Perimetry

U. Schiefer, J. Schiller and W. Hart

Examination of the visual field, whether on a hemispheric surface (in which case one is doing perimetry) or on a flat surface (where one is doing campimetry) is perhaps the single most important diagnostic test in neuro-ophthalmology.

j| Definition

By visual field we mean the sum total of all visual sensation experienced by on observer with a fixed head and torso position, with eyes steadily gazing at a stationary object. This is in contradistinction to field of view and oculomotor fields, in which it is understood that the head and eyes are allowed all freedom of movement. The visual field examination, in contradistinction to testing the pupils with the swinging flashlight test (see Chaps. 2 and 5), is not an objective test, but rather depends on the cooperation of the patient being examined. Given such cooperation, one can determine not only the depth of a defect, but also its size, shape, and location (■ Fig. 4.1). This is fundamentally important for topographic diagnosis (■ Fig. 4.2) and for monitoring the temporal course of lesions affecting the afferent visual pathway. This chapter describes the various methods of perimetric testing, provides a classification system (■ Fig. 4.3) of the visual field defects that are associated with neuro-ophthalmic disease, describes their various differential diagnoses, and shows which additional diagnostic tests they may indicate.

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Visual Field Confrontation
Fig. 4.1. The patient complaint,"My vision is getting worse," is consistent with many potential causes and types of visual field loss. Particularly significant is the fact that this sort of complaint can be produced by either monocular or binocular types of visual dam

age. This makes an initial examination of the visual fields of both eyes a necessary part of a complete neuro-ophthalmic examination

Nasal Step Visual Field Defect

Fig. 4.2. Correlation between type of scotoma and location of disease in the visual pathway: If the defect lies in the superior half of the field, the causative lesion lies in the lower half of the retina or the lower half of the retrobulbar visual pathway, up to and including the primary visual cortex and vice versa. In addition, defects in the nasal half of the visual field correspond to the temporal hemi-retina; the retinal ganglion cell axons originating there do not decussate at the chiasm, but remain in the ipsilateral half of the visual

Fig. 4.2. Correlation between type of scotoma and location of disease in the visual pathway: If the defect lies in the superior half of the field, the causative lesion lies in the lower half of the retina or the lower half of the retrobulbar visual pathway, up to and including the primary visual cortex and vice versa. In addition, defects in the nasal half of the visual field correspond to the temporal hemi-retina; the retinal ganglion cell axons originating there do not decussate at the chiasm, but remain in the ipsilateral half of the visual

Visual Field as seen by the patient (correct side, upright image).

pathway. A defect in the temporal hemifield maps to the nasal half of the retina, and the axons arising there decussate in the optic chiasm, projecting to the contralateral lateral geniculate body and cerebral hemisphere. All lesions of the afferent visual pathway lie in positions that are the opposite of their corresponding visual field defect, i.e., the mapping between field defect and lesion is both horizontal and vertically reversed

Indications for Perimetric Testing

Initial Methods of Testing

Perimetric testing should be considered appropriate when the following problems, findings, or factors are present:

■ The presence of a relative afferent pupillary defect (RAPD)

■ Monitoring of visual field defects already known to exist

■ Signs or symptoms of damage to the afferent visual pathway

■ Visual disturbances of unknown cause (e.g., for nyctalopia, loss of brightness perception, and disturbances of reading or of visual orientation)

■ Abnormalities of ocular fundus (optic disc and retina)

■ Certification of visual function for driving or occupational tasks

Additional Historical Information

It is useful to ask patients specifically whether they have been aware of visual field defects (called positive scotomas) or changes in their field of view, such as metamorphopsia. One should also briefly inquire whether there have been episodic periods of visual loss that might indicate a problem with transient ischemic episodes (see Chap. 14).

T| Pearl

Temporary blurring of vision or passing bouts of darkening can be manifestations of the obscurations of vision associated with papilledema, or may suggest a problem with intermittent optic nerve compression. Not uncommonly, such symptoms begin as monocular, affecting one eye more heavily than the other, can be provoked with only minimal head movements, and indicate a disturbance of axoplasmic flow (see Chap. 3, ■ Fig. 3.3, and Chaps. 9 and 12).

Fig. 4.3. Topologically diagnostic classification of scotomas (schematic diagram). Visual field defects have been divided into seven primary types (left side of diagram); in the right half of the diagram are commonly occurring subtypes. The numerical labels in the left half of the diagram refer to the corresponding numbers of the Compendium of Visual Field Defects. DLS Differential luminance sensitivity

Fig. 4.3. Topologically diagnostic classification of scotomas (schematic diagram). Visual field defects have been divided into seven primary types (left side of diagram); in the right half of the diagram are commonly occurring subtypes. The numerical labels in the left half of the diagram refer to the corresponding numbers of the Compendium of Visual Field Defects. DLS Differential luminance sensitivity

Visual Field Defects Diagram

Swinging Flashlight Test Confrontation Testing of the Visual Field

Although the swinging flashlight test (see Chap. 2) is not a visual field test in the narrower sense, it should be used routinely as a quick and simple means of detecting afferent visual problems prior to a more thorough examination of the visual field. It can detect monocular or significantly asymmetric, bilateral disturbances of vision in the prege-niculate (anterior) portions of the afferent pathway, and is an objective method that does not depend on the patient's cooperation. The swinging flashlight test is indicated by any unexplained disturbance of vision, as well as by any suspicion of a lesion in the afferent visual pathway.

As an initial step in the examination, the hands or fingers of the examiner can be held (under steady control of fixation) in either or both halves of the visual field, but moved in only one hemifield at a time (■ Fig. 4.4 b).

Alternatively, when there is a suspicion of a hemianopic loss of field, the examiner can present both hands - held motionless - to either side of the vertical meridian, and ask the patient to report the total number of fingers being held out. This "finger perimetry" method can detect most widespread losses of the peripheral visual field, including hemi-anopsias and quadrantanopsias (■ Fig. 4.4 c).

Optic Nerve CuresEye Schematic Confrontation Test

Fig. 4.4. Confrontation testing of the visual field a Maintenance of fixation (e.g., on the bridge of examiner's nose) is essential. b Bimanual testing of the visual field: The examiner closes the eye that is directly opposite to the eye the patient has closed and uses the field of his or her open eye's monocular perception as a basis for comparison to the patient's visual field, while presenting visual stimuli (e.g., moving one or both hands) in the opposing portion of the patient's visual field. This serves to reduce problems with fixation. c Counting of fingers held to either side of the vertical meridian tests the central visual field with relatively smaller "test objects'; as compared with the bimanual tests

Fig. 4.4. Confrontation testing of the visual field a Maintenance of fixation (e.g., on the bridge of examiner's nose) is essential. b Bimanual testing of the visual field: The examiner closes the eye that is directly opposite to the eye the patient has closed and uses the field of his or her open eye's monocular perception as a basis for comparison to the patient's visual field, while presenting visual stimuli (e.g., moving one or both hands) in the opposing portion of the patient's visual field. This serves to reduce problems with fixation. c Counting of fingers held to either side of the vertical meridian tests the central visual field with relatively smaller "test objects'; as compared with the bimanual tests

When examining small children, the examiner can maintain control of fixation while an assistant positioned behind the child introduces objects of visual interest into the various quadrants of the peripheral visual field. The child's head movements on detection of these stimuli allow a judgment of the intactness of the visual field.

Confrontation testing should be used routinely, when examining small children or uncooperative patients, whenever there is a suspicion of an advanced degree of visual loss. This preliminary method can also be put to use when circumstances do not allow use of conventional perimetric testing, such as at bedside consultations or in intensive care units. It also serves as a quick check of the plausibility of a patient's responses.

Pearl

A very effective modification of confrontation testing, useful in cases of homonymous visual field loss, has the examiner face the patient at a distance of about 30 in., while the patient fixes on the center of the examiner's face. The examiner then asks the patient whether the entire face is simultaneously visible. Missing portions of the face give an indication as to the extent of a homonymous defect. The closer a defect is to the vertical meridian, and the smaller is the remaining, congruous portion of the paracentral visual field (■ Fig. 4.5), the more likely there is to be a loss of reading fluency (see below).

Amsler Grid Testing

The Amsler grid is a visual field test that encompasses the central visual field to an eccentricity of 10°. It allows for the subjective detection and ongoing monitoring of retinal, primarily macular, diseases. The patient views an orthogonal grid of vertical and horizontal straight lines at a reading distance of about 17 in (40 cm). The grid has a central fixation point and is shown on a featureless background. It is a very effective test for the detection of metamorphopsia (e.g., caused by retinal edema or other disturbances of pho-toreceptor alignment), and its use can be delegated to the patient as a solo screening test.

The Amsler grid test is particularly useful when evaluating subjective symptoms of distorted images, reading problems, or signs of abnormal fundus appearance in the macular region. A small, printed figure of the grid can be given to a patient for self-monitoring of visual changes at home.

Optic Nerve CuresOptic Nerve Cures

Fig. 4.5. Looking at and describing the examiner's face in the case of a homonymous visual field defect (the corresponding values for eccentricity at a distance of 75 cm [approximately 30 in.] are shown). If the patient with a hemianopic defect while fixing attention on the bridge of the examiner's nose can describe details only as far as the inner canthus of the examiner's eye, and all details peripheral to this locus are not seen, the visual field defect extends up to a point within 1° or less of fixation. In this case, reading ability will be significantly impaired

Fig. 4.5. Looking at and describing the examiner's face in the case of a homonymous visual field defect (the corresponding values for eccentricity at a distance of 75 cm [approximately 30 in.] are shown). If the patient with a hemianopic defect while fixing attention on the bridge of the examiner's nose can describe details only as far as the inner canthus of the examiner's eye, and all details peripheral to this locus are not seen, the visual field defect extends up to a point within 1° or less of fixation. In this case, reading ability will be significantly impaired

Reading Ability

Tests of reading ability, with an age-appropriate correction for near, are an effective method for examining a portion of the central visual field. Fluency of reading requires the normal function of a region at least 2° to the left and right and 1° above and below the point of central fixation (see Chap. 24). When a reading problem is found, a careful ophthalmoscopic examination of the macula will often reveal the cause, but if the ocular fundus is unremarkable in appearance, there is likely to be a homonymous hemianopic visual field defect that divides the central visual field, including the point of fixation and the perifoveal field on the affected side. Conversely, if there is a suspicion of a hom-onymous visual field defect, if there are any ophthalmo-scopic signs of macular disease, complaints of reading problems or of amblyopia, tests of reading ability are a necessary part of the clinical evaluation.

Testing of Color Saturation

Testing of the symmetry of color saturation is best done with a brightly colored (red is usually best) object, such as the top of a mydriatic bottle. The object should be shown to one eye and then the other by slowly alternating monocular occlusion. When there is an optic neuropathy causing an acquired color deficit, the object seen by the affected eye will have a relative deficit of color perception, usually described by the patient as a "darker" or "faded" color, particularly when the object is held in an area of relative visual field loss (see Chaps. 2 and 6). Using the same colored object, confrontation perimetry can be done, while monitoring the patient's fixation, by holding the object in various locations in the visual field. Particularly noteworthy responses are descriptions of sudden change in the color's appearance, when the object is moved into or out of the region of a visual field defect. This can sometimes be quite striking, particularly when crossing the vertical meridian in instances of hemianopie deficits in the visual field.

In cases where optic atrophy is seen or an optic neuropathy is likely (as in patients with relative afferent pupillary defects), testing of color saturation by these simple methods is often very helpful (see Chap. 6).

Conventional Methods of Perimetric Testing | Pearl

When correlating visual field defects with lesions in the afferent visual pathway, it is helpful to keep in mind that the optical inversion of images in the eye causes an inversion of the visual field with respect to the afferent pathway. Thus, objects in the superior visual field are imaged in the inferior half of the retina, and the encoded data for that image remain in the fibers of the inferior half the pathway, at least for the optic nerves, the chiasm, and the retrogeniculate path. In addition, the temporal half of the visual field is imaged in the nasal fundus, etc. (see Chap. 3, ■ Figs. 3.4 and 3.7, and ■ Fig. 4.2).

Luminance Sensitivity The Eye
a * Differential luminance sensitivity
Luminance Sensitivity The Eye
b * Differential luminance sensitivity
Kinetic Perimetry Physiological

c * Differential luminance sensitivity

Fig. 4.6. a Conventional methods of perimetric testing: Kinetic perimetry (using test objects that are moved) yields results that are displayed as so-called isopters (lines of identical DLS that are comparable to a weather map's isobars). b Conventional methods of perimetric testing: Static profile perimetry (visual field testing with stationary test objects arrayed along a single meridian in the visual field) produces data that are displayed as a vertical profile of DLS that passes straight through the center of the field. c Conventional methods of perimetric testing. Automated static perimetry, in which static test object locations are distributed across a contiguous area of the visual field produces loci of DLS for each location in the pattern. Using these data, a virtual surface is computed, yielding a "reconstruction" of the hill of vision c * Differential luminance sensitivity

There are two types of perimetry, kinetic and static. In kinetic perimetry, moving test objects are presented, while in static perimetry test objects are held in stationary positions when presented (■ Fig. 4.6). The common goal of these methods is to determine, as precisely as possible, the sensitivity of visual perception as a function of location in the visual field. The correct physiologic term for sensitivity is differential luminance sensitivity ([DLS] also incremental luminance sensitivity). These terms arise from the use of a background light to maintain uniform dark/light adaptation in all visual field areas, while test objects (small spots of light) are projected onto the background, thus adding their luminance to that of the adapting surround. The stimulus strength is expressed as a function of the incremental addition of the stimulus light to the adapting surround on which it is projected. Definitions and descriptions of related physiologic terms in perimetry are listed in ■ Tables 4.1

and 4.2. Three-dimensional reconstructions of the spatial distribution of visual sensitivity produce a conceptual "hill of vision" (■ Fig. 4.6). Radial lines extending from the visual field center are termed meridians (■ Fig. 4.6 c). Their angles of meridional orientation are analogous to those of the geographic meridians of the globe.

The currently accepted definition of the differential luminance sensitivity is related to the perimeter-specific maximum stimulus luminance capacity, complicating any comparison of results between differing instruments (■ Fig. 4.7). It made more sense to adopt a standard of reference in which the adapting background (surround) luminance level was agreed upon and held constant for all instruments. The level of 10 cd/m2 was accepted, which is a low photopic level of retinal adaptation.

Table 4.1. Sizes and luminance intensities of the test objects used by the Goldmann perimeter

Perimetry Instruments

Kinetic Perimetry

When using kinetic perimetry, the examiner presents stimuli that move with a constant angular velocity and a constant brightness and size - the corresponding physiologic terms being luminance intensity and angular subtense (for additional help and definitions, see ■ Tables 4.1 and 4.2). The stimulus is projected onto a matte surface of uniform light intensity that serves as an adapting background. The test objects (the projected light stimuli) are moved from nonseeing to seeing areas of the visual field, as perpendicular as is possible to the expected peripheral boundaries of perception (■ Fig. 4.6 a).

An angular velocity of 4°/s is regarded as an optimal compromise between spatial resolution on the one hand and examination duration on the other.

Multiple presentations with a constant stimulus value detect contours of isosensitivity, comparable to the isobars of meteorological maps, and are called isopters (■ Fig. 4.6 a). Each isopter is tested within at least eight evenly spaced meridians, with stimuli moving from the periphery toward the center of the visual field. By choosing appropriate stimulus sizes and luminous intensities, the hill of vision's (con-ceptional) three-dimensional shape and size can be documented by plotting the various isopters. Kinetic perimetry is suited to the documentation of visual function for certification of visual performance, e.g., as necessary for safe operation of a motor vehicle. It is particularly suited to the examination of patients capable of only poor cooperation, permitting a high level of efficiency when examining large visual field defects.

A common stimulus used for certification examinations is the III/4e test object, which has an angular size of about 26' and a luminance of about 320 cd/m2. This stimulus should be used in all kinetic perimetric tests so that the results of the various examinations can be more easily compared to one another.

When testing the physiologic blind spot, the usual test object is the I/4e, having a size of 6.5' and a luminance of 320 cd/m2. For diagnostic testing, the most discriminating (and therefore most significant) test objects are those of low luminance and small size. These are best suited to the detection of subtle, relative central and paracentral scoto-mas.

The dimensions of the test objects in the Goldmann perimeter are listed in ■ Table 4.1, and their luminance levels are given in ■ Fig. 4.7.

Static Profile Perimetry

For static profile perimetry, test objects are held in stationary positions, and their luminance levels are varied. The various locations are arrayed along straight lines that pass through the field center, and the choices of lines are determined by the regions of interest in the central and paracentral parts of the visual field (usually within 30° of eccentricity). Profiles of the island (or "hill") of vision (■ Fig. 4.6 b) are produced. This method of testing is no longer in general use, but remains a particularly good technique for

Table 4.2. Computational definitions of perimetrically relevant physical and psychophysical terms

Term

Definition

Symbol or abbreviation

Unit

Calculation

Examples

Luminance (/_)

Absolute level of light intensity

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Responses

  • lori williams
    What is a iii 4e test object?
    7 years ago
  • jared
    How to do the confrontation test for visual fields?
    7 years ago
  • Mira
    Where is the problem on the retina of visual field defect?
    7 years ago
  • WOLFGANG
    How to perform confrontation field test?
    6 years ago
  • Maja
    What is a iii4e stimulus?
    6 years ago
  • Graziella
    What is pelvic perimetry?
    2 years ago

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