Variations in charts

Worldwide, Snellen charts are the most popular charts for vision measurement, with many variations using a range of optotypes. For testing vision in children, the Landolt C chart, the Tumbling E chart, Lea Symbols, Lea grating, or other tests may be employed^(2,5,6). Based on the judgment of trained professionals, these charts may also be used for vision testing in adults with impaired mental abilities or low literacy^(2,7,8). Jaeger charts are used to test near VA⁽⁹⁾. The choice of chart varies depending on patient requirement, practitioner preference, testing method, resource availability, and staff expertise.

Variations in notations

Visual acuity in Snellen charts is often represented as a fraction in which the numerator is distance from the chart and the denominator is size of the smallest line that can be read. Feet (ft.) and meters (m) are the most commonly used distance units for notation of VA. However, in clinical settings, a variety of notations are used throughout the world. For instance, the decimal system is used in Algeria, Bulgaria, Belgium, Denmark, Japan, and several other countries^(4,8). Decimals as x/10 fractions are used in several parts of Europe^(3,10) and some parts of Africa and Latin America. Five-score is used in China (personal communication). Vision standards recommended by the Federal Aviation Administration include Military Standard MIL-STD-401E and National Aerospace Standards (NAS) developed by the Aerospace Industries Association (AIA). AIA-NAS-410 uses Jaeger units for vision notation. The International organization for Standardization (ISO) accepts Jaeger and N points of VA to qualify nondestructive inspection and testing (NDI/NDT) personnel using ISO-9712⁽⁹⁾. When the Jaeger chart is used for vision testing, readings may vary depending on testing distance. A J1 measurement based on the revised Jaeger standard testing distance of 14 inches would be equivalent to 20/20 under the Snellen acuity standard. If the NDI/NDT vision standard of a 17.5-inch testing distance is followed, J1 would be equivalent to 20/15 and J2 would be 20/20⁽⁹⁾. Grating tests may be used for patients with severely constricted visual fields or scotomas with notation units of cycles per degree⁽²⁾.

ETDRS for research

In Snellen charts, every line has variable letter sizes and a variable number of letters are present in each line. Good vision lines have up to eight letters and some poor vision lines may have only two letters. They contain an irregular progression of letter sizes between lines, and non-uniform spacing between letters and rows. In clinical settings, variable testing distance, illumination, test chart design, font types, optotype size, optotype spacing, patient variables, and methods of scoring all contribute small margins of error to test-retest variabilities in vision testing^(2,5,11). Building up on research data and theories from as early as 1868, Bailey and Lovie first designed the logMAR chart in 1976. They addressed several known drawbacks of the Snellen chart. Their new design incorporated five optotypes in each row and a uniform logarithmic letter size progression increa­sing in 0.1 logMAR steps where logMAR is an acronym for log₁₀ of minimum angle of resolution (MAR). Spacing between letters and rows was equalized to letter height in the smaller of the two rows⁽⁵⁾. In 1982, the US National Eye Institute developed the Early Treatment Diabetic Retinopathy Study (ETDRS) chart and a protocol for vision testing. British standard letters (rectangular) in the Bailey Lovie chart were replaced with Sloan Letters (square). A testing distance of 4 m was used instead of the earlier 6 m. Readings for these charts could be noted in logMAR, Visual acuity score, Letter score, MAR, and others, adding further to the range of notations.

The use of the ETDRS method, which had a superior chart design and a defined method for testing vision resulted in considerable improvement of VA scores, especially in the low vision range with two-line improvement in patients with ≤20/200 vision, approximately five lines in those with 20/400 vision, and also in patients with exudative age-related macular degeneration, when compared to the Snellen chart⁽¹²⁾. The accuracy of results provided by the ETDRS method helped its establishment as a Gold standard in vision testing for research purposes. E-ETDRS or Electronic ETDRS testing is also now employed in clinical trials and has been proven to reduce test-retest time and technician bias^(5,13). Despite these advances, some researchers still express concern that there might be variation in scores with use of standard ETDRS or E-ETDRS charts in patients with visual disorders such as macular degeneration or amblyopia⁽¹³⁾. It should be noted that the standards of illuminance used in ETDRS protocols, measured in candela per square meter (cd/m²) units, are also variable across countries. The ETDRS protocol approved by the US FDA defines a standard illumination of 85 cd/m², use of a wide chart on a lightbox, and mandates a testing distance of 4 m. The accepted standard in Germany is 300 cd/m² and 120 cd/m² has been adopted in the United Kingdom^(11,14). Nevertheless, even with these caveats, the appeal of the ETDRS method for vision testing in research and academic settings is undisputed.

logMAR for routine use

Though the ETDRS method seems ideal for vision testing in re­search settings, it does not automatically hold appeal among practicing ophthalmologists. Latin (Roman) alphabets on the ETDRS chart were modified to Sloan letters for use in Europe⁽¹⁴⁾. Thus, region-specific optoty­pes in vision charts were desirable for workers in the field. Many found the ETDRS protocol hard to adopt because of the need for special equipment, complex scoring, staff re-training, and the extended time needed to complete the evaluation. Some practitioners were of the opinion that the standard testing distan­ce of 4 m is cumbersome when examining patients with low vision. However, there was general acceptance of the advantages of the ETDRS protocol and several practitioners explored methods to make it more cli­nic-friendly. Reduced logMAR E (RLME)⁽¹⁵⁾, Compact reduced logMAR⁽¹⁶⁾ Simplified logMAR⁽¹⁷⁾, Reduced logMAR¹⁸ and other charts were tested and found acceptable. These charts are all re­cogni­zed as logMAR charts and differ in the use of optotypes, number of optotypes in a row, or number of lines in a chart. The use of logMAR charts with standard methods is expected to cause some test-retest variability based on the use of optotypes, number of lines, illumination, and other factors, but is a considerable improvement over using the Snellen chart. This method can reduce testing time and is much less cumbersome to use than the standard ETDRS research protocol. Though using the compact logMAR chart takes a slightly longer time than using the Snellen chart (5-40 sec vs 3-35 sec), the time taken was almost half that taken for using the ETDRS chart (9-80 sec)⁽¹⁶⁾. The difference in sensitivity and specificity of simplified logMAR compared to the standard is marginal (95 % vs 98%). The cost of a simplified logMAR chart printed on A4 size paper and pasted on a cardboard is less than $1, compared to the standard logMAR chart printed on acrylic material costing approximately $121. Training school teachers in the use of simplified logMAR charts was accomplished in half a day and the only logistics required for screening were a pen, a measuring tape, and the chart itself⁽¹⁷⁾. Thus, from a practical standpoint, using logMAR charts with standard methods would be an excellent compromise.