Visual Acuity Measurement in Treatment Studies

To understand treatment studies, it is necessary to be conversant with multiple forms of VA measurement and multiple forms of macular thickness measurement. Visual acuities are frequently expressed as Snellen ratios (e.g., 20/32), as Snellen decimals (e.g., 0.625), as logMAR decimals (e.g., 0.2), and as number of letters read from the ETDRS chart (e.g., 61 letters). Several useful tips can make the interpretation of these publications easier. First, an ETDRS letter is equivalent to a logMAR increment of 0.02. Therefore, one line on the ETDRS chart equals a logMAR increment of 0.1, or Þve ETDRS letters. A conversion formula allowing a Snellen VA to be expressed in logMAR format is

logMAR visual acuity = −log10 (Snellen ratio or Snellen decimal).

Conversely, a conversion formula allowing a logMAR visual acuity to be expressed in Snellen decimal format is

Snellen decimal visual acuity = 1.0−logMAR visual acuity.

A conversion formula allowing a Snellen visual acuity to be expressed in ETDRS letters is

ETDRS letters = 85 + 50 × log (Snellen decimal visual acuity).86

Here is a conversion table for these different formats (Table 13.2).

Table 13.2 Table for converting visual acuity to various formats

Red font denotes visual acuity levels that are present on the Snellen chart but not on the ETDRS chart. The ETDRS letter score is shifted in some cases. For example, the ETDRS letter score for 20/20 is listed as 100 in some references and as 86 in others. In all cases, however, each letter on a line is equivalent to an increment of 0.02 logMAR units such that one line on the ETDRS chart = Þve ETDRS letters

It is important to distinguish between what happens at the level of an individual patient and what happens at the level of a group of patients in a study. In a study comparing two groups of