Peristat is a device for automated static perimetry. It emulates the conventional visual field testing environment and allows patients to detect scotomas in their visual field through self-testing. A total of 100 points are tested over an area of at least 24 degrees of central visual field. The system has three threshold levels of contrast sensitivity to ensure high-fidelity screening for conditions such as glaucoma, cataracts, strokes, retinal detachments and other. The output of the system is represented graphically as a field covered with color-coded squares. The location of each square corresponds to the location of a probe in user’s visual field, while the shade of it reflects the user’s response to stimulus at that location. Response to the lowest level of stimulus is represented by a white box. Response to higher levels of contrast are represented by shades of gray. If the patient fails to register the brightest stimulus, the box is colored black.
Peristat is as simple to analyze as any perimetry test in the office. Even if you have no experience with perimetry in the past, the read-out of the visual field is easily mastered. The global indexes of reliability are the fixation losses (patient looks around/no focus on target), false positives (patient responds to stimuli when none are presented) and false negatives (patient fails to respond to a previously registered stimulus due to lack of alertness and concentration). In general, all of the following parameters need to be within range for a test to be meaningful
- Fixation Losses
- False Positives
- False Negatives
The Peristat system automatically computes these values and rejects all tests where any of the three error metrics is above 30%.
It is important to realize that anyone can miss a few points on perimetry testing and inter-test variability is common to all perimetry tests. Nonspecific defects and scattered omissions are easy to recognize and differentiate from significant pathology. Glaucoma is the result of progressive loss of the ganglion nerve fiber layer and most visual field defects resulting from glaucoma correspond with the gradual loss of this important nerve layer. The four main types of glaucomatous visual field defects are:
- Nasal steps
- Paracentral scotoma
- Arcuate or Bjerrum defects
- General visual field depression
Please see the examples below for a graphical depiction of these visual field defects.
When the visual field defects do not have the typical appearance of the above patterns, the following guideline may help to determine the significance of the visual field defects.
To qualify for a significant defect, the following should be true:
- Two or more neighboring black squares (they missed two consecutive stimuli at the highest level of brightness)
- Three or more neighboring gray shaded squares (they missed three stimuli at the lower brightness thresholds)
- When the defect is next to the blind spot, then you need three or more neighboring black squares and four or more neighboring gray squares.
Please consult the examples below for demonstration of different clinical scenarios.