Aberropia: the discovery of a new refractive entity

July 1, 2007

Prior to the advent of wavefront-guided LASIK, the only parameters that could be modified in order to obtain optical correction for a given patient's refractive error were the sphere, cylinder and axis. This approach, however, would often not yield the ideal optical correction and, in many cases, the post-refractive surgery patient may be able to read the 20/20 line but not clearly. In some cases, best corrected visual acuity (BCVA) would even decrease following surgery. This situation can usually be attributed to the persistence of significant amounts of higher order aberrations (HOAs).

Prior to the advent of wavefront-guided LASIK, the only parameters that could be modified in order to obtain optical correction for a given patient's refractive error were the sphere, cylinder and axis. This approach, however, would often not yield the ideal optical correction and, in many cases, the post-refractive surgery patient may be able to read the 20/20 line but not clearly. In some cases, best corrected visual acuity (BCVA) would even decrease following surgery. This situation can usually be attributed to the persistence of significant amounts of higher order aberrations (HOAs).

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.

There may therefore be a large group of patients whose BCVA may actually improve significantly by altering optical aberrations that are contributed to by the eye's entire optical system i.e. the cornea, the lens, the vitreous and the retina. Therefore, there exists an unidentified refractive entity, which we first published in 2002, and we label "aberropia."

Until recently, refractive disorders were treated with standard techniques, which only took into consideration the subjective refraction. Wavefront techniques, on the other hand, take into account the patient's subjective refraction, ocular optical aberrations and corneal topography, with the latter not only used for diagnosis, but also for therapeutic treatment, so that a personalized treatment based on the total structure of the eye can be designed. The wavefront technology in Zyoptix (Bausch & Lomb), for example, uses the Hartmann-Shack aberrometer based on the Hartmann-Shack principle,1 demonstrated by Liang et al.,2,3 to measure the ocular aberrations.

Zernike polynomials are useful for describing ocular aberrations.4,5,6 The role of HOAs in defining visual acuity, however, has not yet been fully understood. There have been studies which state that, as aberrations increase, visual performance decreases, but these studies included patients with gross abnormalities such as keratoconus, penetrating keratoplasty and corneal trauma,7 and this inverse correlation between aberrations and acuity was found to a much lesser extent in eyes over a lower aberration range.8,9

Recently, Applegate and associates10 discovered that, for low levels of aberration, the root-mean-square (RMS) wavefront error is not a good predictor of visual acuity. Elsewhere, Levy et al.11 also claimed that the amount of ocular HOAs in eyes with naturally supernormal vision [(UCVA) ≥20/15] is not negligible, and is instead comparable to the reported amount of HOAs in myopic eyes.

It is important to know the effectiveness of correcting HOAs in improving visual acuity over this lower aberration range that compose the majority of normal and refractive surgery patients. In order to do 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.12 Thus, some beneficial aberrations overcome the detrimental effects of other aberrations and help to reduce the point spread function from a large blur to a smaller spot of light.

This interaction, for the better or worse, occurs independently of the increase or decrease in the total wavefront error, i.e., 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 HOA 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 the presence of positively interacting wavefront aberrations is actually beneficial, since these aberrations neutralize the negative wavefront aberrations that deteriorate visual quality.

Selecting the right patients