Next-generation devices offer new approaches to refractions

March 25, 2013

 

Author Info

Kenneth Daniels, OD, FAAO, Diplomate ABO, practices in Hopewell, N.J. Dr. Daniels is adjunct associate professor at Pennsylvania College of Optometry at Salus University and International Studies External Educator for New England College of Optometry. He also is on advisory panels for Science Based Health, Sauflon, Hydrogel, QSpex, CooperVision, and SynergEyes, and has received and performed research projects for Vistakon, Alcon, Allergan, CooperVision, and Hydrogel. Dr. Daniels has no financial interest in Vmax.

 

Take-Home Message

A patient can achieve 20/20 vision in a variety of ways, but optimizing visual quality is the only way to ensure complaint-free 20/20 vision. New-generation refractors ensure that visual optimization can be delivered promptly and effectively.

Callout 1

Optimizing visual quality is the only way to ensure complaint-free 20/20 vision.

Callout 2

Aberrometers provide a very good estimate, but only a patient can voice exactly what he or she sees.

 

By Kenneth Daniels, OD, FAAO

Vision is a twofold experience composed of both acuity and quality. Understandably, acuity remains the primary focus; however, if quality is not optimized, visual complaints may persist even in the presence of 20/20 vision. Eye specialists have developed several approaches to identifying and minimizing the higher-order aberrations that compromise visual quality. But these strategies, which involve the use of wavefront aberrometers and quality-enhancing spectacle lens coatings, still require the initial use of a phoropter to determine acuity. This can produce a drawn-out refraction experience for the patient, aggravated by the lengthy trial and error process many patients undergo to determine the best combination of lens coatings required to achieve aberration-free vision.

Quality, acuity in one sitting

In recent years, a group of next -generation devices has entered the eyecare market and offered a new approach to refractions. Such devices include:

  • i.Profilerplus system (Carl Zeiss Vision) a combined autorefractor, corneal topographer, and wavefront aberrometer

  • iTrace (Tracey Technologies Corp.) ray tracing wavefront aberrometer and corneal topographer

  • Point Spread Function (PSF) Refractor (Vmax Vision Inc.). The PSF Refractor is notable among new-generation devices because it improves the accuracy and speed of a typical refraction by allowing visual quality and acuity to be assessed by one machine in a single sitting.

When the PSF device is used in conjunction with the Encepsion lens (Vmax Vision), the precise measurements captured during refraction can be transferred to a spectacle lens that provides a high level of visual quality and clarity.

Much like the traditional phoropter, the PSF Refractor permits subjective determination of refractive error. But, by using the principles of point spread function, this device calculates refractive error with 5 times greater accuracy than a phoropter.1 This means patients can be refracted down to 0.05 D rather than the standard 0.25 D.

Results from two studies have shown that refractions performed with the PSF Refractor produce more accurate visual outcomes than a phoropter. The first study involved 50 patients who underwent bilateral refraction with the PSF Refractor, followed by phoropter-based refraction. When vision tests, including visual acuity tests (at 100%, 25%, and 12% contrasts) and contrast sensitivity testing (using sine gratings at 6 cycles per degree [CPD], and 12 CPD [charts by Vector Vision]) were performed, with and without spectacle correction, 85% of the group achieved a higher final visual acuity with the PSF Refractor than with the phoropter. These findings were echoed in a similarly executed second study, which consisted of 1800 eyes of 900 patients from seven U.S. clinical sites. In this study, the PSF Refractor produced a visual acuity equal to or better than the phoropter in 90% of patients.1

The different levels of accuracy achieved with the PSF Refractor relate to each device’s choice of refractive target. During phoropter-based refraction, patients are asked to view Snellen chart letters and interpret blur, but this scenario isn’t true to everyday life. PSF-based refractions overcome this limitation by asking patients to interact with a real-life target and provide feedback about the true amount of visual distortion seen. This refractive strategy ensures that the resulting refractive prescription given is one that has been optimized to eliminate all visual aberrations and distortions experienced in everyday life.

Subjective vs. objective refraction

I’ve been using the PSF Refractor for nearly 2 years now. My colleagues and I welcomed the device into our practice because we were already using both wavefront aberrometers and phoropters to assess refraction. Using one technology to measure both quality and acuity was the next logical step. Not only does using one instrument to carry out both parts of the exam save time and increase patient convenience, but study results have also shown that subjective refractions are more accurate than objectively determined refractions, such as those obtained with wavefront aberrometers.2 This may be because objective measurements account only for the eye’s perception of vision but fail to account for the brain’s influence on image interpretation. In other words, aberrometers provide a very good estimate, but only a patient can voice exactly what he or she sees. By using subjective refraction, the PSF Refractor ensures that the patient is the ultimate decider of the best refractive correction.

Wavefront aberrometers are a great benefit when used with a phoropter in a refractive practice. However, room for improvement remains, and this is where the new technologies excel. The i.Profilerplus system, which uses the properties of an autorefractor, corneal topographer, and wavefront aberrometer, is reported to refract with an accuracy of 0.01 D and also determines visual quality, much like the PSF Refractor. However, because it uses an objective method of refraction, it remains susceptible to the limitations encountered with objective refraction.2,3 In my practice, I have used the iTrace system, which offers refractive refining similar to the i.Profilerplus system. I have found that the iTrace system provides refractive outcomes close to that of the PSF Refractor without that device’s speed and patient convenience.

Patient perception

If a patient walks into the exam room and can read the 20/20 line on the Snellen chart without hesitation, there isn’t a quality concern. However, the patient who comes in and says, “I think it’s a T,” or “I’m not sure if this is a V or an E,” is the one with an obvious quality concern. That patient is most likely appreciating higher-order aberrations and that’s the patient the new generation refractive devices were made for.

Prior to using the PSF Refractor, I used aberrometers as well as phoropters to determine my patients’ visual level. Making the change to the PSF Refractor has allowed me to provide an enhanced refractive examination with subjective outcomes that are more consistent with a patient’s visual needs. The patient can now react and say, “That’s a much better image quality,” when he feels that’s the case. More direct involvement in the refractive process has increased patient satisfaction. When the 900 patients involved in the aforementioned PSF Refractor study were asked to compare the refractive experience with a phoropter vs. the PSF Refractor, 90% said they preferred the PSF Refractor.1

Turning problem identification into real results

Identifying how vision can be improved is useful only when an effective solution exists. Standard spectacles attempt to optimize vision quality and acuity via a variety of methods, such as aspheric lenses and anti-reflective coatings. Standard lens prescriptions offer fairly poor vision quality-patients must rely on specialty lenses and coatings to enhance their quality of vision in a stepwise fashion. The Encepsion lens provides a customized solution that automatically corrects all quality and acuity errors identified during the refraction process. The unique aspect of the Vmax system (PSF Refractor and Encepsion lens) is that a standard lens has distortions to the periphery, which mean that curvature effects increase toward the lens periphery. This, in turn, enhances ghosting, aberrations, and image warping.

With the Vmax system, the refraction achieved is so specific that a prescription of –4.00 spherical, –2.00 cylindrical, can be refined to a level as specific as –4.22 spherical, –1.98 cylindrical. And it is recalculated point by point across the entire lens surface so that the power in every single portion of that lens is corrected right to the edge. In addition, the highest level of anti-reflective coating is used as standard, which ensures that glare from exogenous light in the area is instantly eliminated.

Other lenses currently out on the market aim to optimize quality, such as Physio 360 (Essilor) and HoyaLux TrueForm (Hoya). However, unlike the Encepsion lens, these lenses do not come with a complementary refraction technology. In the absence of a tailor-made technology, it can be difficult to ensure that any refractive measurement captured is specifically matched with the lens prescription produced. Even lenses that come with a refraction technology, such as i.Scription, are based on objectively determined refractions, which can be less accurate than subjectively determined refraction.4,5

The nighttime vision dilemma

The responses of patients on follow-up visits have been the greatest testament to the difference that PSF-based refraction and Encepsion lenses can make to everyday vision. The best analogy I’ve heard is that the change is like moving from standard television to high-definition -everything looks crisper and clearer. Patients particularly remark on the manner in which the level of clarity provided by the lens stays the same, even when exogenous lighting is poor. The area in which the PSF Refractor makes the biggest difference to everyday vision is nighttime vision.

One of the most common complaints I receive from refractive patients is that they can’t see well at night. This is due to the increase in pupil size and the defocus of light from the peripheral portions of the pupil that occur in low-light situations. A standard phoropter can’t assess this nighttime-dependent change, which means that standard prescriptions don’t correct for nighttime vision problems. The PSF Refractor has a subjective nighttime vision function, which ensures that a nighttime prescription can be calculated during the refraction process. It does so by allowing the background on which the PSF target is viewed to be darkened to simulate night vision. Once a nighttime prescription has been calculated, the patient can then decide if she wants two pairs of spectacles, one for day and one for night, or if one pair of spectacles with a refractive power balanced between day and nighttime is preferable. Patients have told me that calculating the variance between night and day makes all the difference to their vision. Because the calculation takes only an additional 1-2 minutes, it doesn’t affect practice workflow.

End result

A patient can achieve 20/20 vision in a variety of ways, but optimizing visual quality is the only way to ensure complaint-free 20/20 vision. New-generation refractors ensure that this visual optimization can be delivered in a quick and effective manner. Additionally, they provide a solution to perhaps the most frustrating complaint of all-the understanding but inactive doctor. Most patients have experienced the frustration that accompanies telling their doctor about a visual complaint but receiving little action other than a sympathetic nod. These new-generation refractors and spectacle lenses finally offer the solution that patients have been waiting for: a doctor who not only hears a patient’s problem but also addresses it.ODT

 

References

Lai, ST. A Multicenter Study Of Visual Acuity Measurements And Patient Satisfaction With a Novel Subjective Point Spread Function Refractor a Standard Manifest Refraction. Paper 3600/D1113 presented at the Association for Research in Vision and Ophthalmology (ARVO) May 2012.

Cheng X, Bradley A, Thibos LN. Predicting subjective judgment of best focus with objective image quality metrics. J Vis. 2004 Apr 23;4(4):310-21.

Carl Zeiss company information. Available at: http://www.zeiss.com/4125680f0053a38d/Contents-Frame/8e87c5b2d433d7a6c1256c3a002e811a. Accessed Feb. 14, 2013.

Salchow, D, Zirm, M, Stieldorf, C, Parisi, A. Comparison of objective and subjective refraction before and after laser in situ keratomileusis. J Cataract Refract Surg 1999. Jun 25(6): 827-835.

Carl Zeiss company information. Available at: http://vision.zeiss.com/eye-care-professionals/en_us/products-and-technologies/lenses.html.