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Clinical science
Lateral incomitancy and surgical results in intermittent exotropia
  1. Chang Ho Yoon,
  2. Seong-Joon Kim
  1. Department of Ophthalmology, Seoul National University College of Medicine, Seoul, Republic of Korea
  1. Correspondence to Dr Seong-Joon Kim, Department of Ophthalmology, Seoul National University College of Medicine, 101 Daehak-ro, Jongno-gu, Seoul 110-744, Republic of Korea; ophjun{at}snu.ac.kr

Abstract

Aim To evaluate the prevalence of lateral incomitance (LI) and its association with surgical outcome in intermittent exotropia.

Methods We retrospectively surveyed patients who had been followed up for 18 months or more after surgery for intermittent exotropia conducted from 1 September 2008 to 31 December 2010. Preoperative significant LI (preLI+) was defined as a decrease of >5 prism dioptres (PD) in exodeviation of distant gaze at lateral gaze. Postoperative significant LI (postLI+) was defined as a difference of >5 PD between distant and lateral gaze. Gender, age at surgery, binocular spherical equivalent, preoperative angle of deviation, type of intermittent exotropia, type of surgery, and stereopsis were investigated together with associations with LI and surgical results. Surgical results were analysed using data from a postoperative period of at least 18 months.

Results Of 155 patients, 63 (40.6%) had preLI+. Surgical failure including consecutive esotropia was not associated with preLI+ (p=0.140). In subgroup analysis, bilateral lateral rectus recession (BLR) and unilateral lateral rectus recession (ULR) procedures did not induce significant LI, but non-operated eyes with ULR showed reduced LI after surgery.

Conclusions Surgical outcomes in ULR and BLR for intermittent exotropia correction showed no association with preLI+. The prevalences of significant LI were unchanged after surgery in both groups.

https://doi.org/10.1136/bjophthalmol-2014-305132

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    Introduction

    Lateral incomitance (LI) is defined as decreased exodeviation in right or left lateral gaze compared with the primary position.1 While the mechanism of LI is obscure, pulley instability, impaired vergence facility, superior oblique palsy, mechanical limitation, and changes in muscle fibre type and distribution are considered possible causes.2–5

    The clinical implications of LI are controversial. Although it has been reported that preoperative significant LI (preLI+) is associated with surgical overcorrection in intermittent exotropia,1 ,6 some authors have suggested that preLI+ does not influence the surgical results.4 ,7 A significant degree of unilateral lateral rectus recession (ULR) is a potent risk for postoperative significant LI (postLI+), but there are few studies about change of LI after surgery.8–10

    There are limited studies on associated factors and prevalence of LI, and this is the first study involving statistical analysis of preoperative and postoperative LI and associated factors in intermittent exotropia. This study aimed to investigate influencing factors in preoperative and postoperative LI and to determine whether LI affects surgical outcome in intermittent exotropia.

    Methods

    This study was approved by our institutional review and aethical board (No. H-1306-082-498). The study protocol followed the tenets of the Declaration of Helsinki. Patients who underwent either the bilateral lateral rectus recession (BLR) or ULR for treatment of intermittent exotropia from 1 September 2008 to 31 December 2010 were enrolled.

    The minimum required follow-up period after surgery was 18 months. However, patients who required reoperation within 18 months after the primary surgery were included and only the patient data before reoperation were used for analysis. Ocular examination included measurement of uncorrected and best corrected visual acuities with all refractions performed under cycloplegia. Preoperatively, deviation was measured using the alternate prism and cover test at distance (6 m) and near (33 cm) in primary and lateral gaze, with appropriate spectacle correction when required. A base in prism was placed over the adducting eye in measurements of lateral gaze deviations.

    The minimum required vertical and/or oblique muscle surgery, or vertical transposition of horizontal muscles during intermittent exotropia surgery, or other ocular abnormalities were excluded. Patients were also excluded if they had paralytic or restrictive exotropia, duction and version abnormalities, trauma history including orbital wall fracture, amblyopia, or other systemic abnormalities including mental retardation and cerebral palsy. Amblyopia was defined as a difference of two or more lines in Snellen visual acuity charts between the best corrected visual acuity of each eye. Patients who had recovered from amblyopia before surgery were included.

    All surgeries were performed by a single surgeon (S-JK) using the surgical formula according to the Park's method.11 Surgical dosage was based on the angle of distant deviation. BLR was performed in patients with exodeviation of 20 prism dioptres (PD) or more. ULR was performed in patients with exodeviation of 25 PD or less. It is known that the surgical results of ULR and BLR in mild to moderate intermittent exotropia are equally effective.10 ,12 The selection of surgical procedure in patients with 20–25 PD exodeviation was made by the operating surgeon, who had no preference for BLR or ULR.

    PreLI+ was defined as a decrease of more than 5 PD in preoperative exodeviation at distance at right or left gaze. PostLI+ was defined as a difference of more than 5 PD in postoperative deviation at distance compared with right or left gaze.

    All patients received follow-up examinations at 1 week, 1 month, 3 or 6 months, and approximate 6-month intervals after the operation. Surgical success was defined as esophoria/tropia of <5 PD to exophoria/tropia of <10 PD according to postoperative angle of deviation at distance at the first examination from 18 months after surgery. Consecutive esotropia was defined as esotropia of at least 10 PD at distance at least once within 3 months after surgery.

    Stereopsis was measured using the Titmus Fly Stereo Test (Stereo Optical Co, Inc, Chicago, Illinois, USA). Stereoacuity of ≤100 s of arc was defined as good stereopsis. Preoperatively, standard nine gaze photographs were obtained for all patients.

    Statistical analyses were performed using PASW software for Windows (V.19.0, SPSS Inc, Chicago, Illinois, USA). p<0.05 was considered to be statistically significant. For the statistical analysis, Pearson χ2 test, Fisher's exact test, Kaplan–Meier survival test, independent t test and paired t test were used.

    Results

    Subject characteristics

    During the enrolment period, a total of 155 patients were included. Three patients underwent revision surgery within 18 months after the initial surgery. Of the 76 excluded patients, 41 had not undergone preoperative evaluation of lateral gaze; 13 underwent reoperation; 5 had other ocular disease such as congenital cataract, optic nerve atrophy, retinopathy of prematurity, or congenital ptosis; 4 had amblyopia; and 13 had central nervous system tumours, chromosomal abnormalities or cerebral palsy.

    Among the included subjects, 95 and 60 patients underwent BLR and ULR, respectively. We analysed the data of 81 subjects in whom postoperative lateral gaze deviation was measured. The average age of the patients at surgery was 6.46 (range 2–12) years. The mean preoperative exodeviation was 25.69 (range 15–47.5) PD.

    All three patients who underwent reoperation had consecutive esotropia. Reoperation was performed at 8, 8 and 12 months of follow-up. Type of surgery was not related to consecutive esotropia (p=0.284).

    A total of 63 (40.6%) subjects had preLI+. Type of exotropia, preLI+, surgical success, preoperative and postoperative good stereopsis and follow-up duration did not differ between the BLR and ULR groups. Table 1 gives a summary of the demographic characteristics of the patients.

    View this table:
    Table 1

    Patient characteristics in the bilateral lateral rectus recession group and the unilateral lateral rectus recession group

    Factors associated with preoperative and postoperative LI

    There was a significant difference between age at operation of patients with and without preLI+, with older subjects more likely to have preLI+. Good stereopsis were statistically associated with preLI+ (p=0.014). Overall surgical success was 54.2%, and there were no statistically significant differences (p=0.140) between preLI+ and preLI− (61.9% and 48.9%, respectively). When preLI+ was defined as a decrease of more than 10 PD, or 20%, or 30% of distant deviation at the right or left gaze, the surgical result was also not related to preLI+ (surgical success/preLI+=12/21 (57.1%), 39/67 (58.2%) and 29/48 (60.4%), p=0.818, 0.418 and 0.384 by χ2 test, respectively). There was no statistical relationship (p=0.272, Fisher’s exact test) between preLI+ and consecutive esotropia. None of the consecutive esotropia patients had preLI+. Table 2 gives a summary of the factors associated with preoperative LI.

    View this table:
    Table 2

    Factors associated with preoperative lateral incomitance

    PostLI+ was not associated with preLI+. However, postLI+ was more frequent among patients in the ULR group than the BLR group (21/51 (41.2%) vs 22/30 (73.3%), p=0.006). The incidence of surgical success was not significant when postLI+ was not evident, being present in 68.4% of such patients compared with 46.5% in the postLI+ group (p=0.072).

    Other factors associated with surgical outcomes

    The cumulative probabilities of success at 2 years after surgery were 56.9% in the preLI– group and 63.0% in the preLI+ group according to Kaplan–Meier life-table analysis, and there was no significant difference between the two groups (p=0.359, log-rank test; figure 1A). In subgroup analysis, the BLR group demonstrated cumulative success rates of 53.4% in the preLI– group and 59.8% in the preLI+ group (p=0.335), and the ULR group demonstrated cumulative success rates of 63.3% in the preLI– group and 67.0% in the preLI+ group (p=0.949), at 2 years postoperatively (figure 1B,C).

    Figure 1
    Figure 1

    Kaplan–Meier plots and cumulative probabilities of all, BLR group and ULR group success at 2 years after surgery according to life table analysis. (A) The preoperative LI (–) group showed 56.9% success after surgery and the preoperative LI (+) group showed 63.0% success after surgery. There was no significant statistical difference (p=0.359, log-rank test). (B) The BLR group demonstrated cumulative success rates of 53.4% in the preoperative LI (–) group and 59.8% in the preoperative LI (+) group (p=0.335). (C) The ULR group demonstrated cumulative success rates of 63.3% in the preoperative LI (–) group and 67.0% in the preoperative LI (+) group (p=0.949). BLR, bilateral lateral rectus recession; LI, lateral incomitance; ULR, unilateral lateral rectus recession.

    Changes of LI after surgery

    There was no association between preLI+ and postLI+ in the BLR and ULR subgroups (table 3). Absolute amount of LI was unchanged in BLR eyes (n=104; doubled because two eyes underwent surgery in BLR patients). In the ULR group, absolute amount of LI in the non-operated eye was decreased after surgery (p=0.003) and absolute amount of LI in the operated eye increased with only borderline significance (p=0.055).

    View this table:
    Table 3

    Changes of lateral incomitance after surgery

    Discussion

    The aim of this study was to evaluate the factors influencing preLI+ and postLI+ together with long-term surgical results of intermittent exotropia correction. In this study, preLI+ was not associated with surgical failure including consecutive esotropia regardless of surgical procedure in the treatment of intermittent exotropia.

    There are two ways of defining preLI+ in intermittent exotropia: by percentage and by absolute amount. Some studies define preLI+ as a reduction of the exoangle by more than 40%,13 one-third14 or 20%4 ,6 compared with the primary gaze, while others set the criterion as an absolute deviation difference of >10 PD4 ,7 ,15 or ≥5 PD.16 ,17 We defined preLI+ as a reduction of the exodeviation by more than 5 PD in the preoperative condition and postLI+ as >5 PD of change in the postoperative condition compared with the primary gaze. A fixed angle difference has limited value as a criterion in large-angle intermittent exotropia because a relatively small difference is regarded as significant LI. Therefore, we also analysed criteria of 20% and 30% reductions in preoperative LI to redeem this weakness.

    The prevalence of preLI+ in intermittent exotropia has been reported to range from 2.2% to 60.3%.4 ,15 ,16 ,18 The frequency of preLI+ was 41.4% in this study. This result is lower than Clarke and Noel's study,18 which reported that preLI+ was present in 60.3% of 78 patents when LI was defined as a 25% reduction. In the report by Jang and associates,19 preLI+ was present in 2.2% of 227 patients when preLI+ was defined as a 20% reduction. However, Jang and associates19 reduced the amount of surgery in some patients who were preLI+ and excluded patients who were preLI+ whose surgical dosage differed from those of patients with preoperative non-significant LI, which makes bias in the prevalence of preLI+.

    Concomitant subclinical 6th nerve palsy could lead to LI. However patients who had duction and version abnormalities were excluded in this study. Even if slight paresis is the cause of LI, it is hard to determine whether a healthy child has minor neurological dysfunction or paresis.

    Repka and Arnoldi5 suggested that improper positioning of the neutralising prism induces LI. For example, a base in prism over the abducting eye induces a smaller angle of deviation in the lateral gaze. The authors recommend that the prism always be placed over the adducting eye in measurements of lateral gaze deviations. All patients in this study were examined by one expert (S-JK) using a prism on the adducting eye. Patients in whom exact lateral gaze was not measured because of poor cooperation were also excluded. Thus, our patients’ results might not have included measurement errors.

    There is debate about whether preoperative LI affects surgical results. Clarke et al18 reported that preLI+ is not related to surgical outcome. In contrast to this result, there are a few reports that preLI+ is associated with poor outcome and reoperation.1 ,7 ,19 Moore et al20 suggested that the surgeon should reduce the amount of surgery when the deviation decreases by at least one-third compared with the primary position. Pineles et al7 reported that patients with LI have poor outcome in more than 10 years of follow-up. Although we performed the surgery by considering distant deviation and not LI, the incidence of surgical success and consecutive esotropia was not different with or without preLI+. This result did not change when the definition of preLI+ was changed to a decrease of more than 10 PD, 20% or 30% of distant deviation, at lateral gaze. Furthermore, none of the consecutive patients with exotropia showed preLI+. Our results support the hypothesis that preLI+ is not related to surgical outcomes, including consecutive esotropia.

    PreLI+ was related to older age and good preoperative stereopsis in the present study. Two possible mechanisms are increased cooperation among older subjects, which allowed more adequate judgment of the differences in gazes, and decreased angle at lateral gaze, which facilitates binocular fusion.

    It is reported that some patients had adduction limitation after medial rectus recession procedure.21 Similarly, large ULR reduces the force of abduction which can create LI after surgery. However, there are reports that significant incomitancy following ULR did not occur.17 ,22 Dadeya and Kamlesh8 suggested that marked recession of the lateral rectus can be performed without LI because the arc of contact of lateral rectus muscle is long and the functional equator lies far behind the anatomy. In this study, significant LI was not altered after surgery in the BLR and ULR groups in subgroup analysis. However, the amount of LI was decreased in the non-operated eye (p=0.003) and tended to increase in the operated eye in the ULR group (p=0.055). This can cause a higher incidence of PostLI+ in the ULR group than the BLR group. Considering these results, it is possible to reduce LI after surgery when the patient has asymmetric LI.

    There were limitations in our study. First, this was a retrospective study. Second, the result of the resection procedure was not included. Third, we only included patients in whom primary and lateral gaze could be measured. Young patients who could not perform examination procedures were excluded. Fourth, immediate postoperative change of LI was not detected by examining LI 18 months after surgery.

    To the best of our knowledge, this is the first study that statistically evaluated prevalence and clinically relevant factors of LI in intermittent exotropia in a large sample of patients. In conclusion, 40.6% of patients with intermittent exotropia had preLI+. PreLI+ did not relate to the surgical outcomes, including consecutive esotropia. Surgical procedure did not change the proportion of significant LI in BLR and ULR, but the ULR procedure decreased the absolute amount of LI in the non-operated eye.

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    View Abstract

    Footnotes

    • Contributors All authors made substantial contributions to conception, design, analysis, and interpretation of data. The two authors contributed to writing the article and approved the current version.

    • Funding This research was supported by Basic Science Research Programme through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012R1A1A2004809).

    • Competing interests None.

    • Ethics approval Institutional Review Board for Clinical Research at the Seoul National University Hospital (No. H-1306-082-498).

    • Provenance and peer review Not commissioned; externally peer reviewed.