By Jeffrey Henderer, MD

The retina, the light-sensitive tissue at the back of the eye, is composed of receptors, "photoreceptors," that change light energy into electrical energy.

The central area of the retina is the most sensitive to light, and we see best what is directly in front of us. The more peripheral areas of the retina are less sensitive to light, but they allow us to see, though less clearly, objects off to the side or above or below straight-ahead vision. Just how much we can see of the world around us is known as our visual field. Each part of the retina "sees" a particular part of the visual field. Each photoreceptor’s signal is then picked up by a special nerve cell called a ganglion cell. The ganglion cells then transmit the signal via the optic nerve to the brain, allowing us to see. Thus, each ganglion cells is responsible for "connecting" a portion of the retina to the brain. Any disruption in the function of these cells will block the signal’s transmission and render that portion of the retina and the accompanying visual field less sensitive to light.

The ganglion cells that make up the optic nerve are living entities that require energy and nourishment. A glaucoma is a process by which the optic nerve cells can become damaged and die, at least partially due to the pressure within the eye. There are millions of these cells, and one can lose perhaps as many as 40% of them before being aware of any visual loss. In fact a small number of cells die every year "naturally."

If doctors relied only on reduced visual field to detect glaucoma, they would miss most people with early glaucoma. That is why the field is only one part of a glaucoma evaluation. But the visual field is important: to diagnose and categorize glaucoma, to help create treatment plans, and to establish a baseline for future comparison. If doctors suspect a person may have glaucoma (if, for example, intraocular pressure is not within the normal range and/or the optic nerve looks unusual), a visual field test may help confirm or rule out glaucoma as the cause. Especially in the later stages of glaucoma the visual field provides essential information about whether the glaucoma is stable or is getting worse. In these later stages changes in the optic nerve become hard to detect. Thus, the history and the visual field provide the essential information needed to understand what is happening.

Visual Field Testing (Perimetry)

Visual field tests are designed to map a person’s visual field, to document the level of peripheral vision. As most glaucoma patients know, the test consists basically of responding every time a flash of light is perceived, all the while looking straight ahead. Understanding the various parts of the print-out of the results, such as shown here, is one way of understanding more about visual field testing.

A. Test Type 

 

The ideal visual field test would be easy to take, easy to administer, and 100% reliable. We have no such test, but fortunately for everyone involved, recent years have seen substantial improvements in all of these areas. Especially welcome to the glaucoma patient are tests that are faster and less tedious.

 

The Glaucoma Service of Wills Eye Hospital now relies almost completely on Humphrey automated perimetry, often using a new testing program known as SITA. This test can be completed anywhere from about 3 to 8 minutes depending on whether the SITA Fast or SITA Standard program is used. This new program is not useful for all patients and some are still tested using older, but still excellent, strategies.

 

B. Patient information 

 

The patient’s visual acuity and age are important factors in obtaining reliable results. The visual field test must be taken with the appropriate correction needed for close vision. Also, since the retina of the normal eye becomes less sensitive with age, it is important that the age of the person being tested be taken into account. When reviewing a visual field test print-out it’s worthwhile for the patient to check to see if these figures are correct.

 

C. Reliability Parameters 

The print-out provides three kinds of information to help the doctor assess just how reliably a visual field test reflects the patient’s actual visual field:

 

(1) Fixation losses. It is very important that the patient keep the eye being tested focused straight ahead while taking a visual field test. The doctor wants to know what the peripheral vision is like, that is, vision off to the sides -- up and down, right and left. In practice it is difficult to maintain this eye position for very long, since the natural tendency is to look to the side, towards the flashing light. But because many such movements may make the test unreliable, the machine records how many times the patient moves his eye off center.

 

(2) False Positives Errors. Sometimes the patient will push the button indicating he has seen a flash when in fact no flash has been shown. This misinformation obviously seriously detracts from the test’s ability to determine what the patient is actually seeing. One reason why the patient may indicate he has seen something even though nothing has been shown is that, like all of us, he wishes to do well on tests. The machine is designed to test for this tendency by making the normal beep or whirr but presenting no light, tempting the patient to click the button inappropriately. Even two false positives may make a test unreliable.

 

(3) False Negative Errors. To further gauge reliability, the test repeats flashes at the same spot at the same and at different levels of intensity. If one time the patient reports seeing a flash at a certain spot, but does not report seeing the same intensity flash at the same spot the second time it is shown, the reliability of the test is reduced. People who have glaucoma may have normal fluctuations at the edge of their visual field loss, so not all of these kinds of errors are truly a problem.

 

D. Retinal sensitivity is not an all-or-nothing affair. Sometimes a relatively weak flash at a particular spot that could not be seen becomes visible if the intensity of the light is increased. By flashing lights of varying intensity, the machine can ascertain the level of retinal sensitivity at each representative point in the visual field. The numbers on the print-out diagram indicate the level of intensity required to enable the patient to see the flash. The higher the number, the dimmer the light that could be seen.

 

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E. A nice picture of a patient’s visual field is obtained by assigning a lighter shade of gray to spots on the visual field in which a patient could see relatively weak flashes (the higher numbers in "D"), and a darker shade of gray to spots in which a patient could see only relatively strong flashes. Here it is appropriate to point out that all eyes have a blind spot (scotoma) where the optic nerve connects with the retina. It is "blind" because there are no light receptors at this point. The blind spot in the eye shown is indicated by the dark area in the lower left half in this print-out.

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F. As pointed out in B., retinal sensitivity diminishes with age. The dark boxes in this diagram indicate areas in which the person saw less well than most people his age.

G. Many conditions other than glaucoma can cause poor vision, for example, cataract or corneal edema. So, if the doctor wants to know how much of a patient’s relative insensitivity to light is due to glaucoma rather than to something else, it is important to "subtract out" these other factors. This can be done because these others conditions tend to produce a similar pattern of diffuse visual field loss, while glaucoma tends to produce localized areas of visual field loss.

 

H. These numbers indicate the extent to which the visual field is outside normal limits. They can be followed over time to see the extent to which it is worsening. 

There are many reasons other than glaucoma for an abnormal visual field result: the test was poorly given, the instrument was defective, the patient did not understand how to take the test, the patient was tired, the defect was real but does not indicate pathology, the defect is accounted for by some pathology other than glaucoma, eg, brain tumor, multiple sclerosis, a vascular problem, a congenital defect, an infection, or retinal disease such as macular degeneration, retinal detachment, or inflammation. Or the defect could be a false defect, that is, really not present at all!

Despite all of the shortcomings of visual field testing it is the only way to document actual visual loss and whether such loss is progressing or remaining stable. As such it plays an indispensable role in helping glaucoma patients retain their sight.