Indications and results of exotropia surgical management.

AIM: To study the clinical particularities and the surgical management of the different forms of exotropia, and to analyze the prognostic factors influencing the surgical result.
METHODS: Medical records of 132 patients who underwent exotropia surgery from 1995 to 2015, were retrospectively reviewed. Ophthalmological examination and a complete sensorimotor assessment were performed for each patient. All surgeries were performed by the same surgeon.
RESULTS: We had 57 cases of intermittent exotropia (IE), 22 cases of infantile exotropia (IfE), 43 cases of sensory exotropia (SE) and 10 cases of concecutive exotropia (CE). The average age of onset of exotropia was 6.8 years +/- 8 SD. Amblyopia was noted in 36.4% of cases  of  IfE.  The mean deviation angle was  36.5  DP in (IE),  39.6  DP in (IfE)  and  44.5  DP  in  (SE). A vertical syndrome was frequently found in (IfE) patients. Bilateral recession of  lateral  rectus  was  the  most  performed  surgery  type.  The overall success rate after a  single  surgery  was  72%.  A multivariate logistic regression analysis  showed  that good prognosis  factors  were  the absence  of  amblyopia, the intermittent form of the strabismus,  a low  preoperative deviation angle and the ocular alignment on day one postoperatively.
CONCLUSIONS: IE is the most common divergent strabismus.  Surgery resulted in successful alignment in most of the cases.  Early management and rigorous analysis of  patients sensorimotor status are the best guarantors of long-term success.

INTRODUCTION

Exotropia is an oculomotor syndrome corresponding to the horizontal divergence of the visual axes occurring in 1.25% of the pediatric population 1. The frequency of exotropia is underestimated in our country because of the consultation delay. Exotropia is less common than esotropia; it is a quiet heterogeneous pathology with various specific forms, the most described of which are: Intermittent exotropia (IE), which is the commonest form, infantile exotropia (IFE), sensory exotropia (SE) and consecutive exotropia (CE). Surgical protocols are not standardized, and their results depend on many parameters such as the type of exotropia as well as the preoperative motor and sensory state.

The aim of our study is to describe our surgical protocols in exotropia, their results and the different factors influencing the postoperative results.

METHODS

The study was retrospective including 132 cases of exotropia followed up between 1995 and 2015. Exclusion criteria were patients with insufficient pre-or postoperative clinical or orthoptic report, insufficient postoperative follow-up (less than three months), restrictive syndromes and paralytic strabismus. This study adhered to the tenets of the Declaration of Helsinki.

All of our patients had a complete ophthalmological examination with refractive error correction after topical administration of 1% cyclopentolate or Atropine (0.3, 0.5 and 1%) in order to identify amblyopia which was classified according to its severity.

The angle of exodeviation was measured pre-and postoperatively with Hirschberg Test, modified Krimsky test, prisms and alternating cover test or with synoptophore. Associated vertical syndromes were noted. The Worth-4-dot test, the striated glasses of Bagolini, red glass, synoptophore and TNO test (Lang 1 test in children) were used to study our patients' sensory state (binocular vision, retinal correspondence, fusion and stereopsis). At the end of the motor and the sensory assessments, we classified our patients into the following groups: (IFE),(IE),(SE) and (CE).

All of our patients had a total optical correction, with an adequate treatment of amblyopia. Orthoptic rehabilitation was indicated in some cases of normosensory strabismus or postoperatively if the residual angle was small. Prismatic correction was indicated preoperatively in some cases of normosensory strabismus when the angle of deviation was less than 20 PD. Surgery was performed under a general anesthesia by the same surgeon after obtaining an informed written consent from all parents.

Bilateral lateral rectus muscle (LRM) recession was indicated in alternating strabismus with isoacuity, and unilateral recession-resection surgery was indicated for non-alternating strabismus. Bilateral asymmetric surgery on three muscles was reserved for strabismus with a large angle exceeding 40 prismatic diopters (PD).

Surgery of the vertical element was simultaneously performed. Unilateral or bilateral superior rectus muscle (SRM) recession was performed for a manifest dissociated vertical deviation (DVD), associated or not with posterior myopexy.

For inferior oblique overaction, a surgery according to Gobin or Parks techniques was performed. For alphabetic syndromes without oblique muscles overaction, we performed a vertical transposition of the horizontal rectus muscles. Surgical dosage was based on the maximal deviation angle: we considered that LRM recession of 1mm corrected 2 PD of deviation, and 1mm of medial rectus muscle (MRM) plication corrected 3 PDs.We considered that the result of our dosage increased by 20%, in case of horizontal muscles surgery in one eye.

Patients were examined on day 1, one month, three months and then every six months post operatively, with an orthoptic assessment in the first month. Motor success was defined by an orthotropy or a residual deviation of less than or equal to 10 PD of exotropy, 8 PD of esotropy and 4 PD of height. Sensory success was defined by stable binocular fusion and less than 200 seconds of arc of stereopsis, if the evaluation was possible. The following informations were recorded when available: age at onset, gender, presence of systemic diseases, preoperative maximum angle of deviation, associated ocular motility disorders, surgery delay and motor and sensory outcomes.

The studied factors that might influence the post-operative results were: the age of strabismus onset, surgery delay, amblyopia severity, preoperative deviation, postoperative day 1 angle and associated vertical element. The data were analyzed by SPSS version 21.0. All quantitative data were expressed as mean ± standard deviation, and qualitative variables were expressed as effectives and percentages analyzed using Chi-squared test. Comparisons of means were made using the Student's t test. Correlation between continuous variables was analyzed using Pearson's correlation analyses. P values ˂0.05 were considered to be statistically significant. Correlation between a quantitative and a qualitative variable was studied by the (ANOVA) test. In order to identify the proper role of each factor that could have influenced the motor outcome, we performed a multivariate statistical analysis in logistic regression with the adjusted odds ratio "r" for each factor.

RESULTS

One hundred and thirty-two patients were included; there were 77 males (42%) and 55 females (58%) classified into four groups (table 1) . The mean age of strabismus onset was 6.8 years old±8 SD and depended on the clinical form (Figure 1). Severe amblyopia was noted in all patients with SE, in 36.4% of patients with IFE and in 50% of patients with CE. Retinal correspondence was normal in all cases of IE and in seven cases of CE. Most of the patients with IE showed good fusion but, only six of them had normal stereoscopic vision before surgery (table 2).

TABLE 1 : Patients' distribution according to the clinical form and subform of exotropia

Forms of exotropia	Number and pourcentage of patients	Sub forms of exotropia
IFE	22 (16,7%)
IE	57 (43,2%)	Divergence excess	8 patients
		Convergence insufficiency	3 patients
		Basic exotropia	46 patients
SE	43 (32,6%)
CE	10 (7,75%)
*IFE: infantile exotropia - IE: Intermittent exotropia - SE: Sensory exotropia - CE: Consecutive exotropia

Figure 1. exotropia age of onset according to the clinical form

TABLE 2 : Fusion state of our patients according to the clinical form of exotropia

Fusion	IFE	IE	SE	CE
Good	0	29	0	0
Instable	0	9	0	0
Absent	22	19	43	10
*IFE: infantile exotropia - IE: Intermittent exotropia- SE: Sensory exotropia CE: Consecutive exotropia

The motor assessment revealed that SE and IFE had the largest deviation angle (table 3) . Vertical syndromes were commonly noted in IFE. We had eight cases of inferior oblique muscle overaction, seven cases of DVD, six cases of V syndrome, four cases of Hypertropia and one case of A syndrome. Figure 2 summarizes the mean age at the time of the surgery.

TABLE 3 : The mean maximum deviation angle according to the clinical form of exotropia

Forms of exotropia	Mean maximal deviation angle
IFE	39,6 DP ± 16
IE	36,5 DP ± 9,1
SE	44,5 DP ± 12
CE	32 DP ± 4,2
*IFE: infantile exotropia - IE: Intermittent exotropia - SE: Sensory exotropia CE: Consecutive exotropia

Figure 2. mean age of surgery according to the clinical form of exotropia

Surgical protocols depended on the type of strabismus in addition to the preoperative motor and sensory findings (table 4) . The postoperative motor results were summarized in (table 5) . On the first postoperative day, 105 patients (79.5%) were orthotropic, 22 (16.7%) had a residual exotropia, and five (3.8%) had a consecutive esotropia. Three months postoperatively, 51 patients (38.6%) were orthotropic, and 77 patients (58.3%) had residual exotropia, 42.4 % of whom had less than 10 PD of exodeviation. Only four patients were overcorrected, three of whom had an exodeviation of less than 8 PD. Thus, after a single surgery, motor success was obtained in 72% of cases (59.1% in IFE, 78.9% in IE, 65.1% in SE and 90% in CE). Twelve patients were reoperated because of exotropia recurrence: five had IFE; three had IE and four had SE. The final motor success was of 81.8% in IFE, 84.2% in IE, 74.4% in SE and 90% in CE. Good fusion was observed in 40 patients within the group of IE and in two cases of CE. Normal stereopsis was obtained in 42.1% of IE cases (24 patients).

TABLE 4 : Surgical protocols according to the exotropia clinical form

Surgical protocol	IFE	IE	SE	CE
Unilateral surgery (LRM recession)	2	4	1	2
Unilateral surgery (LRM recession/MRM plication)	6	10	23	6
Bilateral surgery (double LRM recession)	9	36	5	1
Bilateral surgery (double LRM recession/MRM plication)	5	7	14	1
*IFE: infantile exotropia - IE: Intermittent exotropia - SE: Sensory exotropia CE: Consecutive exotropia - LRM: lateral rectus muscle - MRM: medial rectus muscle

TABLE 5 : Surgical protocols according to the exotropia clinical form

Postoperative delay	Motor status	IFE	IE	SE	CE
First day postoperatively	orthophoric	17	50	30	8
	Residual XT	4	6	10	2
	ET	1	1	3	0
3 months after the surgery	Orthophoric	4	33	9	4
	XT<10 Dp	13	13	25	5
	XT>10 DP	4	10	7	0
	ET<8 Dp	0	1	2	0
	ET>8Dp	1	0	0	0
After a single surgery	Orthophoric	1	24	3	4
	XT<10 Dp	11	19	23	5
	XT>10 DP	8	12	15	1
	ET<8 Dp	1	2	2	0
	ET>8Dp	1	0	0	0
	total	22	57	43	10
*IFE: infantile exotropia- IE: Intermittent exotropia - SE: Sensory exotropia CE: Consecutive exotropia

According to a multivariate analysis in logistic regression, factors that influenced the motor result were the intermittent form of exotropia (r = 3.128, p = 0.046), the degree of relative amblyopia (r = 1.033, p = 0.02) and the motor outcomes on day 1 postoperatively (r = 0.184, p = 0.003). Other factors, such as the surgery age, sex, pre- and postoperative sensory state, preoperative angle, surgical dosage and associated vertical syndromes, did not show any significant influence on the probability of motor success.

DISCUSSION

Surgical protocols depended on the type of strabismus.

For IFE, we indicated surgery if the maximum angle of deviation was higher than 15 PD and as soon as a reliable orthoptic assessment became achievable (3 to 4 years old). For IE, surgery was indicated if tropia episodes became frequent (more than 50% of the time), in the presence of functional signs like diplopia or in case of stereopsis deterioration. It is often at this stage that patients consult, which explains the high average age found in our series.

For CE, we operated when the deviation exceeded 15 PD and persisted six months after the first intervention. We used the maximum angle, after dissociation maneuvers in certain cases, to calculate the amounts of recession/ plication. We aimed the orthotropy unlike some authors who indicate a slight surgical overcorrection ( 2, 6). We believe that an exact measurement of the maximum angle allows a stable result and avoids consecutive esotropia.

In case of alternating strabismus, especially in case of IE, we preferred the bilateral recession of the LRM, to which we can associate a plication of a MRM of the most deviated eye under general anesthesia, if the angle exceeds 40 PD. Kushner et al. reported the superiority of unilateral recession/resection compared with the bilateral recession of LRM in the basic type of IE (7). Twenty years later, a second trial published by PEDIG concluded that there was no significant difference between the two approaches (8).

In cases of non-alternating strabismus, we performed a unilateral recession/plication on the amblyopic eye, like most authors.

The vertical component, when it exists, was operated during the horizontal muscles surgery. This allows to promote the binocular fusion and thus, to consolidate the long-term results. A DVD was treated by unilateral recession of 4 to 6 mm of the superior rectus which was optionally combined with a posterior fixation. The inferior obliques overaction as well as the V syndrome, were treated by anteroposition of the inferior obliques according to the techniques of Parks or Gobin. We used sometimes, upward or downward transposition of horizontal recti muscles in some forms of alphabetic syndromes with low incomittance.

Most authors consider surgical success as a residual postoperative angle of less than 10 PD (9). We used, as success criteria, a postoperative angle of less than 10 PD of exotropia and 8 PD of esotropia, since consecutive esotropia is often less tolerated than residual exotropia.

Surgical success rates in the literature were widely variable. This was due to the non-comparability of the series in terms of classification systems, of postoperative follow-up durations and of the definitions used 10. Using the above-mentioned definitions of motor success, we obtained, after a single operating time, a success rate of 59.1% for IFE, 78.9% for IE, 65.1% for SE and 90% for CE.

The surgical success rates reported in the literature were 58 to 82% for IFE( 11 15), 50 to 79% for IE ( 12, 16- 25) and 75 to 90% for SE ( 26, 27) . After a second surgery on twelve patients, the final success rate increased to 81.8% for IFE, 84.2% for IE and 74.4% for SE.

We have demonstrated, by a multivariate analysis, that the factors significantly related to failure were the degree of amblyopia, the preoperative angle and the angle on postoperative day 1. The intermittent type of strabismus was significantly correlated with surgical success. These factors were found in several studies (12, 18, 28- 35) . Other factors were reported to have prognostic value, such as binocular vision status, vertical syndrome and vertical component surgery, but their impact was not significant in our analysis ( 33, 36) .

IE is the most common form of divergent strabismus. Surgery resulted in a successful alignment in most of the cases. Early management and rigorous analysis of patients' sensorimotor status are the best guarantors of long-term success.

The strengths of our study were the size of the sample, the length of the follow-up period and the group analysis by type of strabismus. The limits were the retrospective nature of the study and the heterogeneity of the population. Despite these limits, our results were comparable to those reported in the literature.

Declaration of Competing Interest

There is no conflict of interest in this study.