IXT is the most common type of divergent strabismus in children and adolescents, with a prevalence of 3–4% in Asia3,4. To our knowledge, our study is the first to examine the reading speed in children with IXT in detail. In our study, we found that children with IXT read more slowly than the age matched normal participants. As is the case in a previous study26, reading speed was found to be correlated with age in IXT children (aged 10–14 year) and those with normal vision. Moreover, stereopsis at near was associated with reading speed in IXT group according to a generalized linear model.
Stereopsis was correlated with reading performance in children with strabismus. Kelly et al. (2020) evaluated motor skills in children with strabismus. They found that strabismic children had a poorer stereoacuity and a stronger suppression, both of which could disrupt the development of reading and motor ability27. Our results found there was a worse stereopsis in the IXT group compared with the control group. Furthermore, we also showed a negative correlation between reading speed and LogTNO (r=-0.244, P = 0.054) in IXT patients after adjusting for age and gender. Moreover, LogTNO was found as the factors associated with reading speed in the IXT group. This suggests that stereopsis at near might be related with reading speed in children with IXT.
Previous studies have shown that binocular fusion maintenance (i.e. the vergence function) is related to reading performance28,29. Convergence reserve was reported reduced in IXT than in normal observers30. Besides, Perrin Fievez et al. (2018) and Clotuche B et al. (2016) found that asymmetric accommodative responses between the dominant eye and the non-dominant eye during binocular viewing in IXT11, 13. The asymmetric accommodative response was suggested would impact reading performance in patients with IXT31. Our study found that vergence and accommodative functions as well as the steropsis are reduced in individuals with IXT compared with the normal control. While no significant correlation was found between the binocular functions and the reading speed in IXT patients. Moreover, a similar level of the deviation angle, convergence function, stereopsis and fusion control ability were found between the IXT group with a reading speed below the normal range and those with normal reading speed. It implies that the deviation angle, fusion control and vergence functions may not cause a difference in reading speed between the children with IXT and normal controls. Reading speed could be evaluated again with exotropia correction so that the role of deviation and binocular functions on the reading speed to children with IXT could be studied.
Defining and evaluation the severity of IXT is difficult because there are too many parameters that are associated with the severity32,33. The deviation angle, fusional control score, and stereoacuity are commonly considered as the main factors that affect the severity of IXT24,25. We found no significant correlation between the reading speed and the mentioned characters of clinical “severity”; however, LogTNO may associated with the reading speed after adjusted the age and gender. It seems that a slower reading speed is not correlated with a more severe IXT in children population. Also, our findings suggest that the severity of IXT could not indicate a slower reading speed. Reading performance is important for children’s academic performance in school6,7. Although our findings indicate that the severity of IXT is not correlated with reading speed, we still show that children with IXT have a slower reading speed. Therefore, it might be important to assess reading speed as part of routine clinical examination in children with IXT.
Our study has a few limitations. First, reading speed alone cannot fully reflect reading performance because one can read quickly without comprehension9. Second, we asked the participants to read out loud. The children’s speaking rate could differ from the speed of reading comprehension; extracting meaning from a text could be achieved without verbalizing the text34. Furthermore, we used the same reading distance for all the subjects, as well as the same distance for assessing the clinical characteristics (i.e., stereoacuity). This study design of using the same distance for all subjects enabled us to easily compare the relationship between reading speed and clinical characteristics15. Lastly, saccadic and fixation function have been shown to be associated with reading performance. Disconjugacy saccadic and longer fixation have been found in children with strabismus because the patients often found themselves rereading the same passage to achieve comprehension. The role of fixation and saccadic parameters in reading performance for subjects with IXT should be investigated in the future by using an eye-tracking device14, 16.
In conclusion, reading speed seems to be slower in school-age children with IXT based on the International Reading Speed Texts. After adjusting for the age and gender, we found that poor stereo function at near was related with a slower reading speed and that stereoacuity at near was associated with reading speed in children with IXT.