Wave-front analysis: are we missing something?

Abstract

W AVE-FRONT ANALYSIS OF ABERRATIONS HAS captured the attention and imagination of scientists and surgeons interested in refractive surgery. This exciting technology offers methodology to objectively measure lowand high-order aberrations within the optical system from the tear film to the surface of the retina. A number of different instruments based on Hartman-Shack aberrometry, ray tracing, and other methodologies are now commercially available. Linkage of the analyses obtained from these systems to excimer laser custom corneal ablation in laser in situ keratomileusis (LASIK), laser subepithelial keratomileusis (LASEK), and photorefractive keratectomy (PRK) has been the inspiration for efforts to optimize visual performance or even produce “super vision.” This new technology has already contributed to our understanding of issues such as decreased quality of vision after refractive surgery and the importance of higher order aberrations in vision quality.1–3 There are, however, already signs that this technology, although certainly useful, will have limitations. For example, it is now clear that the measured aberrations are dependent on pupil size, accommodative state, and changes that occur in the lens and other structures with age.1–3 These findings raise issues about long-term stability of the correction and which measurement parameters should be used to direct surgery in a particular patient. Physicians who have begun using the data provided by aberrometers in clinical practice have also observed notso-subtle observations about the clinical utility of the information for something as precise as laser vision correction. One obvious example is the correlation between refractions provided by aberrometers and manifest refractions with subjective refinement. There are significant differences between these measured parameters in many eyes studied. In some cases, the differences are notable. For example, in the wavefront analysis in Figure 1 analyzed over a 4-mm pupil, 1.86 diopters of astigmatism were detected, while the patient would only accept 1.25 diopters during manifest refraction refined with a .25 Jackson cross cylinder. The wavefront-derived refraction provided astigmatism powers of 1.77, 1.86, and 1.87 diopters with analyses at 3-, 4-, and 5-mm pupil sizes, respectively, in this eye. Such differences pose interesting dilemmas for treatment. Should treatments be applied based on the wavefront data alone or wave-front data adjusted according to the manifest refraction with subjective refinement? These differences have been ascribed to imperfections in the relatively new instrumentation, differences in pupil dilation during aberrometry and refraction, errors in registration, poor pupil centration, measurement noise, and other technical factors. However, there is likely another more fundamental factor that also contributes to the discrepancy. Simply put, most aberrometers and ray tracers provide no information about what happens to visual input during central nervous system processing from the retina to the visual cortex, but manifest refractions with subjective refinement do. The information provided by the manifest refraction is limited to lower order aberrations, but correction of lower order aberrations is an important determinant of visual quality after refractive surgery. Actually, clinicians have had some indications of the limitations of objective optical measurements for centuries. From time to time, every clinician examines a patient in whom objective refraction with a retinoscope reveals high astigmatism that the patient will not accept during subjective refinement with a Jackson Cross cylinder. Clinicians also deal with issues related to plasticity in central processing in patients who have adverse reactions to new prescriptions for glasses. Many of these patients adapt to the prescription change over time. However, some patients never seem to adjust to even minor changes. Ophthalmologists have learned to compromise and decrease changes to parameters such as level of astigmatism or astigmatism axis to smooth the patient adjustment. Hubel and Wiesel received the Nobel Prize in 1981 for their pioneering work on the architecture, function, development, and plasticity of the visual cortex.4 Their studies, and subsequent work by these and other authors, have shown that development of the visual pathways of the central nervous system is modulated by visual input Accepted for publication Feb 19, 2003. InternetAdvance publication at ajo.com June 13, 2003. From the Department of Ophthalmology, University of Washington, Seattle, WA. Inquiries to Steven E. Wilson, MD, Department of Ophthalmology, University of Washington, Box 356485, Seatlle, WA 98195.