Tests performed at baseline and each follow-up visit

After obtaining consent and assent, health-related quality of life (HRQOL) will first be assessed using the Intermittent Exotropia Questionnaire (IXTQ) [25]. It will be completed by each participant and their guardians in separate and quiet rooms. Then, each participant will undergo comprehensive ophthalmic examinations, including best corrected visual acuity (BCVA), subjective refraction, slit-lamp biomicroscopy, and fundus examination. Then, strabismic and binocular visual examinations will be performed, including stereopsis, the amplitude of fusional vergence at far and near, monocular and binocular accommodative amplitude, monocular and binocular accommodative facility, near point of convergence, the prism and alternate cover test (PACT) and simultaneous prism and cover test (SPCT). The examination will first test for binocular and then monocular functions without breaking fusion. If the fusion is broken, for example, the monocular items will be used first for examination. The present study includes a baseline visit and 6-, 12-, 18-, and 24-month follow-up visits. Each examination will be completed on the same day. The 4 follow-up visits will fall into a ± 2-week range. The HRQOL will be completed at the first and last examinations. To ensure the quality and repeatability of data acquisition, we will ask two experienced clinicians to perform all testing procedures; they will be blind to the patient’s randomization status. The standard process regarding the important test items is as follows:

We will use the Intermittent Exotropia Questionnaire (IXTQ) [25] to assess the participants’ and their guardians’ health-related quality of life (HRQOL). The IXTQ consists of three components:

Child questionnaire: It will be used to assess the feelings of the child about his or her eye condition. Since the children will be over 7 years old, they will already have a certain level of reading comprehension. It consists of 12 question items and a five-level response scale (never, almost never, sometimes, often, and almost always). It will be self-administered. However, if some children have hard time understanding the question, we will assign staffs for guidance.

Parent proxy questionnaire: This will be used to assess how parents think their child’s eye condition affects their child. It has 12 items and a 5-level response scale.

Parental questionnaire: This will be used to assess the parents’ feelings about their child’s eye condition. It has 17 items and a 5-level response scale.

All questionnaires about the parents are completed by the parents themselves. Parents and children will be separated into two different rooms during the completion of the questionnaire to minimize their interaction.

We will assess the exodeviation control at each follow-up period using the Office Control Score [26]. This assessment will be conducted before all other examinations. The investigator will look at the participants’ eyes when they focus at an accommodative target at distance (6 m) or near (40 cm). Their score will be determined as shown below:

5 is constant exotropia

4 is exotropia > 50% of the 30-s period before dissociation

3 is exotropia < 50% of the 30-s period before dissociation

2 is no exotropia unless dissociated; recovers in > 5 s

1 is no exotropia unless dissociated; recovers in 1–5 s

0 is no exotropia unless dissociated; recovers in < 1 s (phoria).

Near and distance stereoacuity will be assessed using the Titmus circles test and TNO at 40 cm and the Distance Randot Stereotest at 3 m, respectively. Stereoacuity will be assessed with the participant’s refractive correction. The Titmus circles will measure from 40 to 800 s of arc (arcsec). The TNO test (Laméris Ootech B.V., Nieuwegein, the Netherlands) will range from 15 to 480 arcsec. The Distance Randot Stereotest (DRS; American Stereo Optical Company U.S.A.) ranges from 63 to 400 arcsec. Testing will start with the largest disparity, and the inability to correctly identify the target with the largest disparity will be recorded as nil stereo. Nil stereoacuity will be assigned the next highest log level (near 1600 s of arc for Titmus; 960 s of arc for TNO; distance 800 s of arc for DRS) for our data analysis.

The Worth 4 Dot test will be conducted at a 40-cm and 5-m fixation for the assessment of near and far sensory fusion, respectively. The participants will wear red-and-green spectacles and view the Worth 4 Dot flashlight at near and at distance. If participants report 2 red or 3 green lights, it will be taken to represent “suppression” of the left eye or right eye. If they report 4 lights at distance or at near, it means that they have a “normal” peripheral or central fusion. If 5 lights are reported, it will represent “diplopia”.

The amplitude of fusional vergence will be assessed by a 1 to 40 PD prism bar for both near and far fixation. Each participant will fixate on a single letter (20/40 Snellen level) at 33 cm or 5 m and will be asked to report when the fixated letter appears to double (this will be recorded as the break point) as the prism strength gradually increases. Then, the prism power will be gradually decreased, and the point at which the patient regains single vision will be recorded as the recovery point. Participants will be reminded to keep the target single as long as possible. Both the divergence (BI) and convergence (BO) of the break point and recovery point will be recorded at near and at distance. If the subject reports suppression, which will be assessed by the Worth 4 Dot test, then this part will be omitted and recorded as 0/0. For subjects who still retain fusion at 40 PD, the break point will be recorded as 40 PD, and the recovery point will be recorded as “-” and excluded from the recovery analysis. The fusional vergence at distance will be first and then the vengeance near (divergence and then convergence). The PD values of break points will be defined as the amplitude of the vergence, which will be defined as convergence reserve in our study.

For this test, participants will fixate at a single Snellen 6/12 letter as an accommodation target at 40 cm in front of their eyes. The investigator will move the target closer to the subject and observe the subject’s eyes until he or she reports diplopia or one eye drifts outward. This point will be recorded, and the distance from the point to the canthus or the plane of the subject’s glasses will be measured.

The accommodative function that we will measure here will include accommodative amplitude and facility. We will separately measure binocular and monocular function in both examinations. Generally, we will measure accommodative facility first and then amplitude, and we will measure binocular function first then monocular function. When measuring binocular function, we will observe the subjects’ eye position and remind them to maintain binocular single vision. The test will be aborted if the patient has diplopia or an outward deviation.

We will use standard test methods (± 2.00 D flipper lenses) to test this facility. The subjects will be asked to read one line above the best visual acuity at 40 cm with their corrected lenses. Then, the + 2.00 D lenses will be placed in front of the subjects’ eyes, and when the letters are reported to be clear, the flipper (− 2.00 D lens) will be quickly flipped in front of the eyes; 1 cycle is achieved when clear is reported again. This will be continued while the flipper lenses alternate for 1 min, and the cycles achieved will be recorded.

We will use “minus lens” to test the amplitude. A near target will be set at 40 cm, and minus lenses will be gradually added in front of the subjects’ eyes until they report sustained blur. The summation of the number of minus lenses and 2.50 D for working distance will provide the total accommodative amplitude.

Ocular alignment will be assessed by SPCT and PACT both at near (33 cm) and at distance (5 m). The deviation will be recorded as constant or intermittent if a manifest exodeviation is present in at least 3 cover and uncover tests. The amount of manifest exodeviations will be recorded by the values of SPCT, and the total amount of exodeviations will be recorded by the measurement of PACT. If no deviation is present at any time, it will be recorded as “no deviation”.