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Clinical science
The relationship between preoperative and postoperative near stereoacuities and surgical outcomes in intermittent exotropia
  1. Dae Seung Lee,
  2. Seong-Joon Kim,
  3. Young Suk Yu
  1. Department of Ophthalmology, Seoul National University College of Medicine, Seoul, Korea
  1. Correspondence to Dr Seong-Joon Kim, Department of Ophthalmology, Seoul National University College of Medicine, #103 Daehak-ro, Jongno-gu, Seoul 110-799, Korea; ophjun{at}gmail.com

Abstract

Aims To classify patients with intermittent exotropia (IXT) according to the level of near stereopsis, evaluate postoperative stereoacuity and assess the surgical outcomes among groups.

Methods The records of 137 patients with IXT were reviewed. Patients were divided into three groups based on the degree of stereopsis: good (40–60 arcsec), moderate (80–200 arcsec) and poor (>200 arcsec). These groups were compared with each other with respect to preoperative and postoperative clinical features.

Results Poor stereopsis was found in 19 (13.9%) patients, moderate in 62 (45.2%) and good in 56 (40.9%) patients. There were no statistically significant differences among groups in the mean preoperative and postoperative exodeviations or in the rate of successful surgery. The preoperative and postoperative stereoscopic statuses were similar in each group. Patients with better stereopsis tended to be older when the IXT was first detected and showed better best-corrected visual acuity than those with poorer stereopsis (p=0.027, 0.005, respectively).

Conclusions Classification of IXT based on the degree of near stereopsis offers a useful tool for the assessment of initial sensory status as well as postoperative prognosis. Also, a lower age at first detection of IXT and a lower best-corrected visual acuity were associated with poor near stereoacuity.

  • Child health (paediatrics)

https://doi.org/10.1136/bjophthalmol-2013-304853

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    Introduction

    Numerous researchers have investigated the relationship between intermittent exotropia (IXT) and stereopsis. Patients with IXT typically exhibit bifoveal fixation and normal near stereoacuity.1–3 However, some patients with IXT exhibit reduced near stereoacuity.4 A recent cohort study demonstrated the deterioration of near stereopsis in children with IXT.5

    There are also a number of reports about the effect of strabismus surgery on stereopsis in patients with IXT. Some studies have demonstrated improved distance stereoacuity after invervention.6 ,7 The patients with XT who achieved superior sensory outcomes were those with motor realignment before age 7 years, strabismus duration of <5 years, greater stereoacuity before surgery, less central suppression and/or superior surgical alignment.6 ,8 ,9 The broad consensus maintains a relationship between distance stereoacuity, preoperative fusional state and stereoacuity.9 ,10 Nevertheless, there are controversies surrounding the effect of near stereoacuity in IXT. Some authors have suggested that realignment improves near stereoacuity, while others observed no change in near stereoacuity after realignment.7 ,10 ,11 Although the predictive value of preoperative near stereoacuity remains unclear, these results suggest the necessity of classifying IXT according to the degree of stereoacuity present.

    There is still no consensus about the importance of preoperative stereoscopic status among patients with IXT. The factors associated with preoperative stereoacuity have not been defined. There is no definite conclusion with respect to the effects of preoperative stereoacuity and surgical intervention on postoperative visual acuity and/or stereoacuity. This study sought to classify IXT based on the patient's stereoacuity and investigate the associated characteristics and the sensory and anatomical outcomes.

    Patients and methods

    The study included 137 consecutive patients with IXT who underwent unilateral recession of the lateral rectus or recession of bilateral lateral rectus from November 2008 to February 2012. All surgeries were performed by a single surgeon (SJK). Patients were included if they had taken at least two Titmus stereoacuity tests as well as the Worth 4-dot test, preoperatively and postoperatively, and had satisfied the minimum required postsurgical follow-up for at least 1 year. We excluded children who were <5 years old owing to the age-related differences in the normative data on near stereoacuity.12

    We excluded patients if they had a history of prior strabismus surgery; vertical strabismus over 5 prism dioptres (PD); systemic diseases or syndromes affecting visual acuities and/or strabismus; traumatic or paralytic strabismus; poor co-operation during stereopsis testing or the alternate prism cover test; convergence insufficiency type XT; and vertical or oblique muscle surgery. We obtained approval to conduct this study from the Institutional Review Board of the Seoul National University.

    Preoperative measurements

    The following patient characteristics were recorded: gender; age at first detection of strabismus; age at surgery; duration of the deviation; refractive error as determined using cycloplegic refraction; the presence of amblyopia; the presence of anisometropia; the angle of deviation at near and distance; the existence of an A or V pattern; lateral incomittance; the motor fusional state at far and near; and preoperative and postoperative sensory statuses.

    Prism and alternate cover testing were performed at 0.33 m and 6 m for all patients. During alternate cover test, none of the patients were measured after a patch test or on an outside distant target. Cycloplegic refraction was performed with 1% cyclopentolate hydrochloride. Refractive errors as measured by cycloplegic refraction were recorded as spherical equivalents.

    We defined anisometropia as a spherical equivalent difference of more than 1.50 dioptres (D) and amblyopia as a difference of at least two lines between right and left monocular visual acuities. The A pattern was defined as a divergence of at least 10 PD between upgaze and downgaze; the V pattern was defined as a divergence of at least 15 PD between downgaze and upgaze. Lateral incomittance was defined as a deviation of over 5 PD in right gaze or left gaze from the primary position. Sensory status was evaluated using the Titmus stereotest (Stereo Optical, Chicago, Illinois, USA) for measuring near stereoacuity and the Worth 4-dot test at distance and near for the degree of sensory fusion. Motor fusional control was also checked and graded as follows: 5=Constant XT, 4= XT lasting >50% of the exam prior to dissociation, 3= XT lasting <50% of the exam prior to dissociation, 2=No XT unless dissociated, recovers in >5 s, 1=No XT unless dissociated, recovers in 1–5 s, 0=No XT unless dissociated, recovers in <1 s (phoria).13 ,14 Stereotests were performed at least twice, and the best result was selected, in order to reduce variability. The Worth 4-dot criteria were as follows. Patients who detected four lights were considered to have fusion, those who saw five lights were considered as diplopic, and those who saw two or three lights were considered to be suppressing.

    Intraoperative procedures

    Each surgery was performed on the basis of the largest distance deviation among several alternate prism cover tests (at least twice). All procedures were conducted by a single surgeon (SJK), with the patient under general anaesthesia. In cases with XT of <20 PD, patients underwent unilateral lateral rectus recession. We selected the non-dominant eye for unilateral lateral rectus recession. If dominancy was unclear, we selected the eye randomly. Bilateral lateral rectus muscle recession was preferred in all remaining cases.

    Postoperative examination

    The postoperative angle of deviation was examined at the last follow-up examination, which occurred more than 12 months after the date of surgery in each case. The degree of surgical success, postoperative sensory status and the improvement in stereopsis were recorded.

    Surgical success was defined as alignment with less than 10 PD of XT or less than 5 PD of esotropia with alternate prism cover testing. Sensory status was measured as described above. Improved stereopsis was defined as a decrease of 2 octaves or more at the last follow-up examination. In line with previous studies, deterioration was defined as an increase of 2 octaves or more.5 ,15 We defined consecutive esotropia as an esodeviation of 5 PD or more that lasted for more than 4 weeks after the surgery.

    Classification

    We classified the patients with IXT into three groups based on the degree of stereopsis: good (40–60 arcsec), moderate (80–200 arcsec) and poor (>200 arcsec).11

    Statistical analysis

    Statistical analyses were conducted using PASW (formerly SPSS) software for Windows V.18.0 (SPSS, Chicago, Illinois, USA). We used the analysis of variance and Pearson χ2 tests to compare groups classified based on preoperative near stereoacuity or changes in near stereoacuity after surgery. Logistic regression analysis was used to investigate factors influencing preoperative near stereoacuity, first using an univariate model, then with a multivariate model that included variables with p<0.10 in the univariate model. The relationship between motor fusional state and preoperative near stereoacuity was analysed by the χ2 test and linear-by-linear association. p Values<0.05 were considered to be statistically significant.

    Results

    Patient characteristics and demographics are depicted in table 1. Nineteen patients showed poor stereopsis (>200 arcsec, 13.9%), 62 patients had moderate stereopsis (80–200 arcsec, 45.2%) and 56 patients had good stereopsis (40–60 arcsec, 40.9%). Overall follow-up period was 17.9±6.41 months (12–42 months). The overall success rate for surgery was 71.5%. There was no statistically significant difference in the success rate among groups (p=0.291).

    View this table:
    Table 1

    Preoperative and postoperative characteristics of patients with intermittent XT among groups classified by near stereopsis

    Patients in all three groups were not significantly different in any of the parameters measured at the time of the preoperative examination, except for age when the exodeviation was detected, age at the time of surgery and best-corrected visual acuity (BCVA). The average ages at the time of IXT diagnosis as well as at the time of surgery were greater in the group of patients with superior stereopsis than other groups (p=0.027, 0.042, respectively). Those with superior stereopsis also had significantly better visual acuities (p<0.005) (table 1, figure 1).

    Figure 1
    Figure 1

    The distribution of age at surgery, age at first detection and best corrected visual acuity (BCVA) among three groups. The patients with intermittent exotropia with better stereopsis were significantly younger at the period of first detection (1) and surgery (2). (p=0.027 and 0.042, respectively). The patients who had better stereopsis showed better BCVA with significant statistical difference (3). (p=0.005).

    A significantly larger percentage of patients in the good and moderate preoperative near stereoacuity groups than in the poor preoperative near stereoacuity group exhibited superior near sensory fusional status, as measured using the Worth 4-dot test (p=0.045). In all groups near sensory fusion improved postoperatively.

    Among those with poor and moderate preoperative near stereopsis, the majority (68.4% and 62.9%, respectively) achieved greater levels of near stereoacuity after the surgery. The deterioration of stereopsis was observed more frequently in the good preoperative stereopsis group (25%) than in the moderate and poor groups (6% and 0%, respectively) (p<0.001). Overall, the preoperative and postoperative stereoscopic statuses were similar in each group.

    We compared the OR between groups (good vs poor stereopsis) using univariate and multivariate models to investigate the factors affecting preoperative near stereoacuity (table 2). In the univariate model, age at the time of surgery and at the time of diagnosis showed a statistically significant association with preoperative near stereoacuity (OR=1.36, 95% CI 1.01 to 1.84; OR=1.30, 95% CI 1.02 to 1.65, respectively). The patients who showed better BCVA demonstrated a tendency to have superior near stereopsis (OR=0.02; 95% CI 0.00 to 0.54). In the multivariate model, all factors examined exhibited similar trends when analysed using either the univariate model or the multivariate model. The factors with borderline statistical significance were age at the time of diagnosis and BCVA (OR=1.23, 95% CI 0.96 to 1.58; OR=0.05, 95% CI 0.00 to 1.60, respectively).

    View this table:
    Table 2

    Factors affecting near stereoacuity in patients with intermittent XT

    The preoperative far motor fusional control was similar among all groups (p=0.172, figure 2) and had no correlation with near stereoacuity (p=0.701). However, subjects in the group with superior near stereoacuity also had superior motor fusional control at near stereoacuity (p=0.044, figure 2).

    Figure 2
    Figure 2

    The preoperative far and near motor fusional states among subjects with intermittent exotropia classified by near stereoacuity (1) There was no correlation between near stereoacuity and far motor fusional state (p=0.701) and no statistically significant difference among groups (p=0.172). The far fusional state was worse than the near fusional state in patients with intermittent exotropia in any group. (2) Although no statistically significant difference of near motor fusional state was seen among groups divided by near stereoacuity (p=0.373), there was a statistically significant positive correlation between near stereoacuity and near motor fusional state (p=0.044).

    After the intervention, the authors classified patients with IXT as improved, maintained or deteriorated, and then analysed the factors associated with postoperative changes in near stereoacuity (table 3). Near stereoacuity had improved in 42% of patients, remained the same in 45% and deteriorated in 13%. The 39 patients (63%, 39/62) with moderate stereopsis and 13 patients (68%, 13/19) with poor stereopsis showed improvement of near stereoacuity after surgery, and majority (66%, 39/56) of patients with good preoperative stereopsis maintained their stereoacuity. There was no other meaningful difference among the groups. Logistic regression in a univariate model showed that none of these factors was significantly associated with improved near stereopsis.

    View this table:
    Table 3

    Comparisons among patients with intermittent XT classified by changes in near stereoacuity after strabismus surgery

    Discussion

    As reported previously, the degree of near stereoacuity observed preoperatively was predictive of that attained postoperatively.3 ,9–11 ,16 These findings suggest that the level of near stereoacuity measured preoperatively determines postoperative near stereoacuity. Our result exhibited similar tendencies, but many patients showed postoperative improvements in near stereoacuity and near sensory fusional status, especially in the moderate (63%) and poor (68%) stereopsis groups (table 2). It looks like proper intervention for IXT improved the stereoacuity even in the poor preoperative stereoacuity group. In the good preoperative stereoacuity group, their stereoacuity would not to be expected to improve, because the definition of good stereoacuity was already high grade (40–60 arcsec). Furthermore, we also found an association between age at the time of diagnosis with IXT and BCVA for preoperative near stereoacuity (figure 1).

    Among patients with infantile XT or early-onset XT (XT onset before 1 year of age), 0–30% of patients exhibited good stereopsis among previous reports.17–21 Choi and Kim21 suggested that early-onset XT could be divided into primary infantile XT and early-onset IXT, based on the results of stereopsis testing. Yam et al20 reported that patients with infantile XT had significantly poorer stereoacuity than those with IXT. Those results suggest that the ability to maintain tenuous fusion and the onset of XT strongly affect the development of stereoacuity. We therefore hypothesise that patients unable to maintain tenuous fusion are more likely to be diagnosed early and exhibit reduced stereoacuity.

    Furthermore, poor BCVA leads to a blurred image which may render fusion tenuous, contributing to poor stereopsis. Adequate interventions in patients with IXT could maintain or even improve stereopsis by correcting the misalignment and assisting in overcoming the low plasticity potential.

    However, there are controversies surrounding the changes of stereoacuity according to age or after surgery.22–24 Some authors have suggested early intervention showed a high incidence of the postoperative monofixation syndrome in patients with IXT.22 ,23 However, our results revealed that 13.9% of patients showed poor preoperative stereoacuity (worse than 200 arcsec) and patients with earlier-onset IXT showed poorer preoperative stereoacuity. It could mean patients who showed monofixation after early intervention might have poor preoperative stereoacuity due to early-onset IXT, which looks like early intervention induces postoperative monofixation.

    Previous studies have reported the proportions of patients with IXT classified based on the degree of Titmus near stereoacuity tests; 14–49% patients showed bifixation and 27–47% showed monofixation.2 ,5 ,23 Our investigation had similar results despite the use of slightly different criteria compared with these previous studies. Morrison et al,11 whose criteria was similar to ours, reported that a much larger proportion of patients (44.2%) showed poor stereopsis preoperatively (400 arcsec or less). The variability of stereoacuity tests and age distribution of each study could make the difference in proportions among studies.

    One limitation of this report was the retrospective design of the data collection, which was standardised, nonetheless. Furthermore, stereoacuity is variable over time in patients with IXT, which may have affected the validity of our conclusions.2 ,5 We therefore measured near stereoacuity at least twice, chose the superior value and excluded the patients less than 5 years to minimise the chance of misclassification. The Titmus stereoacuity test provides the subject with monocular cues,25 so near stereopsis from 80 s to 200 s of arc was considered as moderate in order to avoid overestimating near stereopsis. Nonetheless, moderate stereopsis and poor stereopsis were sometimes difficult to distinguish. The use of other types of near preoperative stereoacuity tests would be necessary for accurate classification. Despite the relevance of distance stereoacuity in a study such as this, no such test was available at our centre.

    In conclusion, classifying IXT based on the degree of near stereopsis offers a useful tool for the assessment of initial sensory status as well as postoperative prognosis. The patients who showed poor stereopsis (13.9%) at near stereoacuity achieved similar sensory outcome after surgery compared with the preoperative state. It appears that age at the time of diagnosis and BCVA can be used to predict the quality of an individual's near stereoacuity. Additional large, prospective studies are needed to elucidate the connection between these factors and near stereoacuity.

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    Footnotes

    • Contributors All authors contributed towards planning, data collection, analysis and reporting the work in this study.

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

    • Competing interests None.

    • Patient consent Obtained.

    • Ethics approval The Ethics Committee/Institutional Review Board of Seoul National University Hospital.

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