What Is an ETDRS Chart and Why Is It Important?

The Early Treatment Diabetic Retinopathy Study (ETDRS) chart is a specialized tool for measuring visual acuity with high accuracy and standardization. Eye care professionals use it for precise vision assessments, particularly when tracking changes over time in clinical and research settings. The chart’s design provides a reproducible method for quantifying vision, making it a benchmark in ophthalmology.

Origins and Key Design Features

The chart originated from a major clinical trial in the 1980s, the Early Treatment Diabetic Retinopathy Study. The study required a more consistent method for measuring vision to assess treatments for diabetic retinopathy and track patient outcomes across multiple clinics. The ETDRS chart was developed to meet this need for precision and uniformity in vision testing.

A primary design feature is its basis on a logarithmic scale for letter size, the Logarithm of the Minimum Angle of Resolution (LogMAR). This design means the letters on each line progress in a uniform, geometric size ratio. This allows for more precise measurement of visual changes, as each letter read correctly can be given an equal score.

The chart also uses 10 specific letters known as Sloan optotypes (C, D, H, K, N, O, R, S, V, Z), selected for their similar legibility. Each row on the chart contains five of these letters. The spacing is also standardized: the space between letters on a line equals the letter width, and the space between rows equals the height of the letters on the line below.

ETDRS Chart Versus Snellen Chart

The ETDRS chart differs from the more common Snellen chart. A Snellen chart has a variable number of letters on each line, with fewer large letters at the top and more small letters at the bottom. In contrast, the ETDRS chart has a consistent five letters on every line, which helps avoid issues of crowding on the smaller lines.

The progression of letter sizes also differs. The ETDRS chart’s logarithmic progression ensures a uniform change in difficulty from one line to the next, while the Snellen chart’s progression can be more irregular. This standardization makes the ETDRS chart more sensitive for detecting subtle shifts in vision.

Another distinction is the scoring methodology. ETDRS testing allows for a letter-by-letter scoring system, where a patient receives credit for each correctly identified letter. Snellen testing is scored on a line-by-line basis, where a person must read a majority of the letters on a line to get credit. This difference in scoring contributes to the ETDRS chart’s reproducible and precise results.

The ETDRS Testing Procedure

A visual acuity test using an ETDRS chart follows a standardized procedure to ensure consistent results. The test is conducted at a specific distance, commonly 4 meters (about 13 feet), to maintain uniformity. If a non-standard distance is used, the results must be calibrated to account for the difference.

Proper illumination is another part of the procedure. ETDRS charts are designed for use with a special light box, or illuminator cabinet, that provides uniform retro-illumination. This controlled lighting ensures repeatable results across different testing environments.

During the test, patients read the letters on the chart one by one, starting from the top. They are encouraged to guess if unsure of a letter, as this provides a more complete picture of their visual capabilities. The score is calculated from the total number of letters read correctly and can be converted to a LogMAR value or a Snellen equivalent.

Clinical Importance of ETDRS Charts

The ETDRS chart is the gold standard for measuring visual outcomes in clinical trials for new treatments and therapies. Regulatory bodies like the U.S. Food and Drug Administration require its use for this purpose. This is especially true in studies related to retinal diseases such as diabetic retinopathy and age-related macular degeneration (AMD).

For ophthalmologists managing patients with chronic eye conditions, the ETDRS chart is a primary tool for monitoring disease progression. Its ability to detect small changes in vision helps clinicians determine if a treatment is effective or if a condition is worsening. A consistent decrease of four or more letters read correctly between visits can indicate a complication.

The chart is also used in low vision assessment. For individuals with significant visual impairment, the ETDRS chart’s design allows for a more accurate quantification of their remaining vision than many other tests. This precision aids specialists in prescribing low vision aids and developing rehabilitation strategies.