Background Aim of this study was to evaluate the efficacy of a novel algorithm to customise overminus lens therapy in intermittent exotropia (IXT) based on clinical factors associated with control of the deviation.

Methods Clinical parameters in IXT vary among individuals. Based on individual's physiological factors, an algorithm was developed. Children aged between 4 and 15 years with IXT were randomised into OML and observation groups. Participants in the observation group were corrected for any significant refractive error. IXT control score, angle of deviation, refraction, axial length and stereopsis were examined at baseline and follow up ranging between 6 and 15 months and compared. Compliance and tolerance to OML was determined by a symptom survey.

Results The OML power ranged between −1.00D and − 6.25D. Of the total 141 participants (mean age 6.8 ± 2.5 year), 77 were in the OML and 66 were in observation group. IXT control score improved (mean difference − 2.5 ± 1.1; p < 0.001) and angle of deviation reduced (6.9 ± 7.2pd; p < 0.001) significantly in the OML group only. Compliance rate to OML wear was 80%; 90% never or rarely experienced asthenopia symptoms. Slightly greater myopic shift (−0.36 ± 0.53D vs. −0.18 ± 0.55D) and change in axial length (0.17 vs. 0.14 mm) were observed in the OML group, but these differences were not statistically significant.

Conclusions A customised OML, calculated using this novel algorithm was effective in improving distance control, angle of deviation and stereopsis. Glasses wear was highly tolerable.