Slow Binocular Reading in Amblyopic Children is a Fellow Eye Deficit

Optometry and Vision Science

Here is the latest research on Amblyopia and scholastic performance from the Pediatric Vision Laboratory of the Retina Foundation of the Southwest. If that source sounds familiar, it’s because the group spearheaded by Krista Kelly and Eileen Birch has published considerably about impaired motor skills and slowed reading speed in the past. Our colleague, Dr. Dan Fortenbacher, did a review of an article they published in JAMA Ophthalmology toward the end of 2018.

It’s interesting to see this article published in Optometry and Vision Science without any optometric authors (the co-authors are four non-OD staff members at the Retina Foundation of the Southwest and four Texas-based pediatric ophthalmologists). The authors pose the question: Why is binocular reading for comprehension slower in amblyopic children compared to
controls when they can rely on their fellow eye that has normal visual acuity?

One suggestion they offer is that binocular inhibition is actively occurring at the time of reading. Although we typically consider suppression to be an effect of the fellow eye on the amblyopic eye, it is also possible that the amblyopic eye affects fellow eye performance during binocular viewing. For example, the reduced contrast sensitivity of fellow eyes during binocular viewing relative to monocular viewing is one type of binocular inhibition that has been reported in amblyopia.

Another possibility is that disruption of binocular vision by abnormal visual experience early in life due to either anisometropia
or strabismus not only places the child at risk for amblyopia, but also disrupts the development and maintenance of binocular connections at cortical levels resulting in deficits in the fellow eye (as well as in the amblyopic eye). This is the concept that has received increasing attention that the fellow or “non-amblyopic” eye is not “normal” either. If that is the case, then punitive measures such as occluding or atropinizing the fellow eye makes no sense regarding overall performance, as the non-amblyopic eye is functionally substandard to begin with. Hence the trend of late to emphasize interventions that enhance binocular integration to improve performance rather than punishing the better eye with patches or drops.

Fellow eye deficits in reading speed even when the amblyopic eye is occluded for monocular testing would rule out inhibition by the amblyopic eye and pinpoint that binocular reading speed is reduced due to the consequences of binocular dysfunction. In this study, that’s exactly what the authors undertook in examining whether slow silent reading for comprehension in children with strabismic and anisometropic amblyopia is due to binocular inhibition or a fellow eye deficit. In other words, if binocular inhibition is actively occurring during reading, they hypothesized that they would see a faster reading rate during fellow eye reading compared as compared to with binocular reading. Conversely, if slow reading in amblyopic children is due to a fellow eye deficit due to early binocular disruption, slow reading should be found under both binocular and fellow eye (monocular) reading conditions.

The strength of the study, as in the authors’ previous work, is that they used a Readalyzer to record eye movement reading patterns and reading rate under both conditions. Chances are you’re familiar with the ReadAnalyzer system, but in case not, here is a quick primer:

Here is how the testing was conducted: Children, aged 7 -13, sat eye level at a comfortable reading distance (35-
40 cm) and were asked to silently read at their normal reading speed for comprehension to replicate conditions typically used for completing schoolwork. Children read three passages, a grade-level 1 passage as a practice run followed by two passages at the last grade level reportedly completed in school. They read one grade-level paragraph viewing binocularly, and one grade level paragraph viewing with the fellow, non-amblyopic eye. For the monocular condition, an occlusive eye patch was placed on the outside of the goggles on the side of the amblyopic eye. Ten ‘Yes/No’ comprehension questions were asked followed each reading to ascertain that the child comprehended the story. If a child’s comprehension or reliability did not meet the criteria for comprehension, they were given another passage of the same grade-level to read. If after a second try, criteria were still not
met, the experiment was terminated for that child.

Bear in mind that the authors already knew from a prior study that under binocular conditions, control children in this age range read an average rate of 204 words per minute as compared to amblyopic children who read at a reduced average rate of 148 words per minute. The bottom line in this study is that the researchers found no within-groups differences in binocular reading versus fellow eye reading for the amblyopic group in reading rate, forward saccades, regressive saccades, or fixation duration. The researchers conclude that binocular inhibition is not the cause of slower reading for comprehension in amblyopic children because they did not find better fellow eye reading when the amblyopic eye was occluded as compared to binocular reading. Both were similarly impaired.

The conclusion of the paper reads as follows:

“Reading is essential for academic achievement, and slow reading for comprehension has the potential to impact performance on time standardized tests that are used for school admissions, to assess academic achievement, and determine eligibility for academic tracks in the USA. Parents of amblyopic children may be unaware that their child’s reading is affected by their amblyopia because their child has 20/20 vision in the fellow eye. Whether recovery of normal visual acuity with amblyopia treatment improves reading rate is yet to be determined. Academic accommodations, such as more time for tests, for timed standardized tests typically administered in the USA may need to be considered for children with amblyopia who are slower at reading than controls so that they perform at their maximum potential.”

Hopefully the Retina Foundation of the Southwest research group will get around to studying the impact of recovery of normal visual acuity with amblyopia treatment and its effect on improved reading rate. As valuable as PEDIG research has been in shifting the paradigm away from occlusion, and in substantiating the therapeutic impact of the appropriate Rx, is fell short in looking only at monocular visual acuity of the amblyopic eye as the definitive index for successful treatment and not other parameters of amblyopia deficit such as decreased reading rate.

In their Discussion section in this article, the researchers note that fixation instability has been previously correlated with reading rate. They suggest that it is also possible that reduced visual span, crowding, or higher order impairments in visual attention are impacting reading speed, all of which could also be disrupted by fixation instability. One would therefore anticipate that interventions that have a beneficial impact on any or all of those factors would aid reading performance significantly.

8 thoughts on “Slow Binocular Reading in Amblyopic Children is a Fellow Eye Deficit

  1. Great post!
    I would be less interested in seeing what improved acuity in the amblyopic eye has to do with reading than what improved binocular and binocular oculomotor function would produce. After all, when I work with these patients, I am more interested in how they see with two eyes than how they see with one.

    I haven’t read the article but were the effects of strabismic versus refractive amblyopia compared? Both in the dyslexia literature and in my own experience, covering the non-preferred eye often improves reading performance. Would be useful to have a comparison between the two major types of amblyopia that we most commonly encounter and their effects on reading. The refractive amblyopes, being in a sense “more binocular” might perform differently than the strabismics.

    Or not.

  2. Thanks, David. Your first comment is precisely the point. If we’re serious about defining amblyopia as a type of binocular dysfunction, then we have to finally move away from the spotlight on the extent of monocular acuity improvement as the sole index for success. It is not even the most important one. Just like there are “criteria for cure” in strabismus, and not just magnitude of deviation, we have to get serious about “criteria for cure” in amblyopia, and not just magnitude of acuity reduction.

    Regarding your second point, the short answer is no. The breakdown in the article on participants with amblyopia is 26% refractive amblyopia, 26% strabismic amblyopia, and 50% mixed strab + refractive. The results weren’t broken down accordingly on relative slowing of reading rate, but we can infer that by the data which shows that the neither the level of stereoacuity nor of sensory suppression/fusion (both of which would be expected to be considerably worse in strabismic amblyopia) made any difference in the results. So it really seams that the main culprit here is not the level of binocularity at the outset, but that the “better eye” has fundamental oculomotor deficits despite having relatively “normal” acuity. So what we really have are two compromised eyes, one more overtly so than the others, when it comes to the underlying neurophysiology to support optimized reading.

  3. Thanks for sharing this research! I was always curious to know why some of my refractive amblyopic patients showed faster tracking speed and accuracy on timed tracking tasks/tests with their amblyopic eye compared to their non-amblyopic eye. I had expected the non-amblyopic eye to be the stronger and faster of the two, but as with this research, that is not the case! I also postulated that perhaps the amblyopic eye had better peripheral or ambient vision, which is needed for fluent reading and why we emphasize peripheral vision as an important component to our vision therapy program for those with reading problems. Our eyes need to both pay attention to what we are looking at, the letters that make up a word on the page and also know where to go next. Otherwise, with out both central and peripheral vision, it’s like reading through towel tubes, which would be a very slow and tedious process. Thanks again!

    • You’re welcome, Carole. That is very much the point, and why the “cure” for amblyopia can’t just be directed to acuity improvement of the eye with poorer central VA. The latter is largely a function of V1, which is all most of the research studies to date have focused on. This research is rooted in the Frontal Eye Fields which is where saccades are driven. It could very well be that the eye with better acuity and contrast abilities is diminished in key oculomotor variables. (I suspect there’s some of the old “magno/parvo” divide going on here.)

  4. Great post! I do have a concern with the authors analysis. “The researchers conclude that binocular inhibition is not the cause of slower reading…”
    I am not sure that covering the front of a goggle that sits a few inches from the eye eliminates cortical binocular inhibition. If finding the neurologic basis of slower reading for comprehension in children with amblyopia then this conclusion may be counterproductive for the future. Not that it really matters to the main point of the post and what most developmentally oriented OD’s already know which is amblyopia affects people in many ways beyond acuity.

    • Thanks, and your point is well taken. I would agree though that the end justifies the means in the sense that this work underscores the significance of attending to maximizing the function of each eye as well as optimizing their integration. It certainly is a strong argument away from the patching or penalization as the primary treatment.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s