Iris registration positively affects visual outcomes and patient satisfaction with LASIK

Amid high rates of satisfaction with LASIK, iris registration may limit two of the most common reasons for dissatisfaction with surgery.

Kerry D. Solomon

A recent study demonstrates that iris registration, or IR, has a positive effect on visual outcomes. To the degree that IR can limit the induction of higher-order aberration and ensure treatment accuracy, it may also limit some of the most common reasons for patient dissatisfaction after LASIK.

We recently conducted a retrospective study, in which 221 eyes of 147 subjects had CustomVue (Abbott Medical Optics) laser vision correction. Seventy-three eyes were treated without IR; 148 were treated with IR.

Study results

The uncorrected vision was very good for both groups, with no statistically significant difference. However, the accuracy of the treatment was better in the IR group (Figure 1).

The IR group had about 10% to 15% greater reduction in cylinder postoperatively compared with the group that had CustomVue without IR. Iris registration ensures that the torsional axes are properly aligned for astigmatic corrections. However, the degree of cylinder in this study was relatively low, so it would be hard to say whether the difference in cylinder correction between the two groups was clinically meaningful.

The biggest difference we saw between the two groups was in higher-order aberration outcomes (Figure 2). Neither treatment induces much higher-order aberration, but there is much less variance from the mean in the IR group, with a standard deviation of 0.3 compared to 0.7 in the CustomVue without IR group. Total higher-order aberration in the IR treatment group is less than half that of the non-IR group — and we see the difference most notably with the asymmetrical aberrations, coma and secondary astigmatism.

Figure 2. Higher order aberrations comparing custom with IR vs. custom alone. Images: Solomon KD

This is most likely due to normal shifts in the pupil centroid between the time of measurement, when the pupil is dilated, to treatment, when it is more constricted. Iris registration compensates for these shifts to ensure that the treatment is applied to the same pupil centroid that was measured initially and used to guide the treatment (Figure 3).

In a spherical treatment when the pupil center does not shift, IR may not make any difference in the treatment at all. But the key point to understand is that pupil centroid shifts are absolutely unpredictable. I believe they account for much of the (albeit limited) variability we have seen in CustomVue results.

Figure 3. Diagram demonstrating pupil centroid shift. Note: The wavefront is centered on the dilated pupil where it should be rather than the constricted pupil which is where most surgeons center.

In the IR arm of this retrospective analysis, the mean cyclotorsion was 3.5º ± 2.4º, with a range of 0º to 9.5º. Three-quarters rotated counterclockwise and the remainder, clockwise. The pupils tended to shift nasally more than temporally, and superiorly more than inferiorly. So, for example, the mean nasal pupil shift was 259 ± 124 µm, with a range of 10 to 500 µm; and the mean superior pupil shift was 212 ± 125 µm, with a range of 10 to 500 µm. At the upper end of those ranges of cyclotorsion or pupil shift, we would expect to see a definite impact on the accuracy of the correction and the visual outcomes.

There is no way for even very experienced surgeons to manually compensate for pupil centroid shift without IR. Some patients have great outcomes with a custom or optimized treatment without IR, but the problem is that we do not know which patients will experience problems until it is too late, so we are better off to register every treatment with IR, if possible.

Gauging patient satisfaction

Both a retrospective world literature review and a smaller, prospective study we conducted have shown that the majority of patients, about 95%, are satisfied with their visual outcomes after LASIK. That represents the highest satisfaction rate of any elective procedure. The prospective treatments were performed before the availability of iris registration, although all patients did have CustomVue wavefront-guided surgery.

Common reasons for dissatisfaction after refractive surgery include residual refractive error, dry eyes and night vision problems. Of these, only dry eye is unlikely to be affected by the accurate registration of the treatment.

In our study, nine of 103 patients were originally dissatisfied. Of these, five underwent enhancement and were satisfied with their final result. Of the remaining four patients, two were undercorrected but chose not to have enhancements, one had dry eyes and one had night vision problems.

There was a small age variation, with older patients more likely to be dissatisfied. We would expect younger patients with full accommodative ability to be more tolerant of residual refractive error relative to those in the presbyopic age group.

There were no differences by gender or postoperative uncorrected visual acuity, although the satisfied group subjectively rated their postoperative vision as better than their preoperative corrected vision. The dissatisfied group rated their postoperative vision as similar to (no better and no worse than) their preoperative corrected vision. This tells us that refractive error is only a small part of the story when it comes to satisfaction with refractive surgery.

Predicting dissatisfaction

There was nothing that could have predicted trouble ahead for any of our dissatisfied patients. None of them were using antidepressants or any other systemic medications that would have been red flags for postoperative problems.

The patient with dry eye was a young woman with a history of dry eye but normal Schirmer’s scores before and after surgery. Almost 12% of the satisfied patients also had preoperative dry eye. Twenty-three percent of patients had worse Schirmer’s scores after surgery and 13.7% said that dry eye symptoms were made worse by LASIK, but they were nevertheless quite satisfied with their LASIK surgery.

We certainly do not want to belittle post-LASIK dry eye, but the reality is that just having dry eye worsened by LASIK is not in any way predictive of dissatisfaction with surgery. Moreover, in our sample, 54.8% felt that their dry eye was actually better after surgery.

We had a 48-year-old male patient whose postoperative night vision problems contributed to his dissatisfaction. He had no symptoms of night vision problems before surgery. His pupils were 6 mm in dim light. Given that nearly all (86%) our patients had pupils of 6.0 mm or greater, pupil size was not predictive of night vision problems. About one quarter of patients said that nighttime glare was worse after surgery, yet most of these patients were completely satisfied. Subjects were actually twice as likely to see a reduction in glare than an increase, with 49.3% reporting less glare after surgery.

We do not know for sure whether pupil centroid shift played a role in the case of the man with night vision problems. However, had iris registration been used it might have been able to limit the induction of higher-order aberration, perhaps improving his quality of vision at night.

The two undercorrected patients who elected not to have enhancements both had best corrected visual acuity of 20/20, with about –1.00 D refractions. Again, we cannot say for sure whether iris registration would have improved the outcomes in these cases, but IR does increase the accuracy of treatments and thus might have eliminated the residual error and need for enhancement.

Fortunately, there is a very high level of patient satisfaction with LASIK outcomes. We found nothing that could have predicted dissatisfaction in the few patients who were not happy with their results. Dissatisfied patients can become satisfied if the underlying reason for their dissatisfaction is successfully addressed.

Of the most common reasons patients cite for dissatisfaction, both residual error and night vision symptoms, may be prevented or avoided with the use of custom corrections and iris registration.

Reference:

• Solomon KD, Frenández de Castro LE, Sandoval HP, et al. LASIK world literature review: Quality of life and patient satisfaction. Ophthalmology. 2009;116(4):691-701.

• Kerry D. Solomon, MD, can be reached at Carolina Eyecare Physicians, 1280 Johnnie Dodds Blvd., Ste. 100, Mt. Pleasant, SC 29464; 843-881-3937, 888-849-3937; fax: 843-884-8587; e-mail: kerry.solomon@carolinaeyecare.com. Dr. Solomon is a paid consultant for Abbott Medical Optics.