Technology changes, sure– but we’re not sure if there’s been an ophthalmic device that has stood the test of time better than the FDT 710. The unit was first produced in the late 1990s and is still made to this day, practically the same in every aspect by ol’ Zeiss. There’s good reason why it’s been around so long and it’s no secret why it can be found in nearly every pretest room on Earth. The 710 is probably the best way to perform visual field screenings if your horizons don’t include full threshold testing. The unit is quite compact, portable, and inexpensive as far as perimeters go. Patients find it incredibly easy-to-use and your technicians won’t need a full day’s training to learn its concepts. The printout is incredibly modified and simple to read and best of all there’s no need for trial lenses (exceeding 6 diopters of spherical equivalent correction will require a trail frame or the patient’s glasses). The 710 with Frequency Doubling Technology is like an old friend that consistently delivers accurate results and shows up on time everyday.
OUR TAKE: The best and most efficient way to perform visual field screeners. It’s just too bad it’s hardware and design hasn’t changed in over 15 years of manufacturing.
#1. 40 Seconds / Eye
The FDT utilizes a vertical sine wave grating of low spatial frequency (0.25 c/deg) that undergoes counterphase flickering at a high temporal frequency (25 Hz). The stimulus is presented for a maximum of 720 msec, in which contrast is gradually ramped up to the contrast selected, remains there for a period of time, and then gradually decreases to zero. There is a variable interval of time between presentations of up to 500 msec to avoid rhythmic patient response. If the patient response button is pressed from 100 msec to 1 sec after the presentation, the stimulus is recorded as “seen” at that contrast level for that location. If not, the stimulus is recorded as a “non seen” response.
This is the long, scientific way of saying the FDT takes only 40 seconds per eye, and is the most clinically validated visual field screener for ophthalmic referral.
Bring your own trial lens (or don’t) because the FDT doesn’t require patients wear corrective lenses (6 diopters or less it’s not required). The targets are imaged at optical infinity, thus patients should wear their distance corrected lenses while performing the test (bifocals and progressive are acceptable). The large low spatial frequency targets are not significantly affected by refractive errors up to 6D, so optimal near correction is not needed. Unlike the HFA series and other similar perimeters, no trial frame is built-in or deemed necessary.
#3. A Simplified Printout
All screening test results are depicted as a Deviation Plot. The 17 (C-20) or 19 (N-30) VF contrast sensitivity location results compared to the normative database are shown on a gray scale plot based on probability levels. Screening tests preformed with the –1 tests use probability symbols that range from P≥1% to not seen, while the –5 screening test index ranges from P≥5% to P<1%
Full threshold test results are graphically represented, similar to the SAP Humphrey Field Analyzer (HFA), with up to three different plots. The Threshold Plot shows the threshold contrast sensitivity (in HFA equivalent dB) at each location. The Total Deviation Probability Plot is found right under the first, showing whether the sensitivity for each VF location falls within normal limits (compared to the age-matched normative database) or whether the sensitivity falls below a probability of 5% to 0.5% of the normal population. This plot is indicative of diffuse or generalized loss in sensitivity.
The Pattern Probability Plot (accessible for ViewFinder software printouts) depicts localized sensitivity loss once the diffuse sensitivity component is removed. The overall height of the average field of vision is moved up or down to match the average normal visual field sensitivity and then differences from this adjusted height are plotted.
Yes, the patient response clicker is standard.
It depends on the test, however most tests last around 40 seconds per eye.
It depends, however we have successfully billed CPT 92081 for N30 / C-20 tests with standard reimbursement rates.
In general, FDT technology is relatively patient friendly– more so than most full threshold perimeters.