Methods for visual field testing
Automated - visual field test (bowl) - The patient sits in front of a concave dome with a target in the center, and asked to stare/focus on the target at the center of the bowl. The eye that is not being tested is covered. A button is given to the patient to be used during the exam. A computer then shines lights on the inside dome and the patient clicks the button whenever a light is seen. The computer then automatically maps and calculates the patient's visual field. Various manufacturers make automated perimeters based upon computerized projection systems and LED (liquid crystal display) systems.
Automated tangent screen visual field test - PC automated (tangent screen) visual field test: the patient sits comfortably in front of a monitor/projector screen and asked to stare/focus on the target at the center of the screen. The software flashes small lights at different locations within the screen's surface. The lights are bright or dim at different stages of the test. Some of the flashes are purely to check the patient is concentrating. The eye that is not being tested is covered. Each eye is tested separately. Both eyes are refracted to the best corrected vision. Pupils are not usually dilated. A mouse is given to the patient to be used during the exam. The patient clicks the left mouse button whenever a light is seen and the right mouse button to pause the exam. The software records the spot of each flash and if the patient pressed the button when the light flashed in that spot. The responses are used to determine the presence of defects within the visual field. The patient should allow 10-20 minutes to have the whole test. The software then automatically maps and calculates the patient's visual field. Patients changing from one test program to another should establish a new baseline.
To check false-positive responses, the program includes stimulus intervals without presenting a target and records the number of times that the patient responds during these intervals. This helps identify the trigger happy patient.
Fixation losses are monitored by intermittently projecting a stimulus in the location of the blind spot and determining whether or not the patient responds. A patient who sees the blind spot stimulus may be making eye movements and not maintaining fixation. If excessive fixation losses still occur, the examiner should observe the eye directly to determine whether the patient is fixating steadily or looking around. Several multicenter clinical trials and research investigations have shown that most patients can perform reliable visual field tests when carefully tested and appropriately monitored.
Short-term fluctuation (SF) indicates the amount of variability exhibited by the patient during the test procedure. It is determined by performing a second threshold determination at 8 pre-selected visual field locations and calculating the average variation of the repeated measures for these points.
May be the procedure of choice for certain conditions / patients who cannot maintain fixation in a bowl perimeter, patients with severe arthritis or handicap or patients who cannot place chin in chin-rest of bowl perimeters. Automated tangent screen perimetry is much like automated bowl perimetry except that a tangent screen is used instead of a bowl. It is a cost effective method. Despite its name, automated static visual field test is not “automatic”; the examiner cannot assume that the computer will take care of everything. Automated visual field tests are time-consuming, demanding, and often boring. Patients can become misaligned, fall asleep, or daydream, and may require frequent rest breaks. Others feel uncomfortable and anxious if left alone in a darkened room. To avoid these problems, the examiner must carefully monitor alignment and attention level and maintain rapport with the patient throughout the visual field test. Automated perimetry - computerized - methods provide the standardization and reproducibility required for the detailed visual field analysis desired for diagnosis and treatment of diseases such as glaucoma.
Bowl / Tangent screen trigonometric relations
(Video projectors, CRTs and plasma screens give better results than LCDs because they produce deeper black then LCDs)
Setup for visual field test using a monitor
Video projector (short throw) tangent screen visual field test
Small Size Projection Screen 120 cm
(click to enlarge)
Tangent screen visual field test or Goldmann field exam - A simple tangent screen is a relatively easy test to run and is much more sensitive than confrontation fields. Here the patient is asked to sit approximately 3 feet from a screen with a target on the center. The patient should be wearing their Rx. If they are presbyopic they should have a +1.00 D lens placed in a trial frame over their distant correction, assuming the tangent screen is at one meter. Patients should never wear glasses with multifocal lenses when being tested. The eye that isn't tested is covered during the exam. While the patient stares at the target the examiner will move an object toward the patient's visual field. The patient signals the examiner when the object comes into view. This exam allows the patient's visual field to be mapped. 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. The equipment needed is simple. 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 tangent screen is also useful when measuring the effect that a ptosis has on the superior visual field. Cost effective for special case fields or for quick screening. Tangent Screen was the standard visual field test for many years Declined use with advent of Goldmann and Automated perimeters.
Confrontation visual field test - The examiner will ask the patient to cover one eye and stare at the examiner. The examiner will then move her hand out of the patient's visual field and then bring it back in. The patient signals the examiner when her hand comes back into view. This is frequently done by an examiner as a simple and preliminary test. 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.
Kinetic vs. Static methods:
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.
Errors in Visual Field Testing
Errors common to all methods:
inadequate patient instruction
inadequate patient supervision inattentive or uncooperative patient (leading to fixation losses and unreliable responses)
fellow eye not patched or partially patched
improper stimulus selection (size, brightness)
improper fixation target (e.g., large target for low vision)
incorrect lens position,
obstructing lens rim, a droopy eyelid or prominent brow obstructs field 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. These factors can create visual field depression, which resembles a partial nerve fiber bundle-type defect. Using surgical tape can help keep the eyelid from obstructing the superior visual field, yet permits the patient to blink and remain comfortable during testing.
pupil too small (less than 2mm) and improper lens corrections can produce artifactual test results that sometimes mimic pathological sensitivity changes. Small pupils (less than 2 mm in diameter) can greatly restrict the amount of light reaching the retina. This can cause the adaptation state to fall below a photopic level, where Weber's Law no longer pertains, and can significantly alter the apparent visual field sensitivity.
Informed and accurate interpretation of visual fields tests depends on careful consideration of all the information in the visual field test printout. No portion should be attended to the exclusion of others. Determining whether a patient’s visual field has improved, deteriorated, or remained stable over time remains the greatest challenge in visual field test interpretation and a fundamental consideration in the management of glaucoma. Several clinical trials have found that distinguishing true glaucomatous change from random variability requires multiple confirming visual field tests. This was shown by the Ocular Hypertension Treatment Study (OHTS), where 88% of initial glaucomatous visual field deficits were not validated on a repeat visual field test examination. Because sequences of any two visual fields tests within the same patient can indicate worsening, recovery, or stability, long-term trends are best appreciated by examining the patient’s full chronological set of visual fields, rather than simply comparing the two most recent fields tests to each other.
Other errors common to kinetic perimetry:
moving the stimulus too fast
failure to test from non-seeing to seeing
Other errors common to automated :
perimetry poor maintenance (e.g. dim bulb)
motor/projector noise: This was a problem with the older automated projection perimeters. The projector would make noise as it moved just before the stimulus presentation and the patient could anticipate the presentation by the noise.
Other errors common to the tangent screen:
inconsistent illumination of the background and/or target
improper subject to screen distance poor positioning of the examiner
Other errors common to Goldmann perimetry:
background and stimulus illumination not properly calibrated