Surgery to restore the accommodative ability of the eye in patients with either presbyopia or cataracts is gaining increasing attention. Several types of accommodative IOLs are in development, and some surgeons are experimenting with scleral expansion. However, one element impeding progression in this area is the inability to accurately and objectively measure the change in accommodation achieved by intraocular surgery. My colleagues and I have been working with the WASCA Analyzer (Carl Zeiss Meditec AG, Jena, Germany), a Shack-Hartmann wavefront analyzer that is capable of obtaining instantaneous aberrometry data in real time, and which may offer a solution to this challenge.
THE WASCA ANALYZER
Measuring accommodative range by near-card acuity testing or near-fogging techniques is fraught with subjective patient variables. By acquiring data in a free-running mode, instantaneous changes in the wavefront can be followed, recorded, and played back as a movie. This capability allows clinicians to examine wavefront changes dynamically. For custom-ablation planning in particular, we can choose the wavefront most likely to correspond to a state of no accommodation. The exam with the lowest sphere value recorded within the timeframe of acquisition confirms these data, which is important in order not to produce overcorrections in customized corneal ablations.
The WASCA Analyzer is capable of acquiring 210 wavefront measurements over a 30-second acquisition window. At 7 Hz, the temporal resolution, is high enough to allow us to monitor even those wavefront changes caused by the redistribution of the tear film over the cornea.
ABERRATIONS INDUCED BY ACCOMMODATION
Normal ocular accommodations show changes in higher-order aberrations, particularly spherical aberrations. Because of the high frame-rate of dynamic aberrometry in the WASCA Analyzer, we are able to detect that at the start of the accommodative response, spherical aberrations increase before the spherical myopic shift occurs (findings in cooperation with Ioannis G. Pallikaris, MD, of Crete, Greece, and Dan Reinstein, MD, of Cambridge, England). This phenomenon may increase the speed with which the eye can perceive near objects. The preliminary increase in spherical aberration would enhance the depth of focus of the eye, and the near field would be brought into focus in advance of the change in actual sphere (or myopization).
STuDYING PSEUDOACCOMMODATION
We used dynamic aberrometry to examine the voluntary range of pseudoaccommodation in elderly individuals. These patients underwent cataract extraction and implantation of the 1CU Akkommodative IOL (HumanOptics AG, Erlangen, Germany), which is currently in clinical trials. After successful phacoemulsification and implantation of the 1CU lens, patients were tested by video-aberrometry.
We asked the patients to observe a target set at 4 meters. On cue, they focused on a target at 23 cm for 10 seconds and then looked again at the 4-meter target. The WASCA Analyzer was able to objectively demonstrate an amplitude of accommodation, as well as the change of that accommodative effort over the 30-second period. Interestingly, in most cases, the eye achieved a fairly stable accommodative state.
CONTINUED STUDY
We are interested in looking at this phenomenon in more detail, over extended periods of accommodation and over longer follow-up times. It is interesting to note that patients implanted with the 1CU do not appear to experience the increase in spherical aberration during accommodation seen in normal, nonpresbyopic phakic eyes.
If greater spherical aberration is truly an advantage, perhaps this is one element that could be incorporated into future IOL designs. Such an optic could change its aberrational structure and hence increase the depth of focus of the eye in the accommodative state.
H. Burkhard Dick, MD, practices at the Department of Ophthalmology at the University of Mainz in Mainz, Germany. He has no direct financial interest in the products mentioned in this article, nor is he a paid consultant for any companies mentioned. Dr. Dick may be reached at +49 61 31 1751 50; dickburkhard@aol.com. (2):149-154.
