Binocular Rivalry – When One Eye Is Better than Two
By: Stanley Woo, OD, MS, FAAO
Patients with low vision due to central field loss often present with asymmetric visual acuity in the early stages. However, it does not always hold true that the better-seeing eye is the dominant eye, particularly with patients who have relatively good vision in one eye (20/25-20/70). This binocular rivalry may sometimes adversely affect reading performance. There are a few quick ways to evaluate binocular rivalry using near acuity charts and an Amsler grid.
When a patient presents with central field loss and difficulty reading, it’s often useful to ask them if they “lose their place while reading” or if the “text flashes on and off.” This may be an indication of difficulty with eccentric viewing. A follow-up question is to inquire if “they close an eye in order to make it easier to read?” This is the first indication of binocular rivalry, and the patient may have already identified an adaptation.
Next, it is important to measure single letter acuity using an M notation chart and a tape measure. Have the patient hold the chart where they like (i.e. not fixed at 40 cm) since it’ll give you some insight as to their habitual working distance. Watch for any head turns or closed eyelid while testing. Single letter acuity yields the optimal near visual acuity since isolated letters have less crowding effect, and can be found more easily with eccentric viewing. Test the better eye first followed by the fellow eye and note the M size and test distance. Follow this with a continuous text chart or paragraph card in M notation and measure binocular visual acuity at approximately the same test distance. It may be the same size print or worse. Now, occlude the worse eye and ask the patient if the text close to threshold appears “plainer” or “easier to read.” Patients are often averse to the concept of occluding an eye, so reassure them that we’re not giving up on vision in that eye. Improvement may be related to “the feel of their vision” as opposed to an actually decrease in print size that they can read.
Lastly, we can use the Amsler grid in a novel way to confirm the presence of binocular rivalry. Follow the standard instructions beginning with the better eye and then the worse eye. Next use the Amsler grid with both eyes open. Does the better eye dominate the binocular percept or the worse eye? The better eye may have a well-circumscribed scotoma, but the dominant eye may have a wider area of involvement including metamorphopsia that adversely affects reading.
In summary, putting together the history, reading performance, and binocular Amsler grid results may help confirm the presence of binocular rivalry. The solution is to simply occlude the worse-seeing eye during reading. A clip-on occluder is the least expensive (and reversible) way to adapt. After a period of 2-3 weeks of using the occluder, I instruct my patients to try and wean themselves off of it. If they read just as well and for as long without it, we have achieved a successful outcome. If not, I have them continue in an effort “to build up the better seeing eye, so that it can become dominant.”