Postoperative outcome and influencing factors of strabismus surgery in infants aged 1–6 years

The mean preoperative AOD in patients with esotropia was a mean of 21.46 ± 7.63/17.27 ± 8.26 (near/distance) respectively and improved to 5.50 ± 9.69/4.62 ± 7.71 at first follow up and to 5.62 ± 8.16/6.20 ± 5.30 at longterm follow up (p < 0.001) (Fig. 1).

The mean preoperative AOD in patients with exotropia was 15.36 ± 23.35/14.17 ± 16.95 (near/distance), 17.09 ± 16.17/13.71 ± 12.93 at first follow-up and 5.92 ± 5.12/0.43 ± 13.87 at longterm follow-up (p = 0.004) (Fig. 2).

AOD in patients with isolated strabismus sursoadductorius improved from 12.00 ± 7.02/9.14 ± 11.31 (near/distance) preoperatively to 3.05 ± 3.67/1.14 ± 3.23 at first follow-up and 0.03 ± 10.54/0.71 ± 10.24 at longterm follow-up (p = 0.037) (Fig. 3).

The mean age of the patients was 3.72 ± 1.36 years at time of the preoperative BCVA measurement, 3.77 ± 1.38 years at first follow-up and 4.88 ± 1.52 years at longterm follow-up.

The Cardiff test was used for examining 12.90% of the children in this study while the remaining 87.10% were tested through the Lea test.

Change in AOD was significant in all three patient groups. Results also show improvement in other subcategories (elevation in adduction, BCVA and refractive error) but were not statistically significant. Multivariate logistic regression was performed to evaluate influencing factors of strabismus surgery. Table 6 shows the individual parameters and the results after the statistical analysis was conducted.

The age at which strabismus surgery should be performed has been a subject for debate for many years. Early surgery can minimize harmful habits due to squinting and long lasting sensory changes but later surgery can lead to more accurate measurements and avoid unnecessary surgery [11‐13] According to Awadein et al. [14] an age greater than 12 years old correlated with a worse surgical outcome. Richard et al. [12] showed that there was no significant difference in strabismus surgery outcome in the age group of 0–6 years. In our study, 50.00% of patients with a successful outcome were between 3 and 4 years old while 31.25% were younger and 1–2 years old. No significant difference was found in relation to age (p = 0.38).

Abbsoglu et al. [15] and Kampanartsanyakorn et al. [16] linked a larger preoperative AOD to a poorer surgical outcome. In this study, however, the preoperative AOD had no statistically significant influence on the surgical outcome (p = 0.38). A study from 2020 by Waheeda-Azwa et al. [17] reported similar findings to ours with a success rate of 81.60% that did not show a significant correlation to the preoperative AOD, with many patients having a large AOD. 13 out of 46 patients with a successful outcome in this study had a preoperative AOD of more than 20°, which could be explained by the young age group with possible congenital strabismus causes [17, 18].

Improvement in AOD was statistically significant in all patient groups. Patients with esotropia showed the greatest improvement (p < 0.001) followed by patients with isolated strabismus sursoadductorius (p = 0.037) and patients with exotropia (p = 0.004). Surgical success rate was 74.19% (n = 46) which correlates to findings of other similar studies [16, 19, 20]. Another study conducted by Kumari et al. [21] also reported a poorer success rate for exotropia in comparison with esotropia. A possible reason could be the inclination to postoperative drift in exotropia patients which results in a slightly poorer outcome.

Surgical management of exotropia includes the deliberate esotropic overcorrection as the initial postoperative result since it has been shown to provide good longterm outcomes [22‐25]. But several studies have reported that initial overcorrection does not correlate with a more favorable longterm outcome [26‐29]. Additionally, overcorrection in visually immature individuals, such as children aged 1–6 years, may be associated with consecutive esotropia, loss of stereopsis and amblyopia [24, 30]. It was also linked to a higher reoperation rate [28].

Improvement in elevation in adduction was shown in all three patient groups (p = 0.212). As Stager et al. [31] reported, more than 70% of patients with infantile esotropia and more than 30% of patients with intermittent exotropia have primary inferior oblique overaction. Overelevation in adduction has many possible causes that are difficult to correct with one surgical intervention. The study of Stager et al. [31] also reports that residual or recurrent over elevation in adduction is common.

Visual acuity did not show a significance in relation to the surgical success. The results of this study can be explained by the infantile patient group with the strabismus surgery taking place before visual maturity has fully occurred. Kampanartsanyakorn et al. [16] conducted a retrospective study of 304 patients to analyze the outcomes of horizontal strabismus surgery and found that preoperative visual acuity did not significantly influence the surgical success. The results of the study by Ganguly et al. [32] were similar and showed no significant change in visual acuity and binocular vision after strabismus surgery.

The refractive error showed general improvement (p = 0.109) and was measured in the preoperative and long-term follow-up without measurements immediately after surgery. Several other studies conclude that strabismus surgery usually leads to short-term improvement regarding the refractive error with a long-term regression due to compensation or other postsurgical factors [33‐36].

Obtaining the information on binocular and stereovision using the Lang test was only partially possible due to the young age of the examined patient group with a mean of four years. The data did not suffice for an in-depth analysis of binocular vision and stereovision. However, there is a trend toward postsurgical improvement with the percentage of a positive Lang test result rising from 16.67% to 38.46%. Children with a positive postsurgical Lang test result had a diagnosis of esotropia or strabismus sursoadductorius. The findings correlate with those of the study conducted by Enz et al. [37] who observed in their study a general improvement without a significant change in binocular vision and stereovision after surgery.

The critical time period for developing stereopsis starts at around 3 months after birth with further maturity until around 18 months of age [38‐41]. Afterwards stereopsis gradually improves until the child is up to 3 years old [42]. Stereopsis was measured with the Lang test which is based on the random-dot technique and shows a cat, a star and a car representing disparities of 1200, 600 and 550 s of arc. The child then has to correctly name the objects [43]. A study by Fawcett et al. [44] investigated the critical period of susceptibility of stereopsis in patients with infantile esotropia and reported an onset at 2.4 months of age and the peak at 4.3 months of age. Other studies also reported the most rapid stereopsis development during the first 12 months after birth [39, 45]. Since we examined children aged 1–6 years in our study and do not have stereovision test results of all patients, we cannot make statements on the influence of strabismus on the development of stereopsis based on our data. We therefore suggest additional studies to further investigate the susceptibility of stereopsis that focus on the period of rapid maturity during the first year of life.

Certain limitations must be discussed as well. As a retrospective study we could only extract the data that were already collected and could not initiate further testing. But with this study we could show the outcome of strabismus surgery and influencing factors in an infantile age group. As a practical takeaway for the prospective surgical treatment of strabismus, the study shows that the overall success rate of strabismus surgery in infants is satisfactory at 74.19%.