Beyond snellen acuity: the assessment of visual function after refractive surgery.

T HE GOAL of refractive surgery is to improve unaided vision in ametropic patients without the aid of spectacles and contact lenses. Refraction for the prescription of these appliances has traditionally been based on high-contrast distance visual acuity using Snellen charts. It is under such conditions that residual spherocylindrical refractive error and visual acuity following refractive surgery are usually assessed, and from these measures, the patient’s visual function is inferred. It is now generally accepted that high-contrast distance visual acuity and residual refractive error are indeed correlated with overall patient visual function and satisfaction following surgery; however, there are many refractive surgery patients with minimal residual spherocylindrical error and excellent uncorrected high-contrast distance visual acuity who are dissatisfied with their postoperative quality of vision. Refractive surgeons will be familiar with a variety of problems expressed by such patients, ranging from general, unspecific complaints, to specific phenomena, such as halo formation that arises consistently under mesopic conditions. Given that the goal of refractive surgery is to improve unaided vision, understanding the nature of this discordance between simple high-contrast distance visual acuity and visual function is of seminal importance. A major problem, however, is that there are no established objective measures by which the necessarily subjective visual experiences of patients can be characterized and quantified. It is likely that a wide range of distinct symptoms are similarly described using undefined terms such as “glare,” “blur,” or “haze,” but most studies of patient function and satisfaction following refractive surgery make little effort to elicit more specific descriptions that could be used to categorize outcome groups and thereby identify risk factors. In an attempt to identify more sensitive measures of visual function following refractive surgery, psychophysical tests such as low-contrast acuity, lowluminance acuity, disability glare, contrast sensitivity, postoperative corneal opacification or “haze,” corneal topographic irregularities, and intraocular light scatter have been studied. Until recently, perhaps the greatest attention has been paid to measures of contrast sensitivity and low-contrast visual acuity. Numerous reports have documented a decrease in some measures of contrast sensitivity in the first few months following excimer laser surface photorefractive keratectomy (PRK) and laserassisted in situ keratomileusis (LASIK). However, the persistence of reduction in the measure of contrast sensitivity employed by a given study group has been variable, with some reporting an increase in function over time to near-preoperative levels, and others reporting a sustained reduction for up to a year following surgery. Although some studies have found lowcontrast visual acuity to be highly correlated with highcontrast visual acuity, and therefore to provide little additional information, others have suggested that lowcontrast visual acuity might be a particularly sensitive measure of visual function following refractive surgery. In such cases, a decrease in low-contrast visual acuity immediately following surgery has been demonstrated that persists to some degree for as long as 12 months following surgery in some groups, but returns to nearnormal levels in others. Unfortunately, poor performance on these tests of contrast sensitivity has not been shown to correlate with patients’ subjective visual function. In fact, few studies have even attempted to establish a relationship between performance on psychometric testing, the subjective experience, and functional satisfaction of patients with postoperative changes in contrast sensitivity measures or low-contrast visual acuity. In a meticulous study of active-duty US military personnel who underwent excimer laser PRK, Schallhorn et al performed a battery of functional and diagnostic tests, including glare disability, near-contrast acuity with glare, and intraocular light scatter. They also administered a questionnaire designed to elicit patient complaints of overall dissatisfaction with quality of vision, glare or halos around lights, or difficulty with night driving. These investigators found no correlation between the objective measures studied and the subjective responses to quality of vision. Since decentration of the treatment zone has been associated with disturbing visual symptoms and decreased lowcontrast visual acuity, by extension, it has been suggested that untoward visual symptoms might be marked by reduced low-contrast visual acuity, but this has not been proven convincingly. The causes for a decrease in contrast sensitivity function following excimer laser treatment are not clearly understood and probably represent a complex interplay of factors. Forward light scatter is to some extent related to the backscatter seen as haze in corneas following PRK and is expected to degrade the retinal image and affect contrast sensitivity function. Although the degree of corneal haze has been correlated with a reduction in contrast sensitivity, the course of clinically apparent haze does not match that of changes in contrast sensitivity. Contrast sensitivity function is noted to decrease immediEDITORIAL