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

6Visual Fields

evaluated, using a tangent screen or a sophisticated field-testing instrument such as a Goldmann or computerized perimeter. The central field itself must be searched for defective or depressed areas, known as scotomas.

The temporal field constricts with age after the sixth decade of life. This constriction may be due partly to age-related miosis but is more probably due to decreased oxygenation of the peripheral retina with age. Oxygen concentration has been reduced experimentally during field measurements. Peripheral loss was reported by dive-bomber pilots during World War II; they noticed peripheral loss as they went into power dives and increased the G forces, reducing the oxygenation of the head.

1-1-2 Central Fields. Different sizes and intensities in test objects are necessary to evaluate the actual density in limits of any field defect. The use of the smallest or most sensitive test objects is necessary to outline the outside limits of the defect. The largest and brightest test object outlines a field defect that is still demonstrable. If a larger or brighter test object is used, no defect is found. This later isopter is necessary because it is least sensitive and is the first to vary as a field defect improves. Thus, this isopter is useful for monitoring improvement. Vascular lesions generally have steep margins to the defect. In those cases, the outside limits of the defect are almost the same as the most and the least sensitive test objects. Tumors usually have relative defects. The central portion, which is denser at the site of the disease process, is surrounded by an edematous area that represents a variable defect in the field. These statements about vascular lesions and tumors are good general rules of thumb, but they are not absolute. Some vascular lesions can be total infarctions with absolute defects or have areas of surrounding edema and cause relative defects in the outside isopters. Some tumors can present with dense defects when the tumor encroaches on a vascular supply. It can be very difficult to outline a central scotoma when the acuity is minimally decreased. Retinal sensitivity can decrease from 20/20 to 20/40 by moving the test object only 2.5° from the fovea.

To explore the central area of the visual field in detail, the perimeter offers such a short working distance that the usual test objects are too small for practical handling. The accepted solution is an instrument such as a Goldmann or computerized perimeter, which permits exploration of the field out to 30° and allows one to perform central field testing at 0.33 m rather than at 1 or 2 m, which is required for a tangent screen. The tangent screen, on the other hand, allows personal contact with the patient, which aids in judging the quality of responses. Black felt is generally used to cover the screen. As exploration of the field progresses, the various isopters can be drawn directly on the felt with a dark-colored pencil and readily brushed off. This manner of recording has supplanted the use of black-headed pins and will be described later in this chapter.

According to Chamlin and Davidoff,3,4 a 1-mm white test object should be visible over most of the screen at 1 m (except at the physiologic blind spot) if there is no visual field defect present. This, however, is not always the case, particularly if it is a patient’s first field test or if a patient’s level of consciousness is depressed (with such patients, I frequently use a 2- or 3-mm white test object). In patients with glaucoma who are well otherwise and have their fields tested repeatedly, however,

the 1-mm white test object is more valuable for discovering the most subtle defects. After a defect is discovered and charted with a small target, it is explored with larger targets, which may be white test objects fastened to the ends of dull-black wands. The largest target made to disappear indicates the density of the defect. It should be noted that when central vision is greatly reduced, fixation is frequently unsatisfactory. This difficulty can be alleviated by using a pair of crossing lines or a circle of adequate size as a fixation target and asking the patient to look at its center, even if the patient cannot see anything in the center.

Exploration of the visual field with white targets of various sizes is known as quantitative perimetry. The use of colored targets, or qualitative perimetry, is of value in distinguishing disturbance of the percipient elements of the retina from interruption of conduction fibers. Colored test objects are also of value in eliciting and mapping scotomas in the central portion of the field. Charting the fields in a dim light exaggerates all relative defects, especially those resulting from implication of the rods and cones.5,6 (Various uses of colored test objects and their specific applications are amplified throughout this monograph, especially in Chapters 7 & 10.)7 this is for a reference.

1-1-3 Physiologic Blind Spot. Plotting the physiologic blind spot early in the course of the examination will make patients aware of the possibility of other blind areas, as well as help them understand what is expected of them. Demonstrating a physiologic scotoma to patients early in the field examination will help their fixation when a pathologic scotoma is found.

The cause of enlargement of the blind spot as a sign of papilledema has been debated since the time of Knapp’s description.8 Retinal displacement and retinal elevation of percipient elements 9,10 and the Stiles-Crawford effect11 are three of the more traditional explanations. More recently, reports suggesting a relative scotoma on the basis of induced hyperopia were suggested by Dailey et al.12 and Corbett et al.13 The theory is that edematous elevations of the percipient elements away from the blood supply of the choriocapillaris result in a decrease in sensitivity.14 As pointed out by Corbett et al.,13 a purely mechanical displacement of the retinal percipient elements would produce an absolute, not a relative, scotoma. A relative scotoma is the defect produced by papilledema.15 This may not be true if the disc edema is due to another cause such as ischemic optic neuritis, in which the retinal or optic nerve tissues have some additional insult that alters their function.

Corbett et al.13 found that in most cases the scotoma could be reduced by using plus lenses. There is previous experience to support their refractive theory as a cause of field defects, as reported by Young et al.16 in their cases of patients with tilted discs. In the tilted disc, there is a bitemporal hemianopia, which can be reduced by using minus lenses. Regardless of the explanation, these authors pointed out an important basic principle in performing fields: Unless the perimetrist pays scrupulous attention to the patient’s refractive error, false-negative or false-positive responses can be produced.

1-1-4 Recording the Fields. Artificial illumination is desirable if fields plotted on one day are to be compared with those plotted on another day; daylight is too variable

used for smaller targets. Color may also be used as a coding device. The system that is adopted should be indicated at the bottom of the recorded field so that anyone reading the chart can easily interpret it.

The choice of sizes of test targets depends in part on the alertness of the patient and on the nature of the defect in the field. It is generally advisable to start with the smallest target the patient can see (often 2 mm), proceed to one or two targets of medium size, and finally determine the largest target that will disappear in any portion of the defect. With experience, one learns to judge the ability of the patient and to guess in advance the probable or possible extent of visual loss. This approach varies with different diseases. For instance, small test objects are necessary to use to discover subtle and early changes in glaucoma. In neuro-ophthalmology, small test objects are used to find the field defect but are followed up with the largest test object that still demonstrates some of the defect, because the largest test object will show improvement first.

The points at which the target is seen on the perimeter are recorded, in degrees, from the point of fixation. The arm of the perimeter is marked in degrees, making notations easy. Because it is preferable on the tangent screen to start with an unmarked surface, a method of transferring the results from the screen to the chart is needed. This can be done by superimposing on the screen a projected slide on which the degrees are ruled or by pulling down in front of the screen a similarly marked transparent cellophane sheet. Most good perimeters, such as the Goldmann, are self-recording and speed the taking of a field. A pantograph has been designed for transferring the results on the tangent screen to a record card fastened to the wall at the side of the screen. This device is generally not available; however, when used, it produces quite satisfactory results when performing tangent fields.

The perimetrist should be aware that a vast number of methods and instruments have been designed over the years for the purpose of seeking evidence of impaired function of the visual pathway. The confusing array of available techniques, many of them offering distinct, if limited, advantages over others, stems from an uncertainty of exactly what should be measured in perimetry. The subject is still wide open for further fundamental research. The essence of doing a good field lies not in the perimeter used but in the acumen of the perimetrist; a tangent screen used skillfully by an experienced perimetrist is every bit as effective as the Goldmann or Humphrey perimeter.

The introduction of computerized perimetry has meant a significant step forward in measuring defects in the visual field.17 There is no doubt that subtle defects not reported by the patient or not found on routine perimetry are being reliably found with computerized perimetry (Figures 1-2 and 1-3). However, in my experience, computerized perimetry occasionally has one drawback in the routine initial examination of neuro-ophthalmic patients: It is a prolonged procedure and may unduly fatigue a patient who is already ill with a neuro-ophthalmic disease that may impair concentration and the ability to respond reliably. The use of computerized perimetry as a way to follow a field defect or in patients who have experience in field examinations or who are physically well, such as patients with glaucoma, is an invaluable diagnostic instrument. For some of my neuro-ophthalmology patients, I use computerized perimetry as an adjunct, because initially it is more important

B

C

Figure 1-2. (Continued)

to determine the anatomic focus of a defect and to establish a likely differential diagnosis. During the past few years, we have had extensive experience with the Octopus perimeter and more recently with the Humphrey.

In this monograph, for the sake of brevity, the isopters of only one or two test targets are shown in cases using the Goldmann perimeter, although in actual practice several targets are usually used.

The reader is also warned against assuming that the technique selected to demonstrate a particular field defect is the only technique that can or should be used. In any examination, both peripheral and central field examinations should be performed to establish the true limits of the defect.

12 Visual Fields

Figure 1-3. A subtle central scotoma shown on a computerized representation. (Source: Courtesy Humphrey Instruments.)

1-2 STRUCTURE OF THE VISUAL PATHWAY

The visual pathway consists of bundles of nerve fibers connecting the retina of each eye with the visual cortex of the occipital lobes. In the retina, there are three layers of nerve cells: the rod and cone cells with their receptors pointing outward, a middle layer of bipolar cells, and an inner layer of large ganglion cells. Each layer sends a single axon back through the optic nerve, the optic chiasm, and one of the

optic tracts. The fibers of the optic tracts end in the lateral geniculate bodies. Other fibers leave the tract before its termination and turn medially to end in the pretectal region. These latter fibers are concerned with pupillary reflexes and are not shown in Figure 1-4.

The cells of each lateral geniculate body send long axons backward, forming a thick band called the optic radiation. This structure ends in the visual cortex on the medial surface of the occipital lobe, in the region of the calcarine fissure.

The visual pathway lies near the base of the brain. The anterior half of each optic nerve lies within the orbit; the posterior half lies within the optic canal of the sphenoid bone and within the cranial cavity. The chiasm is suspended in the basal cistern 5 to 10 mm above the hypophysis and forms part of the floor of the third ventricle. The optic tracts encircle the cerebral peduncles and are laterally covered by the foreparts of the temporal lobes. These fibers terminate in the lateral geniculate bodies, gray masses of cell bodies situated at the posterior lateral margin of the peduncles. The geniculocalcarine fibers, comprising the optic radiation, lie in the external sagittal stratum close to the outer walls of the lateral ventricles. The geniculocalcarine fibers first extend laterally. The upper fibers soon turn backward, but the lower ones loop forward a variable distance around the inferior horn of the lateral ventricle (forming Meyer’s loop) before they pass backward and join the upper fibers on their way to the visual cortex. They all lie deep within the temporal and occipital lobes. (The importance of the relationship between adjacent anatomy and a specific field defect is graphically outlined in Chapter 2.)

Figure 1-4 is a diagram of the visual pathway viewed from above. The rectangle at the top represents the field of vision; the yellow dot at its center is the point of fixation. A vertical line and a horizontal line divide the field into quarters. The color of each quadrant corresponds to that of the portion of the visual pathway along which that quadrant is projected.

The left half of the visual field (green) is projected onto the right half of each retina. From there, axons pass back through corresponding portions of the optic nerves to the chiasm, where fibers from the nasal half of the left retina cross to the opposite side, while fibers arising from the temporal half of the right retina remain uncrossed. Behind the chiasm, all the nerve fibers concerned with the left half of the visual field lie within the right half of the visual pathway.

The macular fibers (yellow) occupy temporal sector of the optic nerve at first; as they pass backward, they dip into the nerve, lying centrally throughout its posterior portion. Axons arising in the nasal half of the macula cross the chiasm, mostly in its posterior portion; axons arising in the temporal half of the macula remain uncrossed. In the tract, the macular bundle is at first central; as it passes backward, it rises and ends in the upper and posterior portions of the lateral geniculate body. In the optic radiation, the macular bundle occupies the central third and spreads over a large portion of the visual cortex at the occipital pole. The macular bundle can be divided into quadrants similar to those of the more peripheral fibers, but for the sake of simplicity this is not shown in the figure.

The left half of the visual field (green) is projected onto the right half of each retina. From there, axons pass back through corresponding portions of the optic nerves to the chiasm, where fibers from the nasal half of the left retina cross to the

opposite side, while fibers arising from the temporal half of the right retina remain uncrossed. Behind the chiasm, all the nerve fibers concerned with the left half of the visual field lie within the right half of the visual pathway.

The macular fibers (yellow) occupy the temporal sector of the optic nerve at first; as they pass backward, they dip into the nerve, lying centrally throughout its posterior portion. Axons arising in the nasal half of the macula cross the chiasm, mostly in its posterior portion; axons arising in the temporal half of the macula remain uncrossed. In the tract, the macular bundle is at first central; as it passes backward, it rises and ends in the upper and posterior portions of the lateral geniculate body. In the optic radiation, the macular bundle occupies the central third and spreads over a large portion of the visual cortex at the occipital pole. The macular bundle can be divided into quadrants similar to those of the more peripheral fibers, but for the sake of simplicity this is not shown in the figure.

The arrangement of the nerves within the pathway is best demonstrated by tracing axons arising in the various quadrants of the retina. For example, the upper left quarters of both retinas (light blue) are concerned with the lower right quadrant of the visual field. From their ganglion cells, axons pass back through corresponding sectors of the optic nerves. Those axons arising in the nasal portion of the right retina cross in the upper portion of the chiasm; those arising in the temporal portion of the left retina remain uncrossed. The lower nasal fibers of each optic nerve cross the chiasm and turn back into the chiasm and optic tract. In the optic tract, as these fibers pass backward, they turn mesially, ending in the medial portion of the lateral geniculate body. From there, nerve cells send axons back through the upper third of the optic radiation to end above the calcarine fissure on the cuneus. The lower left quarters of both retinas (blue) are concerned with the upper right quadrant of the visual field. They send axons back through corresponding sectors of the optic nerves. Those axons arising in the nasal portion of the right retina cross through the lower part of the chiasm, while those arising in the temporal portion of the left retina remain uncrossed. In the optic tract, as the axons pass backward, they turn outward and end in the lateral portion of the lateral geniculate body. From there, nerve cells send axons back through the lower third of the optic radiation, ending below the calcarine fissure on the lingual gyrus.

The complexity of the internal arrangements of fibers in the optic nerves and tracts may be attributed in part to the intrusion of the macular bundle and to the rotation of the nerves and tracts on their longitudinal axis. The optic nerves rotate mesially as they approach the chiasm, their upper sectors becoming nasal.

In the anterior portions of the tracts, as in the posterior portions of the nerves, fibers that arose in the upper quadrants of the retina lie dorsomedially; those that arose in the lower quadrants of the retina lie ventrolaterally. The rotation continues within the tracts to their termination, where fibers that were originally superior are now medial and those that were inferior are now lateral. The horizontal raphe of the retina is vertically projected on the lateral geniculate body. At the beginning of the optic radiation, a counterrotation returns the fibers to relationships that existed at the forward portions of the optic nerves.