Lessons from our Pupils: A Reflection [Podcast Episode 146]
For the final episode in 2018 (LINK), Jay was joined for Journal Club by Drs. Daniel Chao and Ajay Kuriyan to discuss 3 recent publications: first the PIVOT trial comparing primary vitrectomy and pneumatic retinopexy, then the FLUID study looking at the effect of residual SRF in wet AMD, and finally a study out of Stanford University looking at timing of macula-off retinal detachment repair.
All three articles measured visual acuity as an outcome but did so utilizing different methods. The PIVOT and FLUID trials used ETDRS logMAR chart, and the macula-off retinal detachment repair study used a Snellen chart. Visual acuity, the ability to resolve spatial objects, is a common chosen endpoint of many clinical studies. In this post we will discuss how the methods used to measure it were developed and subsequently improved upon.
The Snellen chart is an iconic image nearly ubiquitous in the doctor’s office. It was created by Dutch ophthalmologist Dr. Herman Snellen in 1862. After his colleague, Dr. Franciscus Donders, had started to diagnose vision problems by asking patients to gaze at a card in a distant wall, he asked Dr. Snellen to help him develop a tool for this purpose. The original chart had shapes of various sizes including squares, circles, and plus signs. This proved to be challenging to use since the patients had to describe the symbols they saw. To simplify the process, letters eventually replaced symbols, giving way to the Snellen chart we use today. It is composed of eleven lines of capital letters that decrease in size as you progress down the rows. The patient is asked to stand 20 feet away from the chart, cover one eye, and read from it without using glasses. The top number seen on the ratio next to the letters represents the distance from the chart, and the bottom number is the distance at which a person with “normal” eyesight can read that same line. A person with normal visual acuity should be able to correctly read line 8 at a 20 feet distance from the image.
The logMAR chart (Logarithm of the Minimum Angle of Resolution) is another chart utilized to test visual acuity in patients and was developed in 1976 at the National Vision Research Institute of Australia. It was designed to be more accurate than the Snellen chart, and therefore, it is commonly used in research. The design allows for a logarithmic or proportional change in the letter size and spacing as it is read. It also improved on some of the previous concerns regarding the Snellen chart. The logMAR chart, unlike the Snellen, has the same number of letters in each line, each with the same degree of difficulty. To test visual acuity using the logMAR method, the patient is asked to stand 4 meters away (13.14 feet) from the chart. Each letter has a score of 0.02 log units, for a total change of 0.1 log units per line. A person with “normal” eyesight should receive a logMAR score of zero, while those with poor vision will receive a positive number.