DIURNAL VISUAL CHANGES IN RADIAL KERATOTOMY - IMPLICATIONS FOR VISUALSTANDARDS

Background. Diurnal variations in vision have been reported in radial keratotomy (RK) patients. Applicants to certain professions may meet a n uncorrected vision standard at the time of testing but fail to meet that standard at another time. Methods. Ten firefighter applicants who had undergone RK and nine normal subjects attended for two morning vi sits, and two afternoon visits. At each visit subjective refraction, u ncorrected visual acuity, best-corrected visual acuity, automated refr action, and keratometry were measured for each eye. Monocular contrast sensitivity was measured using the Pelli-Robson chart with and withou t a glare source (brightness acuity tester, BAT) and for both natural and dilated pupils. Results. The RK group showed a significant myopic shift from morning to afternoon (mean = -0.41 +/- 0.33 D; t = 3.92, p = 0.004), which was well correlated (r = -0.86) with significant steep ening of the corneal curvature (mean = +0.41 +/- 0.36 D; t = -3.65, p = 0.005). The controls showed no change in either refractive error (me an change = +0.06 +/- 0.42 D) or corneal curvature (mean change = +0.0 5 +/- 0.08 D). No significant difference in corrected visual acuity or contrast sensitivity was found between the RK and control groups for natural pupils. For dilated pupils, the RK patients showed significant ly poorer log contrast sensitivity both with and without glare (RK mea n = 1.49 +/- 0.11; control mean = 1.67 +/- 0.11; t = 3.21, p = 0.005). All RK subjects met the firefighter visual acuity standard on the ini tial visit, three subjects did not meet the standard at an afternoon e xamination. Discussion. We propose that, if RK applicants are to be co nsidered for employment, existing visual standards be amended to inclu de visual acuity testing in both the early morning and late afternoon. Clinicians and agencies should also be aware that contrast sensitivit y may be reduced at low light levels.