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C. Kum, J. Hong, A. Doshi; Character Counting Estimates Standard Visual Acuity. Invest. Ophthalmol. Vis. Sci. 2007;48(13):5513.
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Visual acuity is routinely determined by the identification of letters on a standard Snellen chart. Alternatively, acuity may be estimated by character counting using the same chart. A previous study on subjects with induced refractive error observed a relationship between these two methods1. Our study attempts to find a similar relationship in subjects who have true ocular pathologies other than induced refractive error.
Subjects with known primary causes of decreased vision other than simple refractive errors were chosen to participate. Participants had their standard visual acuities evaluated, one eye at a time, using a Snellen acuity chart simulated with a projector and mirror to the usual distance of 20 feet. Standard acuities were determined to be the smallest line where subjects could correctly identify letters. Those who had best-corrected standard visual acuities of 20/20 or better were then excluded from the remainder of the study. A total of 88 remaining subjects, who were unable to obtain a best-corrected standard acuity of 20/20 in either eye, were allowed to continue. The eye with the better standard acuity was covered and character counting acuity was subsequently obtained with the uncovered eye, which had the poorer acuity. Character counting acuity was taken beginning at the line of best-corrected standard acuity for that eye and ending at the smallest line in which the number of unidentifiable characters was correctly reported. This smallest line was recorded as the subjects' character counting acuity. Individuals with standard acuities of at least 20/200 or better (Group 1, n=67) had their character counting acuities evaluated using the same projected Snellen chart used to obtain their standard acuities. Subjects who displayed standard acuities of worse than 20/200 (Group 2, n=21) had their character counting acuities evaluated with a LogMAR chart held at 1 meter.
The following correlations (standard acuity:counting acuity) were obtained: 20/500:20/200, 20/360:20/128, 20/200:20/100, 20/80:20/30, 20/60:20/25, 20/40:20/15, and 20:30/20:15. The character size of character counting acuity is therefore 54% of standard acuity's. (standard/counting ratio = LogMar 1.9 ± 0.6)
Character counting provides a useful alternative to determining standard visual acuity, particularly in malingering patients and low-vision patients.
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