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Module 12 Section 3 |
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Module 12: |
Visual Field Testing | |||
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Section 3: |
Testing Methods | |||
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Common testing methods: Testing methods in disuse: Visual field testing methods have the following in common:
Manual methods require the examiner to regulate and control variables such as background illumination and stimulus selection and presentation. Manual methods are less expensive and can provide basic visual field information in a relatively fast and efficient manner. It is difficult to standardize and reproduce results with manual methods. Automated perimeters provide the standardization and reproducibility required for the detailed visual field analysis desired for diagnosis and treatment of diseases such as glaucoma. All of the above listed methods are manual except for the automated category.
In general, kinetic methods are used in manual perimetry and static methods are used in automated perimetry. However, kinetic perimetry can make use of static methods, and kinetic perimetry can be performed by some automated perimeters.
Screening vs. Diagnostic methods:
Screening tests are fast methods used to detect a defect in the visual field. A diagnostic test is usually much more time consuming and is used to "characterize" defects. The confrontation field test is a screening test. Manual and automated perimeters use screening protocols that test a limited number of points or meridians in specific patterns dependent upon what type of defect is suspected. Visual field testing is not part of a routine eye exam (with the exception of the confrontation VF). Screening tests are performed in response to the probability of a field defect being present in a specific disease process, or in response to a patient complaint.
See Module 15 for more information on the threshold strategy used for automated diagnostic testing and the suprathreshold strategy used for automated screening tests.
Confrontation visual fields: Discussed in Section 2.
This method is still used because it is inexpensive, efficient, and it can yield a fairly accurate measure of the central 30 degrees of the field. |
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The equipment needed is simple and usually includes:
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The basic tangent screen procedure is as follows:
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| The one meter distance is the most often used distance for testing. As pictured above, the area map of the blind spot moves outward and enlarges as the patient-to-screen distance changes from 1 meter to 2 meters and the stimulus size is doubled. The projection concept is pictured below. The area covered by a scotoma enlarges as the view recedes from the eye. | ||||
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The ability to use the tangent screen to test at two different distances is particularly useful when testing the malingerer who claims to have a field defect. The field defect (usually tunnel vision) can be tested at both the one meter and two meter distances. The stimulus used at two meters is twice the size of the stimulus used at one meter. If it is true tunnel vision, then the central area of seeing will double in size as the test is performed at one meter and then at two meters. If the patient is malingering, he will probably not be aware of this effect and will give responses that keep the area of central vision the same size no matter what the test distance.
True tunnel vision can be the result of end stage glaucoma or end stage retinitis pigmentosa.
The tangent screen is also useful when measuring the effect that a ptosis has on the superior visual field. Insurance companies usually require this documentation before surgery is approved, and the tangent screen field can be performed quickly and easily. In this situation, the blind spot would not need to be mapped.
As pictured below, the red line indicates the normal field appearance with the lids held up, and the green line indicates the boundary of the superior field as affected by the droopy lid. The shaded area indicates how much of the field is affected. |
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| Below is pictured what a normal tangent screen visual field would look like when drawn on a tangent screen chart. The inner blind spots are shaded in, indicating that the field was performed at a distance of one meter. The outer boundary is drawn beyond the outer boundary of the tangent screen, indicating there were no field cuts within the central 30 degrees when tested with the particular target size used. The target size and color should be recorded on the chart paper along with the name and date. Boundaries from different target sizes can be drawn with different colors. | ||||
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A crude form of static perimetry can be used in a systematic search for scotomas. The black side of the stimulus disk is moved to the desired position on the field. The disk is then flipped with the wand to the stimulus side of the disk and the patient indicates if the stimulus is seen or not. |
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The patient with a central scotoma (low vision) can be aided by using tape to create a large "X" that goes through the central fixation dot (left). The patient is instructed to maintain fixation by concentrating on the point where the lines would intersect, even though the center of the screen is blocked by the scotoma (below). | |||
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The nice thing about the tangent screen concept is that the procedure can be performed without any of the standard equipment. All you really need is a blank wall and an improvised stimulus. The fixation target can be a small dot of paper taped to the wall.
The Auto-Plot visual field machine is essentially a glorified tangent screen. It consists of a screen that is held by two arms at a specific distance from a stimulus projection setup. There may be some Auto-Plot machines still in use, but I have not seen one for years. The unit does offer more standardization than the tangent screen, but they essentially perform the same functions. The tangent screen costs much less and takes up much less space.
Goldmann Perimetry is discussed in Section 1. The basic concepts of kinetic perimetry are the same no matter what method is used. The advantages of the Goldmann Perimeter as compared to the tangent screen are as follows:
The major disadvantages of both the tangent screen and Goldmann perimetry have to do with stimulus presentation. Neither performs static perimetry efficiently, which is the preferred method for exploring scotomas. Kinetic perimetry involves moving the stimulus from a non-seeing area to a seeing area. With the tangent screen and Goldmann perimetry, the speed of the stimulus presentation is controlled by the examiner and cannot be standardized. This factor limits the reliability of serial fields because a faster moving target will be soon sooner than a slower moving target. It is important to move the target at a constant speed of about 5 degrees per second to minimize this source of error.
This is another dinosaur that I have not seen for years. It is a kinetic perimeter that uses a metal band formed in an arc that rotates 360 degrees. It was used to mark the boundaries of the peripheral field before the Goldmann perimeter came along. A stimulus is mechanically moved across the arc and patient responses are punched onto recording paper. It was a clever machine that never got much use.
The Harrington-Flocks screener is a visual field screening machine that fits in a suitcase. The device flashes a series of dot patterns to the central 25 degrees of each eye. The patient tells the examiner how many dots were seen in each presentation. If the patient does not see the correct number, then she describes the pattern so that missed dots can be recorded by the observer. If the screener detects a defect, then the field is further explored with a diagnostic field exam such as an automated threshold test. |
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Two Harrington-Flocks dot patters. |
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Automated perimetry was developed to standardize visual field testing and to therefore increase the reliability of visual field tests. Various manufacturers make automated perimeters based upon computerized projection systems and LED (liquid crystal display) systems. Automated perimeters can perform screening and diagnostic field tests and can use kinetic and static methods. These perimeters are most often used for static threshold testing.
By far, the most widely used automated perimeter is the Humphrey Field Analyzer, which has become the standard for visual field testing. The HVA and automated perimetry are discussed in detail in Modules 13, 14, and 15.
One of the most useful and effective visual field tests also happens to be very simple .
The Amsler Grid is just that, a square with a grid work of lines forming smaller squares within the large square, with a fixation dot in the center. The grid tests the central 20 degrees of the visual field and is used to test macular function. It can be used in a white on black pattern, or a black on white pattern. The black lines on a white background pattern is more useful for drawing or marking on. |
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The grid is held at reading distance in good light. The patient wears his near correction, and the non-tested eye is occluded. The patient maintains fixation on the fixation mark at the center of the grid and is asked if he notices any abnormal or missing squares, or if any of the lines are missing, distorted, or abnormal in appearance.
The Amsler Grid is particularly useful in two situations:
Errors in Visual Field Testing
Errors common to all methods:
The lens rim, a droopy eyelid, and a small pupil can obstruct vision and cause what appears to be a significant field defect. This is termed artifactual field loss, meaning it is due to a "mechanical" obstruction and is not due to decrease retinal sensitivity.
Other errors common to kinetic perimetry
Other errors common to automated perimetry
Other errors common to the tangent screen
Other errors common to Goldmann perimetry
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