Faster technology makes autorefractors a fundamental part of today’s busy office

Issue: October 1999

When autorefractors first came on the scene in the early 1970s, they had difficulty achieving true refraction because of problems with accommodation, accuracy and the overall speed of the test. Today, manufacturers are addressing each of these problems, as autorefractors get faster and come closer than ever to the results obtained with subjective refraction.

Peter Leadem, of Lombart Instruments’ sales division, said that while manufacturers share the goal of the autorefractor becoming a sufficient substitute for the subjective method, the chances are fairly remote. “Every manufacturer, obviously, is working to bring autorefractors as close to the final Rx as possible,” he told Primary Care Optometry News. “There’s probably never going to be a time when an autorefractor is going to replace the actual refraction the doctor performs. Everybody is trying to make them faster so the acquisition speed is quicker or the printers are quicker. Right now, they’re all very good, so manufacturers are working on smaller benefits such as faster speed and ease of operation.”

Same principles, different technologies

Sue Corwin, CO, COMT, director of education and training at Marco Technologies, told Primary Care Optometry News that the company uses a unique system to obtain the refraction. Instead of sampling from 180°, averaging the samples and using an algorithm, she said, the Marco ARK760A employs all 180° with which to pinpoint the cylinder access and cylinder power with greater accuracy. The instrument has evolved, she said, by striving for the same goals but advancing with progressive technology. The high-speed autotracking feature, for instance, will track a roving eye until it is in the correct position to take the refraction.

“We have an infinity target for patients to look at, which is important so they don’t accommodate,” she said. “There is a little hot-air balloon at the end of a highway, and even if patients move their eyes around, the unit will track them. So if you have someone who’s chewing his or her dentures or a child is wiggling around, the unit will keep tracking him or her to get the measurement.”

The difference between the Leica KR450 and earlier models is “like night and day,” Mr. Leadem said. The SR-IV, he said, was a subjective autorefractor, something that has lost popularity with the advent of objective autorefractors. “When the SR-IV was out, there was less emphasis placed on an instrument’s ability to take a good objective autorefraction and more emphasis placed on the fact that refraction could be refined after the fact with subjective testing. That’s where that unit earned its success,” he said. “The new units are strictly objective and now involve the keratometer, also.”

Pick up the pace

Current autorefractors are much faster than those of the past, said James Jarmusch, national sales manager for Topcon America Corp., making the process simpler for children and adults alike. “You’re taking readings in less than one-tenth of a second,” he noted. “You can obtain multiple readings within a second, present averages to the operator or present all of the readings to the operator. Every single autorefractor goes so quickly these days that a short attention span isn’t really an issue anymore,” he said.

That’s not necessarily so, said David Guyton, MD, Zanvyl Krieger professor of pediatric ophthalmology at the Wilmer Eye Institute at Johns Hopkins Hospital. He said that while today’s autorefractors are much quicker than their predecessors, working with the brief attention spans of children still proves a challenge. “You’ve got to have them look at the target,” he said. “Two seconds is a long time for a child to look at a target.”

Dr. Guyton, who developed the SR-IV programmed subjective refractor for American Optical (now Leica) about 20 years ago, said he is currently developing an objective screening device that will “give an indication of what the child’s refractive error is more from a screening viewpoint than from a final refraction viewpoint.”

While current photographic screening instruments are fast, he said, the level of accuracy is still a concern. “They’re not accurate enough or reliable enough to be cost-effective just yet, but they will be,” he said. “We’re trying to come up with a way to get four or five readings of defocus each second, every second. By also sensing alignment of both eyes simultaneously, we can tell when the readings are valid.”

Preventing accommodation

Fogging techniques have improved over the years to increase accuracy, Mr. Jarmusch noted, so that patients — especially children — cannot accommodate during the exam. “In the past, the target was simply shown to the patient, but young patients and myopes would just accommodate for the minus, and you’d have readings that were a little bit off,” he said. “But since the new autorefractors are so fast and employ fogging techniques so the patient cannot accommodate for the target, you get a much more accurate reading.”

The ARK760A has an automatic fog that inserts a +1.50 lens, Ms. Corwin said. “If the patient has a very poor acuity or a very high refractive error, sometimes when you put him or her in the unit, he or she won’t see the balloon target,” she said. “We tell the patient to look straight ahead, and once we take a reading, it puts the corrective lenses in front of the patient. He or she can then see the balloon, and we take more readings — up to 10, depending on what setting you want to put it on — to get a more accurate reading.”

Using autorefractors for unusual cases

Rotary prism technology, the defining feature of Topcon’s KR-8000, addresses some of the more unique cases, such as cataract and postrefractive surgery patients, Mr. Jarmusch said. “The target itself is projected into the patient’s eye but then is rotated within the pupil,” he said. “During that rotation, you’re able to go through a 2-mm pupil size, which is prevalent in older glaucoma patients. For the postrefractive surgery patients who have irregular corneas, the autorefractor is able to go through those as well.”

Marco’s “confidence index reading” will display a number between 5 and 9 denoting what percentage of light that was shone into the eye was reflected back. While a higher number is a good reading, a number on the low end of the scale may mean a cloudy cornea, a dirty contact lens, a poster subcapsular cataract or an opaque posterior capsule after IOL implantation, some edema in the macula or something else causing the light to be blocked. For patients with IOLs, Ms. Corwin said, an infrared light bounces off of the retina and takes the measurement. “Some times, when you shine a light into an eye, that light is distorted from either an implant or some kind of pathology. We can use a slightly different method of getting that light in there and measuring the eye,” Ms. Corwin said, adding that a specific button may be pressed to take the measurement in IOL patients.

Mr. Leadem said that all autorefractors have an IOL feature, which, in essence, reduces the sensitivity of the machine. This can sometimes cause misconceptions about IOLs, he added. “In the early days, with anterior chamber IOLs, sometimes the autorefractors did not work too well through the lens,” he said. “Nowadays, it has become a non-issue, as anterior chamber lenses are rarely used, so most of the manufacturers still have an IOL switch on them. It has nothing to do with the fact that there’s an IOL in there, and actually it serves more to confuse than to help.” When a doctor or technician selects the IOL switch unnecessarily for psuedophakes, it may result in less accurate results, he said.

While autorefractors can provide a reading for these types of patients where one otherwise may not be obtained from a retinoscopy or subjective refraction, said Dr. Guyton, this is where the instrument may encounter accuracy problems. “Any irregularity in the optics can throw autorefractors off, some more than others depending on how they work and what part of the pupil they look at,” he noted. “Some use the whole pupil; others use only small areas. Those that use small areas of the pupil are prone to be thrown off by irregular refraction across the pupil, as happens after refractive surgery and often after cataract surgery.”

Autorefraction vs. subjective refraction

Mr. Jarmusch said that measuring a distance prescription with the instrument usually comes within one-fourth of a diopter to a subjective refraction. “From a refraction standpoint, the practitioner will always go through the subjective refraction and refine it from the baseline,” he stated. “With the autorefractor, when you start within one-quarter of 1 D, if that’s dialed into the phoropter for the doctor to use, the doctor’s refraction time is reduced, thereby enabling him or her to actually spend time with the patient and less time asking which lens is better.”

Although autorefraction is continually nearing the subjective method, it is something that should be reserved primarily to check the doctor’s refraction, said Tim Neitzke, OD, in private group practice in Perham, Minn. “I don’t necessarily prescribe off of it, although my prescriptions have matched it many, many times,” he said. “The K-readings on it have been very accurate and very helpful to us. It speeds up taking keratometry. I’ve come to rely on that almost more than I do on my manual for a lot of things.”

Autorefractors may come close to subjective refraction, but they lack the human element necessary to make the two equal, Dr. Guyton said. “The autorefractors evaluate the optical image falling on the retina,” he said. “The brain is able to pick out groups of rays or images that are formed through only part of the pupil and pretty much ignore the rest of the pupil. There’s no way the autorefractor knows what portion of the pupil the brain pays most attention to. It’s going to be a long time before a purely objective instrument can do that.”

Before taking the autorefraction, Ms. Corwin said, “we recommend that patients blink their eyes before they are measured, because you want to have a nice, wet cornea to get a better K reading and a better autorefraction,” she said.

The future of autorefraction

In the future, autorefractors may offer refined technology to use information provided by the brain to provide the patient’s refraction, Dr. Guyton said. “There have been attempts in the past to use the visual-evoked potential, which is the EEG signal over the back of the brain, the occipital cortex, because you can tell when an image is in focus,” he said. “That would, objectively, automate the subjective refraction. Those attempts were pilot studies only and were never really developed. Eventually, maybe 20 or 30 years from now, that probably will be doable.”

Dr. Neitzke said that, while autorefraction still has a way to go, the pressure of managed care companies to expedite the daily schedules of optometrists makes an autorefractor more important than ever before. “I wouldn’t put it on par with subjective refracting, but I would say that it has made great inroads,” he said. “Autorefracting has come a long way. It’s something that I trust more now than ever before. I’m not afraid of taking those readings and implementing them into our routine. It’s obvious; managed care is pushing us to become more efficient. This is a tool that every office needs.”

For Your Information:

Peter Leadem is in the sales division at Lombart Instruments. He may be reached at 5358 Robin Hood Rd., Norfolk, VA 23513-2407; (757) 853-8888, ext. 303; fax: (757) 855-1232.

Sue Corwin, CO, COMT, is the director of education and training at Marco Ophthalmic/Technologies. She may be contacted at 11825 Central Pkwy., Jacksonville, FL 32224; (800) 874-5274, ext. 142; fax: (904) 642-9338.

James Jarmusch is the national sales manager for Topcon America Corp. He can be reached at 37 West Century Rd., Paramus, NJ 07652; (800) 223-1130, ext. 207; fax: (201) 634-1365; e-mail: jarmusch@topcon.com.

David Guyton, MD, is the Zanvyl Krieger professor of pediatric ophthalmology and director of the Zanvyl Krieger Children's Eye Center at the Wilmer Eye Institute. He may be reached at Johns Hopkins Hospital, 600 N. Wolfe St., Baltimore, MD 21287-9028; (410) 955-8314; fax: (410) 955-0809; e-mail: dguyton@jhmi.edu. Dr. Guyton is not a paid consultant for any products mentioned in this article. He has a direct financial interest in the objective screening device currently in development (patent pending).

Tim Neitzke, OD, is in private group practice at the Minnesota Eyecare Network. He may be reached at 340 Fox St., Perham, MN 56573; (218) 346-3310; fax: (218) 346-9064. Dr. Nietzke has no direct financial interest in the products mentioned in this article, nor is he a paid consultant for any companies mentioned.

The ARK760A is available from Marco Ophthalmic/Technologies, 11825 Central Pkwy., Jacksonville, FL 32224; (800) 874-5274; Web site: www.marcooph.com.

The Leica KR450 is available from Lombart Instruments, 5358 Robin Hood Rd., Norfolk, VA 23513-2407; (757) 853-8888; e-mail: lombart@lombartusa.com.

The KR-8000 is available from Topcon America Corp., 37 West Century Rd., Paramus, NJ 07652; (800) 223-1130; Web site: www.topcon.com.