Slow Binocular Reading in Amblyopic Children is a Fellow Eye Deficit

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.