Ординатура / Офтальмология / Английские материалы / Visual Fields Examination and Interpretation_Walsh_2011

Figure 1-15. An arcuate defect follows the direction of arcuate fibers coming out of the blind spot and stopping at the horizontal raphe, as shown in the left eye. The defect is not always continuous but may be segmental along the same meridian, as shown in the field of the right eye. (A) Tangent screen representation. (B) Computerized representation.

the patient’s response. It is also important that the examiner learns to twist the handle of the test object between the fingers so the examiner knows without looking when the test object is white or black. Turning one’s hand or head may give an unwanted clue to the patient.

The projection perimeter machines, such as the Goldmann or a computerized perimeter (Figure 1-18), can switch the test object on and off and can project the same-size spot of light in all quadrants. An additional advantage of projection perimeters is that the examiner can move the test object randomly from one part of the field to another without the examiner moving or the patient being aware of the change. This feature eliminates the otherwise obvious clue to the patient that the test object will always appear from the side on which the examiner is standing.

There is no absolute, standard way to conduct a central field examination, but there are general guidelines that should always be observed:

1.Before the examination begins, the patient is told what a field examination is, what is going to be done with the test object, and what is expected of him or her. This is especially important for the patient who is having a field examination for the first time: The patient will perform more accurately if he knows what is being asked of him. There is no question that the more often a patient performs a field, the more precise he will become. However, an initial examination can be improved immeasurably if the patient thoroughly understands the process.

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Figure 1-16. (A) The apex of this defect goes to the fixation point and represents a lesion behind the lamina cribrosa in the nerve. (B) The apex of this defect goes to the blind spot and is due to a vascular lesion in the retina that divides the field at the blind spot, not the fixation point.

2.On a tangent screen examination, the patient is shown the test object and is told to give a verbal response when the white side is exposed and to give no response for the dark side. The patient is then asked to fixate on a target on the screen. At this point, before mapping the patient’s blind spot, the examiner explains that there are certain areas within which the patient will not be able to see the test object and that this is normal. The examiner then proceeds to map the blind spot as an example. The advance warning relieves the patient’s anxiety or curiosity about not seeing something essentially in front of him and allows him to fixate better. So when another scotoma is found, the patient should be able to fixate well during exploration of its limits.

3.After the blind spot is mapped, the peripheral limits of the field are examined. A 1-mm white test object is usually used, but a 2-mm white test object will be required if there is too much peripheral contraction. Recordings should be made about every 15°. In the usual testing format, the test object is brought from beyond the visible limits of the field to a point at which the patient just sees it. At this point, a recording mark that will later be transferred to a permanent record is made on the screen; it is here where a cardinal mistake is often made. The perimetrist must watch the patient all the time to make sure small lapses in fixation do not mislead him. Special care should be taken when using the Goldmann perimeter because the patient’s fixation must be watched through a small tube. With any perimeter, a lapse in fixation of which the patient is unaware may cause a more significant error than missing putting a mark on the tangent screen by 1°. Computerized perimeters have built-in fixation sensors.

4.After the outer limits of the central field are examined, the inner aspects are explored. Once a defect is found, the examiner tests for the defect in all directions to map its limits. This procedure again uses the principle of going from a nonseeing, or scotomatous, area to a seeing area. The density of the scotoma is also examined by using different sizes of test objects until the defect can no longer be found. During the testing of any area, the dark side is shown periodically so that the validity of the patient’s response can be evaluated.

The usual tangent screen has subtle lines sewn in, marking out the degrees from fixation and the blind spot. The use of pins or chalk marks on the screen is clumsy as well as difficult. In addition, if the marks are too obvious, they act as a clue for the patient as to the limits of the field. A gray, No. 966 Eagle Prismacolor pencil is

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Figure 1-17. (A) The correct way is to present two fingers to a patient so they are side by side in a frontal plane. (B) The incorrect way is to show two fingers, with one behind the other so they appear as one finger to the patient. (Source: Photographs courtesy of Pamela Ossono.)

an excellent marker. If the pencil is used lightly, the patient cannot see the marks. Different symbols can be used to outline the different sizes of test objects. When the examination is finished, all the room lights are turned on and the marks can be seen and transferred to a permanent record. The marks can easily be erased from the felt screen with a felt blackboard eraser, and the screen is ready for another examination.

1-4-3 Chamlin Step Technique. When the examiner finds a minimal peripheral field defect, he or she needs to determine whether it is truly an early defect or merely an artifact. Chamlin3,4 developed a theory and a method for evaluating this problem. He postulated not only that the peripheral defect affects the peripheral fibers but also that there is hemiretinal suppression. Therefore, all the examiner has to do is find a method to compare one half of the field with the other. This is done at the vertical meridian, which separates the two homonymous or bitemporal fields. In the experience of many perimetrists, a significant defect is one that differs at least 10° from one side to the other. If the difference is less, the test is not necessarily negative, but it is questionable.

If the points of reference are at 15° on either side of 90° and at 90°, the line of the field can look like that in Figure 1-19. If, however, one measures 2° on each side of the vertical meridian, the difference between the two fields is obvious (Figure 1-20). The examiner, therefore, should always measure on each side adjacent to the vertical meridian and not exactly at the vertical or at 15° or 30° nasal and temporal to the 90° meridian.

There are several ways to evaluate the problem. The examiner can simultaneously bring down on either side of the vertical meridian two small white test objects of the same size. The patient tells when and in which field he sees one test object and when he sees both. The patient’s fixation must be scrupulously watched, since recognition of the first object will encourage him to shift fixation. If the patient truly sees the two objects at the same time, there may be no difference in the two fields. The test can also be performed by comparing one side at a time using a single test object. Any difference of 10° or more is considered significant, and the defect, although subtle, is real.

If the defect is consistently present but is equivocal such as 5° or 6°, a further refinement can be added—the use of color recognition. The perimetrist brings a colored test object down from the periphery and along each side of the vertical meridian until the color of the test object is recognized. The patient’s fixation must again be constantly and scrupulously watched; when he sees the test object as colorless, he may lose fixation and invalidate the test. The examiner should make sure the patient maintains fixation until he recognizes a color. For example, the usual Bausch & Lomb colored test objects are red and blue, back to back. By repeating the test and varying the color from red to blue, the perimetrist can keep the patient from identifying red automatically regardless of whether red is present. Perhaps the previous defect to a small white test object was 7°; now when the patient is asked to recognize red, he may miss an entire quadrant (Figure 1-21). The examiner shows the patient blue as well as red but only records responses to

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Figure 1-19. The “X” marks are the points recorded and suggest only a right homonymous defect. The minimal slope leaves the perimetrist wondering if the defect is real. (A) Tangent screen representation. (B) Computerized representation of the left eye. (C) Computerized representation of the right eye.

the red test object. The field to a blue test object is greater than that to the same size of red test object.

If the patient has a congenital color defect or even a brunescent cataract that changes his color perception, the test will not be seriously affected because the entire field has the color defect. If there is a hemianopic defect, there will be an even more pronounced color defect in the hemifield.

A mistake that many perimetrists make is to select a colored test object that is too small. They assume that if the field defect is equivocal when a 1-mm white object is used, the smallest red test object available (usually 3 mm) must be used. This is fallacious and will produce disappointing results, because it is color, and not size, that is being evaluated. The perimetrist is measuring the decreased sensitivity of cells in the central field that are predominantly cones and that will show defects

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Figure 1-20. Reference points are on either side of the vertical meridian. Note that the test object in the left homonymous field is seen at 30°, while just 4° away it is seen at 20°. (A) Tangent screen representation. (B) Computerized representation of the left eye. (C) Computerized representation of the right eye.

to colored test objects before defects to white test objects. All too often, patients with 20/20 or 20/25 vision miss all the colored testing plates because of optic nerve disease. This is a frequent and well-known phenomenon. A 9- or 12-mm red test object is much more effective in achieving valid results.

Mindel et al.20 asserted that it is not the color but the intensity of the object that identifies the defect. They believed that equal intensity of red and white test objects should produce equal results. This view is supported by Safran and Glaser.21 However, it is much easier clinically to use the red test object than the white test object of equal intensity that I use the red test object in a patient with optic neuritis and 20/25 vision. The perimetrist avoids the difficulty of identifying the scotoma with a small white test object and demonstrates a central scotoma more easily with a red color test object.

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Figure 1-21. A Chamlin step to a 1-mm white test object is only 7°. A defect to a larger, red test object in color recognition, however, is 90°.

A simple method of comparing the two homonymous fields is to use the red caps from two bottles of mydriatic eye drops or red test objects cut from red paper. The examiner holds one test object in the nasal field as the patient fixates centrally on the examiner’s nose. The patient is asked to comment on whether there is a difference in the color of the two test objects. If there is no defect, the patient will describe them as the same (Figure 1-22A). If there is a defect, the patient will describe one as red and the other as light red, barely pink, or colorless (Figure 1-22B). This suggests a defect on the side described as pink or colorless. If the defect is more subtle, the patient may just comment that there is a qualitative difference between the two test objects. Although subtle, this distinction is valid if it is reproducible. If the patient is then asked to compare one test object centrally and one or two peripherally at the same time, the central one may appear faded or colorless, suggesting a central scotoma (Figure 1-22C and D).

If a patient’s test results suggest a particular homonymous defect but are mildly inconsistent, another technique is available. The premise on which the testing of the Chamlin step defect is based is the difference between one homonymous field and the other. Therefore, instead of smaller and more subtle test objects being brought down on either side of the vertical meridian, the test objects should be brought across the vertical meridian. As the test object comes from the supposedly defective side to the normal side, there will be a sudden change in color or in intensity. Repetition of this test will demonstrate that for at least part of the field there is not only a difference but also a vertical line dividing it. In exploring the characteristics of the peripheral field, Damgaard-Jensen22 found that about 50% of normal persons have a slight step at the vertical meridian but not 10°. He also found that the field was always larger on the temporal side of the vertical meridian. Therefore, any suggestion of a Chamlin step with the smaller field occurring on the temporal side should be considered pathologic.

1-4-4 Peripheral Field Technique. The examination of the peripheral field is performed in somewhat the same way as that of the central field. The peripheral limits of the field are examined every 15° and recorded. The area inside that peripheral limit is also examined for defects. The selection of a field machine usually depends on one’s training. The Goldmann perimeter is self-recording. It has the additional advantage

of being able to perform both central and peripheral field examination. The instrument has a variety of sizes, colors, and intensities of test objects available.

The Riddoch phenomenon, in which the patient perceives movement before light in the peripheral field, was originally described as a sign of a recovering field defect in the occipital lobe. This can be demonstrated when present by the projection perimeters. In one case, the patient perceives the movement of the projected test light but does not see that same object when it is not in motion.21 They regarded its use as not as anatomically specific—that it could be demonstrated in most field defects using kinetic red stimuli.

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Figure 1-22. (A) Two colored test objects presented simultaneously to the nasal and temporal fields appear equal when the fields are normal. (B) The same test with test objects of equal intensity do not appear the same to this patient. In this instance, the temporal field in the right eye is defective. The temporal field test object is colorless or less color-saturated. (C) In this instance, three identical test objects can be used to compare the nasal, temporal, and central fields simultaneously. (D) The central target in this instance is much lighter to the patient than the peripheral field, indicating a central field deficit. (Source: Photographs courtesy of Pamela Ossono.)