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I. L. Bailey, J. Jackson, H. Minto, R. B. Greer; Visual Acuity Measurement Beyond the Limits of the Letter Chart. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4724.
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With more severe levels of visual impairment, it becomes impractical or inappropriate to measure visual acuity with a letter chart. We propose that the limit of letter chart acuity be LogMAR = 1.6, when the top (40M) row of an ETDRS chart, or similar, cannot be read correctly at 1 meter. After that, single optotypes, such as the Single Tumbling E (STE) may be used to cover a range up LogMAR = 2.0 (100M at 1 m), or up to LogMAR = 2.6 (100M at 25cm). When STE acuity is poorer than this, Basic Form Vision (BFV) may be tested using gratings, or discrimination of standard black-and-white large-area target fields presented at 25 cm. We have a simple set of test cards and procedures for testing form vision beyond the limits of the letter chart.
The Berkeley Rudimentary Vision Test has 3 hinged card-pairs that are 25cm square. The STE card-pair has E targets at sizes of 100M, 63M, 40M and 25M. The Grating Acuity card-pair has gratings with stripe-widths 60, 38, 24 and 15 mm (LogMAR 2.9-2.3 at 25 cm) The Black-White Discrimination (BWD) and the White Field Projection (WFP) card-pair present target areas that are 25 or 12.5 cm wide (53, 26 deg at 25cm) The 100M and 25M STE panels are used to decide which test should be used. If the 25M E can be seen at 1m or at 4 m, then Letter Chart acuity should be measured at that distance. If the 100M E can be seen at 1 m, STE acuity is determined at 1m; otherwise all further testing is at 25cm. If the 100M E is seen at 25 cm, the STE acuity is measured. Otherwise, the Grating Acuity, WFP and BWD tests are conducted at 25cm. A PowerPoint version of the test is also available.
Testing takes about 3 minutes. Data from 30 subjects with substantially reduced vision illustrate that common clinical practices, using Count Fingers and Hand Motion tests to classify impaired vision, lack clinically-needed precision, and often under-estimate the patient’s visual resolution capabilities.
The BRVT provides a quick and simple procedure for quantifying visual acuity poorer than the Letter Chart limit of LogMAR = 1.6 (20/800 equivalent), It measures STE acuity up to LogMAR = 2.6 (20/8000), Grating Acuity up to LogMAR = 2.9 (20/16000), and includes White Field Projection and Black-White Discrimination tasks with fields of 3.2 and 3.5 LogMinArc. The BRVT has application for testing eyes with very poor vision, adding precision for quantifying change and categorizing visual capabilities.
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