Exotropia

The following clinical assessments are undertaken at each visit:

Binocular single vision (BSV) testing: simultaneous perception, motor fusion, and distance stereopsis using synoptophore; near and distance stereopsis using Titmus test and Randot test

Best corrected visual acuity (BCVA)

Measurement of ocular alignment using the prism and alternative cover test (PACT) at near and distance

Ocular motility examination

Measurement of control of alignment using revised Newcastle control score (NCS)

Evaluation of quality of life using Intermittent exotropia questionnaire (IXTQ) [19 (link), 20 ]

Cycloplegic refraction

Routine ocular examination of anterior segment, fundus, and ocular motility

Strabismus surgery (group A)

Surgery is performed by specialized strabismus surgeons (Yan, Deng, Lin, Kang, Chen, Wu, Wang, Qiu, and Shen) according to agreed surgical formulae tailored to the clinical characteristics of the surgeon. Principles involved in the surgical procedure have been agreed as follows:

General anesthesia

Bilateral lateral rectus recession to be performed in divergence excessive IXT; unilateral recess/resect surgery to be performed in convergence insufficient IXT; either surgery in basic type of IXT

Standard sterile preparation of the operative sites

Conjunctival incisions

Standard isolation and cleaning of muscle to be operated

Muscle secured with 6/0 vicryl suture

Amount of recession and resection assessed on the basis of the maximum distance deviation angle (Table 1), modified according to standard practice of surgeon

Conjunctival incisions closed with 8/0 vicryl suture

Antibiotic ointments give at the end of procedure

Table 1

Surgical amounts for SOMIX trial

Deviation angle	Unilateral/bilateral recess surgery		Unilateral recess/resect surgery
(PD)	ULR recession (mm)	BLR recession (mm)	LR recession (mm)	MR resection (mm)
15	6
20	8		4	3
25		5.5	5	3.5
30		6	5.5	4
35		6.5	6	4.5
40		7	6.5	5
45		7.5	7	5
50		8	7	6

PD Prism diopter, ULR Unilateral lateral rectus, BLR Bilateral lateral rectus, LR Lateral rectus, MR Medial rectus

b) Follow-up visit    
The follow-up visits schedule of the study and corresponding clinical assessments for each group are showed in Fig. 2.Fig. 2

Standard protocol item. The follow-up visits schedule and corresponding clinical assessments. D, day; W, week; M, month; Y, year; BSV, binocular single vision; BCVA, best corrected visual acuity; PACT, prism and alternative cover test; NCS, Newcastle control score; IXTQ, intermittent exotropia questionnaire

Children in the observation group will be offered surgical treatment if a constant strabismus appears to be developing or parents request surgery and the responsible clinical team agrees that this is appropriate.

The baseline distributions of the participants’ characteristics, such as age, height, weight, SER, and exo-deviation were expressed with means (standard deviation, SD) or medians (interquartile range, IQR), depending on whether the data were normally distributed or not. We checked for the normality of our datasets using a Shapiro–Wilk test and found that our data were normally distributed (p’s > 0.05). Categorical variables were expressed as the rates (or proportions).
We determined the necessary sample size based on previous studies [14 (link),23 (link)] by performing a power analysis with 80% power and a type I error probability (alpha) of 0.05. Finally, we concluded that a sample size of 136 would be adequate to answer our main research questions.
We considered both motor alignment (the deviation between 10 PD of exophoria/exotropia to 5 PD of esophoria/esotropia by SPCT at distance or near) and sensory status (loss of 2 octaves or more of stereopsis from baseline) when defining the successful outcome criteria. For the primary outcome, we cited a recent PEDIG study which used the “suboptimal surgical outcome” as the primary outcome. The cumulative suboptimal surgical outcome proportion of patients by 12 months was compared between the two groups using the Kaplan–Meier method. An intergroup difference and a corresponding 95% confidence interval (CI) were also calculated. Suboptimal surgical outcomes were defined as: (1) exodeviation of ≥10 PD at distance or near using SPCT, (2) constant esotropia ≥6 PD at distance or near using SPCT, or (3) loss of 2 octaves or more of stereopsis from baseline [14 (link)].
Secondary outcomes were surgical motor alignment success, stereopsis, exodeviation speculated by PACT, fusional control score, and fusional convergence parameters. Surgery motor alignment success was defined as exotropia of <10 PD and esotropia of <5 PD [30 (link)] According to the results of DRS and TNO stereopsis, we divided the patients into three groups: “Good” denotes DRS ≤ 100″ or TNO ≤ 60″, “moderate” indicates 100″ < DRS ≤ 400″ or 60″ < TNO ≤ 480″, and “nil” indicates that they were unable to recognize the cues [31 (link),32 (link)]. Stereopsis was transformed into log units for data analysis. Patients with “nil” stereopsis were assigned to the next highest 0.3 log increment level (i.e., 800 arcsec for DRS and 960 arcsec for TNO) [33 (link)]. A fusional exotropia control score of ≤2 indicates that the patient can be controlled in an exophoria state [28 (link)]. We divided the patients into two groups according to the exotropia control scores at the 12-month follow-up.
For all IXTs who completed the 12-month follow-up, we conducted a repeated-measures analysis of variance (ANOVA) to compare the difference in exodeviation and fusional convergence amplitude between the two groups. We used a two-tailed chi-square test to compare the difference in stereopsis grouping and exotropia control grouping between the orthoptic therapy and control groups. Statistical analysis was performed using SPSS software version 20.0 (IBM Corp., Armonk, NY, USA). A p value of 0.05 was established as statistically significant.