Corresponding Author: Seung-Hyun Kim, MD, PhD. Department of Ophthalmology, Korea University Anam Hospital, 73 Goryeodae-ro, Seongbuk-gu, Seoul 02841, Korea. In the surgical management of intermittent exotropia, widespread agreement exists that an initial overcorrection is required because of the tendency towards postoperative exotropic drift [1,2]. Early overcorrection has been ascribed either to the induction of diplopia and stimulation to moving the fixated image out of previously induced temporal suppression scotoma [3,4]. Cho and Kim [5] reported that the greater degree of initial overcorrection lowered the recurrence rate, but it induced higher possibility of consecutive esotropia. However, consecutive esotropia did not occur only on overcorrection below 5 prism diopters (PD) in the early stage after the surgery. They also observed that the recurrence rate was not different according to the amount of overcorrection. Raab and Parks [6] found that an overcorrection of 10 to 20 PD gave the best outcome, although good outcomes were also obtained with overcorrections in the range of 0 to 10 PD [7].
We hypothesized that the modified Park’s normogram by Wright of bilateral lateral rectus recession (BLR) is for targeting the initial overcorrections of 10 PD or higher [8]. We have performed BLR with reduction of 1.0 mm from the original normogram since 2014. The purpose of this study was to identify the amount of initial overcorrection resulting from our modified normogram and to assess whether the overcorrection is appropriate in the surgical management of intermittent exotropia in terms of the long-term success rate, recurrence rate, and overcorrection rate.
Materials and Methods
Ethics statement
This study was approved by the Institutional Review Board of Korea University Anam Hospital ( No. 2023AN0509). The requirement for informed consent was waived due to the retrospective nature of the study. The study adhered to the principles of the Declaration of Helsinki.
Study design and setting
We retrospectively reviewed the clinical records of patients who underwent BLR between October 14, 2014, and June 14, 2020, for basic type intermittent exotropia. Basic type intermittent exotropia was classified when the degree of misalignment between deviations measured at distance and near was within 10 PD. Patients who have been diagnosed with intermittent exotropia accompanied by other strabismus or infantile exotropia, who have been diagnosed with other eye diseases including optic nerve diseases and amblyopia, who have undergone any other intraocular surgery or strabismus surgery other than BLR, who underwent surgery with a reduced amount of correction due to lateral incomitance, who over 10 years of age at the time of surgery, and patients with follow-up less than 3 years after surgery were excluded.
Preoperative examination
We analyzed the patient’s age at the time of surgery, sex, preoperative angle of deviation at distance and near, surgical amounts, postoperative angle of deviation at each visit, duration of postoperative diplopia, distance suppression, and follow-up period. Suppression was evaluated using vectogram (vectographic projector test, L29; Luneau Technology). Alternate prism cover test for assessing near and distance fixation was performed at 1/3 and 6 m in primary gaze. Ocular dominance was assessed with the cover-uncover test. The negative numbers mean esodeviation and exodeviation was denoted by positive values.
Surgical procedures
All surgeries were performed by a single surgeon (SHK) under general anesthesia. The postoperative goal for all patients included in this study was a small overcorrection around 5 PD of esodeviation [5]. All surgeries were performed by reducing the total amount by 1.0 mm from the correction level of original normogram. In addition, surgery was performed with a slight reduction in the surgical dosage for the dominant eye compared to the nondominant eye. For example, when a 25 PD exodeviation at distance was shown, the BLR was corrected by 6.0 mm for each eye according to the original normogram. However, in our modified normogram of BLR, recession was performed by reducing the total amount by 1.0 mm, which means 6.0 mm in the nondominant eye and 5.0 mm in the dominant eye (Table 1).
Postoperative management
We allowed for a reduction in the period of maintaining alternative patching, which accelerated the recovery of patients’ daily life after strabismus surgery [9,10]. Patients who experienced recurrence during the follow-up period were frequently followed up at 1- to 2-month intervals and were treated with nonsurgical methods such as refractive correction or patching to stimulate fusional convergence [11]. Reoperation for recurrent patients was performed when patients had distant suppression in either eye and ≥15 PD exodeviation. Reoperation for overcorrected patients was performed if >10 PD esodeviation lasting for >6 months postoperatively.
Surgical outcomes
In this study, the surgical outcome was evaluated at 1 day, 1 month, 6 months, 1 year, 2 years and 3 years postoperatively. A satisfactory outcome was defined as cases in which distant angle of deviation in primary gaze ranged between ≤10 PD of exotropia and ≤5 PD of esotropia. Recurrence was defined as the presence of >10 PD of exotropia, and overcorrection was defined as the presence of >5 PD of esotropia.
Statistical analysis
We used IBM SPSS ver. 21.0 (IBM Corp) for the statistical analysis. Kaplan-Meier survival analysis was used to evaluate cumulative probabilities of recurrence. Linear regression analysis was performed to analyze the correlation between initial overcorrection and long-term angle of deviation at distance. A statistically significant difference was defined as a case where the p-value was less than 0.05.
Results
Of the 242 patients enrolled in the study, 113 (46.69%) were male, and 129 (53.31%) were female. The mean age at surgery was 5.40 ± 1.63 years (range, 4-9 years). A total of 242 patients underwent BLR by our modified normogram and had follow-up at least more 3 years after surgery. Their mean preoperative distant angle of deviation was 29.32 ± 3.92 PD (range, 20-40 PD) and mean preoperative near angle of deviation was 29.88 ± 4.41 PD (range, 20-40 PD). The postoperative follow-up period was 38.60 ± 7.99 months (range, 36-96 months) (Table 2).
Mean surgical amount was 12.50 ± 1.23 mm bilaterally. The patients’ mean postoperative angle of deviation at distance and near on postoperative day 1 was −6.22 ± 1.44 and −6.22 ± 1.49 PD, respectively. The duration of postoperative diplopia was 12.64 ± 4.61 days. At postoperative day 1, overcorrection within 6 PD of esodeviation and overcorrection by more than 6 PD of esodeviation was seen in 168 (69.42%) and 63 patients (26.03%), respectively. In addition, undercorrection with exophoria was noted in 11 patients (4.55%) at postoperative day 1. Only two cases exhibited an initial overcorrection with an angle of esodeviation ≥10 PD on the first postoperative day.
Surgical outcomes at each postoperative time point showed that recurrence rate increased until 2 years postoperatively and overcorrection rate decreased from postoperative 6 months. Patients with overcorrection were 10 (4.13%) at postoperative 1 month and 18 (7.44%) at post-operative 6 months. At postoperative 1, 2, and 3 years, 13 (5.37%), 10 (4.13%), and seven patients (2.89%), respectively, experienced overcorrection. From overcorrection patients, four patients underwent medial rectus recession, 27 patients were treated with base out prism treatment, and one patient was treated with patching. Long-term success rate, recurrence rate, and overcorrection rate at postoperative 3 years was 77.27% (187 of 242), 19.83% (48 of 242), and 2.89% (7 of 242), respectively (Table 3 and Fig. 1). Ka-plan-Meier survival analysis demonstrated cumulative probability of recurrence at 3 years after surgery was 34.71% (Fig. 2). A significant positive correlation between the angle of deviation at postoperative 1 day and 3 years was observed (r = 0.22, p = 0.001) (Fig. 3).
Among 48 recurrent exotropia patients, only nine patients who had suppression at either eye underwent reoperation. Among the cumulative cases with consecutive esotropia during the follow-up period in this study, delayed-onset consecutive esotropia, which initially reached orthotropia within the first postoperative month and was followed by the esotropia, was observed in 87.5% (28 of 32), while continuous consecutive esotropia with persistent esotropia without achieving orthotropia was seen in 12.5% (4 of 32). We achieved the final success rate of 83.06%, including patients who experienced a relapse but reached the success range through nonsurgical treatments during the follow-up period.
Discussion
Postoperative initial overcorrection in intermittent exotropia has been accepted over time due to exotropic drift [1]. However, the degree of postoperative intentional overcorrection is still controversial for each surgeon. Raab and Parks [6] determined the surgical amount to be about 10 to 20 PD esodeviation for BLR. We performed BLR by reducing 1.0 mm for one eye, with the goal of small overcorrection around 5 PD esodeviation.
Our modified normogram, which involved slightly reducing the surgical amount for dominant eye, resulted in asymmetric surgical amounts in both eyes. Cho et al. [12] thought that intermittent exotropia would have a higher recurrence rate in the deviating eye preoperatively. Therefore, they performed an asymmetric BLR method, which involved more augmented recession on the deviating eye compared to the dominant eye. They reported a success rate of 78.5% at 24 months postoperatively, suggesting better outcomes compared to symmetric BLR. Inspired by this approach, we reduced the amount of recession in the fixing eye while maintaining the surgical amount in the deviating eye in our modified normogram.
Basic type intermittent exotropia is a type in which there is more concern about the optimal surgical approach and also appears to be the more recurrence rate than the overcorrection rate [13]. Kushner [14] recommended that augmenting the dose of BLR could offer potential benefits to patients with basic type intermittent exotropia. However, McNeer [15] suggested that the overcorrection of 0 to 10 PD is proper, similar to our study. A survey study targeting strabismus specialists who had performed intermittent exotropia surgery for over 15 years found that, in basic type intermittent exotropia cases, 40% (10 of 25) aimed for an initial overcorrection of 11 to 15 PD on postoperative day 1 [16]. Cho et al. [16] reported that 28% (7 of 25) aimed for esotropia of 10 PD or less.
Previous studies have reported success rates of basic type intermittent exotropia ranging from approximately 50% to 80% [17]. In a study by Wang et al. [18], long-term recurrence rate of basic type intermittent exotropia was 23.7% (9 of 38) after a mean follow-up of 14.8 ± 9.5 months. They determined surgical dose according to conventional normogram and classified groups as following: orthophoria was defined as 5 PD esodeviation to 8 PD exodeviation, undercorrection was defined as >8 PD exodeviation and overcorrection was defined as >5 PD esodeviation. Surgical outcomes on postoperative 1 to 3 days showed 7.9% (3 of 38) belonged to undercorrection group. Bang et al. [19] reported recurrence rates of 0% (0 of 37) until 6 months, 5.4% (2 of 37) for 1 year, 16.2% (6 of 37) for 2 years, 40.5% (15 of 37) for 3 years, and 45.9% (17 of 37) for 5 years, including 37 patients with basic type intermittent exotropia who underwent BLR based on conventional normogram. In our study, the recurrence rate of basic type intermittent exotropia undergoing BLR according to our modified normogram was 19.83% (48 of 242) at postoperative 3 years. Despite undergoing reduced surgical amounts of BLR, the results of our study were comparable to those of previous studies, indicating that lesser recession did not result in a markedly higher incidence of undercorrection or recurrence.
In this study, the peak recurrence rate was observed in the second year after surgery. Several studies have shown that the exotropic drift occur within 2 years of the surgery after patients recover most of orthotropia, especially during the several months after the surgery, angle of deviation is unstable and variable [2]. Bang et al. [19] also reported that the recurrence of exophoria or exotropia most commonly occurred within the 2- to 3-year period following BLR. The majority of consecutive esotropia cases in this study occurred within 6 months postoperatively. Instability of the angle of deviation appears in the early stage of intermittent exotropia surgery because of period of readaptation to the changed fusional divergence [20]. Ruttum [21] also mentioned the uncertainty of initial eye alignment after surgery for exotropia.
A study, which retrospectively analyzed 111 patients who underwent surgery for intermittent exotropia and divided into four subgroups according to the amount of postoperative overcorrection (ortho, ≤5, 6-10, and >10 PD subgroups), found that the lower the degree of postoperative initial overcorrection, the higher the recurrence rate and the lower the overcorrection rate [5]. In addition, Oh et al. [22] retrospectively reviewed 365 patients who underwent surgery for exotropia and analyzed the relationship between the angle of deviation at postoperative day 1 and the last visit, with an average follow-up period of 18.4 months. They reported that the subgroup with an initial overcorrection of >10 PD at postoperative day 1 had the lowest recurrence rate at the last visit. Similarly, a positive correlation between the degree of postoperative initial overcorrection and the long-term angle of deviation at postoperative 3 years was observed in this study (Fig. 3).
Our study had several limitations. First, this study was a retrospective design. Second, it was a single-center study. Furthermore, this study does not include a group of patients who underwent surgery based on the modified Park’s normogram by Wright. Thus, this study cannot claim superiority of our modified normogram over the conventional normogram. Future research that includes a comparison with patients undergoing surgery according to the conventional nomogram will be needed. Despite these limitations, the present study holds significance in being the first to present, through long-term outcomes, the feasibility of reducing surgical amounts without significant complications.
In conclusion, we achieved successful long-term surgical outcomes in the management of basic type intermittent exotropia using a modified normogram of BLR for targeting small overcorrection. Importantly, this modified normogram effectively reduced the incidence of consecutive esotropia resulting from unnecessary overcorrection.
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