After 3 months of topical cycloplegics, stereoacuity improved following the esotropia decreases among the SAACE patients. However, the administration was found to be less effective in patients with longer untreated periods.
Three major types of AACE have been defined : (1) Swan type: esotropia due to the disruption of fusion following monocular occlusion or vision loss in young children; (2) Burian-Franceschetti: esotropia associated with physical or psychological stress in young patients; (3) Bielschowsky type: esotropia in young uncorrected myopia with excessive near work. Excessive near work with excessive accommodation followed by convergence spasm has been suggested to be the mechanism of this esotropia . Since the convergence spasm in this condition cannot be relaxed at distance fixation, there is development of esotropia with diplopia. The imbalance between the convergence and divergence forces also leads to the development of increases in the tonus of the medial rectus, which leads to esotropia. Patients enrolled in this study exhibited the onset of AACE after several hours of digital device usage. Only 1 out of the 23 patients had emmetropia. The other 22 patients had myopia, with 8 of these patients exhibiting high myopia over -6.0D. The characteristics found for these patients were similar to those of Bielschowsky esotropia, which exhibits equal deviation at near and distance fixation, myopia and adolescent onset after excessive near work.
Changes in the tonus of accommodation and binocular vergence have been reported after near work , especially after hours of work with digital devices [18, 19]. As compared to that found for ordinary hard copy workers, digital devices workers exhibit a higher ratio of abnormal accommodation and convergence . Over the last few decades, the usage of handheld devices, including smartphones and tablets, has increased, especially during lockdowns due to the COVID-19 pandemic. Several ocular symptoms, including dry eye and binocular disabilities have also been reported after excessive usage of handheld devices [21, 22]. When using a smartphone, the working distances are shorter than that found for typical near working distances , with these distances becoming even shorter when there is over 60 min of usage . Furthermore, some subjects will view text on their smartphones using a smaller font size . Deterioration of fusional vergence has been reported after 30-min smartphone usage . When smartphone or tablet usage was evaluated in AACE patients, nearer working distances were found as compared to that observed in age-matched controls . Thus, closer working distances and smaller font sizes may increase the demands on accommodation and convergence, thereby resulting in a spasm of the near reflex that leads to dynamic activation of the medial rectus. It has been suggested that spasm of the near reflex could be one of the causes in patients presenting with AACE .
Anatomical anomalies could also potentially contribute to the onset of SAACE. It has been reported that there are shorter distances between the limbus and the insertion of the medial recti in AACE . Hypertrophic medial recti were also observed in some of the patients in our surgery group. In addition, the changes of the resting states of accommodation and vergence in near work have been reported to depend on the oculomotor resting tonus . Additional investigations into the possibilities of anomalies in the extraocular muscles, such as the distance between the limbus and insertion, will need to be undertaken in the future.
In our current study, in addition to imposing limitations on digital device usage, a prescription of 0.4% tropicamide at bedtime was found to improve the angles of deviation and diplopia among patients with untreated SAACE that had persisted for over 1 year. As we have described earlier, excessive accommodation and convergence could potentially contribute to the SAACE onset. Cycloplegic eye drops, which include cyclopentolate and atropine, have been reported to be effective on releasing spasm of the near reflex, especially the accommodation component . Moreover, the prescription of atropine has also been reported to resolve Bielschowsky esotropia . It has additionally been reported that 2 cases of AACE with accommodative spasm responded to 1% cyclopentolate eye drops  and 1% atropine sulfate eye ointment . Cycloplegics inhibit spasm of the ciliary muscle and near reflex, and thus, can further improve esotropia. Tropicamide has been reported to have the similar cycloplegic effects as cyclopentolate [30,31,32], although tropicamide has a shorter duration as compared to that observed for cyclopentolate and atropine sulfate. The onset of SAACE is thought to be due to the continuity of the convergence spasm after excessive near work. A 0.4% tropicamide prescription at bedtime appears to inhibit the accommodative spasm during the night. Although these effects do not persist during the following day, once the accommodative spasm stops, the continuity of accommodative spasm is interrupted. Accompanying convergence spasms also stopped at night when the continuity was interrupted. As a result, the diplopia disappeared in conjunction with decreases in the esotropia. Moreover, a prescription of 0.4% tropicamide at bedtime will not induce mydriasis or cycloplegia during the daytime. Thus, this could reduce patient distress and increase treatment compliance.
There were several limitations in the present study. First, the current results suggest both imposing limitations on the digital device usage and the administration of topical cycloplegics could be beneficial in these types of patients. Further evaluation of the individual effects of 0.4% tropicamide will need to be undertaken. However, studies done to examine the amount of time spent using digital devices can only be performed based on self-reported data. In this study, there were equal proportions of patients in both the study and control groups who reported the same daily average time of digital device usage, which was over 80%, even after being advised to limit their usage. Thus, duration of smartphone usage affected the study group in the same way as that observed in the control group and therefore, it is not likely that this influenced the results of the present study. Furthermore, our results also suggest that topical cycloplegics can help resolve SAACE. Although decreases in the angles of deviation were less in patients using topical cycloplegics, the angle of esotropia decreased to near 50%. Accompanying this decease, there was an improvement in the stereoacuity, with patients achieving the same level as that observed for patients undergoing surgery. Furthermore, using glasses with built-in prisms was able to correct diplopia in most patients (13 out of 14 patients) in conjunction with cycloplegic eye drops. Thus, overall, the findings of our present study indicate the beneficial effects of topical cycloplegics in treating SAACE, especially in patients who are not able or do not agree to undergo any operations. The small sample size is another limitation of the present study. Although SAACE has recently garnered more attention, SAACE is still considered to be an unusual presentation of esotropia. Several case reports [5, 8] and case series [4, 9,10,11] have been published, with the largest series evaluating 15 patients . Although our present study enrolled 23 patients with SAACE, which is a much larger number than that reported in previous studies, further evaluation of additional patients will be necessary in order to better investigate the characteristics and treatment issues associated with SAACE.
In our current study, 26% of the enrolled patients were over 25 years of age, which was higher than that reported in the other studies. Recently, excessive near work has become a more widely investigated issue, as this is now considered to be a major problem in not only teenagers but also in adults. According to our current results, there was no correlation between the age of onset and the decreases in the angles of esotropia after the administration of local short acting cycloplegics. Furthermore, tropicamide eye drops appear to be effective in patients up to the 4th decade. Further studies that focus on patients of different age groups will need to be undertaken.