Reported success rates of strabismus surgery vary widely, ranging from 35.6 to 80.5% [3-8]. 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 dioptre esotropia or 10-15 prism dioptre exotropia [3-4,6-7]. Table 4 summarizes published reports of strabismus surgery outcome in Asian countries over the past 15 years, including our study.
We documented a success rate of 81.6% at six months post-surgery. Our outcome criteria were similar to criteria used by Kampanartsanyakorn et al. from Thailand . Success rates of strabismus surgery tend to be higher (60 to 80%) when satisfactory alignment is the sole criterion for a successful outcome [3-8]. We did not include measurement of sensory outcome during the post-operative period. We experienced significant difficulties in performing tests of stereopsis in our cohort of young children, especially in those aged less than seven years old.
Age at surgery [3,6,9], pre-operative deviation [3-4,6-7,10-11], amblyopia , refractive error [5,10], and post-operative deviations  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 six month postoperative period.
We found no significant association between age of onset and outcome of surgery at six months post-operative 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 years old. Good post-operative outcome has been reported in published studies involving Asian children [3,5,8]. Jung et al. and Raiyawa et al. also revealed satisfactory post-operative results in adult patients with exotropia [4,7].
In contrast to the above studies, Yam et al. observed that older age at surgery was associated with early surgical success . This was explained by more accurate measurement of pre-operative deviation for older children. Secondly, children with intermittent deviation usually need surgery later because they have better control, stereopsis and fusion . We agree that inaccuracy in measurement of the angle may lead to unpredictable and unfavourable results in uncooperative young children. Thus, more than a single clinical assessment is essential when planning for strabismus surgery.
Our analysis showed absence of significant association between 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 dioptre exotropia or esotropia before the surgery. This was probably due to the large number (83.7%) of patients with congenital or infantile onset of strabismus. Our findings are consistent with Jung et al. and Raiyawa et al., who achieved a high success rate of surgery (72-75%) despite larger pre-operative deviations (51.40 ±17.9 PD and 62.10 ± 10.8 PD respectively) [4,7].
In contrast, Yam et al. noted that a smaller pre-surgery distance deviation was associated with a smaller final distance deviation at one year post surgery, and thus a more favourable long-term surgical outcome . Kampanartsanyakorn et al. believed that this was partly because small angle deviations can be more accurately measured than large angle deviations . They reported that successful surgery was related to pre-operative deviation less than 30 prism diopters . Gezer et al. also concurred that patients with smaller degrees of pre-operative deviation tended to have more favourable outcomes of surgery .
We also noted the absence of any significant association of amblyopia and outcome at six 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 2 hours per day and were monitored closely during follow-up visits. Their pre-operative 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 . 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 post-operation period . 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 post-operative 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 six 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 pre-operatively; 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.