Visual outcome and factors influencing surgical outcome of horizontal strabismus surgery in a teaching hospital in Malaysia: A 5-year experience.

PURPOSE: The available data on strabismus surgery in South East Asian countries are scarce. This study aimed to identify visual outcome and factors influencing surgical outcome of horizontal strabismus surgery in a Southeast Asian cohort.
MATERIALS AND METHODS: A retrospective review of patients who underwent horizontal strabismus surgery between 2013 and 2017 in Hospital Universiti Sains Malaysia was conducted. Surgery was considered successful if the post-operative deviation was within 10 prism diopters at 6 months' postoperative period. Factors influencing the outcome of surgery at 6 months were identified. Chi-square and Fisher's exact tests were used in data analysis.
RESULTS: Ninety-eight patients were included. Both genders were equally affected. Exotropia (58.2%) was the most common type. About 65.3% of patients had alternating strabismus, while 51% had an angle of deviation of more than 45 prism diopters. Amblyopia was documented in 14.3% of patients. Those operated on below 10 years of age comprised 64.3%. Ninety-four patients completed follow-ups at 6 months after the surgery. The success rate was 81.6%. Approximately 92% of the patients had best-corrected visual acuities of 6/12 and better at 6 months' postoperative period. There was no significant association between age of onset, gender, presence of amblyopia, type of deviation, amount of deviation, and postoperative best-corrected visual acuity with surgical outcome at 6 months' postoperative period (P > 0.05).
CONCLUSION: The success rate was good. Postoperative best-corrected visual acuity was promising. Age of onset, gender, presence of amblyopia, type of deviation, amount of deviation, and postoperative best-corrected visual acuity did not influence the outcome of horizontal strabismus surgery in our review. Copyright:

Introduction

Strabismus surgery aims to improve the ocular alignment of the visual axis and the cosmetic appearance of patients with strabismus. The most common functional benefits are the restoration of binocular vision, with the elimination of diplopia and compensatory head posture.[1] Significant improvement in interpersonal interactions has also been observed following strabismus surgery.[2]

The outcome of strabismus surgery in Asian countries has been reported by researchers in Thailand, Korea, Hong Kong, and China.[345678] However, apart from Thailand, we found no other data from Southeast Asian countries.[3] We describe the surgical outcome in our institution, which acts as a referral center for strabismus consultation and surgery in the north-eastern states of Peninsular Malaysia. This study aimed to identify the visual outcome and factors influencing the outcome of horizontal strabismus surgery at 6 months' postoperative period.

Methods

We conducted a retrospective record review study of 98 patients (aged 2–49 years old) with strabismus who underwent horizontal strabismus surgery in Hospital Universiti Sains Malaysia, Kelantan, Malaysia, from January 2013 to December 2017. This study adhered to the tenets of the Declaration of Helsinki and was approved by the Human Research Ethics Committee of Universiti Sains Malaysia (USM/JEPem/16030105). Permission to access hospital records and waiver of the need for informed consent was also granted by the aforementioned ethics committee.

The inclusion criteria were all patients with strabismus who underwent horizontal strabismus surgery within the study duration. All surgeries were performed by a single surgeon trained in strabismus surgery. The exclusion criteria were (a) associated vertical muscles involvement in horizontal surgery such as superior or inferior oblique muscle; (b) pre-existing ocular disease such as corneal opacity, cataract, congenital optic atrophy, or glaucoma; (c) history of ocular surgery; and (d) presence of systemic disease such as Down syndrome, Marfan syndrome, and cerebral palsy.

The following data were documented and analyzed; demographic characteristics including age at onset and gender. Preoperative details including laterality, cause of strabismus, type of strabismus, amount of deviation, best-corrected visual acuity before the surgery, presence of refractive error, amblyopia, and binocular single vision. Patients with refractive errors were corrected with glasses. All patients with amblyopia had maximum patching treatment of 2 h/day. The optimum best-corrected visual acuity was documented at 2 weeks before the surgery.

Number of muscles operated and postoperative details were also documented, including magnitude of strabismus (if any), best-corrected visual acuity, and presence of complications. Outcome at 6 months was classified as straight (orthophoria), under-correction or overcorrection. All patients with a postoperative deviation <10 prism diopters at 6 months were considered to have a successful outcome (orthophoria). Best-corrected postoperative visual acuity was also documented.

A consultant pediatric ophthalmologist completed the Strabismus Registry Forms. Chi-square test and Fisher's exact test in Stata 14 software (StataCorp LLC, USA) were used to analyze the association between identified variables and surgical outcome of strabismus surgery.

Results

Ninety-eight patients were recruited in this study. Of these, only 94 participants had complete data at 6 months after surgery. The majority were below 10 years of age (63 patients, 64.7%). The population sample was evenly distributed among males and females. Only 10.2% (10 patients) had a positive family history of strabismus. Most patients had alternating strabismus (65.3%, 64 patients). The majority (83.7%, 82 patients) had congenital/infantile strabismus, followed by 8.1% (8 patients) with accommodative type, 5.1% (5 patients) with sensory deprivation strabismus and 3.1% (3 patients) with abducens nerve palsy due to trauma.

The most common type of deviation was exotropia (58.2%, 57 patients). The majority (85.7%, 84 patients) had no amblyopia or had been successfully treated for amblyopia before surgery. Half of our cohort (51.0%, 50 patients) had large-angle deviations (more than 45 prism diopter). Binocular single vision was absent in most patients (63.3%, 62 patients). Approximately 85% (83 patients) had a best-corrected visual acuity of 6/12 and better before the surgery. The above data are presented in Table 1.

Table 1

Demographic and preoperative clinical characteristics

Characteristic	n (%)
Gender
Male	47 (48.0)
Female	51 (52.0)
Age at surgery (years)
0-5	31 (31.6)
6-10	32 (32.7)
11-15	11 (11.2)
16-20	6 (6.1)
>20	18 (18.4)
Family history
Yes	10 (10.2)
No	55 (56.1)
Unknown	33 (33.7)
Laterality
Alternating	64 (65.3)
Unilateral	34 (34.7)
Cause of strabismus
Congenital	82 (83.7)
Accommodative	8 (8.2)
Sensory deprivation	5 (5.1)
Paralytic	3 (3.1)
Amblyopia
Yes	14 (14.3)
No	84 (85.7)
Type of squint
Esotropia	41 (41.8)
Exotropia	57 (58.2)
Preoperative deviation (PD)
<20	8 (8.2)
21-45	40 (40.8)
>45	50 (51.0)
Binocular single vision
Present	22 (22.4)
Absent	62 (63.3)
Not able to examine	14 (14.3)
Preoperative best-corrected visual acuity
6/12 and better	83 (84.7)
6/18-6/60	11 (11.2)
6/60 and worse	4 (4.1)

PD=Prism diopter

Eighty-two patients (83.7%) had surgery performed on two muscles, which were a recession of medial or lateral rectus on both sides. The remaining patients had eye deviations of more than 50 prism diopter and required three/four muscle surgeries. These included recessions of medial rectus on both sides and unilateral/bilateral resection of lateral rectus (for esotropia) and lateral rectus recession on both sides and unilateral/bilateral medial rectus resection (for exotropia). None of these patients had concurrent vertical muscle surgery, either inferior oblique, superior rectus, or superior oblique muscles.

The success rate was 81.6%, in which 80 patients achieved satisfactory alignment at 6 months postsurgery. Eighty-seven patients (88.8%) had an uneventful postoperative outcome. Suture granuloma (4.4%, 4 patients) was the main complication observed during the first 1 month postoperative period. A transient postoperative diplopia was observed in an adult patient (1%) with preoperative deviation of 45 prism diopter and had achieved a successful surgical outcome. Six months' poststrabismus surgery, 91.8% (90 patients) had best-corrected visual acuity of 6/12 and better. These figures are tabulated in Table 2.

Table 2

Intra- and postoperative characteristics

Intra- and postoperative details	n (%)
Number of muscles operated
1	0
2	82 (83.7)
3	12 (12.2)
4	4 (4.1)
Postoperative deviation
≤10 PD	80 (81.6)
>10 PD	14 (14.3)
Incomplete data	4 (4.1)
Postoperative complication
Postoperative infection	1 (1.0)
Conjunctival cyst/scar	1 (1.0)
Suture granuloma	4 (4.4)
Diplopia	1 (1.0)
Nil	87 (88.8)
Incomplete data	4 (4.1)
Postoperative best-corrected visual acuity
6/12 and better	90 (91.8)
6/15-6/60	4 (4.1)
6/60 and worse	0
Incomplete data	4 (4.1)

PD=Prism dioptre

We found no significant association between demographic (i.e., age of onset and gender), preoperative factors (i.e., amblyopia before surgery, type of deviation, and preoperative amount of deviation), and surgical outcome at 6 months' postoperative period (P > 0.05). Table 3 shows the P values for each of these variables. Likewise, postoperative best-corrected visual acuity was not statistically associated with the surgical outcome at 6 months' postoperation (P = 0.588).

Table 3

Analysis of factors affecting horizontal strabismus surgery at 6 months

Factors	Outcome			Fisher’s exact value	P
	Straight, n (%)	Under-correction, n (%)	Over-correction, n (%)
Gender
Female	36 (50.7)	12 (54.5)	1 (100.0)	0.982	0.901
Male	35 (49.3)	10 (45.5)	0
Age at surgery (years)
0-5	24 (33.8)	5 (22.7)	1 (100.0)	9.298	0.314
6-10	24 (33.8)	7 (31.8)	0
11-15	6 (8.5)	5 (22.7)	0
16-20	6 (8.5)	0	0
>21	11 (15.5)	5 (22.7)	0
Amblyopia before surgery
No	59 (83.1)	21 (95.5)	1 (100.0)	2.508	0.387
Yes	12 (16.9)	1 (4.5)	0
Type of deviation
Esotropia	33 (46.5)	6 (27.3)	1 (100.0)	3.774	0.087
Exotropia	38 (53.5)	16 (72.7)	0
Preoperative deviation (PD)
<20	6 (8.5)	2 (9.1)	0	4.510	0.365
21-45	33 (46.5)	6 (27.3)	0
>45	32 (45.1)	14 (63.6)	1 (100.0)
Postoperative best-corrected visual acuity
6/12 and better	67 (94.4)	22 (100.0)	1 (100.0)	2.202	0.588
6/15-6/60	4 (5.6)	0	0

PD=Prism diopter

Table 4 describes the characters of 14 patients who have postoperative deviation larger than 10 prism diopter. Majority are female (64.3%, 9 patients) and age <15-year-old (92.9%, 13 patients). Exotropia is more common (64.3%, 9 patients). 71.4% (10 patients) had preoperative deviation of more than 45 prism diopter. None of them had amblyopia.

Table 4

Characteristics of patients with postoperative angle deviation larger than 10 prism diopter (n=14)

Factors	n (%)
Gender
Female	9 (64.3)
Male	5 (35.7)
Age at surgery (years)
0-5	4 (28.6)
6-10	4 (28.6)
11-15	5 (35.7)
16-20	0
>21	1 (7.1)
Amblyopia before surgery
No	14 (100.0)
Yes	0
Type of deviation
Esotropia	5 (35.7)
Exotropia	9 (64.3)
Preoperative deviation (PD)
<20	0
21-45	4 (28.6)
>45	10 (71.4)
Refractive error (spherical equivalent) D
−1.00 DS-0.00	3 (21.4)
0.00-+1.00 DS	9 (64.3)
+1.00-+2.00 DS	2 (14.3)
Postoperative best-corrected visual acuity
6/12 and better	14 (100.0)
6/15-6/60	0

PD=Prism dioptre, D=Diopter, DS=Dioptre sphere

Discussion

Reported success rates of strabismus surgery vary widely, ranging from 35.6% to 80.5%.[345678] However, the criteria for successful surgery vary among researchers. Most researchers defined according to motor criteria, or a post-operative deviation of 5–10 prism diopter esotropia or 10–15 prism diopter exotropia.[3467] Table 5 summarizes published reports of strabismus surgery outcome in Asian countries over the past 15 years, including our study.

Table 5

Published articles regarding outcome of strabismus surgery in Asian countries

Authors	Years	Country	Type of strabismus	Age (years), mean±SD	Success rate (%)	Preoperative deviation (PD), mean±SD	Complication rate (%)	Recurrence rate (%)	Amblyopia (%)	Refractive error (diopter), mean±SD
Kampanartsanyakorn et al.[3]	2005	Thailand	Esotropia, exotropia	10.50±10.2	60.2	44.90±16.6	0.7	12.9	-	-
Jung et al.[4]	2016	South Korea	Exotropia, intermittent exotropia	37.70±13.6	72.0	51.40±17.9	-	10.0	3.0	Right eye: −2.20±3.0 Left eye: −2.00±2.8
Kim et al.[5]	2015	South Korea	Group 1: Intermittent exotropia with hyperopia	7.02±4.48	60.4 (motor) 55.1 (motor and sensory)	27.03±6.20	-	-	-	+2.05±1.47
			Group 2:Intermittent exotropia with emmetropia	7.73±3.94		26.85±6.14				0.07±0.44
			Group 3: Intermittent exotropia with Myopia	10.47±6.35		26.54±7.86				−2.39±1.42
Yam et al.[6]	2013	Hong Kong	Exotropia, intermittent exotropia	70.50±46.7 (months)	64.0 (6 weeks) 48 (1 year)	31.00±9 (successful group) 39.00±13 (unsuccessful group)	-	10.0	16.0	−0.27±1.6
Raiyawa et al.[7]	2015	Thailand	Exotropia	31.20±14.5	69.0 (last follow up) 75.0 (after 2 years)	62.10±10.8	-	-	-	-
Yang et al.[8]	2016	South China	Intermittent exotropia	13.70±8.8	80.5 (motor) 35.6 (motor and sensory)	40.00±16 (near) 41.00±14 (distant)	-	-	4.2	Right eye: −0.84±2.69 Left eye: −0.72±2.58
Present study	2019	Malaysia	Esotropia, exotropia	12.73±12.03	81.6 (6 months)	46.13±16.93	6.8	2.2	14.3	+0.10±2.05

SD=Standard deviation, PD=Prism dioptre

We documented a success rate of 81.6% at 6 months' postsurgery. Our outcome criteria were similar to criteria used by Kampanartsanyakorn et al. from Thailand.[3] Success rates of strabismus surgery tend to be higher (60%–80%) when satisfactory alignment is the sole criterion for a successful outcome.[345678] We did not include measurement of sensory outcome during the postoperative period. We experienced significant difficulties in performing tests of stereopsis in our cohort of young children, especially in those aged <7-year-old.

Age at surgery,[369] preoperative deviation,[34671011] amblyopia,[4] refractive error,[510] and postoperative deviations[4] have been reported to influence the outcome of strabismus surgery. In contrast, our study found no significant association between these factors and a successful outcome of strabismus surgery at 6-month postoperative period.

We found no significant association between the age of surgery and outcome of surgery at 6 months' postoperative period (P = 0.314). The outcome of surgery was similar in both younger and older patients in our review. Approximately 64.3% of our patients were children younger than 10-year-old. Good postoperative outcome has been reported in published studies involving Asian children.[358] Jung et al. and Raiyawa et al. also revealed satisfactory postoperative results in adult patients with exotropia.[47]

In contrast to the above studies, Yam et al. observed that older age at surgery was associated with early surgical success.[6] This was explained by a more accurate measurement of preoperative deviation for older children. Second, children with intermittent deviation usually need surgery later because they have better control, stereopsis, and fusion.[6] We agree that inaccuracy in the measurement of the angle may lead to unpredictable and unfavorable results in uncooperative young children. Thus, more than a single clinical assessment is essential when planning for strabismus surgery.

Our analysis showed the absence of a significant association between the amount of deviation and success rate of ocular alignment (0.365). The majority (91.8%) of our patients had angle deviation of more than 20 prism diopter exotropia or esotropia before the surgery. This was probably due to the large number (83.7%) of patients with strabismus of congenital or infantile causes. Our findings are consistent with Jung et al., and Raiyawa et al., who achieved a high success rate of surgery (72%–75%) despite larger preoperative deviations (51.40 ± 17.9 PD and 62.10 ± 10.8 PD, respectively).[47]

In contrast, Yam et al. noted that a smaller presurgery distance deviation was associated with a smaller final distance deviation at 1-year postsurgery, and thus a more favorable long-term surgical outcome.[6] Kampanartsanyakorn et al. believed that this was partly because small angle deviations can be more accurately measured than large-angle deviations.[3] They reported that successful surgery was related to preoperative deviation <30 prism diopters.[3] Gezer et al. also concurred that patients with smaller degrees of preoperative deviation tended to have more favorable outcomes of surgery.[10]

We also noted the absence of any significant association of amblyopia and outcome at 6 months after the surgery (P = 0.387). Our study documented a small number of patients (14.3%) who had amblyopia before they underwent surgery. These patients had monocular patching of a maximum of 2 h/day and were monitored closely during the follow-up visits. Their preoperative best-corrected visual acuities ranged from 6/60 to 6/18.

These findings are in agreement with Jung et al., who observed a relatively high success rate (72.0%) in a group with a low prevalence (3%) of amblyopia.[4] Kampanartsanyakorn et al. explained that older patients undergo surgery for cosmetic reasons, and the presence of amblyopia in these patients might affect the ocular alignment during the postoperation period.[3] We strongly believe that all patients require optimum time for amblyopia treatment before strabismus surgery to reduce the chance of strabismus recurrence.

We found that postoperative visual acuity had no significant association with surgical outcome (P = 0.588). 91.8% of our patients had best-corrected visual acuity of 6/12 or better at 6 months after the surgery. This is likely related to a high percentage (65.3%) of alternating type of strabismus in our review. The remaining subjects (34 patients, 34.7%) with unilateral strabismus underwent intensive amblyopia treatment preoperatively; 20 patients showed improvement of best-corrected visual acuity before the surgery, while 14 patients (14.3%) were still on patching treatment when the surgery was performed.

Conclusion

The success rate of strabismus surgery in our institution was parallel with published reports from other Asian countries. The postoperative best-corrected visual outcome was good. Age of onset, gender, presence of amblyopia before the surgery, type of deviation, amount of deviation, and postoperative best-corrected visual acuity did not influence the outcome of horizontal strabismus surgery in our patients.

Financial support and sponsorship

This study was supported by Universiti Sains Malaysia Usains Grant (No. 0310000998520). The funding organization had no role in the design or conduct of this research.

Conflicts of interest

The authors declare that there are no conflicts of interest of this paper.