‘Aberropia’ identifies new refractive error based on higher-order aberrations

The decrease in visual quality may be corrected through wavefront-guided refractive surgical treatments.

Issue: August 15, 2007

ByAmar Agarwal, MS, FRCS, FRCOphth

BySoosan Jacob, MS, FRCS, DNB

ByAthiya Agarwal, MS, FRCS, FRCOphth

Before the advent of wavefront-guided LASIK, the only parameters that could be modified to obtain optical correction for a given patient’s refractive error were sphere, cylinder and axis.

Amar Agarwal

Often, this would not give the ideal optical correction, resulting either in the common refrain by the post-refractive surgery patient that he could read the 20/20 line but not clearly or in a decrease in best corrected visual acuity. This situation usually occurs because of the persistence of significant amounts of higher-order aberrations. The ideal optical system should be able to correct the optical aberrations in such a way that the spatial resolving ability of the eye is limited only by the limits imposed by the neural retina and by the neural transfer function.

Therefore, there may be a large group of patients whose BCVA may actually improve significantly on altering their optical aberrations. These optical aberrations are contributed to by the eye’s entire optical system, ie, the cornea, the lens, the vitreous and the retina. There exists a hitherto unidentified refractive entity that we are calling “aberropia.” We first published this term in 2002.

Wavefront

Until recently, refractive disorders were treated with standard techniques, which took into consideration only the subjective refraction. Wavefront techniques, however, take into account the patient’s subjective refraction, ocular optical aberrations and corneal topography, with the latter not only for diagnosis, but also for therapeutic treatment, in order to design a personalized treatment based on the total structure of the eye. The wavefront technology in Bausch & Lomb’s Zyoptix uses the Shack-Hartmann aberrometer based on the Shack-Hartmann principle demonstrated by Liang and colleagues to measure ocular aberrations.

Zernike polynomials are useful to describe ocular aberrations. The role of higher-order aberrations in defining visual acuity has not yet been fully understood. There have been studies that state as aberrations increase, the visual performance decreases, but this study included patients with gross abnormalities such as keratoconus, penetrating keratoplasty, corneal trauma, etc., and this inverse correlation between aberrations and acuity was found to a much lesser extent in eyes over a lower aberration range. Recently, Applegate and associates found that for low levels of aberration, the root mean square wavefront error is not a good predictor of visual acuity. Levy and colleagues also stated that the amount of ocular higher-order aberrations in eyes with natural supernormal vision (uncorrected visual acuity greater than or equal to 20/15) is not negligible and is comparable to the reported amount of higher-order aberrations in myopic eyes.

Figure 1. Comparison of RMS values preop and postop aberropia

Source: Agarwal A

It is important to know the effectiveness of correcting higher-order aberrations in improving the visual acuity over this lower aberration range that compose the majority of normal and refractive surgery patients. For this, we need to consider the effect of aberrations when they interact with each other. Combinations of Zernike polynomials have been known to improve or worsen visual performance. Thus, some beneficial aberrations overcome the detrimental effects of other aberrations and help in reducing the point spread function from a large blur to a smaller spot of light.

This interaction, for better or worse, occurs independently of the increase or decrease in the total wavefront error, ie, they may interact for the better, leading to better visual performance, despite an overall increase in the wavefront error. Logically, in the perfect scenario, a zero higher-order aberration would provide the best visual performance. If this zero-order aberration would extend over the entire scotopic pupil size, then variations in pupillary size would also not have any effect on changing the wavefront error. It is only in the less than perfect situation that it is an advantage to have positively interacting wavefront aberrations to cancel the wavefront aberrations that deteriorate visual quality.

Figure 2: Pre- and postoperative aberrometer maps of a patient
Figure 2. Pre- and postoperative aberrometer maps of a patient. The reduced higher-order aberrations can be seen postoperatively in the images above.

Images: Agarwal A.

Study

A retrospective study was conducted on patients with BCVA of 6/12 or worse who improved by two Snellen lines or more after refractive correction of their wavefront aberration and with no other known cause for decreased preoperative BCVA including abnormal topography, amblyopia or hitherto known amblyopiogenic factors. Sixteen eyes of 10 patients satisfied the inclusion criteria. Root mean square pre- and post-laser correction showed a reduction in the higher-order aberrations: 6.25% of patients achieved 6/9; 31.25% patients achieved 6/6 (1); 37.5% achieved a BCVA of 6/5; and 25% achieved a BCVA of 6/4 (Figures 1 and 2).

Analysis

In our retrospective study, the patients had an improvement in their BCVA that could not be explained by any other reason. The improved visual acuity in these patients must have been due to either removal of their aberrations or by obtaining a set of aberrations that interacted positively to improve visual performance. The question of a magnification factor improving visual acuity does not arise, as these patients preoperatively did not improve with contact lenses. Further, the refractive error in many of these patients was not large. Our series included only patients with an improvement in BCVA after laser surgery. Aberropia need not be limited to this but would refer to any significant improvement in the visual acuity or quality that is brought about by altering or removing the wavefront error.

Considering the reverse of our series of cases, there are also post-LASIK patients who have a decrease in their BCVA or visual quality that is not correctable by spherocylindrical combinations and cannot be explained because of media opacities, such as epithelial ingrowth. These could be due to decentered aberrations or central islands, which are conditions that operate in the higher ranges of aberrations. There is also a group of post-LASIK patients whose total wavefront error is not too large but still do not improve with spherocylinders. Despite being in the lower aberration range, these may have net detrimental aberrations. It might, at least theoretically, be possible to treat them either by inducing aberrations that interact positively with these detrimental aberrations or, more ideally, by bringing the total wavefront error to zero, ie, making it an aberration-free optics.

In healthy eyes with pupils larger than 3 mm and clear media, the wavefront error is the main contributor to image degradation. The aberrations present in the eye can be anywhere in the optical media and can be corrected at the corneal plane. Thus, correction of any significant wavefront error in patients with unexplained decreased visual performance might give them the benefit of improved visual acuity or quality. Identification of this subgroup of patients, possibly with preoperative testing with customized wavefront glasses, adaptive optics or some other such modality might allow one to perform customized wavefront-guided LASIK, and if the treatment is applied over at least the scotopic pupil size, it should neutralize their optical aberrations and allow them improved visual performance. It would be necessary to develop ways of testing these patients in the clinical set-up.

Classification of aberropia

Keeping all of these in mind, we venture forward to propose a new refractive error – aberropia. We define this as a refractive error that results in a decrease in the visual quality that can be attributable to higher-order aberrations. This is either because of helpful aberrations being inadequate in overcoming the detrimental aberrations or a situation where there are only detrimental aberrations present in the eye. Thus, we have coined this term for a refractive error that is due to a net detrimental higher-order aberration, post-interaction between different types of aberrations. Correction of this refractive error could be changing the different aberrations so that they interact differently, thus having a net effect of clear visual performance, despite the fact that the total wavefront error is not zero. Correction could also be achieved by bringing down the total wavefront error to zero. Both these situations would yield improved visual performance. Further research is required to make this kind of wavefront correction a reality, and this would be possible only by further improvements in diagnostic modalities as well as refinements in the present wavefront-guided LASIK systems available. We have also proposed two systems of classification for this newly defined refractive error based on the etiology and also on the magnitude of aberropia. These are shown in Tables 1 and 2.

Conclusion

We know that not all wavefront aberrations may be bad for the eye and that these aberrations may sometimes interact positively or negatively to increase or decrease visual performance. We propose a new refractive entity — aberropia — where there is a net negative effect of the higher-order aberrations so that there is deterioration in the visual performance of the patients. We hypothesize that selected cases of unexplained poor vision are due to aberropia, and that these are treatable and these patients obtain an improvement in the BCVA/visual quality.

Thus, similar to the conventional hyperopia, myopia and astigmatism, which are corrected with spherocylinders, it is only logical that this higher-order aberration induced loss of vision would be a new refractive entity – aberropia, which could be correctable by removing or altering these aberrations. It does cause one to think what happens when a treated person presents with age-related changes in his wavefront aberrations, either due to loss of accommodation or due to lens changes or any other cause. This would be similar to the present problem of calculating IOL power for post-LASIK patients. One may have to treat the patient further for his wavefronts or use customized wavefront glasses, contact lenses or even IOLs. There may also develop, in the future, some newer modality of optical correction utilizing adaptive optics as done in astrophysics, currently. This field definitely deserves to be researched a great deal more, and it will be interesting to see what the future holds.

Table 2: Classification of aberropia based on magnitude

For More Information:

Amar Agarwal, MS, FRCS, FRCOphth is director of Dr. Agarwal’s Group of Eye Hospitals. Dr. Agarwal is author of several books published by SLACK, Incorporated, publisher of Ocular Surgery News, including Phaco Nightmares: Conquering Cataract Catastrophes, Bimanual Phaco: Mastering the Phakonit/MICS Technique, Dry Eye: A Practical Guide to Ocular Surface Disorders and Stem Cell Surgery, and Presbyopia: A Surgical Textbook. He can be reached at 19 Cathedral Road, Chennai 600 086, India; fax: 91-44-28115871; e-mail: dragarwal@vsnl.com; Web site: www.dragarwal.com.

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