

There is no special preparation for a visual field evaluation using a perimeter. If you normally wear glasses, make sure to bring them with you for the exam.

What can I expect during the test?

You will be sitting for the visual field evaluation. The perimeter has a place to rest your forehead. Once you are comfortable, you will cover one eye. You will be asked to stare a point that appears on the illuminated screen in front of you. One or more points that are the same size, but a different color, will then appear on the screen. Some might be close to the original point, while others might be far away.

Your task will be to count the number of additional points that you can see while focusing on the original point. You will record the number of additional points through a device that is something like a joystick used in video games. For example, if you see three additional points on the screen, you will squeeze the joystick three times.

Once you have finished, the screen will clear and the original dot will move to a new position. The process will be repeated several times. You then will switch to the other eye and proceed through the test again. The result is printed as a matrix that defines the scope and sensitivity of your visual field. The test lasts about 10 to 15 minutes.

Results of a visual field evaluation of a patient with glaucoma using a Goldmann Perimeter. From: Kwon YH, Fingert JH, Greenlee EC (2008). A Patient's Guide to Glaucoma. Free Educational Publications International.

Glaucoma and other eye diseases can cause abnormalities in the visual fields. Each has particular characteristics. Perimetry is a visual field evaluation tool that I use to monitor the progress of glaucoma.

I usually recommend that patients diagnosed with glaucoma have a perimetry test yearly. But some patients require more frequent visits than others.

My opthalmologist told me that a recent perimetry test revealed a slight deterioration in my visual field compared to last year's examination. I'm scared. I was diagnosed with glaucoma two years ago. Since then, I have followed my treatment regimen and schedule for checkups without fail, and my intraocular pressures have been normal over the past year. What's going on?

Periodic checkups and adherence to treatment are extremely important. It's great that you are following the recommendations of your doctor. But keep in mind that intraocular pressure is one tool among others, such as perimetry, ophthalmoscopy and OCT, for diagnosing and monitoring glaucoma.

Intraocular pressures tend to change throughout the day and night. You are probably experiencing some elevation in your intraocular pressure, but this has not been evident when you have come in for checkups. The fact that you are recently diagnosed with glaucoma (2 years), and you probably have had your intraocular pressure measured only 6 or 7 times during this period, also supports my suspicion that you probably are experiencing some increase in intraocular pressure but it has not been detected yet.

The important thing to understand is that visual field evaluation using perimetry can detect small changes in the progression of glaucoma even when other tests do not necessarily show evidence of the changes. Your doctor probably will recommend changes in your treatment to take tighter control of your intraocular pressure.

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