When determining the visual field, the perimetrist attempts to make a two-dimensional map of a patient’s entire area of vision. Normally in everyday living, we do not place much importance on the width of our vision, the emphasis being directed on seeing clearly straight ahead. However, those people who have lost much of their peripheral field are just as incapacitated functionally as those who have lost much of their central field. Try rolling up two sheets of paper and placing them before your eyes so that you are basically looking through two large-diameter straws. Although you can see directly ahead clearly, it is very difficult to walk through a room without bumping into the furniture.
The circumference of the visual field depends on many factors. Obviously, the field of vision needed for seeing a mosquito flying about would be different than for seeing a jumbo jet. Thus the size of an object is important in referring to the dimensions of the seeing area. Also the ability to perceive at the sides is not as great when the visibility is poor, as opposed to when it is clear; therefore illumination is an important factor in mapping the field of vision. The state of adaptation of the eye, whether light-adapted or dark-adapted, although not a critical factor, influences the measurable size of the visual field area.
These factors are objective and can in all instances be controlled by the perimetrist. The difficulty in qualitative or quantitative perimetry is not in assessing the factors of size, illumination, and adaptation but in assessing the patient. Perimetry depends entirely on obtaining an accurate and rapid subjective response. How does one compare the replies of a 72-year-old belligerent, slightly confused patient who has recently had a stroke to those of an intelligent 25-year-old woman with no cerebral disease? Because it is difficult to evaluate such factors as reaction time, fatigue, and general health, what is done and noted in every case is a simple evaluation (good, fair, or poor) of the patient’s cooperation and reliability.
Although perimetry is not an exact science because it is entirely dependent on the subjective replies of the patient, the visual field for a given reliable patient should be reproducible. Many methods have been devised for estimating visual fields. We discuss only those methods that have survived the tests of time and their applications and limitations.
Preliminary procedures
A general statement should be recorded about the patient’s visual behavior. If it is noted that the patient tends to bump into objects located on either the right or left, a right- or left-sided total loss of visual field (homonymous hemianopia) may be present.
Visual acuity, with and without glasses, should be noted before taking a visual field. As a rule, it can be stated that the poorer the patient’s visual acuity, the larger is the test object that must be used to plot an accurate visual field. The manner in which the patient responds to visual acuity testing should be recorded. If the patient appears to see only the last three letters of all the lines on the chart, a loss of one-half of the visual field may be present.
Color vision should be checked, especially if colored test objects are going to be used.
The purpose of the visual field examination should be noted so emphasis can be given to specific areas. Such instruction must come from the ophthalmologist, who has made a complete ocular examination and a tentative diagnosis. For instance, for a patient with papilledema, the tangent screen might be thoroughly explored, with particular attention paid to the state of the blind spots, as opposed to the patient with retinitis pigmentosa, in whom a ring scotoma might be anticipated.
Facilities for field testing
Ideally, every office or clinic should have a visual field room that is quiet and out of the way of the normal practice traffic. This room should be of simple design so that distraction is kept to a minimum. Approximately 7 footcandles of illumination (see Ch. 3 ) are necessary for adequate visual field tests if the test apparatus does not contain its own light source.
Realistically, most offices do not have special visual field quarters because of the cost involved and lack of available space. Similarly, the equipment used varies from place to place. Although it is helpful to have the best ophthalmic equipment, the room and equipment always take a subordinate position to the skill and ingenuity of a competent perimetrist. If the patient understands what is expected and has rapport with the examiner, adequate perimetry can be performed.
Confrontation test
Of all methods of perimetry, the confrontation test is the most widely used because it requires no special facilities or equipment and can be performed in the home, on bedridden patients, and in hospitals. This is essentially a screening test. Any pathologic condition discovered requires a more sophisticated test when possible to determine the exact nature of the visual defect.
In this test, the examiner compares the range of the patient’s field with his or her own, which is presumed to be normal. The examiner stands facing the patient at a distance of approximately 2 feet (60 cm). Opposite eyes are occluded; that is, the patient’s left eye is covered while the examiner closes his or her right eye. Each of them then fixes the exposed eye of the other. The examiner moves a finger or a white test object, such as a small hatpin mounted on a handle, from the extreme periphery midway between examiner and patient and notes when it comes into the field of view; the patient and examiner should see it simultaneously ( Fig. 18.1 ). The test is best performed while the patient’s back is to the light and the background behind the examiner is uniform and dark.
All four quadrants of the visual field should be tested and two different approaches should be used in each quadrant. If any defect is indicated or suspected, the field should be accurately mapped and recorded with the perimeter and tangent screen. When vision is extremely poor, a small penlight may be used for a rough test.
A modification of this test is to have the patient count fingers. While one eye is occluded, the examiner brings in from the periphery one, two, or three fingers and asks the patient to count the number of fingers brought in from each quadrant.
The confrontation test is an excellent method of screening patients and, if used skillfully, can be surprisingly accurate. It may be the only method of examining children, people who cannot read, and the cognitively challenged. With children, a small article of interest, such as a brightly colored plastic toy may be used as a test object. The preservation of a field in a particular area is indicated when the child makes a quick glance at the object of interest detected in the peripheral field. Finally, when fixation is lost or essential vision is grossly impaired, this method may be more valuable than a more refined technique. For example, in a patient with a cataract, the accurate perception and projection of light or a hand in all four quadrants may be the only method of determining retinal function.
However, this should be regarded only as a rough test, and failure to demonstrate a field defect does not imply a normal field. Defects of large size may easily be missed by this method.
Perimeters
Many perimeters are constructed in such a manner that the eye is at the center of rotation of a hemisphere that has a radius of curvature of 33 cm. Some perimeters consist of an arc of a circle that is rotated, and the test object is moved either manually or mechanically along this arc from the periphery toward the center. The more elaborate perimeters, such as the Goldmann perimeter, are constructed from a half shell, in which the test object is projected ( Fig. 18.2 ). In this type of perimeter, the intensity of the illumination of the test object can be controlled and the patient’s fixation can be continually checked by a viewing device behind the perimeter.
Before use, the Goldmann perimeter should be aligned and calibrated as follows:
- 1.
Level the instrument so that the projector arm will swing back automatically into the protected position on the sphere when the instrument is not being used. Insert the chart paper and position the vertical and horizontal lines with the V-notch on the frame. If a fixation light projector is used, the chart is positioned 5 degrees from the center line.
- 2.
Use the 15-degree position to the right and left for examination when the central scotoma device is indicated.
- 3.
Adjust the telescope so that the reticule and the patient’s eye are clear and in plain focus. Proper adjustment of the light within the sphere and of the projected target is most important. Position the recording arm at the 70-degree mark on the chart. This allows you to lock the projector arm in the proper position by pushing the centering pin on the operator’s side into the socket on the upper left side of the instrument. When the arm is locked in, light from the projector will fall onto the light meter.
- 4.
Position the size and brightness control levers to the far right. Turn the instrument on and the room lights off. Turn the appropriate control knob and adjust to a reading of 1000 apostilbs on the light meter.
- 5.
Set the gray filter level to the 0.0315 position. This produces an illumination of 31.5 apostilbs.
- 6.
Interpose the white photometer shade between the projected light and the light meter.
- 7.
At this point, look at the photometer screen through the cut-out on the opposite side of the sphere. Match the sphere’s brightness to the brightness on the photometer shade or screen by moving the diaphragm up or down. To achieve reliable and accurate fields, calibration of the instrument should be performed before each examination, but from a practical standpoint once monthly is adequate. You are now ready to proceed with the visual field test.
When the visual field has been mapped out on a perimeter, the following notations should be made: (1) the size of the test object, which can vary from 1 to 25 mm, (2) the test distance, which is always 330 mm in a perimeter, (3) the color of the test object, and (4) the cooperation and reliability of the patient.
Measuring a field on the perimeter
Just before the actual perimetric examination, the patient should be told in detail what is expected during this test. It is essential that the patient be comfortable, relaxed, and alert. Glasses are not required. The patient is brought to the perimeter and his or her chin is set on the chin rest. The chin should be placed comfortably on the rest so that the patient’s face is held vertically and not tilted to one side. One eye is covered. The other eye, situated in the center of the arc, fixes on the white fixation target at the center of the perimeter around which the arc revolves.
The size of the test object chosen depends on the accuracy of the patient’s fixation and reaction time. If the patient is young and alert and has 20/20 vision, the examination is begun with a 1- or 2-mm white target (I-2-e or I-3-e on the Goldmann perimeter). However, if the patient is confused and suffers with early stages of dementia, or has dementia, and has vision no better than 20/200, it would probably be best to use a 10 mm, 20 mm, or even larger test object (III-4-e) as the initial stimulus. The test object is always brought in from the periphery toward the center (from the nonseeing to the seeing area). The test object should be brought in slowly so that the time lag from the patient’s response to the mark of the examiner will not be great. In many instances, the ophthalmologist will request a perimetric examination to be performed with certain test targets. These different targets show characteristically larger fields with the larger target. In follow-up visits, it is important to reuse the same targets to show any regression or progression of the field changes. The different targets show a different-sized field at a common illumination and distance; each of these completed mappings of a single target size and brightness is called an isopter . The ophthalmologist will inform the ophthalmic assistant which isopters are desired.
The patient should be taught to buzz or tap when the stimulus is seen. Conversation during the test should be kept to a minimum because it only serves to distract the patient and cause head movement.
Ideally, 24 meridians should be tested. The normal physiologic response to an object in the peripheral field is to turn the eyes toward it. When the field of vision is charted, this normal response has to be suppressed because fixation by the patient must be rigidly maintained on the central target. Therefore it is imperative to observe the patient’s fixation at all times. In some of the more elaborate perimeters, a viewing system at the back of the shell enables the examiner to constantly watch the patient’s eyes during the entire examination.
Field testing is taxing. Accuracy depends on the subject’s accurate and quick response. It should not be laborious because prolonged visual field testing will tire a patient and cause erroneous results. Drooping eyelids or eyeglass frames interfering with a clear view of the test object can cause these erroneous results.
Errors in field testing can occur on the part of either the patient or the examiner. Errors attributable to the patient may include following the test object rather than maintaining proper fixation, tilting the head or moving the chin off the chin rest, not understanding the test or being generally uncooperative, responding slowly, most often done by mentally challenged patients with low visual acuity or obvious field defects, and physical, mental, or psychologic handicaps. Errors attributable to the examiner may include poor patient instruction, too-rapid movement of the test object, incorrect monitoring of the patient’s fixation, poor or inaccurate marking of the chart, and poor or incorrect adjustment of the perimeter.
Charts
The visual field chart is merely a permanent record of the patient’s responses at tangent screen and perimeter examinations. The best type of tangent screen chart is that in which both fields are represented on a small pad that can be fastened to the patient’s record so that the whole picture of the patient’s visual status can be seen at a glance. The right eye and left eye should be indicated and the chart should be as the patient sees the visual field; that is, the field for the right eye is on the right side of the chart. A notation should be made of the patient’s name, the date, and the examiner’s name. In addition, the size and color of target used, the corrected visual acuity, the pupillary size, and the patient’s cooperation and reliability should be noted.
An isopter is the map of the circumference of a visual field determined by a test object of a certain size, with the patient at a certain distance from the tangent screen or perimeter. The isopter, as indicated on the visual field chart, should be noted as a fraction, the numerator indicating the size of the test object used in millimeters, and the denominator indicating the distance of the patient from the field chart in millimeters. Thus the fraction 5/1000 indicates to anyone what test object was used and at what distance in millimeters.
Special perimetric techniques
Visual field screening
Visual field screening is a good method for rapidly determining the presence or absence of a field defect. It is useful as a preliminary procedure in offices or in testing large groups, such as military personnel or students.
Automated visual fields
The automated perimeter or visual field plotter is a quick, randomized test to determine field defects ( Fig. 18.3 ). A complete discussion of automated visual field equipment is found in Chapter 18.
