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

16 Visual Fields

1-3 INTERPRETATION OF DEFECTS IN THE FIELDS

Each of the many types of defects has a specific anatomic location. Some are peripheral, such as general contraction, quadrantanopsia, hemianopsia, altitudinal defects, and others. Those inside the peripheral limits of a normal field are central, paracentral, cecocentral, and arcuate scotomas. Some defects are best seen at the vertical meridian such as temporal lobe and pituitary lesions. Arcuate lesions may be seen along the horizontal meridian and are called nasal step defects. Not all defects of the central field cause loss of acuity. Therefore, good acuity does not rule out defects near the center that do not break out to the periphery. Hence, it is important to examine the central as well as the peripheral field.

Lesions that interrupt the visual pathways behind the chiasm produce a homonymous hemianopia; that is, they impair the function of both eyes, causing defects in either the right or the left half of both visual fields without affecting the other half-fields. The term hemianopia is not restricted to its literal sense. Rather, it implies a loss in one of the half-fields, a loss that is not necessarily complete, but that may be quadrantic, partial, or even relative. Defects of the fields that are similar in the two eyes are called congruous (Figure 1-5). Exact congruity suggests that the location of the lesion causing the defects is in the posterior portion of the optic radiation. Lesions that interrupt the anterior portion of the radiation may produce defects that are slightly incongruous (in my experience, however, they are usually congruous). Lesions that interrupt the tracts cause incongruous homonymous defects that are grossly dissimilar (Figure 1-6). The reason is that while fibers from the corresponding points on the two retinas are projected onto identical areas of the visual cortex, they do not pursue exactly the same course in arriving there. Near the cortex, corresponding fibers lie close together. Farther forward in the optic tract, they are only roughly sorted out. Consequently, a lesion situated in the tract may interrupt fibers from a segment of the retina of one eye and from a larger or smaller segment of the retina of the other eye.

Lesions situated at the chiasm, by interrupting the crossing nasal fibers, bring about a loss in the temporal portion of the field of each eye. This loss is called a bitemporal hemianopia (Figure 1-7). Such lesions are commonly tumors, which may grow toward one side more than the other and appear as incongruous defects. By interrupting an optic nerve, a central scotoma may be added to the bitemporal hemianopia, or one eye may be completely blind, leaving only a nasal half-field in the opposite eye (Figure 1-8). If the tumor grows posteriorly or there is an anteriorly placed chiasm, the tumor encroaches on the tract and causes an incongruous homonymous hemianopia (Figure 1-9).

A scotoma is a field defect surrounded by a normal field (Figure 1-10). This differentiates it from a hemianopia or a quadrantanopia, both of which break out to the periphery and have no field remaining beyond their peripheral limits.

Lesions that affect the retina or the optic nerve induce a defect in the field of the corresponding eye and spare the field of the other eye. The characteristic loss is in the form of a central scotoma, but contractions also occur and complete blindness may ensue. Certain poisons and inflammations may affect both optic nerves and

C

Figure 1-6. (Continued)

produce defects in the fields of both eyes. In these instances, the defects may be present in both the nasal and the temporal half of the fields and are not limited by the vertical midline, as are defects caused by postchiasmal lesions.

Defects vary in their densities. In some cases, large targets disappear; in others, only small targets fade out. A defect so dense that not even light is perceived within it is said to be absolute; all other defects are relative. The density of a defect indicates the degree of interruption of the nerve fibers involved.

The term sloping refers to the variation of the size of the field defect according to different sizes of test objects. The smaller the test object, the larger is the defect, so the edge of the defect looks like a children’s slide rather than a cliff. This type of defect is more commonly seen with a tumor that has a central absolute defect and surrounding tissue that is variably affected (Figure 1-11). This damaged tissue produces a defect resulting from that involvement to varying sizes of test objects or different shades of gray in the Humphrey perimeter that depicts the same variations.

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Figure 1-7. A bitemporal hemianopia is incongruous, typical of chiasmal tumors. (A) Tangent screen representation. (B) Computerized representation of left eye. (C) Computerized representation of right eye.

The term sharp margin indicates that the edge of the defect can be demonstrated by large or small test objects; the site of the defect remains the same regardless of the size of the test object (Figure 1-12).This is shown as no variation of the grayscale and numeric readout. Such a demonstration indicates a sharp margin of tissue involvement, which is usually seen with vascular infarctions. For this reason, test objects of several different sizes should be used to plot any field defect. Merely finding the defect is not sufficient. This is done in computerized perimeters by using a brackening technique.

The defects affecting the optic nerve and the retina differ from those affecting the chiasm, the tract, and the optic radiation. In addition to the central scotoma (see Figure 1-10), there are defects of the following types: paracentral (Figure 1-13), cecocentral (Figure 1-14), and arcuate (Figure 1-15). Quadrantic defects also arise in the retina as a result of vascular disease, but they differ in one important aspect: Unlike quadrantic defects behind the globe, a quadrantic defect caused by

Figure 1-8. A chiasmal tumor not only involves the crossing fibers from both eyes but also encroaches on the left optic nerve. (A) Tangent screen representation. (B) Computerized representation of left eye. (C) Computerized representation of right eye.

an infarction of the retina has the central apex of the lesion pointing toward the blind spot rather than toward the fixation point. A large test object that reveals a partial quadrantic defect may not demonstrate this, so smaller and smaller test objects should be used to enlarge the defect until it extends to the blind spot or to the fixation point. Neurologically, the field of vision divides at the fixation point (Figure 1-16A), whereas the vascular supply to the retina divides into quadrants at the optic nerve or at the blind spot (Figure 1-16B).

1-4 TECHNIQUES OF FIELD TESTING

1-4-1 Confrontation Technique. The easiest and most rudimentary method of field testing is the confrontation technique.18 The procedure may involve the examiner holding up fingers, which are the test object, and asking the patient whether they

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Figure 1-9. A tangent screen examination demonstrating an incongruous defect. Different isopters would still show the incongruous defect to a greater or lesser degree. (A) Tangent screen representation. (B) Computerized representation of left eye. (C) Computerized representation of right eye.

are moving. Because the patient has only two choices; however, this is not the best way to perform the test.

Usually, the examiner stands several feet in front of the patient.18,19 The patient’s right eye is covered and the examiner’s left eye is closed, enabling the examiner to use the right eye as a check against the patient’s left eye for field size. The patient is to fixate on the examiner’s nose. The examiner shows fingers in all four quadrants of the patient’s left eye out to the limits the examiner can see them with the right eye. If the patient counts them correctly, there is presumably no gross defect. Most people can distinguish one, two, or five fingers accurately, but many cannot accurately tell three and then four fingers. If three and four fingers are used consecutively, incorrect answers should not be counted. How the examiner holds his or her hand is important. What appears to the examiner as two fingers

Figure 1-10. This defect is found only in the left eye and is presented to several sizes of test objects. The defect does not respect the vertical midline and has a normal field peripheral to it in all quadrants. (A) Tangent screen representation. (B) Computerized representation of right eye.

(Figure 1-17A) may be perceived as only one finger if one finger is behind the other in the patient’s field of vision (Figure 1-17B).

For the patient who has trouble fixating, finger counting offers an advantage over other methods of field testing. In this technique, the number of fingers can easily be shown only briefly before the patient is forced to change fixation. If the finger-movement technique is used, however, the fingers will theoretically be moving almost constantly as the examiner briefly projects a few fingers into one of the patient’s fields.

The second step in the confrontation technique is to project a series of fingers into the nasal and temporal fields of one eye simultaneously. The combinations are one and one, two and two, one and two, and one and five fingers. Other combinations, such as one and three or three and four fingers, should not be used, because even a patient with normal fields will frequently miss an accurate numeric identification. The patient who fails this test of simultaneous stimulation may have a more subtle field defect than could be demonstrated by testing each quadrant separately. Failure of the test could also represent the extinction phenomenon (discussed in Chapter 10) as the radiation passes through the parietal lobe, or it could reflect the patient’s inability to add one finger and two fingers (a condition termed dyscalculia).

The last step of the confrontation technique is to compare the temporal and nasal fields for subtle differences. This requires judgment on the part of the patient;

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Figure 1-11. As smaller or subtler test objects are used, the defect becomes larger and is described as having a sloping margin with a relative defect. (A) Tangent screen representation. (B) Computerized representation of the right eye, which demonstrates a variable defect, as does the left eye. The left eye has a congruous defect to the right eye.

therefore, the interpretation is not as clear-cut as are the results of the first two steps. The examiner shows one hand to each half-field of the patient’s left eye as the patient continues to fixate on the examiner’s nose. The patient then compares the clarity of the two hands. If the patient says the hand in the temporal field is blurred compared with the hand in the nasal field, a field defect may exist. When the right eye is similarly examined, the patient may give a similar response for the temporal field, suggesting chiasmal involvement. Blurring of the nasal field would suggest a homonymous defect, indicating a location in the optic radiation. If the patient gives no such response for the right eye, the possibility exists that the defect is not a true defect in the left eye—that it is instead in the nasal retinal portion of the left eye or in the nasal portion of the left nerve—or that there is a defect in the right eye that is much more subtle and was not picked up. The simultaneous comparison of nasal and temporal fields can further be refined by using colored test objects, such as the tops of mydriatic bottles. If you use two bottle tops at a time as comparison, it is important to repeat the test and switch bottle tops, because the difference noted by the patient may be a difference in manufacture.

1-4-2 Central Field Technique. The area encompassed by the central field is the central 30°, and in the central field technique this is examined at a distance of 1 m with a

Figure 1-12. The defect as shown is the same for the three test objects. If the defect is found with all test objects, it is called absolute. (A) Tangent screen representation. (B) Computerized representation of the right eye. The left eye, which is not shown, is a mirror image of the right eye, as demonstrated (A).

1-mm white test object. Chamlin4 found that 30° was the average limit for seeing a 1-mm white test object.

There are many sizes, colors, and shapes of test objects for performing central field examinations on the tangent screen. The advantage of a spherical test object is that it projects an image of the same size on the retina regardless of how it is held or in what part of the field it is held. The disadvantage is that the sphere cannot be turned off or turned over so that the patient cannot see it, because it is all of one color.

The ability to hide a test object or to turn it on and off during the field test is important in testing the validity of the patient’s response. The flat test objects of the Bausch & Lomb type have different colors on each side. These test objects can be modified by blackening one side and turning them over so that the color is hidden and the dark side now blends into the black tangent screen. Unless the test object is held just right, however, it is seen at different angles and, therefore, presents a different size at different times. Even if the test object is held flush with the tangent screen, it projects a slightly different size on the retina as it is moved farther from fixation. This small problem, however, is overshadowed by the ability of the examiner to turn the test object on and off to test the validity of the patient’s response. It is important that the examiner learns to test the validity of