Pupil size doesn't matter, study says

June 1, 2014

In this article, Dr Schallhorn reveals the results of a large case series that provides evidence that there is no need to reject candidates for LASIK on the basis of pupil size or to increase the treatment zone size for such patients.

Despite this, the belief that mesopic pupil size is a critical risk factor for glare and halo after laser vision correction surgery persists. In fact, it remains quite widespread among refractive surgeons and referring clinicians, which has had a significant impact on clinical practice. Many patients who are otherwise good candidates for LASIK are disqualified altogether or discouraged from having the procedure by dire warnings of their increased risk. Others undergo LASIK with a larger optical zone treatment that removes more tissue than is really necessary.

Simple optical modelling suggests there should be a relationship between pupil size and outcomes. As light rays enter the eye, one presumes they would bend differently through ablated tissue versus the virgin periphery, causing visual side effects in those whose pupils expand beyond the treated optical zone. However, the human optical system is designed to support central vision. Photoreceptors in the retina are preferentially stimulated by light rays entering the central cornea and much less sensitive to peripheral light rays. This mechanism, the Stiles-Crawford effect, is one of the mitigating factors against visual symptoms in eyes with larger pupils.

Large series

We retrospectively analysed a large case series of 14686 eyes of 7448 young myopes (18–40 years old, mean age 29.7 years) who underwent bilateral primary wavefront-guided (WFG) LASIK at Optical Express centres.5 All treatments were performed using the Star S4 IR excimer laser system (Abbott Medical Optics, Santa Ana, California, USA) with a WFG ablation profile (Advanced CustomVue). In all cases, the optical zone diameter was 6.0 mm with a transition zone out to 8.0 mm. For patients with astigmatism, 6.0 mm was the size of the minor axis of the elliptical ablation.

Preoperative pupil size was measured under low-light conditions with an infrared pupillometer. Visual and refractive outcomes were evaluated one month postoperatively, when a questionnaire was also administered to assess patient reported outcomes, including satisfaction with the procedure, night driving, glare and halo.

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