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Rijo Hayashi,1 Shimmin Hayashi,1,2 Tomoharu Nishimura,1 Shigeki Machida1 1Department of Ophthalmology, Saitama Medical Center, Dokkyo Medical University, Koshigaya, Saitama, Japan; 2Lively Eye Clinic, Soka, Saitama, JapanCorrespondence: Rijo Hayashi, Department of Ophthalmology, Saitama Medical Center, Dokkyo Medical University, 2-1-50, Minamikoshigaya, Koshigaya, Saitama, 3438555, Japan, Tel/Fax +81-48-965-8659, Email lhayashi@dokkyomed.ac.jpPurpose: Acute acquired concomitant esotropia induced by excessive digital device usage, especially smartphones (SAACE), has been increasing over the past few years. Convergence spasm induced by excessive near work has been suggested as a mechanism. Anatomical differences could also potentially contribute to SAACE onset. The present study investigated the conformation of horizontal recti between SAACE patients and normal subjects.Patients and Methods: In 15 SAACE patients (SAACE group), the distances between the limbus and insertion of the horizontal recti (LI distance) and the widths of horizontal recti on the insertion (insertion width) were measured. The control group consisted of 30 patients who underwent retinal detachment surgery. Differences in LI distances and insertion widths were compared between SAACE and control groups.Results: While there were no differences between the two groups for LI distances and insertion widths of lateral recti, there were significantly shorter LI distances for the medial recti in the SAACE group (P< 0.05). Moreover, the SAACE group tended to exhibit larger insertion widths of the medial recti. Medial/lateral ratio of LI distances were significantly lower and insertion widths were significantly higher in the SAACE compared to the control group (P< 0.05).Conclusion: Based on the observations of more anterior insertion and larger muscle widths, this suggests there are stronger forces of medial recti in SAACE. In addition to excessive accommodation followed by increases in medial recti tonus, the results also suggest that an anatomical imbalance between lateral and medial recti contributes to esotropia onset following excessive near work.Keywords: acute acquired concomitant esotropia, digital devices, extraocular muscles, insertion distance, insertion width