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Mark I. Crosby, Tyson Brunstetter, David Tanzer, Sandor Kaupp; Repeatability of a Low Light, Low Contrast Acuity Test. Invest. Ophthalmol. Vis. Sci. 2012;53(14):4797. doi: https://doi.org/.
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Test the repeatability and any learning effect of a commercially-available low-light, low-contrast acuity chart, namely the backlit Logarithmic SLOAN Visual Acuity Low Contrast Chart 25% filtered by a two log neutral density filter (Precision Vision™, Cat No. 918) by measuring inter- and intra-subject variation.
Fifty subjects were enrolled in this study from a pool of patients already electing to undergo corneal refractive surgery (i.e., PRK or LASIK) at NRSC San Diego. During a normal pre-op clinical visit following standard-of-care clinical testing (i.e., visual acuities, manifest refraction, measurement of ocular aberrations, and pupil size), the subject’s dominant eye was tested for low-light, low-contrast acuity in two sessions spaced approximately five minutes apart. During each session, the patient was asked to read three different 25% contrast logarithmic visual acuity charts placed four meters from the phoropter under mesopic (dark room) visual conditions (luminance = 1 cd/m2). Each patient was given five minutes to dark-adapt once seated in the testing room before each of the tests were administered. These six threshold measurements made in two sessions allowed the calculation of per subject variability and analysis by a 2X3 ANOVA for the group.
There was no significant change in variability for the group between the first and second session of three determinations (F=0.05, p=0.95). There was a significant improvement of one letter of acuity (change in logMAR=0.019, F=3.76, p=0.003) over the six determinations for the group. The median of the 50 standard deviations was 0.034 logMAR for the six determinations of mesopic 25% contrast acuity. Consequently, 2 SDs or 95% limits on the determination of acuity would be 0.078 logMAR, or ±4 letters of the 5 letter line of acuity. This would represent a 95% confidence interval of ±0.8 lines of Snellen acuity on the ETDRS style chart.
The learning effect with mesopic contrast acuity testing is small: 0.017 logMAR or 1 letter. Repeatability of the test is excellent, with a median estimated SD of just under 2 letters of acuity: SD=0.034 logMAR or 95% CI=0.078 logMAR. This is comparable to our center’s repeatability for high (100%) contrast acuity which has been previously reported as 1.5 letters (0.028 logMAR).
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