Prevalence of visual stress in dyslexia and controls (3)

Henderson LM, Tsogka N, Snowling MJ. Questioningthe benefits that coloured overlays can have for reading in students with and without dyslexia. J Res Spec Educ Needs. 2013 Jan;13(1):57–65. 

It is a little unfair to include this very good paper here because it does not really claim to be an epidemiological study examining the prevalence of visual stress in dyslexia and controls.
The important conclusion of this study, which will be discussed in more detail on the in a future post, is that improvements in reading the Wilkins Rate of Reading Test (WRRT) obtained with coloured overlays are neither sustained nor matched by improvements in reading naturalistic text.
However, in the process of conducting the study, the authors tested for visual stress using the WRRT and intuitive overlays in students at a higher education establishment. There were 16 students with dyslexia and 26 controls with no reading impairment.
56% of the dyslexic students had previously been exposed to coloured overlays illustrating the problem of conducting epidemiological research in the area. Because of enthusiastic marketing of coloured overlays, treatment naive subjects are now hard to find.

The diagnostic criterion for visual stress was reading the WRRT 5%,8% or 10% faster using the chosen overlay.

Results

It can be seen that the 95% confidence intervals are very wide and straddle the odds ratio of 1- no matter which criterion is used.













Conclusion
The data could be consistent with visual stress being more common among controls or subjects with dyslexia. This reflects the small sample size.
Because of the possible sources of bias in this study.

• 56% of dyslexics previously exposed to overlays 
• Screeners not blinded to the reading status of the individuals   

This data could not be included in a meta-analysis
In contrast to the previous study, the authors are open about the shortcomings as well as the strengths of their data.