Point spread function could replace phoropters

Take-Home:

Early reports are positive for point spread function technology, which is the latest in a long line of attempts to replace the phoropter for refraction.

Dr. Gordon

By Fred Gebhart; Reviewed by Alison Gordon, MD

La Jolla, CA-The jury is still out on the next generation of devices to replace phoropters for refraction, but early experience with a point spread function device shows promising results, according to Alison Gordon, MD.

“This new device offers the potential for more accurate refractions, faster refractions, a very short learning curve, and strongly positive reception by patients,” said Dr. Gordon, a refractive surgeon, Gordon-Weiss-Schanzlin Vision Institute in La Jolla, CA. “Phoropters are clearly the standard of care, but point spread function technology has the potential to replace them.”

A comparison study, of which Dr. Gordon is lead author, utilized measurements obtained using one device (PSF Refractor, Vmax Vision) and compared the measurements obtained using standard phoropters.

About the study

The prospective, multicenter study enrolled 900 patients across five clinical sites.

The phoropter was introduced in 1921 and has changed very little over the intervening decades, Dr. Gordon said. It also still measures only defocus (myopia and hyperopia) and astigmatism.

Results are highly subjective and depend on both operator and patient cooperation. Detection limits are +/- 0.25 D, but human visual resolution is far finer than 0.25 D. And the bracketing design is far from ideal.

There have been multiple attempts to replace phoropters-starting with auto refraction and topographers in the 1980s, a second round of auto refractors in the 1990s, wavefront aberrometers in the 2000s, and followed by combination topography + wavefront.

All are objective technologies and answer some of the longstanding objections to phoropters, but reliability is only 50% to 80%.

There is also the problem that Snellen acuity has more to do with recognition and identification of the appropriate target than true visual acuity, Dr. Gordon said. Point spread function provides the additional sensitivity needed.

About the technology

Point spread function shows how a point of light focuses to the retina and provides insight into how patients visualize their aberrations. Instead of the Snellen letter chart, the technology uses a small dark circle with spoke-like projections to assess resolution and a moving dot to assess astigmatism.

The VMax PSF Refractor is not an objective measuring device. Patients have to choose between two different images, but it is far more sensitive than phoropters, with resolution of 0.05 D myopia or hyperopia and 0.03 D cylinder power correction.

“Point spread function takes less time and is less confusing to the patient,” Dr. Gordon said. “It is easier to detect the difference between the two choices you are shown, so you get more accurate results.

“You don’t have patients making responses solely based on recognition,” she said.

The device is finding some use in optometrist offices, she said.

It was the practice optometrist who initially suggested the increased accuracy, speed, ease of use, and patient satisfaction might make the unit a good fit with presurgical refraction.

(Figure 1) A point spread function-based refraction system has been found to be a useful tool that provides better or equal accuracy and reliability in refractive endpoints, according to Alison Gordon, MD.

More evidence

An analysis of 900 patients across five clinical sites found that the point spread function refractor provided equal (63%) or better (28%) visual acuity results compared with a standard phoropter manifest refraction (p < 0.001).

The point spread function device provided equal (33%) or better (67%) visual quality results compared with a standard phoropter.

Patient satisfaction scores strongly favored the point spread function device. Just three percent of patients preferred the phoropter, seven percent said the two procedures were equal and 90% preferred the point spread function device (p < 0.001).

The point spread function unit offers multiple advantages over a phoropter, Dr. Gordon said.

Snellen letter charts almost invite patients to game the system and react more to recognition than to acuity, she said. This tendency can lead to overminusing and an inaccurate refraction.

With a point spread function refraction, overminusing produces a blurry target, which patients are quick to notice.

Changes in specific point targets are easier for patients to detect than changes in Snellen letters, which leads to faster results.

The average refraction time for the point spread function device was 90 seconds.

Point spread function also works well in complicated eyes, such as those with age-related macular degeneration or keratoconus.

Point spread function technology is easy to adopt as well, Dr. Gordon said.

Moving from a phoropter to the new device takes just 2 hours of training and 10 patients to get a good feel for the system and use it effectively. The increased speed of refraction can improve patient flow and increase the efficiency of the practice.

The device-and the targets used-are easier for patients to understand than a phoropter and Snellen charts, there is no more gaming of the system or asking, “which is better, one or two?” Dr. Gordon said.

“I didn’t see any real down side to the (device) for my surgical patients,” she said. “It takes less time, it is less confusing for patients, and it provides accurate results that give us a good baseline for planning surgery.

“It has worked well for us,” she said.

Alison Gordon, MD

E: alisonngordon@gmail.com

Dr. Gordon has no financial interest in the VMax PSF Refractor.

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