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Up Front | Feb 2009

Aberropia Defined

Higher-order aberrations negatively affect visual performance.

Prior to the advent of wavefront-guided LASIK, the only correctable parameters for refractive correction were sphere and cylinder. These did not provide an ideal solution; in some instances, BCVA actually decreased postoperatively, leaving patients unable to clearly read at 20/20 because of significant amounts of higher-order aberrations (HOAs). The ideal refractive system should correct optical aberrations so that the eye's spatial resolving ability is limited only by the neural retina and neural transfer function.

There may be a large group of patients whose BCVA may improve significantly after optical aberrations are altered. Therein exists an unidentified refractive entity, which we call aberropia. In this article, we will discuss the definition of aberropia and its effect on the optical system.

Until recently, refractive disorders were treated with standard techniques that accounted only for subjective refraction. Wavefront-based techniques also use subjective refraction, but they also include ocular optical aberrations and corneal topography—with the latter used not only for diagnosis but also for therapeutic treatment—to design a personalized treatment based on the entire eye. For example, the Zyoptix (Bausch & Lomb, Rochester, New York) system measures ocular aberrations with a Hartmann–Shack-type aberrometer.1-3

Zernike polynomials help describe ocular aberrations.4-6 The association between HOAs and visual acuity is not yet fully understood. One study found that as aberrations increased, visual performance decreased; however, this study included patients with gross abnormalities, such as keratoconus, penetrating keratoplasty, and corneal trauma.7 In studies of eyes over a lower aberration range, there was less of an inverse correlation.8,9 Recently, Applegate et al10 found that for low levels of aberration, the wavefront error (root mean square) was an unlikely predictor of visual acuity. Levy et al11 concluded that the amount of HOAs in eyes with natural supernormal vision (ie, UCVA better than or equal to 20/15) is not negligible but rather comparable with HOAs in myopic eyes.

The more effective we are at correcting HOAs, the better we become at improving visual acuity for the majority of our patients who are in the lower aberration range. Therefore, we must consider the interactions between aberrations and what affect these have on the optical system. Combinations of Zernike polynomials may improve or worsen visual performance.12 Thus, some beneficial aberrations overcome the detrimental effects of other aberrations, helping to reduce the point spread function from a large blur to a smaller spot of light.

The interaction between aberrations is independent from the increase or decrease in total wavefront error. The best visual performance would result from zero HOA extending over the entire scotopic pupil. Only in the less-than-perfect situation is it advantageous for positively interacting wavefront aberrations to cancel wavefront aberrations that deteriorate visual quality.

A retrospective study including patients with BCVA of 6/12 or worse was conducted to study the possibility of a new refractive entity, aberropia. Sixteen eyes (10 patients) were included; all showed an improvement in visual acuity by at least two Snellen lines after refractive correction of their wavefront aberration and had no other known cause for decreased preoperative BCVA (ie, abnormal topography, amblyopia, or already known amblyopiogenic factors). Preoperatively, no patient's BCVA improved with the aid of contact lenses and no patient's refractive error was large.

Root mean square pre- and postlaser showed a reduction in HOAs. A total of 6.25% of patients achieved 6/9, and 31.25% of patients achieved 6/6 or better. Additionally, 37.50% achieved a BCVA of 6/5, and 25% achieved a BCVA of 6/4 (Figures 1 and 2).

In this case series, the improvement in BCVA was unexplainable. We propose that the visual acuity improvement was due to either removing the aberrations or activating aberrations that interacted positively to improve visual performance. We concluded that aberropia refers to any significant improvement in visual acuity or quality of vision that is brought about by altering or removing the wavefront error.

Decrease in BCVA or visual quality that is not caused by media opacities, such as epithelial ingrowth, may also occur after LASIK. Additionally, spherocylindrical combinations may leave the error uncorrected, most likely due to decentered aberrations, central islands, or other conditions that operate in the higher ranges of aberrations. Post-LASIK patients with a total wavefront error in the lower range of aberrations also may not improve with spherocylinder combinations, possibly due to net detrimental aberrations. Theoretically, it may be possible to treat these patients either by inducing aberrations that interact positively with the detrimental aberrations, or, more ideally, by bringing the total wavefront error to zero (ie, aberration-free optics).

Correcting any significant wavefront error in patients with an unexplained decrease in visual performance may provide an improvement in visual acuity or quality of vision.

In a healthy eye with a pupil size larger than 3 mm and clear media, the wavefront error is the main contributor to image degradation. Thus, correcting any significant wavefront error in patients with unexplained, decreased visual performance may provide an improvement in visual acuity or quality of vision.

Identifying this subgroup of patients, possibly with preoperative use of customized wavefront glasses or adaptive optics, may result in proper use of customized wavefront-guided LASIK. If the treatment is applied over at least the scotopic pupil size, it should neutralize optical aberrations and provide improved visual performance. Clinical testing methods are still needed.

We define aberropia as a refractive error resulting in decreased visual quality attributable to HOAs.13-20 Two scenarios fit this description: Either the helpful aberrations are inadequate to overcome the detrimental aberrations, or only detrimental aberrations are present in the eye. Aberropia is due to net detrimental HOAs and interaction among the aberrations. We have also proposed two systems of classification for aberropia, based on the etiology and magnitude of aberropia (Tables 1 and 2).

By changing the interactions among aberrations, we create a net effect of clear visual performance—even though the total wavefront error is not zero. Correction may also be achieved by decreasing the total wavefront error to zero. Both situations would yield improved visual performance. Further research and improvements in diagnostic modalities and refinements in present wavefront-guided LASIK systems are required to complete this kind of wavefront correction.

Not all individual HOAs may be detrimental to visual function. In fact, sometimes the interactions among aberrations positively affect visual performance. We propose that a new refractive entity, aberropia, describes the net negative effect of HOAs on the deterioration of a patient's visual performance. We hypothesize that selected cases of unexplained poor vision are due to aberropia and that these are treatable cases in which BCVA and visual quality can be improved.

Thus, similar to conventional hyperopia, myopia, and astigmatism—all correctable with spherocylinders—it is only logical that this HOA-induced loss of vision is also a refractive entity. Aberropia may be corrected by removing or altering these aberrations.

Athiya Agarwal, MD, DO, practices with the Eye Research Center and Dr. Agarwal's Group of Eye Hospitals, Chennai, India. Dr. Agarwal states that she has no financial interest in the products or companies mentioned. She may be reached at tel: + 91 44 2811 6233; fax: + 91 44 2811 5871; e-mail: dragarwal@vsnl.com.